Displays

The High Agri Valley (HAV) is an axial zone of the Southern Apennines thrust belt chain with a strong seismogenic potential as shown by different stress indicators and space geodesy data that suggest an NE-SW extensional stress regime still active. Moreover, the HAV hosts the Europe’s largest onshore oil and gas field, which give it further strategic importance.

There is a certain ambiguity concern the causative fault of the large event (M=7.0) occurred in 1857 in Agri Valley, although two well-documented fault systems are recognised as potentially seismogenic: the Monti della Maddalena Fault System (MMFS) and the Eastern Agri Fault System (EAFS).

With the aim to bring new information on identification and characterization of the principal structures, on the fluids distribution and their possible relationship with the developed of kinematics in upper fragile crust, several multiscale and multidisciplinary surveys are currently running in the HAV. Here we present the first results of a 3D Magnetotelluric (MT) investigation composed of 58 MT soundings in the period range [10^-2 Hz, 10³ Hz] which cover an area of approximately of 15 km x 30 km. All the 3D results were obtained by using the 3D inversion conde ModEM: Modular EM Inversion Software.

The work carried out so far has been mainly focused on the definition of the best mesh to adopt, both in terms of cell size and orientation, and on the correct choice of the inversion parameters: resistivity of the starting model, smoothing model parameter, minimum error floor attributed to the data, regularization parameter (trade-off).

The 3D MT preliminary model obtained shows a good agreement with 2D models previously realized using a part of the same dataset and defines the main geo-structural features of the HAV.

The resistivity variations in HAV subsurface will be jointly interpreted with accurate seismic data collected by seismic broadband network INSIEME (composed by 8 stations distributed throughout the Agri Valley) and other available geophysical and geological data.

The interconnection between the conductivity and seismicity information will be useful to implement the knowledge on the role that fluids play in fault zones and in earthquake processes. 

How to cite: Pastoressa, A. E., Balasco, M., Ledo, J., Queralt, P., Romano, G., Siniscalchi, A., and Tripaldi, S.: Three-dimensional Magnetotelluric Crustal Model of High Agri Valley seismic area to identify and to quantify the resistivity variation in depth , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21279, https://doi.org/10.5194/egusphere-egu2020-21279, 2020.

We monitor the relative variations of crustal velocity during a stop of water injection at the Val d’Agri oilfield (Italy) in January-February 2015 from the analysis of the ambient seismic noise cross-correlations. This technique allows the continuous estimations of the relative velocity variations occurred in the superficial layers of the Earth crust independently from the earthquake occurrence. Our results show a relative decrease in seismic velocity of about 0.08%, detected seven days after the injection restart of fluids injection and can be compatible with an increase of fluids in the medium. We estimate the medium diffusivity from this delay time obtaining a value of about 2.0 m²/s. Independently, we compute diffusivity from the observed delay time of small-magnitude (M_L ≤ 1.8) seismicity induced by the first injection tests in June 2006, finding a similar value. The high diffusivity values found from the two independent analysis are compatible with the hydraulic properties of the hydrocarbon reservoir. Finally, we estimate the spatial distribution of the observed variations finding that the largest changes are located in the North-West direction, where the oilfield is elongated. Our results show that fluids propagate efficiently from the wellbore in the reservoir direction through a strongly fractured medium following efficient hydraulic pathways, and that the noise-based monitoring technique adequately map in time and space this perturbation.

How to cite: Berbellini, A., Zaccarelli, L., Morelli, A., Faenza, L., Garcia-Aristizabal, A., Improta, L., and De Gori, P.: Evidences of high diffusivity near a waste water injection well in the Val d’Agri oil field (Italy) from noise-based monitoring, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7061, https://doi.org/10.5194/egusphere-egu2020-7061, 2020.

         Anomalously high seismic P- to S-wave velocity ratios (Vp/Vs) have been observed in subduction zones, in locations where varieties of earthquakes and slips are expected to occur. From qualitative laboratory knowledge of rocks Poisson’s ratio, these results were interpreted as evidence of near-lithostatic pore fluid pressure. Because most laboratory data did not document such high Vp/Vs values, these were further linked to additional constrains of anisotropy or the dominance of minerals of very high intrinsic Vp/Vs, e.g. mafic rocks.However, does high Vp/Vs necessarily imply anisotropy and/or mafic composition?

         Recently, the measuring frequency (f) was shown to play a major role on rocks’ resulting Poisson’s ratio, so that usual laboratory results (at f = 1 MHz) might not directly transfer to field ones (at f = 1 Hz). From this consideration, we investigate Vp/Vs of a variety of crustal rocks in the elastic regime relevant at the field scale, the undrained elastic regime.Accounting for rocks dispersive properties, this work aims to show that:

• In the laboratory, in isotropic rocks, one might attain Vp/Vs values as high as the anomalous ones observed in subduction zones.
• No mineralogical control is needed for such high Vp/Vs values, which could be consistent with the inherent mineral variability in different settings across the globe.
• High pore fluid pressure is a major parameter, but not alone: such high values cannot be achieved without very high degree of micro-fracturing of the rock, opened by high fluid pressures, an information of potential importance to understand those seismogenic zones.

How to cite: Pimienta, L., Schubnel, A., Fortin, J., Guéguen, Y., Lyon-Caen, H., and Violay, M.: Anomalous Vp/Vs in highly pressurized rocks: Evidence for anisotropy or mafic composition?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15845, https://doi.org/10.5194/egusphere-egu2020-15845, 2020.

Understanding how the permeability of a fault evolves during injection induced fault reactivation process is of great interest. The interactions between fluids and faults can be complex, as the confining pressure, effective stress and shear slip can affect the hydro-mechanical properties of the fault. The relationship between induced slip (reactivation) front and fluid front requires a better understanding of what controls hydraulic diffusivity as well.
In this study, we investigate shear induced fluid flow and permeability enhancement during fracture shearing. We used a series of laboratory injection reactivation tests on saw cut Andesite rock sample, under triaxial conditions, at different confining pressures (30, 60 and 95 MPa). The sample was connected to two pressure sensors, at two opposite locations of the fault, and equipped by strain gauges along strike.
We thus propose a numerical method, in the context of deterministic and probabilistic inversion approaches, that allows to estimate the temporal evolution of the effective hydraulic diffusivity (and its associated uncertainties) of an experimental fault throughout an injection test, using the pressure history at two points on the fault.
The numerical method was able to reproduce the experimental data for a wide time range of the different experiments. The hydraulic diffusivity was found to largely depend on the confining pressure and to increase (by one order of magnitude) throughout the injection experiment with the reduction of the mean effective stress acting along the fault plane. As well, the shear slip was observed to have an effect on the hydraulic diffusivity evolution. Instantaneous short term diffusivity enhancement accompanied slip events with large slip velocities, while long term increases accompanied slow slip events.

How to cite: Almakari, M., Chauris, H., Passelègue, F., and Dublanchet, P.: Induced Fault Reactivation and Hydraulic Diffusivity Enhancement : Insights from Pressure Diffusion Inversion in Laboratory Injection Tests, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19708, https://doi.org/10.5194/egusphere-egu2020-19708, 2020.

The stability of a critically stressed fault depends on the surrounding stresses acting on it. Fluids, by reducing the effective normal stress, play a major role. It has been observed that in karstic regions, an increase in groundwater pressure following significant recharge (precipitations and/or seasonal snowmelt) can result in a fault re-activation, inducing microseismicity. This study combines the natural microseismicity and the groundwater level fluctuations observations to estimate the fault criticality. The research is carried out on two major strike-slip faults in the folded Jura in Switzerland – La Lance Fault and La Ferrière Fault – most likely critically stressed according to their position in the global stress-regime. Data acquisition mainly consists in hydrogeologic and seismic monitoring. The objectives are to have continuous discharge rates of the major karstic springs and to produce a seismic catalog for the area of interest. Combining both data sets will allow to determine relations between increasing spring discharge rates and low magnitude earthquakes and eventually to acquire a quantitative knowledge on what pressure change is affecting the fault’s stability. This knowledge will be used to develop a straighforward methodology to assess fault criticality.  In addition, the study of a possible time lag between aquifer response and fault activation, as well as back-analysis of seismic events can provide, respectively, important information about the deep-seated fluid circulation and the local stress-regime.

How to cite: Perrochet, L., Preisig, G., and Valley, B.: Assessing fault criticality using seismic monitoring and fluid pressure analysis , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3446, https://doi.org/10.5194/egusphere-egu2020-3446, 2020.

