Displays

The present study focuses on the Western Gulf of Corinth (WGoC), which is one of the most seismically active sites in Europe and also the region where the Corinth Rift Laboratory (CRL) Near Fault Observatory (NFO) has been installed. The WGoC exhibits high extension rates and intense microseismicity, with frequent occurrence of clustered seismicity, as in the cases of the 2001 Agios Ioannis swarm, the seismic sequences of 2003-2004 and 2006-2007 in the central part of the Gulf and the 2013 Helike swarm. These outbreaks of seismicity, lasting a few days to months, are characterized by a high frequency and density of earthquakes, with magnitudes generally not exceeding 4.5, with the strongest ones usually occurring in the middle or towards the end of the sequence. These short-lived seismic crises often exhibit patterns of spatio-temporal migration in their hypocentral distribution, which has been associated with the effects of diffusion and circulation of fluids to the seismogenic crust of the WGoC. Fluids appear to play an important role in both triggering and evolving seismic sequences. In the framework of the present study, earthquake hypocenters, relocated with high resolution by employing waveform cross-correlation and the double difference-method, are used to perform an upper crust Shear-Wave Splitting (SWS) study at the WGoC area. The temporal variation of the SWS is investigated, in relation with the temporal evolution of seismicity, to possibly identify patterns related to changes of the stress-field due to fluid migration, or before the occurrence of moderate to strong earthquakes. In addition, the relocated catalogue is analyzed using nonlinear statistical physics for the definition of the spatio-temporal scaling properties of clustered seismicity, as well as for the quantification and modeling of seismic diffusion phenomena associated with fluid circulation at the upper crust of the WGoC.

Acknowledgements

We would like to thank the personnel of the Hellenic Unified Seismological Network and the Corinth Rift Laboratory Network (https://doi.org/10.15778/RESIF.CL) for the installation and operation of the stations used in the current article. The present work was co-funded by the European Union (ESF) and Greek national funds through the Operational Program "Human Resources Development, Education and Lifelong Learning", project title “Τhe effect of fluids on the seismicity of the Western Gulf of Corinth” (project code MIS: 5048127).

How to cite: Michas, G., Kapetanidis, V., Kaviris, G., and Vallianatos, F.: The role of fluids in the seismicity of the Western Gulf of Corinth (Greece), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5996, https://doi.org/10.5194/egusphere-egu2020-5996, 2020.

The Corinth Rift (Greece) is one of the most seismically active regions in Europe and has been studied extensively during the past decades. It is characterized by normal faulting and extension rates between 6 and 15 mm yr−1 in an approximately N10E° direction. The seismicity of the area is continuously monitored by the stations of the Corinth Rift Laboratory Network (CRL Net). The availability of a dense permanent seismological network allows the extensive analysis of the seismic swarms which occur frequently. In this study, the September 2014 swarm located at the western part of the Corinth Gulf is analyzed. Initially, more than 4000 automatically located events, of a two month period, were relocated using the HYPODD algorithm, incorporating both catalogue and cross-correlation differential traveltimes. Consequently, the initial seismic cloud was separated into several smaller, densely concentrated clusters. Double difference relocation was also applied to 707 manually located events in order to investigate the Vp/Vs ratio variation, due to its sensitivity in pore fluids. The swarm’s parameters such as seismicity distribution and moment tensors were combined with the seismotectonic data of the area. The results indicate an initial activation of the Psathopyrgos normal fault; afterwards the seismicity extended both towards East and West, while most events occurred at the western part of the study area. The seismicity distribution revealed a main activation of the North – dipping faults. The seismicity migration with respect to pore pressure changes due to fluid movements was investigated through diffusivity calculations. The diffusivity value was found to be 4.5m²s^-1, which is consistent with results of previous studies in the area. The results of the investigation of the fault- zone hydraulic behavior provide evidence for the fluid – triggered earthquake swarms and the related rock physical properties.

How to cite: Serpetsidaki, A., Sokos, E., Lambotte, S., Bernard, P., and Lyon-Caen, H.: Seismotectonic analysis of the 2014 seismic swarm at the Western Corinth Gulf (Greece), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20869, https://doi.org/10.5194/egusphere-egu2020-20869, 2020.

