SM4.1

Surface deformation associated with continental earthquakes separates into a localized component occurring on faults, and distributed deformation affecting the surrounding medium, referred to as off-fault deformation (OFD). This OFD includes both displacement discrete on secondary faults and cracks, and more diffuse deformation affecting the bulk volume of the crust. Although the deformation occurring on faults and cracks can be observed and measured in the field, diffuse deformation is more challenging to detect because it generates kilometer-scale continuous gradients of displacement without any visible disruption of the ground surface. Consequently, surface displacements measured in the field generally underestimate the total surface displacement of the earthquakes. Moreover, results from inversions of geodetic and/or seismic data suggest that, for many earthquakes, the amount of coseismic slip occurring at depth (3-7 km) is larger than what occurs in the shallower part (<3 km). This is referred to as the shallow slip deficit (SSD). So far, because diffuse deformation is not explicitly considered in earthquake displacement budgets, and because the origin of the SSD remains debated, it is difficult to directly compare directly surface observations with modeling results. In this study, we use a set of complementary geodetic data (InSAR, GPS, high-resolution optical data) to jointly invert for the coseismic slip of the 2019 Ridgecrest earthquake sequence in Southern California (M_w6.4 and 7.1). To reproduce the rupture complexity observed in the high-resolution optical data, we use a complex fault model with increased resolution in the uppermost crust. We pay special attention that our preferred model fits both with the fault slip distribution observed at the surface in the high-resolution optical imagery data, and regional-scale displacement data from InSAR and GPS. In our best model, we estimate a 30% SSD in the upper 3 km. This value of 30% matches the amount of diffuse deformation we measured around the ruptures at the surface directly on the high-resolution optical data. From these observations, we propose that SSD is entirely balanced by the volumetric diffuse deformation, and more generally, that diffuse surface deformation is proportional to SSD. Finally, based on a compilation of published data, we show that SSD and diffuse deformation are both inversely proportional to the earthquake magnitude. Indeed, for large magnitude earthquakes, SSD and diffuse deformation are close to 0%. Conversely, for earthquakes that do not break the surface, diffuse deformation might be close to 100%. However, in this latter case, it still needs to be determined whether the diffuse deformation is only elastic, or not.

How to cite: Antoine, S. L., Wang, K., Klinger, Y., Bürgmann, R., and Delorme, A.: Kilometer-wide volumetric deformation of the shallow crust associated with strike-slip continental earthquakes and its relation with coseismic shallow slip deficits, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2661, https://doi.org/10.5194/egusphere-egu22-2661, 2022.

Thrust faults are commonly known to produce significant amounts of slip, damage and ground acceleration, especially close to the free surface. The effect of the free surface on faulting has always been a standing issue in theoretical mechanics. While static solutions exist, they still cannot explain the large amounts of slip, damage and ground acceleration observed on low dipping faults. Dynamics effects raised by the presence of a free surface were first evaluated by Brune [1996] using analog experiments, which hinted at a torque mechanism induced in the hanging wall leading to a natural reduction in elastic compressive normal stress as the rupture approaches the surface. This solution was recently supported by preliminary work from Gabuchian et al. [2017], which, combining numerical and experimental simulations, also showed that the earthquake rupture, propagating up dip, induces rotation of the hanging wall, and might promote fault opening.

In this work, we use enhanced numerical solutions for earthquake rupture, based on the Combined Finite-Discrete Element Methodology (FDEM), which were recently developed by the Los Alamos National Laboratory, to carry out dynamic rupture simulations on thrust faults to characterize this opening effect and investigate the physical mechanism responsible for it. Through a systematic analysis of case studies, we explore the effect of fault geometry and friction properties on rupture behavior and its associated deformation pattern. We observe that fault opening occurs in all our simulations and increases significantly as the rupture reaches the free surface and for low dip-angle faults.We document the evolution of different metrics such as slip, slip rate, fault-normal displacement and velocities, as well as the displacements and velocities on the free surface to identify near-field deformation features that will serve as synthetic data when comparing with recorded surface deformation from real-case earthquakes.

How to cite: Villafuerte, C., Bhat, H. S., Okubo, K., Rougier, E., and Dubernet, P.: Fault opening on thrust faults due to free surface interaction and its near-field deformation features, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12223, https://doi.org/10.5194/egusphere-egu22-12223, 2022.