In northeastern Japan, low-frequency earthquakes (LFEQs) occur preferentially at depths from the lower crust to the uppermost mantle near the active volcanoes. Many researchers have suggested the contribution of geofluid to the occurrence of these unusually deep LFEQs. Recent observations show that relatively low-frequency earthquakes occur even in the upper crust as well. Investigation of the generation mechanism of shallow LFEQs is quite important because it is directly related to the mechanism of closely located high-frequency earthquakes in the brittle upper crust. One of the areas of enhanced shallow LFEQ seismicity is the aftershock zone of the 2008 Iwate-Miyagi Nairiku Earthquake (Mw 6.8) located to the west of the 2011 great Tohoku earthquake. We detected LFEQs by using the frequency index (FI) defined by the logarithm of a ratio of high- and low-frequency spectral amplitudes. We used 2–4 Hz and 10–20 Hz bands for low- and high-frequency ranges. We analyzed more than 4000 events observed by a dense temporary seismic network deployed just after the occurrence of the mainshock. Our detection revealed that there are five LFEQs dominant clusters in the aftershock zone trending NNE-SSW with a length of about 40 km: the northern and the southern edge of the aftershock zone, to the north of the mainshock epicenter, the eastern and western edge of the central aftershock zone. In the area near the mainshock epicenter, hypocenter distribution shows two planes: mainshock fault dipping to the west and a conjugate fault dipping to the east. The previous study has shown that the events with N-S trending largest principle stress axis are distributed along the conjugate plane. In contrast, the events along the mainshock fault have E-W trending largest principle axis that is consistent with the relative motion of the subducting Pacific plate beneath the Tohoku region. The former anomalous groups are interpreted to be caused by local stress change by the mainshock applied to a neutral stress field with high pore pressure suggested by high Vp/Vs ratio. An interesting feature is the preferential distribution of LFEQs along the conjugate plane. Also, the hypocenter of LFEQs migrated with time from deeper to the shallower part of the plane. These observations strongly suggest that the existence and movement of geofluid are responsible for both the unusual stress field and the occurrence and migration of LFEQs. The location of LFEQs at the northern and eastern edge of the aftershock zone is close to the areas of postseismic slip detected by GNSS observation, which is suggestive of the increased pore pressure in the area. The LFEQs at the southern and western edge of the aftershock zone occur in calderas, suggesting that these LFEQs occur in hotter and/or fluid-rich areas where the ductile deformation occurs. Thus, though the interpretation of the cause of LFEQs is not unique, the distribution of LFEQs plays a crucial role in understanding the contribution of geofluids not only to the seismogenic processes of aftershocks but to the faulting mechanism in the upper crust.

How to cite: Kosuga, M.: Spatial distribution of low-frequency earthquakes suggestive of geofluid among the aftershocks of the 2008 Iwate-Miyagi Nairiku Earthquake in northeastern Japan, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13654, https://doi.org/10.5194/egusphere-egu2020-13654, 2020.

The Yellowstone volcanic field, in western United States, is well known for intense seismic activity, abundant geothermal features and a violent volcanic history that includes a caldera-forming eruption 640 ka ago. Even though the recentmost eruption dates back to 70 ka ago, a very high seismicity, quasi-continuous surficial deformation through uplift and subsidence stages (at rates of up to 70 mm/yr) and intense hydrothermal activity are clear evidences of a still very active volcanic field. Thanks to a recently improved seismic network, here we analyze the rate of occurrence of 19’538 relocated earthquakes belonging to the temporal window between 1988 and 2016. Starting from this dataset, we identify and characterize the seismic swarm activity occurring in the study area after 2007. We also evaluate the analogies and differences of their seismic behavior through the analysis of frequency-magnitude distribution of seismic events. We investigate the identified seismic swarms clustered in space and time, their relation with active volcanic and tectonic processes and stress field variations caused by the migration of magmatic and hydrothermal fluids. Calculated b-values associated with the recentmost seismic swarms have been related to past swarms that occurred in the area, thus revealing the temporal and spatial evolution of such phenomena. Our study gives new crucial insights to understand the relation between seismic and magmatic activity in the Yellowstone volcanic plateau, with important implications for a better comprehension of the local seismic and volcanic hazards.

How to cite: Carbone, L., Russo, E., de Nardis, R., Lavecchia, G., Tibaldi, A., and Bonali, F.: New insights on temporal and spatial evolution of Yellowstone earthquake swarms: a multidisciplinary geological-seismological approach, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9809, https://doi.org/10.5194/egusphere-egu2020-9809, 2020.

The 2018 Lombok earthquake sequence, which took place ~10km to the north of the Rinjani volcano on the Flores thrust fault, are distributed beneath the northern coast of the island, composing of two Mw6.4 and two Mw6.9 earthquakes and numerous aftershocks. The first Mw6.4 earthquake was followed by the first Mw6.9 event in a week, which was located only a few kilometers to the west of the Mw6.4 event, characterized with strong westwards rupture directivity and multiple asperities (rougher source time function). Two weeks later, the second Mw6.4 event took place a few km to the east of the first Mw6.4 event and triggered the second Mw6.9 event 12 hours later. In contrast, the second Mw6.9 ruptured towards east with a single major asperity, with a centroid depth of ~18km, ~5km shallower than the first Mw6.9 event. The seismicity was well captured by 7 broadband stations and 6 short period nodes deployed just before the first Mw6.9 event, mostly concentrated within a depth range of 5km. Relocated seismicity shows shallower depth to the west and deeper to the east, in consistent with the coseismic rupture of the largest events. Aftershocks are shallowest below the volcano due to an elevated Brittle-Ductile-Transition (BDT) zone depth controlled by the thermal structure. A few anomalous earthquakes were identified between the M_w6.9 events below the BDT zone that could be related to the basaltic conduit of the volcano. Several sets of repeating earthquakes were identified and are mostly located in the rupture area of the first Mw6.9 event, indicating a highly heterogeneous friction on the fault that is probably caused by to the stronger thermal gradient compared with the second Mw6.9 event. The earthquake sequence highlights the strong interaction between the volcanic system and the tectonic faulting process. 

How to cite: Wei, S., Lythogoe, K., Muzli, M., Hugraha, A. D., Bradley, K., and zulhan, Z.: Fault geometry and rupture patterns of the 2018 Lombok earthquakes – complex thrust faulting in a volcanic retro-arc setting, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10012, https://doi.org/10.5194/egusphere-egu2020-10012, 2020.

Large continental earthquakes necessarily involve cascading rupture of multiple faults or segments (e.g. El Mayor-Cucapah 2010). But these same critically-stressed systems sometimes rupture in drawn-out sequences of smaller earthquakes over days or years (e.g. Central Italy 2016), instead of in a single large event. Due to the similarity in the initial conditions of both scenarios, seismic sequences may be considered as ‘failed’ multi-segment earthquakes, whereby cascading rupture is prematurely halted before all available slip deficit is released.

These two modes of strain-release have vastly different implications for seismic hazard. Recent work on the 2016 Central Italy earthquake sequence, which is the first seismic sequence to be studied with modern high-quality geodetic and seismological datasets, showed that complexity in fault structure appeared to exercise a dual control on both the timing and sizes of events throughout this sequence. However, it is unclear if this structural control is common for all continental seismic sequences, how important seismic sequences are for the global seismic moment budget, and how this contribution to moment budget may vary between different tectonic regions.

Here we select shallow crustal continental earthquakes from the Global Centroid Moment Tensor catalog, and identify seismic sequences as agglomerates of clustered pairs of earthquakes where the summed moment (M₀) of all aftershocks is greater than 50% of the M₀ of the first event in the sequence. We analyse the relative number of seismic sequences compared to other earthquakes for normal, reverse, and strike-slip faulting regions, and also calculate the relative M₀ release of seismic sequences and other earthquakes in these three regimes.

We find that although seismic sequences are equally common by number in all continental tectonic regimes, seismic sequences account for a much higher proportion of M₀ release for normal faults (~20%) than for reverse faults (~10%), with strike-slip faults intermediate between these two end-members. We also find that the proportion of M₀ release in seismic sequences is higher for events that occur in regions characterised by a diversity of different earthquake types (e.g. both reverse and strike-slip faulting) than for events that occur in regions characterised by a single earthquake type (e.g. strike-slip faulting only). Together these findings imply that complexity of fault network is an important factor in controlling the occurrence of large-M₀ seismic sequences, and that ‘failed’ multi-segment earthquakes and therefore large-M₀ seismic sequences are more likely to occur in regions with complex fault networks.

How to cite: Walters, R., Craig, T., Gregory, L., and Azad Khan, R.: The Global Importance of Continental Seismic Sequences, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13584, https://doi.org/10.5194/egusphere-egu2020-13584, 2020.

Swarms of microearthquakes on a network of conjugate strike-slip faults in the rift zone in Central Iceland have been detected and located using a dense local seismic network operational since 2007. These swarms have been recorded since the 1970s, but the cause of their clear swarm-like nature remains enigmatic.