Monitoring of CO2 degassing in seismoactive areas allows the study of correlations of gas
release and seismic activity. Reliable continuous monitoring of the gas flow rate in rough field
conditions requires robust methods capable of measuring gas flow at different types of gas
outlets such as wet mofettes, mineral springs and boreholes. In this paper we focus on the
methods and results of the long-term monitoring of CO2 degassing in the West
Bohemia/Vogtland region in Central Europe, which is typified by the occurrence of
earthquake swarms and emanations of carbon dioxide of magmatic origin. Besides direct
flow measurement using flowmeters, we introduce a novel indirect technique based on
quantifying the gas bubble contents in a water column, which is capable of functioning in
severe environmental conditions. The method calculates the mean bubble fraction in a water-
gas mixture from the pressure difference along a fixed depth interval in a water column.
Laboratory tests indicate the nonlinear dependence of the bubble fraction on the flow rate,
which is confirmed by empirical models found in the chemical and nuclear engineering
literature. Application of the method in a pilot borehole shows a high correlation between the
bubble fraction and measured gas flow rate. This was specifically the case of two coseismic
anomalies in 2008 and 2014, when the flow rate rose during a seismic swarm to a multitude
of the pre-seismic level for several months and was followed by a long-term flow rate decline.
However, three more seismic swarms occurring in the same fault zone were not associated
with any significant CO2 flow anomaly. We surmise that this could be related to the slightly
farther distance of the hypocenters of these swarms than the two ones which caused the
coseismic CO2 flow rise. Further long-term CO2-flow monitoring is required to verify the
mutual influence of CO2 degassing and seismic activity in the area.

How to cite: Vlček, J., Fischer, T., and Lanzendörfer, M.: Monitoring natural CO2 flow in the mofettes of the West Bohemia seismoactive region, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19156, https://doi.org/10.5194/egusphere-egu2020-19156, 2020.

The Eger Rift (Czech Republic) is an intraplate region without active volcanism but with emanations of magma-derived gases and the recurrence of mid-crustal earthquake swarms with small to intermediate magnitudes (M_L < 5) in the Cheb Basin. To understand the anomalous earthquake activity and CO₂ degassing, an interdisciplinary well-based observatory is built up for continuous fluid and earthquake monitoring at depth.

The fluid observatory is located at the Hartoušov Mofette (Cheb Basin), an area characterized by intense mantle degassing with a subcontinental lithospheric mantle (SCLM) contribution of He that increased from 38% in 1993 to 89% in 2016. Two drillings with depths of 30 and 108 m (F1 and F2, respectively) are being monitored since August 2019 for the composition of ascending fluids. Additionally, the environmental air composition is monitored. Gas concentrations were determined in-situ at 1-min intervals, while direct sampling campaigns took place periodically and samples were analyzed for their chemical and isotope composition. Samples of gases emerging in the mofette were also collected. During this period, a third borehole (F3) with a depth of 238 m was drilled.

At Hartoušov, carbon dioxide is the prevailing gas component (concentrations above 99.5%), with helium presenting a mantle origin (up to 90% considering a SCLM-type source). The atmospheric contribution is negligible, even though during drilling of F3 enrichments in atmospheric components such as Ar and N₂ have been observed. An increase in both CH₄ and He has been noticed in F2 (108 m borehole) at 40 m depth, whilst a decrease in He has been observed at 193 m depth in both F1 and the natural mofette. Enrichments in less soluble gases (eg. He and N₂) at various depths accompanied by a minor CO₂ decrease have also been noticed. Such variations may have been caused by the different solubilities of gases in aquatic environments. Moreover, a decrease in CO₂ followed by a subsequent enrichment of CH₄ and C_xH_y during the first days after the initial drilling could promote the hypothesis of the generation of microbialy derived CH₄. Diurnal variations were observed for the majority of the gas components during the last phase of the F3 drilling, when the well reached a depth >200 m.

This research is a part of the MoRe - “Mofette Research” project, which is included in the ICDP project “Drilling the Eger Rift: Magmatic fluids driving the earthquake swarms and the deep biosphere”). This work was supported by the DFG grant# WO 855/4-1 and BA 2207/19-1.