The analogy between earthquakes (i.e. frictional ruptures) and shear crack motions is commonly used to investigate and understand the mechanics and occurrence of such natural phenomena. However, if on one side experimental works showed how shear cracks (obeying to a square root singularity, in the framework of Linear Elastic Fracture Mechanics) describe the onset of frictional ruptures, on the other side recent models suggested that frictional ruptures can be controlled by unconventional singularities (i.e., singularity orders that deviated from the square root singularity) if weakening occurs behind the rupture tip.

To study this, we performed stick-slip experiments with a biaxial apparatus working in a direct shear configuration. The tested samples consist of two polymethylmethacrylate (PMMA) blocks generating, once put into contact, an artificial fault interface. Normal load (1 to 5 MPa) and an increasing shear load were applied, leading to spontaneous ruptures nucleation. Rupture was captured through a strain gauge rosettes array along the fault, allowing the measurement of local strain and stress fields at high recording frequency (2 MHz).

Different events occurring at different rupture speeds (100 to 900 m/s) were studied. At the strain gauge location, a dual strength weakening is observed, reflected in a scale dependent evolution of breakdown work with fault’ slip, contrarily to fracture energy which is, by definition, scale independent. This behavior, probably caused by thermal weakening (i.e. flash heating) activated during slip, is well described by the recently developed unconventional theory of frictional ruptures (i.e. rupture driven by a non-square root singularity). We demonstrate that such unconventional singularity emerges from velocity strengthening behavior, related to heat diffusion far from the rupture tip. Moreover, these experiments suggest that an analysis of the propagating rupture in the framework of Linear Elastic Fracture Mechanics, which assumes a square root singularity, could prove to be not always sufficiently exhaustive when frictional weakening occurs far from the rupture tip.

How to cite: Paglialunga, F., Passelègue, F., Lebihain, M., and Violay, M.: Contribution of thermal weakening in the frictional rupture dynamics, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6939, https://doi.org/10.5194/egusphere-egu22-6939, 2022.

With friction experiments, we investigate the rupture dynamics process, in particular the nucleation part, of laboratory earthquakes conducted on a periodically heterogeneous interface between two polycarbonate plates. Thanks to photoelasticity we follow the evolution of rupture front along the fault over time and we estimate the stress-drop with a strain gauge localized at the center of the fault.

We observe that the nucleation process generally does not consist of a monotonic growth observed on homogeneous cases, but of an alternation between slow and fast parts that accelerates until it reaches a point at which fast propagation dominates. Those alternations are correlated with the position of heterogeneities on the interface. Moreover, we observe that nucleation process of ruptures with smaller stress drop last longer than ruptures with a higher stress drop. Finally, we also point out a large variability in the rupture process due to the balance between the friction heterogeneity and the uncontrolled stress heterogeneity.

How to cite: Gounon, A., Latour, S., and Letort, J.: Experimental observations of rupture nucleation phase on heterogeneous interface, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7600, https://doi.org/10.5194/egusphere-egu22-7600, 2022.

During an earthquake, the elastic energy released from the Earth’s crust is partially radiated as seismic waves and partially dissipated along the tectonic fault. The dissipated energy can be divided into heat, which is produced by friction and other nonlinear mechanisms, and breakdown energy, which is associated with the dynamic weakening process of the fault. This breakdown energy is a key fault property as it directly affects nucleation, propagation and arrest of earthquake ruptures, and, hence, may control the size of an earthquake. However, the breakdown energy is difficult to measure directly on the fault and, therefore, it is routinely inferred from seismological measurements. A common observation is that the inferred breakdown energy, if positive-valued, scales with relative slip along the fault. In other words, larger earthquakes appear to dissipate more energy per unit rupture area through the weakening process, which typically occurs over very short slip distances. This would suggest that the earthquake rupture contains information about the final size of the earthquake starting from a very early stage of the earthquake, which is reasonably disputed in literature. In addition, the inferred breakdown energy is frequently observed to be negative-valued, which would violate thermodynamics. Therefore, we note that our current understanding of the seismologically inferred breakdown energy remains inconsistent. Here, we introduce a self-similar earthquake model that presents a similar scaling of the inferred breakdown energy despite constant and scale-independent fault properties (including the locally dissipated energy). We will show that the observed scaling is the result of a scale-invariant stress drop overshoot that distorts the global energy balance used for determining the breakdown energy. Therefore, our results suggest that the overall rupture mode – whether it is a crack-like or a pulse-like rupture – is a crucial factor for the inferred breakdown energy. Consequently, a pulse-like rupture, which is typically associated to stress drop undershoot, may explain the observed negative breakdown-energy values.