We use the QuakeMigrate earthquake detection and location software – which is able to detect earthquakes separated by very small inter-event times – to produce a highly complete catalogue. Automatic hypocentre locations have been refined using waveform cross-correlation and double-difference relocation, and focal mechanisms and manual earthquake locations have been produced for a subset of events by manual picking. Analysis of the resulting high-resolution earthquake catalogue reveals systematic migration of hypocentres at velocities of ~ 1 km/day along sharply defined fault planes ranging from 1 – 10 km in length. In the majority of swarms we also observe clusters of identical repeating events, providing evidence for re-loading of the brittle asperities that produce earthquakes.

For a selection of swarms, our high resolution seismic observations are complemented by GPS and InSAR measurements, allowing us to constrain the amount of fault slip. Comparing this, and the area of the fault plane activated in the swarm, to the seismic moment release reveals a significant contribution of aseismic slip, or very low effective stress drop. Analysis of swarms triggered on these faults by the static coulomb stress increase induced by the 2014 Bárðarbunga-Holuhraun dike intrusion provides a further estimate of the amplitude of the stress cycle.

We combine our observations with comparisons to numerical & laboratory modelling studies, observed swarm scaling properties and knowledge of the geological and permeability structure of the Icelandic crust to determine the nature of the transient forcing driving these exceptionally well-recorded tectonic earthquake swarms.

How to cite: Winder, T. and White, R. S.: Slowly migrating tectonic microearthquake swarms in the Icelandic Rift Zone: driven by pore-pressure or aseismic slip transients?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19827, https://doi.org/10.5194/egusphere-egu2020-19827, 2020.

The Tjörnes fracture zone (TFZ) in N-Iceland is a seismically active zone with on average 4000 earthquakes detected annually since 1993 by the regional seismic network operated by the Icelandic Meteorological Office (IMO). Most of the earthquakes occur offshore and with only one seismic station on the Grímsey island north of Iceland, the seismic network detects earthquakes down to magnitude M-0.5. The fracture zone, essentially a transform between the northern volcanic zone of Iceland and the Mid-Atlantic Ridge north of Iceland, has three major segments; the Grímsey Oblique Rift (GOR) farthest to the North which accounts for 60% of the seismicity of the TFZ, the Húsavík-Flatey Fault (HFF) in the middle, where 38% of the TFZ earthquakes occur and the least active Dalvík Lineament (DL) farthest to the south (only 2% of TFZ seismicity). The IMO’s seismic catalogue clearly draws up the most active segments of the TFZ, where each extends laterally roughly 100 km. The largest earthquakes occur on the HFF where the accumulated seismic moment release is an order of magnitude higher than the GOR and three orders of magnitude higher than the DL.

There are other interesting differences between the segments. There are several known central volcanoes aligned along the GOR and the oblique rifting is likely to cause both tectonic and volcanic seismicity which shows up as a catalogue of many but similarly sized earthquakes, in other words a catalogue with a higher b-value than the neighbouring HFF. Despite these differences, seismic swarms, without a clear mainshock or aftershock sequences, counting thousands of earthquakes with a duration of a few days upto weeks, are recorded every 2-3 years both in GOR and HFF. In late March 2019, one of this seismic swarms took place on GOR, mostly on a single NNE-SSW striking fault near Kópasker. Relative earthquake locations draw the fault up nicely and in addition a few shorter faults with similar strike of 15°deg. The temporal evolution of the swarm shows an upwards migration and how the seismicity starts at the middle of the fault, jumps a little to the north and migrates in two days to the southern end of the fault over 7 km. When that point is reached, the largest earthquake in the swarm takes place, M4.2, however in the very northern end of the fault. The focal mechanism of this largest event shows a left-lateral strike-slip as do the smaller earthquakes. A b-value plot of the 2300 earthquakes that were recorded during the swarm reveal a value of 1.2, which is typical for volcanic seismicity. The size of active fault is considerable larger than expected from a M4.2 earthquake and the question rises if part of the motion is taken up as aseismic slip.

We will present examples of recent swarms in the TFZ along with new results of a cross-correlation study of the waveforms recorded during the swarm activity.

How to cite: Jónsdóttir, K., Guðmundsson, G. B., Passarelli, L., Jónsson, S., Cubuncu, Y., Lecocq, T., Caudron, C., and Rodriguez Cardozo, F.: Recent seismic swarms in the Tjornes fracture zone, N-Iceland, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19267, https://doi.org/10.5194/egusphere-egu2020-19267, 2020.

The Central-Southern Apennines of Italy are a region with high seismic risk zone and experienced destructive earthquakes both in historical and in instrumental time. Geological data and historical seismicity indicate that the fault structures in this area are able to produce earthquakes with magnitude greater than 6.5. In fact the sector, stretching between the Irpinia 1980 (Mw 6.9) and the Accumuli-Visso-Norcia 2016 (Mw 6.5) seismic sequences, was struck by eleven events (MW ≥ 6.5) among the largest historical and early-instrumental earthquakes  since 1349. On the contrary, if we exclude the Barrea seismic sequence occurred in 1984 (Mw 5.9), the instrumental catalogue shows that this area is predominantly characterized by a low background level of seismicity and by earthquake clustering characterized by low release of strain energy.

We analyzed the seismicity occurring in this area from 1985 to 2018 (0.0 ≤ ML ≤ 5.0)  and  by a declustering algorithm  we indentified a set of 45 spatio-temporal clusters where the earthquake number stem out significantly from the background seismicity rate. The background seismicity (6196 events, 0.0≤ML≤4.1) is characterized by a b value of 0.96 ± 0.4, a magnitude of completeness of 1.4 and it is strictly controlled by known fault patterns. The earthquake clusters accounts for a non-negligible (45%) part of the total seismicity. A close inspection to the individual clusters allowed us to identify 4 seismic sequences characterized by isolated mainshock-aftershocks behaviour and 41 tectonic earthquake swarms (TESs). TESs have a duration ranging 2-12 days, 2.5-3.0 characteristic magnitude and 1.2 km/d migration rate. They are constituted by mono and/or polyphase episodes and they do not show a spatial complementary along the system of activated fault rather they are often spatially overlaid occupying the same fault segment. The latter behavior seems to indicate TES occurrences be driven by an underlying transients loading of the fault faster than the few mm/yr long-term extension active along the Apennines chain. The best candidates to explain these transients are likely presence of pressurized fluid abundant and/or  possible small scale creeping. The focal mechanisms and the depth of foci well correlate with the mapped normal fault systems and TESs illuminate regions of these faults adjoining ruptures of past large earthquakes. The spatio-temporal distribution of TESs suggests that the system of faults in the southern and central Apennines is characterized by heterogeneous rheology where small fault patches systematically release strain through TESs and other parts are to date locked. These findings are of great importance to better improve models for the assessment of seismic risk in the area.

How to cite: de Nardis, R., Carbone, L., Pandolfi, C., Passarelli, L., and Lavecchia, G.: Quantitative analyses of background seismicity and earthquake clustering in extensional seismogenic settings - case studies from the Central-Southern Apennines of Italy, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9480, https://doi.org/10.5194/egusphere-egu2020-9480, 2020.

Earthquake hypocenter migration is the most characteristic pattern which indicates aseismic processes triggering the observed seismicity. These processes can involve creep, fluid migration or similar. While interactions among earthquakes can also lead to some expansion of seismic clouds, these expansions are rather small and not comparable to migration patterns related to pore-pressure diffusion, slow slip events, or growing hydraulic fractures. Thus, identification and modeling of migration patterns, which has not been studied in detail, is important for the characterization of fault dynamics.

Advance of the triggering front is usually analyzed using distance-time plots that show the time dependence of the distance of individual events from the origin. If event order is used instead of time as the argument on the horizontal axis, event migration is analyzed in dependence on the seismic activity itself, which brings a new view to the running seismicity. We applied this approach to the relocated earthquake swarm catalogs from West Bohemia, California and Iceland and found a striking linear growth of the triggering front. This indicates that the advance of the front is likely to be driven by the rupture of individual earthquakes rather than by the running time. It also turned out that the growth velocity measured in meters per event increases with the magnitude of the data set.

Using the basic concepts of earthquake physics, we propose the relation of the growth velocity on earthquake magnitudes and compare it with measurements on the analyzed swarm catalogues. We show that the spreading velocity of the triggering front is closely related to source parameters, which gives hints to the understanding of the background mechanism.

How to cite: Fischer, T. and Hainzl, S.: Migration patterns of earthquake clusters, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10865, https://doi.org/10.5194/egusphere-egu2020-10865, 2020.