How to cite: Daskalopoulou, K., Woith, H., Zimmer, M., Niedermann, S., Bağ, C. D., Bauz, R., Trubač, J., and Fischer, T.: Variations of gas compositions during a drilling process: A key study on the Hartoušov Mofette, Czech Republic, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5735, https://doi.org/10.5194/egusphere-egu2020-5735, 2020.

Romanian National Institute of Earth Physics (NIEP) develops a gas emission monitoring network as part of a multidisciplinary activity. The goal is to help organizations specialized in emergency situations with short-term earthquakes forecast and information related to pollution and effects of climate change. In Romania, the important seismic area is Vrancea where there are seismic and multidisciplinary monitoring stations. The methods and monitoring solutions are general and they could be applied in any place. The main part of our system is related to CO₂, CO, radon, air ionization in correlation with earth radiation, air ionization, telluric currents, ULF radio waves disturbance, magnetic field, temperature in borehole, infrasound, acoustic waves and meteorological data. The monitoring stations are located on the faults in the curvature of the Carpathian Mountains. The first step is to determine the daily, seasonal and annual evolutions of gas emissions and ionization of the air for at least one year. We are looking for time intervals during which the seismic activity was reduced to determine the normal evolutions of the measured parameters. Then we can determine the effects of active seismic periods on gas emissions. We will apply several methods of analysis and will correlate the particularities of the geological structure in which the monitoring stations are located and the position of the epicenters of earthquakes. The present results are favorable to the analysis by integrating the values measured on variable time windows according to the case. Instantaneous values also include local effects that are not related to tectonic stress. Current measurements indicate the presence of CO at certain times of the day and at certain stations. This is not possible due to tectonic stress, but may be the result of pollution in short-distance cities and air currents that spread it.

Key words: gas emission, multidisciplinary analysis, radon concentration, air ionization, multi-parametric monitoring, earthquake forecast, earthquake precursors

How to cite: Toader, V.-E., Moldovan, I.-A., Ionescu, C., Marmureanu, A., and Mihai, A.: Complex system for earthquakes forecast using gas emission observations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13025, https://doi.org/10.5194/egusphere-egu2020-13025, 2020.

The Irpinia Near Fault Observatory (NFO) is an EPOS infrastructure located in the Campania-Lucania Region (Southern Italy). Its goal is to characterize the microseismicity and to understand the underlying chemical and physical processes occurring along the fault systems that can potentially generate large earthquakes in the area. The Irpinia NFO is currently composed of ISNet – Irpinia Seismic Network, with associated products and services. ISNet is a local network of 32 accelerometric, short-period and broad band stations, that cover the seismogenic areas related to the main earthquakes that occurred in the region in the last centuries, including the 23 November 1980 , Ms = 6.9 event.  Also, ISNet provides real-time analysis and it represents the prototype network for the testing of early warning systems in Italy.

Here we present tools and techniques to accurately locate and characterize the seismicity within the Observatory. Accurate event detection of weak signals from small magnitude events is based on the coherence of arrival times and migration techniques. This method is then coupled with advanced picking and double differences techniques to accurately locate events with a sub-kilometric scale resolution. Source parameters are thus computed inverting the displacement amplitude spectrum, with a probabilistic approach based on the conjunction of states of information in the data and model spaces. This technique is able to automatically rule out unconstrained solutions, while accounting for correlation among parameters. Source location and characterization is here used to investigate the role of the fluids in the region embedding the fault systems; space and time changes in the medium properties or in the source parameters can be used to detect a deviation from the present mechanical state of the faults owing to changes in fluid pore pressure and migration.

Finally, local dense arrays are used here to improve our capability to detect events within the noise level and to move from a point-source to an extended source description of small events in the area.

How to cite: Festa, G., Picozzi, M., Adinolfi, G. M., Caruso, A., Colombelli, S., De Landro, G., Elia, L., Riccio, R., Scala, A., Supino, M., and Zollo, A.: Detection and characterization of microseismicity and its relation with fluids at the Irpinia Near Fault Observatory, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11381, https://doi.org/10.5194/egusphere-egu2020-11381, 2020.