How to cite: Kammer, D., Ke, C.-Y., and McLaskey, G.: Breakdown energy scaling in a self-similar earthquake model, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2184, https://doi.org/10.5194/egusphere-egu22-2184, 2022.

It is relatively simple to calculate the magnitude of an earthquake after it has happened.  However, it is unclear if an earthquake 'knows' its final magnitude before rupture ends.  We are interested in whether earthquakes are deterministic: whether features of the initial stages of an earthquake make accurate predictions about the earthquakes' final size.

A major piece in the puzzle of determinism was proposed around 15 years ago by Olson and Allen (2005), who found a relationship between the predominant period of the early stages of an earthquake and its final magnitude. However, the results remain controversial, partly because Olsen and Allen (2005) analysed only 71 events. Here we aim to test their prediction in a statistically robust way using many more earthquakes, from a variety of settings. 

We calculate the predominant and average periods for several thousand earthquakes from around the world. Our preliminary results find a deterministic relationship, where both parameters increase with earthquake magnitude, but with a large scatter. They highlight the importance of filtering, and the parameters used to filter, as these have a significant effect on your final result. We are therefore now analysing the spectra of these earthquakes to look for patterns amongst them, and to better understand the physical basis of the predominant and average period calculations.

How to cite: Colquhoun, R. L. and Hawthorne, J. C.: Using seismograms to investigate earthquake determinism, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8306, https://doi.org/10.5194/egusphere-egu22-8306, 2022.

How to cite: Kemna, K. B. and Harrington, R. M.: Source Parameters for the Community Stress Drop Validation Study, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4302, https://doi.org/10.5194/egusphere-egu22-4302, 2022.

A spectral decomposition of the Fourier amplitude spectra is applied to determine the source parameters of earthquakes (source spectral shape, stress drop) that have occurred in central-southern Europe. About 52 million waveforms recorded in the target area since the late ‘90s have been downloaded from the European Integrated Data Archive (EIDA) within the tool stream2segment (Zaccarelli et al., 2019), by using the event catalog of the International Seismic Centre (ISC) and innovative data quality assessment. A non-parametric decomposition approach in this study introduced a regionalization for the attenuation models into two spatial domains (southern and “active” Europe, northern and “stable” Europe). For each domain, a spectral attenuation with hypocentral distance model is simultaneously determined and used to remove regional specific propagation effects from the spectra of recordings. Once isolated from local site effects, the obtained source spectra of 4380 earthquakes of magnitude larger than 2.5 are fitted to a standard -model to determine the seismic moment, corner frequency and Brune stress drop. The scaling relationship, spatial variation and variability of these source parameters are finally derived and discussed.

How to cite: Yen, M.-H., Bindi, D., Zaccarelli, R., Oth, A., Edwards, B., and Cotton, F.: Source Parameter Determination Using a Spectral Decomposition Approach in the central-southern Europe, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7759, https://doi.org/10.5194/egusphere-egu22-7759, 2022.

With the aim to investigate the rupture complexity and the radiated wave field of 2017, Mw 3.9, Ischia earthquake, south-west of Naples (Italy), we used finite-fault modeling to invert the near-source (<1-km epicentral distance) horizontal component velocity records of the accelerometric station (IOCA)
and searched for the best-fit kinematic rupture parameters. This analysis showed that the rupture nucleated at about 600 m west of IOCA and 1.1-km depth, along a 1 km, NW-SE striking fault (i.e., thrust with right-lateral component), with a rupture velocity of about 0.7 km/s. The retrieved rupture model coupled with multipath reverberations effects related to a thin, low-velocity near-surface volcanic sedimentary layer, well explains the observed long ground motion duration and the large amplitudes recorded all over the island. Finally, the apparent source time function (STF), obtained from inverse modeling using a theoretical Green’ function (GF), is validated by implementing an empirical GF (EGF) analysis.

How to cite: Nazeri, S., Zollo, A., Maria Adinolfi, G., Amoroso, O., and Picozzi, M.: The 2017 Ischia Earthquake (Southern Italy): Source Mechanism and Rupture Model From the inversion of a Near-Source Strong Motion Record, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1495, https://doi.org/10.5194/egusphere-egu22-1495, 2022.