Aftershock cascades and aftershock zones play an important role in forecasting seismic activity in both natural and human-made situations. While their behavior including the spatial aftershock zone scaling has been the focus of many studies in tectonic settings finding, for example, long-range earthquake-earthquake triggering in the near-field, this is not the case in situations where the seismic activity is primarily driven by fluids and the diffusion of excessive pore pressure. Here, we probe three different seismic settings that are believed to be influenced by fluid diffusion. The natural swarm in i) the Long Valley Caldera and the suspected swarms in ii) the Yuha Desert, both located in California, and associated earthquake-earthquake triggering behavior are compared against induced seismicity related to large scale wastewater disposal in iii) Oklahoma and southern Kansas. All settings exhibit a significant amount of event-event triggering highlighting the importance of secondary processes for the overall seismicity. We find an almost identical temporal event-event triggering behavior including the Omori-Utsu relation and the associated productivity relation. In terms of the spatial triggering density, both cases i) and iii) show a rapid decay beyond their rupture length. This proves that narrow spatial “aftershock” zones are not specific to induced seismicity but also occur in natural settings. Typical of most tectonic settings, a relatively long-range behavior is observed in case ii) suggesting that fluid migration might not be the dominant driving mechanism of the seismic activity and/or that the underlying structure of the fault network may control the secondary earthquake-earthquake triggering and its spatial evolution.

How to cite: Karimi, K. and Davidsen, J.: Earthquake-earthquake triggering in natural swarms and fluid-induced seismicity, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10497, https://doi.org/10.5194/egusphere-egu2020-10497, 2020.

The aim of this study is to identify precursors related to fluid pressure changes at depth to large earthquakes that occured in central Italy such as the 2009 Mw6.3 L'Aquila and the 2016 Mw6.2 Amatrice earthquake. To that end, we monitor the temporal evolution of the crust using a new method called Coherence of Correlated Waveforms [CCW] that uses seismic noise autocorrelation measurements. This allow us to look for changes in the medium with a high temporal resolution of 5 days. Our measurements of the CCW show that the L'Aquila Earthquake (2009-M6.3) is preceded by a 150-day oscillation whose amplitude and frequency progressively increases until the rupture. The high Vp/Vs ratios measured on the foreshocks of L’Aquila earthquake correspond to the CCW drop periods, suggesting the sensitivity of the measurement to crusty fluids.

Analysing 17 years of data, we found that this signal occurred only before the L'Aquila and the Amatrice earthquakes. This suggests the existence of a unique nucleation process.

Finally, for the 2016 Amatrice Earthquake, using an array of 25 seismic stations we are able to map the geographical extension of this precursory signal. This pattern, evolving over time, suggests diffusion phenomena in the upper crust.

How to cite: Delouche, E., Stehly, L., and Voisin, C.: Seismic Precursors in Central Italy using Autocorrelation Measurements, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20892, https://doi.org/10.5194/egusphere-egu2020-20892, 2020.

In 2014 t In the framework of the MEDSUV project, the Solfatara area, part of the Campi Flegrei caldera, was chosen as test site for the RICEN (Repeated InduCed Earthquakes and Noise) experiment mainly oriented to the use of seismic waves (both in passive and active mode) as a diagnostic tool to investigate the changes in the properties of the medium at small scales. Besides the study of seismic waves (both in passive and active mode), part of the RICEN activities was focused on the detection and characterization of the Seismo-Electromagnetic signals (SES) associated with their propagation.

Considering the abundance of fluids that characterize the shallow hydrothermal system of Solfatara area, SES arewere expected to be detectable and informative of the subsoil structure. On May 2014, Hence for their detection during RICEN experimthree magnetotelluric (MT) stations were installed outside the seismic grid and close to the main volcanic fumaroles in the Solfatara area. Thusing electrical and magnetic components concurrently with seismic and geochemical measurements were recorded. As a result, SES related to Vibrosesis seismic source energization in 100 sites, distributed on an almost regular grid on an area of about 115m x 90m, were analyzed. The m.

Although Unfortunately the electrical part of the SES could not be extracted by the recorded time series due to the severe effects of the Solfatara volcanic environment on the unmpolarizable electrodes. ,Converselyagnetic components, instead of the electrical ones, were generally better appreciable, in terms of amplitude, with respect to the natural electromagnetic fluctuations(s?). This circumstance allowed to verify the strict causality relationship between of the SES with and seismic signals for interstation distances (Seismic source -MT stations) ranging between 100 m and 200 m..

Focusing on the magnetic part, a comparative Wavelet analysis on SES and on seismic source permitted to evaluate that in the time domain that SES signals are mainly associated with Rayleigh wave, due to relatively large average distance between shot and MT sites, ranging between 60 m and 200 m.

Once defined SES characteristics, in terms of frequency content and amplitude, possible information on the subsurface status and new inferences on fluids characterizing the subsoil of the studied area were obtained. This was possible by investigating the spatial distribution of SES amplitude and by comparing it with 3D model of Vp, Vs and resistivity as well as with temperature and CO₂ flux maps.

How to cite: Siniscalchi, A., Balasco, M., Romano, G., and Tripaldi, S.: Study of Seismo-Electromagnetic signals in an area characterized by an intense hydrothermal activity: a case study from the Solfatara area (Campi Flegrei, Italy), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19048, https://doi.org/10.5194/egusphere-egu2020-19048, 2020.

After the occurrence of the 1999 magnitude 7.3 Chi-Chi earthquake, a cluster of NE-SW trending earthquakes,
almost along the surface trace of the Lishan fault, has been detected in the northern portion of the Central Range
in northern Taiwan. From the spatiotemporal distribution of hypocenters based on cluster analysis, the Lishan
fault cluster (LFC) can quantify the evolution of seismicity as aftershocks of the 1999 Chi-Chi earthquake. The
results of seismic tomographic inversion indicate that the LFC extends down to about 10 km depth and seems
to be distributed in high Vp areas rather than in low Vp areas. This temporal expansion is attributed to fluid
diffusion. Seismic activity in the upper crust tends to be high above broad zone with low Vp in the lower crust. Our
tomographic images demonstrate a series of relatively high Vp/Vs anomalies dipping to the east which seems to
form a fluid upwelling conduit beneath the Central Range. We thus suggest that the Lishan Fault might play a role
of an active fluid conduit, fluid or fluid fluxed a partial melt of the Philippines Sea plate would be released along
the east-dipping conduit and rise gravitationally to the upper crust.

How to cite: Cheng, W.-B.: Tomographic imaging of a seismic cluster in northern Taiwan and its implications for crustal fluid migration, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2006, https://doi.org/10.5194/egusphere-egu2020-2006, 2020.

We investigated temporal changes in the waveform of wave packets in S-coda associated with a swarm-like earthquake sequence, and estimated the original location of the wave packets via an array analysis. The earthquakes are located around the Moriyoshi-zan volcano in northeastern Japan, and were triggered by the 2011 off the Pacific coast of Tohoku earthquake, forming the largest cluster to the north of the volcano. A notable feature of seismograms from the triggered earthquakes is the appearance of the distinct scattered wave-packets (DSW) that are S-to-S scattered waves from the localized strong heterogeneity in the mid-crust. The DSW appear about 2–3 s after the onset of S-wave with a dominant frequency of 8–24 Hz and with a duration of around 1 s. Furthermore, the DSW show the variation in their shapes even in the roughly near events. 
To investigated the variation of DSW in detail, we first grouped events in the largest cluster with short inter-event distances and high cross-correlation coefficients (CC) in the time window of direct waves. Then we focused on the DSW part. Even in the same group, DSW showed temporal changes in their amplitudes and shapes. The change occurred gradually in some cases, but temporal variation were much more complicated in many cases. For example, the shapes of DSW changed from unclear peak to clear double peaks and suddenly back to the unclear. We also found that the shape of DSW changed in a very short time interval, for example, within ~ 12 h. 
Next, we estimated the location of DSW origin by applying the semblance analysis to the data of the temporary small-aperture array deployed to the north of the largest cluster. The DSW origin is located between the largest cluster within which hypocentral migration had occurred and the low-velocity zone depicted by a previous tomographic study. These observations imply the existence of crustal fluid and the DSW origin was composed of crustal fluid accumulated midway in the upward fluid pathway from the low-velocity zone to the earthquake cluster. 
Though we could not entirely exclude the possibility of the effect of the event location and focal mechanisms, the remarkable temporal changes in DSW waveforms possibly reflect the temporal changes in and/or near the origin. The short term change in DSW implies that fast movement of crustal fluid can occur at the depth of the mid-crust. 

How to cite: Amezawa, Y., Kosuga, M., and Maeda, T.: Temporal changes in the distinct scattered wave packets and their origin associated with triggered earthquake swarm beneath the Moriyoshi-zan volcano, northeastern Japan , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2243, https://doi.org/10.5194/egusphere-egu2020-2243, 2020.

