Displays

The recent finding of prompt elastogravity signals (PEGS) before the arrival of P-waves, associated with the M9.1 2011 Tohoku earthquake (Montagner et al., Nat. Comm., 2016; Vallée et al., Science, 2017) and a few earthquakes of magnitude larger than 8.5  (Vallée and Juhel, JGR, 2019) opens the new field of speed-of-light seismology.  The systematic detection of PEGS on real-time might help saving a few seconds before the arrival of destructive seismic waves and to obtain an accurate determination of the magnitude of the earthquake at the end of rupture. So the potential application to earthquake early warning is obvious.

However, the use of classical broadband seismometers for detecting PEGS has severe limitations for detecting earthquakes of magnitude smaller than 8.5: first of all, the background seismic noise and second of all, the partial cancellation of the gravitational perturbation by the inertial induced acceleration recorded by seismometers (Heaton, Nature Comm., 2017). Two different approaches can be explored for detecting for earthquakes of magnitude smaller than 8.5. Either, by using a dense array of broadband seismometers  (more than 100 receivers) or by designing completely new instruments such as gravity strainmeters. These new detectors must be able to measure the difference in gravity acceleration between two masses, making this instrument isolated from the seismic noise. A sensitivity of 10^-15 Hz^-1/2 at 0.1 Hz is required for detecting earthquakes of M>7 (Juhel et al., JGR, 2019) and the technology developed by the gravitational wave physicists can be used for reaching such a sensitivity. The simulation of the expected gravity strain signals based on analytical model of gravity perturbations associated with a network-based matched filter approach show that a network of 3 gravity strainmeters might make it possible to reach such a challenging goal. Gravity strainmeters could therefore open new ways to investigate the first seconds of the earthquake rupture, speed up the estimate of earthquake magnitude, enhance tsunami warning systems and  complement other EEWS in the future.

How to cite: Juhel, K., Montagner, J.-P., Ampuero, J.-P., Barsuglia, M., Bernard, P., Losurdo, G., and Vallée, M.: A new instrument in earthquake early warning system by detection and modeling of prompt gravity signals, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5587, https://doi.org/10.5194/egusphere-egu2020-5587, 2020.

Coseismically displaced rock fragments (chips) in the near-field (less than 5 km) of the 2016 moment magnitude (M_W) 6.1 Petermann earthquake (Australia) preserve directionality of strong ground motions. Displacement data from 1437 chips collected over an area of 100 km² along and across the Petermann surface rupture is interpreted to record combinations of co-seismic directed permanent ground displacements associated with elastic rebound (fling) and transient  ground shaking, with intensities of motion increasing with proximity to the surface rupture. The observations provide a proxy test for available models for directionality of near-field reverse fault strong ground motions in the absence of instrumental data. This study provides a dense proxy record of strong ground motions at less than 5 km distance from a surface rupturing reverse earthquake, and may help test models of near-field dynamic and static pulse-like strong ground motion for dip-slip earthquakes.

How to cite: King, T., Quigley, M., and Clark, D.: Near-field directionality of earthquake strong ground motions measured by displaced geological objects, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12092, https://doi.org/10.5194/egusphere-egu2020-12092, 2020.

The objective of our study is the improvement of shallow rupture characterization in kinematic rupture models used in strong ground motion simulations. Based on geological investigations, earthquake stress drop, depth-variation of seismicity, as well as recorded near-fault ground motion, there is clear evidence for depth variation of frictional properties of crustal materials. The material ductility in the weak zone (upper 3-5 km of the crust) and the transition from ductile state to brittle state in the upper seismogenic zone, determine how the fracture energy is consumed by the earthquake rupture, and  how generated seismic energy is distributed in space and time.

Using plausible stress models for crustal ruptures, we performed dynamic rupture simulations on vertical strike slip faults that break the free surface. We used a 3D staggered-grid finite-difference method (Pitarka and Dalguer, 2009) and regional 1D velocity model. The stress drop as a function of slip was modeled using a linear slip weakening frictional law that reflects the depth and lateral variations of frictional properties of crustal materials. Through dynamic rupture modeling we were able to extract kinematic rupture characteristics, such as changes in the shape of slip rate functions, rupture velocity, and peak slip  rate across the weak zone, and in the slip asperity areas. These results were then used to refine our existing rupture generating model (Graves and Pitarka, 2016) for crustal  earthquakes. The modifications to the rupture generator code include changes to the shape of slip-rate function at shallow depths, rise time variation with depth and stronger correlation with slip at shallow depths.

The effects of the new characterization of shallow rupture kinematics on simulated ground motion was thoroughly investigated in broad-band (0-10Hz) simulations of the M7.1 2019 Ridgecrest California earthquake. The ground motion time histories were computed using the hybrid method of Graves and Pitarka (2010. In our simulations we considered several slip distributions, including two that were obtained by inverting recorded velocity and displacement ground motion, respectively.  Finally, through comparisons with recorded data, we analyzed the sensitivity of computed near-fault broad-band ground motion characteristics, including amplitude of ground motion velocity pulse, peak acceleration, and response spectra, to shallow slip characterization and location of strong motion generation areas for each rupture model. The proposed modifications to kinematic rupture models of crustal earthquakes provide improved simulation of broadband strong ground motion and seismic hazard assessment.

This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344

How to cite: Pitarka, A. and Graves, R.: Characterization of Shallow Rupture Kinematics in Strong Ground Motion Simulations of Strike-Slip Earthquakes Constrained by Dynamic Rupture Modeling, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4180, https://doi.org/10.5194/egusphere-egu2020-4180, 2020.