The Alto Tiberina fault (ATF) in the Northern Apennines (Central Italy) is a low-angle normal fault (mean dip 20°) that is the target of TABOO (The Alto Tiberina Near Fault Observatory), a state-of-art research and monitoring infrastructure based on multidisciplinary sensors. With the STAR’s project, we intend to deploy a strain- and seismo-meter array in six shallow boreholes to complement and enhance TABOO. This will happen with the active contribution of US National Science Foundation and International Continental Scientific Drilling Program (ICDP Project ID: ICDP-2018/05).

Existing seismic data from TABOO reveal microseismicity, at a consistently high rate on the ATF fault plane, including repeating earthquakes (RE). REs together with a steep gradient in crustal velocities measured by GPS and transient surface motion lasting for few months and coinciding with seismic swarms, support the hypothesis that portions of the ATF are creeping aseismically.

Recent studies document that any given patch of a fault can creep, nucleate slow earthquakes, and also host large earthquakes. Thus, these observations are forcing a revolution in our way of thinking about how faults accommodate slip. However, the interaction between creep, slow, and regular earthquakes is still poorly documented by observation. The ATF fault is perhaps the best place in the world to understand at local scale the mechanisms and implications of stress transfer process between seismic and aseismic fault segments. With STAR we will collect the Open Access data to illuminate the physics that allows for both seismic and aseismic slip on a single fault patch, with potentially transformational implications for seismic hazard and risk assessment globally.

STAR will consist of six 80-160m deep vertical boreholes covering the portion of the ATF that exhibits REs at shallow depth (~4 km), identified with waveforms analysis. The observatory will provide the international community an opportunity to study creep at local scale and over periods of minutes to months poorly constrained by other geophysical instruments. We will also deploy downhole seismometers and pressure transducers co-located with the strainmeters, and each station will be equipped with surface GPS and a meteorological instrument. The suite of instruments will enable the collection and calibration of strain records with exquisitely high precision, allowing for a quantitative characterization of ATF creep (~1mm over <1km2), enhanced monitoring of microseismicity (below Mc 0.5), and allowing correlation between degassing (CO2, Rn) measurements and subsurface strain.

How to cite: Chiaraluce, L., Bennett, R., Mencin, D., Barchi, M., and Bohnhoff, M.: A Strainmeter Array Along the Alto Tiberina Fault System, Central Italy, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19792, https://doi.org/10.5194/egusphere-egu2020-19792, 2020.

Seismic arrays are instruments capable to lower the magnitude threshold of earthquake detection by improving the signal-to-noise ratio of recordings. Seismic arrays have been used since 1960s, to investigate global earthquakes or small-scale structure of the Earth’s interior. We designed a cost-effective seismic monitoring array-system made by autonomous stations specifically designed for studying local micro-seismicity. The data collected by this system allow to develop and apply a new method for earthquake detection and location of micro-seismicity based on a frequency-wavenumber (f-k) domain analysis of continuous data recorded by small seismic arrays, in order to separate the coherent signal of low magnitude events from the surrounding noise.
Field surveys have been carried out in two Italian regions, which are seismically active and already monitored by high quality, standard seismic networks, so to allow us to test the performance of the proposed array configuration and processing algorithm.
The first survey was carried out in the Irpinia region (Southern Italy), near the main fault segment activated during the Ms 6.9 Irpinia earthquake occurred in 1980. The natural seismicity of the area features occasional small seismic sequences, with magnitude (ML) less than 3, that are recorded by the local seismic network that monitors the Irpinia fault-system (ISNet - Irpinia Seismic Network). Three small aperture seismic arrays (few hundred meters wide) were deployed at distance of few tens of kilometers each other for three months. Each array was made up of seven 3-component stations, arranged in irregular geometry.
The second experiment was carried out in the Montello-Collalto area (Veneto region, North-East Italy), where an underground gas storage concession, known as “Collalto Stoccaggio”, exists. The gas storage activity is monitored by a local seismic network named “Collalto Seismic Network” (Rete Sismica di Collalto, or RSC). This dense network was designed and is managed by the National Institute of Oceanography and Applied Geophysics (OGS) on behalf of Edison Stoccaggio S.p.A., the storage concession holder. A seismic array composed of eight 3-component stations with 2 km of maximum aperture was deployed for a couple of months to monitor the micro-seismicity occurring nearby.
Our preliminary results suggest that the f-k earthquake detection algorithm of micro-seismicity using seismic arrays can become a valid tool to complement standard seismic networks in monitoring and studying natural and induced seismicity. In the Irpinia region, for instance, we have detected and located about six times the earthquakes recorded by the local network lowering the magnitude down to 0.1, smaller than the catalogue minimum magnitude that is equal to 1.2.