We show the results of a multidisciplinary study on hydrologically-induced deformation in the Southern Alps (Italy) developed integrating geodetic, seismological and hydrological observations. The study region, located across the Belluno Valley and the Montello Hill, is part of the Adria-Eurasia boundary, where ~1 mm/yr of N-S shortening is accommodated across a S-verging fold-and-thrust belt. GNSS time-series show the occurrence of non-seasonal horizontal transient displacements, characterized by a sequence of extensional and contractional deformation episodes oriented along the direction of the tectonic shortening. This signal is temporally correlated with water storage changes that are estimated using a lumped hydrological model based on precipitation, temperature, potential evapotranspiration and Piave river flow measurements. Geodetic and hydrological information are integrated in a 2D mechanical model with the goal of defining possible geological structures responsible for the measured subcentimetric geodetic displacements. Our interpretation implies that precipitation water rapidly penetrates the epikarst developed at the hinge of the anticline associated with the Bassano-Valdobbiadene thrust, converging toward a sub-vertical, deeply rooted hydrologically-active fracture (associated with its back-thrust), which tend to focus groundwater fluxes and pressure changes, generating ground displacements. Accordingly, seismic velocity changes computed from the analysis of ambient seismic noise cross-correlation show a temporal (anti) correlation with the evolution of water storage changes, suggesting that fluid increase in the aquifer perturb the Earth crust at depth by decreasing the seismic velocity (and vice-versa, during water storage decrease phases). Finally, by analyzing the seismicity recorded between 2012 and 2017 by a local network using a covariate model, we found that seismicity rates from a cluster of background seismicity correlate with changes in water storage. Although a spatial correlation between these seismic events and Coulomb stress changes associated with transient deformation episodes is not clear, it is worth noting that our model suggests stress perturbations of the order of 5-10 KPa down to 5-10 km of depth.

How to cite: Pintori, F., Serpelloni, E., Longuevergne, L., Almagro-Vidal, C., Zaccarelli, L., Garcia-Aristizabal, A., Faenza, L., Belardinelli, M. E., and Bragato, P. L.: Groundwater changes affect crustal deformation, elastic properties and seismicity rates in the Southern Alps (Italy), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15110, https://doi.org/10.5194/egusphere-egu2020-15110, 2020.

With the development of shale gas in the Changning-Zhaotong play in the southern Sichuan basin of China, which is the largest shale gas prospect in China, the frequency and magnitude of earthquakes in this region have increased significantly in recent years. For example, a M5.7 earthquake occurred on December 16, 2018, and a M5.3 earthquake on January 6, 2019 in addition to many M4.0+ earthquakes in this area. Some studies argue the large magnitude earthquakes are triggered by hydraulic fracturing in for the local shale gas development, which commenced in 2011. The frequency of the earthquake occurrence has been on steady increase in the past few years that local residents often reported felt quakes. To further understand the correlation between the shale gas development and local seismic activity, we conducted a two-phase dense array seismic monitoring with about 200 Zland 3C and SmartSolo 3C 5 Hz seismic nodes, from late February to early May, 2019 for a period of 70 days. The survey consists of roughly 340 deployments at 240 sites, with an average interstation distance of 1.5 km, covering 500 km² in total. We have processed seismic records from late February to early April, 2019 (phase I), and picked some 600,000 P- and S-wave arrival times from 4385 detected local earthquakes. The earthquake hypocenters and the subsurface velocity structure of the Changning-Zhaotong area are inverted for using the double-difference tomography method. The relocation results show that the majority of hypocenters were located at depths ranging from 1.0km to 4.0km, in the proximity of the horizontal hydraulic fracturing wells. The tomographic results (< 3 km) correlate well with the known surface geological units, and most earthquakes occurred along the velocity discontinuities, likely characterizing a large hidden fault which, interestingly, is where the January 2019 M5.3 occurred. Our study is very important for understanding the seismic potentials in this area, and should provide useful information for the shale gas development in this region and other areas in China with similar geological, tectonic and stress conditions.

How to cite: Yang, W., Li, J., Tan, Y., Li, Y., Qian, J., and Zhang, H.: Microseismic and Induced Seismicity Monitoring and Tomography of the Changning-Zhaotong shale gas play in China using dense nodal arrays, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3505, https://doi.org/10.5194/egusphere-egu2020-3505, 2020.

This study focuses on the the active fault system that caused the 1980 M_S 6.9 Irpinia earthquake (Irpinia fault zone (IFZ)) that is presently interested by a continuous and frequent micro-earthquake activity occurring within the volume in the volume enclosed by two antithetic faults. It is therefore important to improve the knowledge of the IFZ dynamics, with reference to potential future occurrence of moderate to large earthquakes, especially in terms of earthquake triggering mechanisms. Several previous works evaluated the spatial distribution of elastic/anelastic fault-embedded medium properties and related rock physical micro-parameters in connection with the seismicity rate. These studies showed a spatial correlation between high Vp/Vs, low seismic attenuation in rock volumes where most of seismicity occurs, suggesting that fluid-driven pore-pressure changes may plays a key role in controlling the seismicity production at the IFZ.
Here we reconstruct accurate 4D seismic velocity images of the volume embedding IFZ which allows to detect and track space-time changes of medium elastic properties possibly induced by fluid pore pressure migration and investigate the related seismicity production.
We analyzed the arrival time phase catalogue of about ten years (2005-2016) of Mw < 3.1 events recorded by the ISNet (Irpinia Seismic Network) and INGV network. We divided the catalog in 5 not-overlapping epochs by selecting in each of them , approximately the same number of events and an uniform volume coverage, in order to ensure that the 3D P and S velocity models could be equally well resolved for each epoch. By comparing the Vp, Vs and Vp/Vs images at each epoch in the equally resolved volume, we are able to detect medium velocity changes. Some regions, in the first 6 km of depth of NE part, do not show velocity changes with time, which is interpreted as the main effect of unperturbed lithology mainly controlling the average seismic velocity. In other regions, in the central part of the model at about 8-10 km depth, we clearly detect velocity changes causing an up to 10% Vp/Vs variation between different epochs. Based on the rock physical modelling, we associate the time-varying Vp/Vs and the observed amplitude of variation to fluid-driven changes in rock physical properties related to their spatial migration or pore-pressure induced changes. The regions where large Vp/Vs changes occur appear correlated with the largest seismicity production volumes, suggesting a direct link between the physical processes associated with fluid mobility and/or pore pressure migration and earthquake generation at the IFZ.

How to cite: De Landro, G., Esposito, R., Ortensia, A., and Zollo, A.: Time-lapse tomographic images of the Irpinia Fault System (Southern Italy) reveal Vp/Vs ratio changes that correlate with micro-seismicity production and evolution, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-14615, https://doi.org/10.5194/egusphere-egu2020-14615, 2020.

Sub-surface operations for energy production may originate various environmental risks among which, of great relevance is the seismic risk due to the induced seismicity associated with field operations.

In the framework of the H2020 Science4CleanEnergy project, S4CE, a multi-disciplinary project aimed at understanding the underlying physical mechanisms underpinning sub-surface geo-energy operations and to measure, control and mitigate their environmental risks, we have investigated the role of fluids in the generation of the seismicity induced during the deep geothermal drilling project close to the city of St.Gallen, Switzerland. To this aim we applied the Focal Mechanism Tomography (FMT) technique and the velocity and attenuation tomography using data collected by the Swiss Seismological Service in 2013 while realizing well control measures after drilling and acidizing the GT-1 well. The dataset consists of 347 earthquakes with magnitude (M_L^corr) between -1.2 and 3.5. P and S phases were initially hand-picked on three-component ground velocity recordings. As an additional enhancement, a refined re-picking algorithm based on the waveforms cross-correlation was applied providing accurate travel-times data set. The revised picks and P polarities were used both to re-locate the events, using probabilistic approach considering both the absolute both the differential arrival times, and to estimate fault mechanisms using the FPFIT code. Only those events having at least 6 clear P-wave polarities have been analysed. To better constrain the focal mechanisms, for the larger magnitude events the BISTROP code (Bayesian Inversion of Spectral-Level Ratios and P-Wave Polarities) has been also applied.

Using the FMT technique we estimated the 3D excess pore fluid pressure field at the events hypocentre. Basically, the technique assumes that fault strength is controlled by Coulomb failure criterion and, under the hypothesis of uniform stress field, it ascribes the focal mechanism variations to pore fluid pressure acting on faults.

The velocity model and the attenuation model have been estimated by using an iterative tomographic inversion of P and S arrival times and t* quantities, which are defined as the ratio of the travel time and quality factor (Q). The t* measures for both P and S wave have been obtained from the analysis of the displacement spectra. We found that fault mechanisms do not fit a uniform stress-field. Based on the events depth, at least two different stress-fields are required. FMT results indicate that fluids contributed to the generation of the induced events. Taking into account for the uncertainties, the inferred excess pore fluid pressure is consistent with the wellhead pressure. Moreover, a correlation exists between the high excess pore fluid pressure and the high Vp/Vs values.

This work has been supported by S4CE ("Science for Clean Energy") project, funded from the European Union’s Horizon 2020 - R&I Framework Programme, under grant agreement No 764810 and by MATISSE project funded by Italian Ministry of Education and Research.

