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.

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.

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.

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.