At the northern Chilean subduction zone, where the IPOC network has been monitoring seismicity since 2007, we have identified multiple families of repeating earthquakes. High data quality and long observation time allow analyzing these sequences in detail.
Often repeaters are searched to be used as creep proxies and their spatio-temporal cumulative displacement is compared with the tectonic plate convergence rate or GPS based slip rate estimates for smaller fault patches. Repeaters can be classified into periodic, pseudo-periodic or aperiodic types. Put into relation with large earthquakes such as the 2014 M_W8.1 Iquique earthquake, repeaters may be described as continuous or burst type families. A precondition for such an analysis is that events are collocated and show highly similar mechanism. This is usually ensured via high cross correlation values between waveforms or by catalog location, or both. Errors in grouping would heavily bias the analysis for individual groups.
Therefore, we not only use cross correlation values, but we analyze the intra-family relations in detail. Events are relocated relative to each other based on phase based cross correlation refined s-p travel time differences. Rupture sizes are estimated and intra-family rupture histories are resolved. Having confirmed the characteristics of true repeating earthquake families in this way, we make the classifications and compute the slip rates mentioned above.
This study shows that the concept of repeating earthquakes holds beautifully in the case of the northern Chilean subduction zone. Repeater families repeatedly rupture the same patches, and they are observed to respond different to the 2014 Iquique with a strong dependence on their location. Particularly, the time around the Iquique megathrust event shows very interesting patterns in several families. We observe clear precursor patterns, burst reactions and unresponsive families simultaneously.

How to cite: Folesky, J., Hofman, R., and Kummerow, J.: On Repeating Earthquakes in the Northern Chilean Subduction Zone, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7316, https://doi.org/10.5194/egusphere-egu22-7316, 2022.

The 2011 Tohoku earthquake in northern Japan triggered thousands of aftershocks within a few days. The 2016 Amatrice-Visso-Norcia (AVN) earthquake sequence in the central Apennines (Italy) triggered hundreds of thousands of aftershocks in the first year, and the 2021 earthquakes in Greece (March 3, 2021 and in Crete on September 12, 2021) triggered numerous sizable aftershocks within a few days. In contrast, an earthquake 100 km east of the Crete earthquake (Oct. 12, 2021) generated almost no aftershocks. Additionally, great earthquakes in Pakistan (M7.8, 2011) and Iran (M7.7, 2013) also spawned no aftershocks.  These observations contradict generally accepted physical models for aftershock genesis.  

In this talk, I compare the rich AVN earthquake sequence with earthquakes that generate few aftershocks and demonstrate through modeling that aftershocks are driven by co-seismically generated (high-pressure) fluid sources through thermal decomposition. Earthquakes without trapped fluid sources at depth, or without thermal decomposition generate few, if any aftershocks.

The AVN sequence showed dramatic differences in aftershock rates along strike, with non-Omori type aftershock behavior. Using a non-linear diffusion model that captures permeability dynamics in the crust combined with a source term to account for thermal decomposition, we show excellent agreement between model and observations for the entire Italy AVN sequence.

How to cite: Gunatilake, T. and Miller, S. A.: The role of decarbonization and dehydration in aftershock genesis. , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10277, https://doi.org/10.5194/egusphere-egu22-10277, 2022.

Rupture source directivity and its potential frequency dependence remains an open question in seismology, especially for small-to-moderate earthquakes. 

In this research, we first calibrate a non-ergodic empirical model of the acceleration Fourier Amplitude Spectra (FAS), and then we adapt our tool in Spectral Amplitude (SA). Thanks to the large amount of high-quality seismic recordings (consisting of more than 400 earthquakes from magnitude 3.4 to 6.5, 460 stations, thus involving more than 30’000 waveforms in the time frame 2008-2018), we provide a statistical overview based on empirical evidence of seismological observations in the Central Italy area, which represents a unique natural laboratory for earthquakes occurring on normal faults. The non-ergodicity enables to remove all the other components of variability (i.e. the event-, site- and path-related) in the ground motion model (GMM) and hence allowing to better isolate the effects connected to source-directivity, that are not unaccounted in the epistemic variability of the ground motion. 

According to our criteria, about 36% of the analyzed events (162 out of 456) exhibits directivity. The distribution of the rupture directions is aligned, as expected, with the strikes of the major faults of the Central Apennines. We find that the directivity is a band-limited phenomenon, which spans from corner frequency (f_c) up to approximately 5 times the event’s f_c; in case of not very pronounced directivity, this band tends to be narrow, suggesting that the complex rupture processes at high-frequency are mainly stochastic. Moreover, we observe directivity not only during seismic sequences, but also in background seismicity. 

Preliminary results provide a useful hint regarding directivity’s parameterization as a frequency-dependent band-limited phenomenon, to be implemented in future ground motion modeling and scenario’s predictions. 