How to cite: Capuano, P., Convertito, V., De Matteis, R., Amoroso, O., Napoli, U., Massa, B., De Landro, G., and Donnarumma, G. P.: Seismic imaging of St Gallen (Switzerland) deep geothermal field medium properties, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8620, https://doi.org/10.5194/egusphere-egu2020-8620, 2020.

Fluids permeate and diffuse within the shallow crust being as originated by internal or external natural sources or by industrial activities for modern energy exploitation and production.

Fluid-induced stress changes can reactivate faults and cause earthquakes. In volcanic environments fluids play a key role in controlling the evolution of magmatic processes and eruption. The reliable imaging of fluid storages and accurate tracking of their movements is therefore critical in evaluating the nature and likelihood of future natural/induced earthquake or volcanic activity and their relative hazard monitoring and assessment.

The project FLUID has been recently approved by the Italiam Ministry for Research and has the ambitious goal to build up and experiment the next generation of deep ( crust and mantle-derived) fluid monitoring systems aimed at their timely detection and space-time tracking. This objective is achieved by developing and applying an integrated, multi-parametric and multi-disciplinary approach for mapping and tracking fluid movements in volcanic, tectonic and industrial exploitation, sub-surface, geological environments. Innovative methodologies and technologies will be developed to reconstruct the 4D (space and time) variations of rock properties in the fluid-filled porous medium and to detect and characterize fluid-triggered natural effects as well as the induced micro-seismicity, electric crustal properties changes, earth surface ground deformation and geochemical signatures of fluid presence and diffusion.

The project will develop activities and scientific products in the following research directions:

• Multi-parametric data acquisition and management aimed at data acquisition, integration and sharing/publishing for scientific and public information purposes;
• 4D multi-parametric crustal imaging aimed at setting up and testing different geophysical/geological methodologies to image the underground in space and time, and at comparing the obtained images for an effective and reliable tracking of fluids;
• Induced phenomena and/or triggered effects by fluid diffusion aimed at investigating fluid properties and movements, developing new methods & technologies for their detection and tracking through their triggered effects and finding their correlation with geophysical observables;
• Characterization and modeling of fluids migration at test-sites from regional to the local scale: through the application of the developed multi-parametric and multi-disciplinary approaches to different test-sites in volcanic, tectonic and industrial exploitation geological environments in Italy.

Results of this project are expected to have a broad scientific-technological impact through the development and application of new, integrated multi-parametric methods & technologies for fluid detection and space-time tracking. As for its socio-economic impact, the project will deliver “best practice” recommendations for managing fluid-induced seismicity for a sustainable and safe exploitation of all the energy resources that involve injection/withdrawal of fluid into/from the subsoil. Forecasts of induced seismicity using multi-parametric observed systems of induced seismicity represent the planning and decision-making tools for mitigating the associated risk for population living nearby industrial sites but also active hazardous tectonic and volcanic regions.

How to cite: Zollo, A., De Landro, G., Caracausi, A., Castaldo, R., D'Agostino, N., Paternoster, M., Siniscalchi, A., Stabile, T. A., and Tallarico, A.: The project FLUIDS: Detection and tracking of crustal fluids by multi-parametric methodologies and technologies, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13536, https://doi.org/10.5194/egusphere-egu2020-13536, 2020.

The Chiles - Cerro Negro Volcanic Complex (CVCCN) is located in the Western cordillera at the Ecuador – Colombia border. This volcanic complex has showed an anomalous seismic activity since late 2013, with high activity peaks in 2014, specially in October and November with up to 6000 earthquakes per day mostly volcanic-tectonics events. The most important earthquake in this sequence occurred on October 20, 2014 with a 5.7 Mw. In order to obtain a better characterization of the seismic source in the CVCCN area, a new 1D velocity model was computed using 300 earthquakes with magnitudes larger than 3.0 MLv, and high quality of P and S pickings.  This model has 8 layers over a semi-space and starts with a Vp = 2.96 Km/s and  Vs = 1.69 Km/s highlighting strong variations at 7km with Vp = 5.87 Km/s and  Vs = 3.52 Km/s and at 24 km Vp = 6.58 Km/s and  Vs = 3.79 Km/s . A value of 1.73 of Vp/Vs was determined, which is a normal for the continental crust. Computed hypocenters with the new velocity model highlighted two sources: one is defined by a concentration of shallow earthquakes on the southern flank of Chiles Volcano, and the second one contains events deeper than 7 km and follows a N-S tectonic structure that crosses the CVCCN and matches the Cauca-Patía fault. This structure obtained with this new model is confirmed by interferograms from Sentinel images after the earthquake MLv 4.2 of September 27, 2019 where a mostly dextral movement is defined. Focal mechanisms were computed for earthquakes larger than MLV 4.0 using waveform inversion (SeisComp3). Most events show ~N-S planes and dextral with inverse component. Focal mechanisms exhibit a Non-Double Couple component (CLVD), which in most of these events is more than 40 percent including the CLVD = 71% calculated for the earthquake of Mw 5.7 on October 20, 2014. This value suggests the presence of a volumetric component that could be induced by magma or fluid movements. This is corroborated by the presence of LP and VLP events inside of the CVCCN system.

How to cite: Córdova, A., Espin, P., and Pacheco, D.: Characterization of Tectonic - Magmatic Seismic Source at Chiles - Cerro Negro Volcanic Complex (CCNVC), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12453, https://doi.org/10.5194/egusphere-egu2020-12453, 2020.

The off Nicobar region in the Andaman Sea is witnessing frequent earthquake swarms after December 2004 Tsunamigenic earthquake in January 2005, March and October 2014, November 2015 and April 2019. In this study, we present the geophysical evidence of active volcanism in the Off Nicobar back-arc region on 21^st and 22^nd March 2014 based on a passive Ocean Bottom Seismometer (OBS) experiment. We detected a series of hybrid earthquake events characterized by the onset of high–frequency signal (1-10 Hz) which is followed by a long period waveform of up to 600s having a range of 0.1-1 Hz. The waveforms appear to be emergent and lack the onset of a distinct S-phase. We also observed a very high frequency (10-40 Hz) hydro-acoustic phase in the coda of long-period events.  These hybrid events are considered to be volcano-tectonic (VT) events that may trigger magmatic activities in the Off Nicobar region. We have identified and located 141 high-frequency events on 21^st and 22^nd March 2014 using hypocent v.3.2 program and they are distributed along NW-SE direction aligning with the submarine volcanoes defining the volcanic arc as observed in the high-resolution bathymetry data. The fault plane solution of the major high-frequency events suggests strike-slip faulting with the strike, dip and rake values of 334^°, 89^° and 171^°, respectively along the direction of the prevalent sliver strike-slip faulting in the Andaman back-arc region. We propose that the upward movement of magma is a plausible mechanism which can explain the frequent occurrence of earthquake swarms in the off Nicobar region. The stress generated from magma movement may initially trigger shallow VT events such as faulting or dike intrusions and later generate long period ringing associated with the resonance of the magma chamber. The shallow nature of the events also generates a hydroacoustic wave which is detected in the OBS experiment as the source region is in the SOFAR channel.

How to cite: Aswini, K. K., Dewangan, P., Kamesh Raju, K. A., Vadakkeyakath, Y., Singha, P., Reddy, R., and Arya, L.: Hybrid long-period volcanic events observed in off Nicobar region, the Andaman Sea from a passive OBS experiment, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-654, https://doi.org/10.5194/egusphere-egu2020-654, 2020.

The knowledge of the dynamic of the Campi Flegrei calderic system is essential to mitigate the volcanic risk in one of the most densely populated volcanic areas in the world. From 1950 to 1985 three bradyseismic crises occurred with a total uplift of almost 3 m (Del Gaudio et al., 2010). After more than 20 years of subsidence, at the end of 2005 the uplift started again accompanied by a low increment in the seismicity rate. In 2012 a further increment in the seismicity rate was observed and a variation in the gas composition of the fumaroles of Solfatara (central area of the caldera) revealed the injection of magmatic fluids into the hydrothermal system (Chiodini et al., 2017). This suggests that the investigation of the seismicity can represent a very useful tool for the risk mitigation. Here we analyze the seismic catalogue of Campi Flegrei (collected by INGV - Osservatorio Vesuviano) to check for any variation of the observed seismicity. This can be eventually associated with geochemical monitored parameters. In addition, we analyzed the most energetic swarms recorded in this period by comparing their locations, waveforms and source mechanisms. We find that occurrence rate, location and b-value change in time. The seismicity occurs in swarms, which, in the last years, tends to became closer but with a smaller number of events. The observed variations are correlated also with the geochemical monitoring parameters suggesting that the uplift process has probably modified the elastic and permeability properties of the shallow part of the crust. 

How to cite: Tramelli, A., Godano, C., Giudicepietro, F., Ricciolino, P., and Caliro, S.: Volcano-tectonic earthquake statistics for Campi Flegrei monitoring, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8498, https://doi.org/10.5194/egusphere-egu2020-8498, 2020.