How to cite: Adinolfi, G. M., Picozzi, M., Priolo, E., Romanelli, M., Riccio, R., Parolai, S., and Zollo, A.: Seismic arrays and frequency-wavenumber spectrum analysis to detect and monitor natural/induced microseismicity, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22582, https://doi.org/10.5194/egusphere-egu2020-22582, 2020.

Within EPOS IP project (H2020, 676564) four organizations (ZRC SAZU as EPOS IP partner, University of Trieste as member of EPOS Italy, and Jožef Stefan Institute and Slovenian Environment Agency as members of consortium EPOS-SI) started with development of Postojna Cave as possible Near Fault Observatory (NFO). Intensive geological, meteorological, hydrogeological, seismological and karstological studies are taking place in Postojna Cave. Being a show cave, it has good infrastructure (electricity, cave train, optical cable etc.) what is necessary for on-line scientific measurements inside the cave and transfer of data.

Postojna Cave is situated in SW part of Slovenia in External Dinarides with tectonically active Alpine thrusts and Dinaric (NW-SE) and cross-Dinaric (NE-SW) faults. It belongs to NE part of Adria microplate. Postojna Cave is situated between regionally important Dinaric-oriented Idrija and Predjama Faults. Idrija Fault is supposed to be responsible for 1511 earthquake (M=6.8), which is the strongest earthquake in the territory of Slovenia. In Postojna Cave there are numerous broken speleothems, some of them can be due to tectonic activity others due to karst processes.

Postojna Cave NFO includes regular micro-climatic monitoring as cave air temperature, water temperature, rock temperature, CO₂, humidity, air pressure, wind speed and direction. At several locations such measurements are going on since 2009 to assess impact of tourism on cave environment and to study natural cave meteorological conditions.

Radon (²²²Rn) monitoring in cave atmosphere started in 1995. In the first period seasonal measurements of radon activity concentration, equilibrium factor, radon progeny activity concentrations in attached and unattached form (EQF-3020-2, Sarad) have been carried out to establish the reliable methodology for dose estimates of cave workers. Contemporary measurements of radon progeny activity concentrations (EQF-3020-2, Sarad) and number concentrations and size distribution of general (non-radioactive) aerosol particles (SMPS, Grimm) started in 2010 and were carried out periodically. In the period 2011-16 continuous radon monitoring (once an hour) was conducted (Radon Scout, Sarad), using radon as a tracer for cave ventilation.

3D micro-displacement monitoring on two Dinaric oriented fault zones in the cave is performed with four TM 71 extensometers. First two instruments were installed in 2004 and the second ones in 2016. Small micro-displacements of up to 0.08 mm in one month are registered.

Seismic station in Postojna Cave is operating since 2010, with periods of inoperability due to power supply problems and hardware malfunctions. The station in the Tartarus tunnel (TTPJ) recorded more than hundred earthquakes of the sequence near Ilirska Bistrica that started on 15 September 2010, with two MLV=3.5 earthquakes and lasted till the end of the year 2010, without accurate timing at that time. A fibre optic cable was installed later on and a Quanterra Q330 data logger with accurate timing and real-time telemetry was installed with an Episensor accelerometer and a Streckeisen STS2 seismometer.

In the future we are planning to enlarge the underground seismic station with equipment provided from RI-SI-EPOS project (EU Cohesion Funds). Micro-deformation monitoring sites will be additionally studied with methane and radon measurements (H2020 871121 EPOS SP).

How to cite: Šebela, S., Costa, G., Vaupotič, J., Živčić, M., Viršek Ravbar, N., and Năpăruș-Aljančič, M.: Postojna Cave as Near Fault Observatory site in SW Slovenia, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4657, https://doi.org/10.5194/egusphere-egu2020-4657, 2020.