How to cite: Colavitti, L., Lanzano, G., Sgobba, S., Pacor, F., and Gallovič, F.: Empirical evidence of frequency-dependent directivity effects from small-to-moderate normal fault earthquakes in Central Italy, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5976, https://doi.org/10.5194/egusphere-egu22-5976, 2022.

Geomechanical modelling is widely used to simulate the triggering of induced earthquakes in a gas-producing region, such as in the Groningen gas field. Dynamic simulation can provide information on the process of dynamic rupture during earthquake nucleation and on the generated seismic wavefield. Through geomechanical modelling, one can investigate the effects of the model parameters, e.g., depletion pattern and friction parameters. In the modelling, the dynamic rupture at a finite fault is simulated both in space and time. The resulting seismic wavefield from such a finite source should be more realistic than that from a point source. Previous studies on the inversion of induced-earthquake data in the Groningen area usually assumed a point source. In the present research, we implement the full moment tensor inversion of the synthetic waveforms caused by the dynamic rupture of a geomechanically simulated finite fault. We then link this moment tensor to the moment tensor obtained from the inversion of field-seismic data for an earlier earthquake, using the same inversion approach in both cases. The inverted moment tensor from the field seismic observation serves as a constraint to our geomechanical simulation. This enables us to perform a more realistic simulation of an induced earthquake.

How to cite: Ruan, J., Masfara, L. O. M., Ghose, R., and Mulder, W.: 3D geomechanical modelling of induced seismicity: simulated finite-source to moment tensor inversion, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7296, https://doi.org/10.5194/egusphere-egu22-7296, 2022.

The Central Apennines is characterized by the presence of several active Plio-Quaternary normal faults, potentially capable of generating damaging earthquakes (as occurred in the recent past).

The seismicity registered for central Italy in the last 20 years by the seismic network of INGV (National Institute of Geophysics and Volcanology) highlights the presence of a regional seismic gap in the Sulmona and Caramanico Plio-Quaternary intermountain basins. In the study area, the magnitude of historical earthquakes ranges of from 5 to 6.8 Mw (from the 2nd century A.D. to 1933), while paleoseismological studies assigned a possible magnitude of 6.7 ± 0.1 to 4 earthquakes in the Sulmona basin (based on the fault length and the average of slip rate per event, estimated to be 1m). The high magnitude recorded for the destructive 1915 Avezzano earthquake (about 7 Mw), located in the Fucino basin (about 25 km to the west of the Sulmona basin), could suggest a similar potential seismic hazard also for the Sulmona and Caramanico normal faults. However, uncertainties remain on the activation mechanism related to the possible earthquake, expected in the study area. To reduce these uncertainties, we use a 3D modelling approach to perform a detailed calculation of the “active” rock volume of the hanging wall of the Sulmona and Caramanico faults (brittle volume), making an estimation of the possible maximum magnitude associated with these normal faults (testing different scenarios on the earthquake enucleation).

To reach this goal, a 3D structural and geological model was carried out starting from the available geological cartography, exploration wells, geophysical data (such as seismic sections and relocated earthquakes), and geological models from the literature. As a first step, several 2D balanced geological cross-sections were built across the Central Apennines to define the main structural picture at the regional scale (still discussed in literature). Cross-sections were built using MOVE (Petroleum Experts), while 3D modelling was completed using Petrel (Schlumberger) software. For the 3D modelling phase, the brittle-ductile transition (BDT) was used to localize the bottom of the potential brittle volumes at depth (assumed as maximum depth of the hypocenter). Following this methodology, the maximum magnitudes were estimated of the Sulmona (7.1 Mw, BDT at 17 km) and Caramanico (7.2 Mw, BDT at 20 km) normal faults. With the aim of simulating a more conservative scenario, the effects of a possible shallower structural cut-off for the Sulmona (8 km) and Caramanico (10 km) areas were investigated. The resulting reduced brittle volumes led to lower magnitude values estimate (6.6 Mw for the Sulmona, and 6.8 Mw for the Caramanico faults).

This approach allowed to make an estimate of the expected magnitudes for future seismic events associated to the Sulmona and Caramanico regional extensional faults, considering two different fault activation models (because of the regional structural uncertainties). Our work also demonstrates the importance of implementing robust 3D geological models to support seismogenic potential evaluation and seismic hazard studies.

How to cite: D'Ambrosio, A., Carminati, E., Doglioni, C., Lipparini, L., Anselmi, M., Cassola, T., and Derks, J. F.: 3D modelling approach for the mitigation of central Italy seismic hazard: the Sulmona and Caramanico case studies., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8117, https://doi.org/10.5194/egusphere-egu22-8117, 2022.