Iberia, located at the southwestern end of Europe, displays a complex pattern of seismic activity, with most known active faults slipping at low rates (< 1 mm/yr). However, the seismic activity is remarkable, with numerous earthquakes in the historical record proving destructive. The earthquake cluster in mainland Portugal that has a highest rate of seismic activity is very localized (small spatial extent), extends vertically from 5 to 20 km depth and lays on the Monchique late Cretaceous magmatic intrusion, in SW Portugal. This magmatic intrusion forms strong rheological contrast between the intruded magmatic rocks and surrounding Paleozoic rocks. Furthermore, it is the locus of abundant natural water springs. Several pertinent questions remain to be answered concerning earthquakes in Monchique: Are earthquakes in Monchique simply a response to tectonic stresses (given the proximity of Monchique to the EU-AF plate boundary), with the localization of brittle failure in the region facilitated by the rheological contrast between the Cretaceous intrusion and surrounding Paleozoic rocks? Do fluids play a role in facilitating slip in existing fractures? Or, conversely, is the circulation of fluids facilitated by the faulting that results from the rheological contrasts? Are there hazardous faults in Monchique? In this presentation, we re-analyze in detail the seismic data recorded by the regional permanent seismic network, in order to better understand the relationship between seismic activity and igneous intrusion. In particular, we re-locate earthquakes using NonLinLoc and PRISM3D, a 3D velocity model for the region. At a subsequent step, we re-locate earthquakes using HypoDD. We also perform a clustering analysis based on waveform similarity and compute focal mechanisms for the region. The results show that earthquakes align along two main directions, E-W and NNE-SSW, coinciding with surface features of the magmatic intrusion. Focal mechanisms indicate dominantly strike-slip faulting, with the possible fault planes coinciding with the favored directions of earthquake lineations. We investigate the spatio-temporal evolution of seismicity and address possible forcing mechanisms, including tidal forcing.

The author would like to acknowledge the financial support  FCT through project UIDB/50019/2020 – IDL and PTDC/GEO-FIQ/2590/2014 - SPIDER.

How to cite: Soares, A., Custódio, S., Neres, M., Vales, D., and Matias, L.: Seismic activity along a Cretaceous magmatic intrusion in Monchique, SW Iberia, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20361, https://doi.org/10.5194/egusphere-egu2020-20361, 2020.

Slow slip events (SSEs) are slow fault ruptures that do not excite detectable seismic waves although they are often accompanied by some forms of seismic strain release, e.g., clusters of low- and very-low frequency earthquakes, and/or episodic or continuous non-volcanic tremor (i.e. tremor-genic SSEs) and earthquake swarms (swarm-genic SSEs). At subduction zones, increasing evidence indicates that aseismic slip and seismic strain release in the form of non-volcanic tremor represent the evolution of slow fracturing. In addition, aseismic slip rate modulates the release of seismic slip during tremor-genic SSEs. No general agreement has been reached, however, on whether source duration-moment scaling of SSEs is linear or follows that of ordinary earthquakes (cubic). To date, investigations on the source scaling has been based on global compilations of tremor-genic SSEs while no studies have looked into the source scaling of swarm-genic SSEs.

We present the first compilation of source parameters of swarm-genic slow slip events occurring in subduction zones as well as in extensional, transform and volcanic environments. We find for swarm-genic SSEs a power-law scaling of aseismic to seismic moment release during episodes of slow slip that is independent of the tectonic setting. The earthquake productivity, i.e., the ratio of seismic to aseismic moment released, of shallow SSEs is on average higher than that of deeper ones and scales inversely with rupture velocity. The inferred source scaling indicates a strong interplay between the evolution of aseismic slip and the associated seismic response of the host medium and that swarm-genic SSEs and tremor-genic SSEs arise from similar fracturing mechanisms. Depth dependent rheological conditions modulated by fluid pore pressure, temperature and density of asperities appear to be the main controls on the scaling. Large SSEs have systematically high earthquake productivity suggesting static stress transfer as an additional factor in triggering swarms of ordinary earthquakes. Our data suggest that during the slow slip evolution the proportion of seismic strain release is always smaller than the aseismic part although transient changes in stress and fault rheology imparted by swarm-genic SSEs can lead to delayed triggering of major and devastating earthquakes like in the Tohoku, Iquique and L’Aquila cases. The evidence of source scaling reported here will help constraining theoretical models of SSEs rupture propagation and seismic hazard assessments that should take into account the new scaling between aseismic and seismic moment release. 

How to cite: Passarelli, L., Rivalta, E., Selvadurai, P. A., and Jónsson, S.: The source scaling of swarm-genic slow slip events, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2626, https://doi.org/10.5194/egusphere-egu2020-2626, 2020.

Natural earthquake clusters are often related to a mainshock, which triggers the sequence by its induced stress changes. These clusters are called mainshock-aftershock sequences and statistically well explained by earthquake-earthquake interactions according to the Epidemic Type Aftershock Sequence (ETAS) model. Additionally, aseismic processes such as slow slip, dike propagation or fluid flow might also play a role in the initiation and driving of the earthquake sequence. Earthquake swarms, which lacks a dominant earthquake, are often believed to indicate such transient aseismic forcing signals. However, swarm-type clusters can also occur by chance in ETAS-simulations and thus not necessarily related to aseismic drivers. Thus, more sophisticated quantification of the space-time-magnitude characteristics of earthquake sequences are required for discrimination. Migration patterns are one of those properties which can be indicative for aseismic triggering. We suggest simple measures to identify and quantify migration patterns and test those for synthetic data, data from fluid injection experiments, and natural swarm activity related to fluid flow in NW Bohemia and Long Valley caldera. We analyze their potential to discriminate from ETAS-type clusters and compare it with those of time-magnitude characteristics of the activity such as seismic moment ratios and skewness. Our results are finally used to discriminate earthquake clusters in California and elsewhere.

How to cite: Hainzl, S. and Fischer, T.: Detection and characterization of fluid-driven earthquake clusters, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10252, https://doi.org/10.5194/egusphere-egu2020-10252, 2020.

Along the Chilean subduction segment, the seismicity tends to display characteristics of mainshock-aftershocks sequences. However, besides large and destructive earthquakes, central Chile has been also characterized by the occurrence of localized seismicity clusters with weak to moderate magnitudes, appearing either in form of repeated short-duration swarms or in form of sustained long-lasting activity. Seismic swarms were observed prior to large earthquakes and were hypothesized as possible precursors, although they did not always develop into major earthquakes. The origin and driving processes of this localized seismic activity have not yet been identified. Here, we characterize the seismicity at two seismic clusters in Central Chile, by analyzing hypocentral locations, spatio-temporal migration, magnitude, and inter-event time distributions and moment tensors. Both clusters are characterized by weak to moderate seismicity and manifest as clear seismicity rate and Benioff strain anomalies. We discuss these seismic clusters over a period of 18 years (2000-2017) and investigate their interactions with the Maule earthquake. We find repeating thrust earthquakes on the slab interface at one cluster beneath Vichuquén slipping at a rate comparable to the tectonically accumulated one. At the offshore Navidad cluster, the seismicity occurs in forms of swarms, with the largest episodes in 2001, 2002, 2004, 2012, 2014, 2016 and 2017 showing some rough temporal recurrence. Moment tensor indicates the occurrence of similar thrust mechanisms along a west-dipping structure across the subducting plate. Clusters persist before and after the Maule earthquake. However, at the Vichuquén cluster, the increased seismicity rate following the Maule earthquake remains to date higher than the background rate and the system is still far from recovery.

How to cite: Valenzuela Malebrán, C., Cesca, S., Ruiz, S., Passarelli, L., Leyton, F., and Dahm, T.: On seismic clusters, swarms and repeating earthquakes in Central Chile., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5153, https://doi.org/10.5194/egusphere-egu2020-5153, 2020.

In October 2019 an earthquake swarm initiated in the Mineral Mountains, Utah near the Roosevelt Hot Springs. The area has been characterized as swarm-genic after the recording of an energetic swarm (1044 microearthquakes, M less than 1.5) during the summer of 1981. This study primarily aims to investigate the spatio-temporal properties of the newly detected earthquake swarm and compare its occurrence to prior seismic activity. The October, 2019 earthquake swarm lasted four days and consists of forty-three shallow earthquakes that were cataloged by the University of Utah Seismograph Stations (UUSS) with magnitudes -0.7 to 1.31. All the events were recorded by a dense local broadband seismic network located around the Frontier Observatory for Research in Geothermal Energy (FORGE) in southcentral Utah, ~10 km west of the activated area. The close proximity of the seismic network along with the density of the seismicity allows us to apply techniques for improving the detection level and earthquake location. To achieve this, we use the earthquakes detected by the UUSS as templates and scan the continuous data for new events by applying a matched filter technique. To perform a detailed spatial analysis of the earthquake swarm and look for migration patterns, we create a high-resolution earthquake catalog using a double difference technique and differential times from both catalog and cross correlation data. To gain insight into the stress regime, we compute fault plane solutions from first motions for individual events and composite focal mechanisms for families of similar events. We further attempt to explore the underlying mechanism by examining the presence of repeating earthquakes comprising the earthquake swarm and their relation to aseismic slip. Such observations may shed insights into the role of fluids and the influence of the high heat flow, due to the geothermal system, on earthquake triggering and migration.