This study presents observations and analysis from a high-sampling-rate micro-seismic network, located at the north of the Sea of Galilee, Israel. Stations’ locations were chosen following the seismic swarm at the North of the Sea of Galilee, in October 2013, aiming to perceive a better understanding of the seismicity and structure of this area, in light of that anomaly seismic swarm, and of the seismic activity along the Dead Sea Fault. The micro-seismic network was active between May 2016 to August 2018, with six stations altogether, in distances of 3-5km around the northern Sea of Galilee.  Each of the micro-seismic stations had two collocated sensors: 1) GS-1 Geospace, 1 Hz vertical seismometers, sampled at 500 samples per second, and 2) 3-channel Episensor embedded in a Rock+ Kinemetrics datalogger, sampled at 200 samples per second. Towards the dismantling of the network, another swarm, stronger in magnitude, and longer in duration, has occurred in July-August 2018, roughly at the same location. Meanwhile, a significant upgrade of the Israel Seismic Network (ISN) was taking place, also densifying the number of stations around the Sea of Galilee.

The seismic processing presented here has many steps of verification, at all levels: detection, association, and location.  Processing begins with the local high-sampling-rate micro-seismic stations, tuning the most appropriate micro-seismic detectors, and association, location and magnitude parameters. Then this new generated micro-seismic catalogue is used to reveal lower magnitude events within the ISN stations, followed by relocation and re-magnitude estimations, done to those events that have additional information from the ISN stations. Running this process for increasing time-windows, it is demonstrated how the use of micro-seismic instrumentation can increase the seismic catalogue by an order of magnitude, providing higher resolution of the seismicity, both in space and time.

These efforts, of increasing the seismic catalogue, and improving their locations, are utilised for two main goals: a) obtaining a clearer picture of the seismicity and structure in the area before and during the seismic swarm of July-August 2018, b) Zooming into the interesting micro-seismic activity just before the initiation of the swarm.

How to cite: Kurzon, I.: Micro-seismicity of the Northern Sea of Galilee - Before and During the July-August 2018 Seismic Swarm, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20684, https://doi.org/10.5194/egusphere-egu2020-20684, 2020.

In Enhanced Geothermal Systems pressurized injections play a role in developing fracture networks and enhancing the water transmissivity. However, the fractures may also coalesce into undesired pathways for fluid migration to enable the fluids to reach pre-existing faults. The properties of observed seismicity can shed some light on the fracture network development and from the standpoint of the possibility to form such undesired pathways. However, to reach this goal the seismic events should be well parameterized. In particular, the information on fault plane mechanisms is essential, which is often not readily accessible. In this study, we use the rupturing process with an accurate P-wave velocity model, which is obtained by the first arrival P-wave tomography approach, to compensate for an eventual lack of source mechanisms of micro-events. For this purpose, four characteristics of the sources (final/average displacement on the fault, the dimension of fault, rupture velocity and particle velocity) can be considered. A 3D model is defined around the hypocenter of each event, so that the size of this model directly depends on the event magnitude. After calculating the arrival time of the selected phase (e.g., P, S, p or s) for each station, all waveforms are then aligned, and stacked by different stacking (e.g., phase weight, N^th-root) methods. By considering the maximum amplitude of the stacked waveform which is stimulated by each grid, the rupturing plane and the average velocity of rupturing can be obtained. This information of source can be replaced by the double-couple mechanism to investigate the fractures linking and tracking.

This work was supported under the S4CE: "Science for Clean Energy" project, which has received funding from the European Union’s Horizon 2020 research and innovation program, under grant agreement No 764810.

How to cite: Shirzad, T., Lasocki, S., and Orlecka‐Sikora, B.: Tracking the development of seismic fracture network by considering the fault rupture method, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13079, https://doi.org/10.5194/egusphere-egu2020-13079, 2020.