How to cite: Mesimeri, M., Pankow, K., Baker, B., and Hale, M.: The October 2019 earthquake swarm in the Mineral Mountains, Utah and its relation to the geothermal system, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5767, https://doi.org/10.5194/egusphere-egu2020-5767, 2020.

Earthquake swarms occurred worldwide in diverse geological units, however, their origin is still unclear. West Bohemia-Vogtland represents one of the most active intraplate earthquake-swarm areas in Europe, South-West Iceland is characterized by intense interplate earthquake swarms. Both these areas exhibit high activity of crustal fluids.

We investigated earthquake swarms from W-Bohemia and from different areas in SW-Iceland: the Hengill volcanic complex, Ölfus transition zone (the edge of the SISZ), and Reykjanes Peninsula, from the perspective of their magnitude-time development, seismic moment release with time, the magnitude-frequency distribution and distribution of the inter-event times, and the space and time distribution of the foci. The aim was to determine the swarm characteristics that are dependent or vice-versa independent on the tectonic environment, and also the characteristics which should help us to distinguish more precisely earthquake swarms from mainshock-aftershock sequences.

We found that the frequency-magnitude (b-values) and inter-event-time distributions are similar for both areas, while total seismic moment release and its rate are much larger for the SW Icelandic activities compared to the W-Bohemia ones. One dominant short-term swarm phase with one or a few dominant events in which significant part of M_0tot released, is typical of the SW Icelandic swarms, whereas the W-Bohemia swarms are characterised by stepwise seismic moment release, which is manifested by several swarm phases. MFDs of the SW-Iceland swarms indicate significantly lower a-value (number of M_L > 0 evens), particularly of those on the Reykjanes Peninsula, compared to W-Bohemia swarms; it is due to the fact that considerable amount of M_0tot released in quasi-mainshocks and the rest in aftershocks; lower a-value was also found for the W-Bohemian mainshock-aftershock sequence in 2014. The W-Bohemian swarms took place in a bounded focal zone consisting of several fault segments but the SW-Icelandic swarms correspond well to tectonic structures along the Mid Atlantic Ridge. We conclude that most of the W-Bohemia earthquake swarms were series of subswarms with one or more embedded mainshock-aftershock sequences, while the SW-Icelandic swarms (particularly those on the Reykjanes Peninsula appear to be a transition between earthquake swarm and mainshock-aftershock sequence. The W-Bohemia and SW-Iceland focal zones are characterized by complex system of short, differently oriented faults/fault segments; interestingly, the W-Bohemia and some SW-Icelandic focal zones exhibit coexistence of faults susceptible to earthquake swarms and differently oriented faults predisposed to common earthquakes (mainshock-aftershocks).

How to cite: Horálek, J., Jakoubková, H., Doubravová, J., and Bachura, M.: Earthquake swarms in West Bohemia-Vogtland and South-West Iceland: are they of similar nature?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7904, https://doi.org/10.5194/egusphere-egu2020-7904, 2020.

The IG CAS in cooperation with IRSM CAS operates two local seismic networks deployed to monitor the seismic swarms in West Bohemia/Vogtland, Czechia and Reykjanes Peninsula, Iceland. 

WEBNET monitors the region of West Bohemia since 1991 developing from 4 short period stations to 24 broadband stations today. The seismoactive region West Bohemia/Vogtland lies in the border area between Czechia and Germany in the western part of Bohemian Massif. It is an intra-continental area with persistent swarm-like seismicity but rarely also main-shock after-shock sequences may occur. 

REYKJANET local seismic network is situated in Reykjanes Peninsula on Southwest Iceland. The area is an onshore part of the mid-Atlantic plate boundary between the North America and Eurasia Plates. The seismic activity of Reykjanes peninsula is represented by typical main-shock after-shock sequences as well as earthquake swarms. The REYKJANET network was built in 2013 and it consists of 15 stations placed around the epicentral area.

Both networks have been substantially upgraded during the last years. In case of REYKJANET the replacement of old sensors and digitizers with new ones made the operation easier and ready for near future plan to stream the waveform files in real time. WEBNET network which was long years divided into two subnets – on-line permanent stations and off-line autonomous stations, was recently homogenized by eco-powering and 4G LTE data connecting of the off-line stations. Additonally, the micro network HORNET was deployed within the WEBNET epicentral zone to monitor Horka water dam.

Data from both above mentioned networks are automatically searched for seismic events by the neural-network-based detector designed by Doubravová et al. (2016, 2019) providing event list with completeness magnitude Mc=0 for REYKJANET and Mc=-0.5 for WEBNET. The main difference of sensitivity is given by different noise levels of the two networks.

How to cite: Klicpera, J., Doubravová, J., and Horálek, J.: Advances in earthquake-swarms monitoring networks WEBNET and REYKJANET, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8391, https://doi.org/10.5194/egusphere-egu2020-8391, 2020.

Repeating earthquakes, sequences of microseismic events with highly similar seismograms and magnitudes, suggest quasi-periodic rupturing of the same asperity. They are observed on creeping fault segments surrounded by aseismic slip area and also in earthquake swarms. However, so far, they have not been documented in the West Bohemia/Vogtland seismic swarm area. These local swarms consist of thousands of M_L < 4 events occurring along a small area of fault zone with repeated activation of some patches during the swarms and weak background activity in the intermediate periods. Detecting and analyzing the repeating earthquakes would help revealing the continuing background activity and identifying fault areas that are active permanently. This could point to the possible sources of fluids or aseismic creep that are supposed to play significant role in swarm generation. Repeating earthquakes are identified by waveform cross-correlation analysis comparing waveforms of repeaters with continuous seismic data set. We developed efficient detection algorithm to identify repeating earthquakes using selected event templates to reveal continuing seismic activity along the main Nový Kostel fault zone, namely in the areas with only episodic activity. The results provide a robust basis for routine application to the long-term seismic dataset that will allow also for further applications including analysis of the source parameters of the repeaters and/or detecting possible seismic velocity variations in the focal zone.

How to cite: Salama, A. and Fischer, T.: Detection of repeating earthquakes in the West Bohemia swarm region, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16840, https://doi.org/10.5194/egusphere-egu2020-16840, 2020.

In the Hualien area, two Mw6.4 and Mw6.2 earthquakes, 20 km apart, occurred in February 2018 and April 2019 respectively. The former to the northeast, located offshore to ​​the Liwu river, triggered several earthquake clusters along the Milun fault and southward to the Longitudinal Valley, the suture of the Eurasian and the Philippine Sea plates; the latter to the southwest, located in the Central Range, also triggered several seismic swarms in the Central Range,  along the Liwu river to the northeast and at Ji'an to the southeast. Except for the Milun fault, neither GPS nor InSAR observations detects significant surface deformation after the occurrence of these two main shocks, indicating that the earthquake ruptures mainly developed within the crust. Therefore, seismic observation becomes an efficient tool for revealing the seismotectonics of the two earthquake sequences. For monitoring the aftershock sequences, two days after the main shocks, we deployed two geophone arrays, 70 Z-component RefTek 125A TEXANs for two weeks in 2018 and 47 three-component Fairfield Nodal Z-Lands for one month in 2019, with 1-5 km station spacing around the Hualien City. These earthquake swarms were well recorded and analyzed through the dense seismic networks. The numbers of aftershock sequences manually identified are two-fold more than that issued by the Central Weather Bureau, Taiwan. The seismicity of the 2018 aftershock sequence, to depths of between 5-15 km, was significantly reduced within 10 days after the main shock. however, the seismicity of the 2019 aftershock sequence, to depths of between 2-50 km, was still above background seismicity rate 30 days after the main shock. The spatial distribution of the 2018 aftershock sequence could be related to a fault zone of the plate boundary, but that of the 2019 and the relocated 1986 aftershock sequences show a conjugate thrust fault pair beneath the eastern Central Range. Our results clearly depict several local tectonic structures that have not been observed at the northern tip of the Longitudinal Valley, not only a suture but also a transitional area from collision to subduction.

How to cite: Sun, W.-F., Kuo-Chen, H., Guan, Z.-K., and Chang, W.-Y.: Seismotectonics of the northern Longitudinal Valley, Taiwan, inferred from aftershock sequences of 2018 Mw6.4 and 2019 Mw6.2 Hualien earthquakes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21398, https://doi.org/10.5194/egusphere-egu2020-21398, 2020.