The 2014 Orkney earthquake (M5.5) occurred below the Moab Khotsong gold mine in South Africa. The shallowest aftershocks were located only several hundred meters below the deepest level of the mine. Two boreholes (Holes A (817 m) and B (700 m)) were drilled toward the upper margin of the aftershock zone from a specially excavated chamber at 2.9 km depth by the ICDP-DSeis project. Hole A deflected from the aftershock zone, while Hole B intersected it. Hole C was branched from Hole B to recover more samples from the aftershock zone. Except for the intersection in Hole B, the drill core recovery was ~100%. In-hole geophysical logging, including the surveys of the borehole wall geometry were carried out along the entire length of Hole A, while they could be done only as far as the intersection with the aftershock zone in Hole B due to hole closure. Hole C was not logged.

The focal mechanism solutions of mining induced earthquakes shallower than 3 km are usually of the normal faulting type, while those of the Orkney earthquake and its aftershocks deeper than 3.5 km have a strike-slip signature. In this study, we applied the Deformation Rate Analysis (DRA) and the Diametrical Core Deformation Analysis (DCDA) techniques to rock cores recovered from Holes A, B and C to explore the depth variation in the stress state that would cause the depth variation in the faulting regime. In the DRA, a cyclic loading is applied to a sub-sample cut from a drill core to determine the normal stress in the loading direction from hystereses of the stress-strain curve. We determined the normal stresses in 9 directions at each depth to recover the principal stress state redundantly. However, because it takes much time for sub-sample preparations and loading, we applied this technique only at 3 depths in Hole A. With the DCDA, the differential stress in the plane normal to a borehole is evaluated from the ellipsoidal cross-sectional shape of the rock cores. Though only the differential stress can be measured by the DCDA, it takes only several minutes for measurement at each depth. We evaluated the differential stress as densely as every several meters along Holes A, B and C.

Rock cores of Hole A were oriented by comparing joints and veins identified on the borehole wall optical-televiewer images and in the cores. Thus, the stress orientations in the plane normal to Hole A can be determined as the orientation of the maximum and the minimum core diameter. The stress orientation is obtained also from the breakout of borehole wall identified by the acoustic televiewer. Further, by combining the differential stress magnitude evaluated by the DCDA and the width of the breakout, magnitudes of the maximum and the minimum compression are estimated. We introduce the depth variations in the stress state along Holes A, B and C, as well as those of in-hole logging data to discuss spatial heterogeneity of stresses in the source region of the Orkney earthquake.

How to cite: Yabe, Y., Kanematu, M., Higashi, M., Tadokoro, R., Yoshida, S., Sugimura, K., Ogasawara, H., Ito, T., Funato, A., Ziegler, M., Liebenberg, B., Watson, B., Mngadi, S., Manzi, M., and Durrheim, R.: Stress state in the upper margin of the aftershock zone of the 2014 Orkney earthquake (M5.5), South Africa, estimated from analyses of drill cores and borehole breakouts of ICDP-DSeis drillings, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3288, https://doi.org/10.5194/egusphere-egu2020-3288, 2020.

The potential seismic hazard in South Korea is known to be that mid-sized earthquakes could occur nationwide. Because the damages of mid-sized earthquake are concentrated only in the vicinity of the epicenter, on-site EEW technology is known to be effective as a means to reduce absence of alarm near the epicenter and to ensure safety from earthquake threat. This study aims to simulate on-site EEW suitable for South Korea's seismic observation environment and verify its reliability. Seismic observations of 267 events occurred in South Korea, have been collected over the past five years for the simulation . Filter Picker was utilized to detect P-wave features from more than 37,000 data sets using a time window suitable for mid-sized earthquakes. The ground noises are removed from the detected P-waves, and a linear empirical relationship between the maximum P-wave amplitudes in vertical direction and observed PGVs on the base rock are derived. Convert the forecasted and observed PGVs to MMI, respectively. Assuming a successful prediction within the MMI±1 margin of error by comparing the two values, the results of this study showed an 80% success rate in the range above MMI 4. Through this study, feasibility and performance of on-site EEWS using domestic earthquake records were verified in South Korea. It is expected that this will contribute to the reduction of earthquake damage near the epicenter through an on-site warning in Korea.

How to cite: Lee, H. J., Seo, J. B., Lee, J. K., and Jeon, I.: Application of on-site EEW technology in South Korea. , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4155, https://doi.org/10.5194/egusphere-egu2020-4155, 2020.