How to cite: de la Puente, J., Monterrubio-Velasco, M., Rodríguez-Pérez, Q., Zúñiga, F. R., and Rojas, O.: Stress transfer process in doublet events studied by numerical TREMOL simulations: Study case Ometepec 1982 Doublet., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6782, https://doi.org/10.5194/egusphere-egu2020-6782, 2020.

How do earthquakes start? What are the parameters influencing fault evolutions? What are the local parameters controlling the seismic or aseismic character of slip?

To predict the dynamic behaviour of faults, it is important to understand slip mechanisms and their source. Lab or in-situ experiments can be very helpful, but tribological experience has shown that it is complicated to install local sensors inside a mechanical contact, and that they could disturb the behaviour of the sheared medium. Even with technical improvements on lab tools, some interesting data regarding gouge kinematics and rheology remains very difficult or impossible to obtain. Numerical modelling seems to be another way of understanding physics of earthquakes.

Fault zone usually present a granular gouge, coming from the wear material of previous slips. That is why, in this study, we present a numerical model to observe the evolution and behaviours of fault gouges. We chose to focus on physics of contacts inside a granular gouge at a millimetre-scale, studying contact interactions and friction coefficient between the different bodies. In order to get access to this kind of information, we implement a 2D granular fault gouge with Discrete Element Modelling in the software MELODY (Mollon, 2016). The gouge model involves two rough surfaces representing the rock walls separated by the granular gouge.

One of the interests of this code is its ability to represent realistic non-circular grain shapes with a Fourier-Voronoï method (Mollon et al., 2012). As most of the simulations reported in the literature use circular (2D)/spherical (3D) grains, we wanted to analyse numerically the contribution of angular grains. We confirm that they lead to higher friction coefficients and different global behaviours (Mair et al., 2002), (Guo et al., 2004).

In a first model, we investigate dry contacts to spotlight the influence of inter-particular cohesion and small particles on slip behaviour and static friction. A second model is carried out to observe aseismic and seismic slips occurring within the gouge. As stability depends on the interplay between the peak of static friction and the stiffness of the surrounding medium, the model includes the stiffness of the loading apparatus on the rock walls.

The work presented here focuses on millimetre-scale phenomena, but the employed model cannot be extended to the scale of the entire fault network, for computational cost reasons. It is expected, however, that it will lead to a better understanding of local behaviours that may be injected as simplified interface laws in larger-scale simulations.

How to cite: Casas, N., Mollon, G., and Daouadji, A.: A small-scale numerical study of fault slip mechanisms using DEM, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8976, https://doi.org/10.5194/egusphere-egu2020-8976, 2020.

Off-fault damage is observed around fault cores in a wide range of length scales, which is identified as an aggregation of localized fractures via geological and geodetic observations, or as low-velocity zone via seismological tomography. However, its seismological observables in earthquake traces, e.g. change in source spectra and/or radiation pattern, remains to be investigated. 

Okubo et al. (2019) proposed an approach framework of physics-based dynamic earthquake rupture modeling with coseismic off-fault damage using the combined finite-discrete element method (FDEM). It shows a non-negligible contribution of coseismic damage to rupture dynamics, high-frequency radiation and overall energy budget, whereas the model domain is limited in the near-field region. This study efficiently computes intermediate- and far-field radiation propagating from earthquake sources with coseismic off-fault damage, and to identify its signature in the seismic traces.

We first conduct the dynamic earthquake rupture with coseismic damage and compute synthetic near-field radiation using FDEM-based software tool, HOSSedu, developed by Los Alamos National Laboratory. We then couple the output of HOSSedu to SPECFEM2D in order to compute intermediate- and far-field radiation. The HOSS-SPECFEM2D coupling can resolve complexities over wide range of length scales associated with earthquake sources with coseismic damage and wave propagation.

We conduct 2D dynamic earthquake rupture modeling with a finite planar fault as canonical simplest model. The comparison between the cases with and without allowing for coseismic off-fault damage shows differences in intermediate- and far-field radiation. 1) High-frequency components in ground motion are enhanced all around the fault. 2) The rupture arresting phase, which clearly appears at the stations located orthogonal to the fault for the case without off-fault damage, is damped due to the smoothed rupture arrest by coseismic damage around fault edges. 3) Radiated energy is enhanced in the direction parallel to the fault due to the substantial damage around fault edges.

These fundamental observables will help identify the existence of coseismic off-fault damage in real earthquakes. It would also contribute to resolve the mechanisms of earthquake sources and the potential distribution of aftershock locations. We also attempt to replace the planar fault to the real fault geometry, e.g. the fault system associated with the 2019 Ridgecrest earthquake sequence, and will investigate the signature of off-fault damage in the seismic traces recorded in intermediate- and far-field range.

How to cite: Okubo, K., S. Bhat, H., Rougier, E., and A. Denolle, M.: Signature of coseismic off-fault damage in intermediate- and far-field radiation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10243, https://doi.org/10.5194/egusphere-egu2020-10243, 2020.

We present simulations of dynamic ruptures in a continuum damage-breakage rheological model and waves radiated by the ruptures observed in the far field. The model combines aspects of a continuum viscoelastic damage framework for brittle solids with a continuum breakage mechanics for granular flow. The brittle instability is associated with a phase transition between a damaged solid with distributed cracks and a granular medium within the generated rupture zone. The formulation significantly extends the ability to model brittle processes in structures with complex volumetric geometries and evolving elastic properties, compared to the traditional models of pre-existing frictional surface(s) in a solid with fixed properties. A set of numerical simulations examines the sensitivity of dynamic ruptures, seismic source properties and radiated waves to material properties controlling the coupled damage-breakage evolution, the thickness and geometry of the damage zone, and fluidity of the granular material. The simulations are performed in two stages. First, details of the rupture process are simulated using adaptive fine grid model. The results of these simulations include source parameters such as rupture velocity, potency, stress and strain drop, heat generation, and others. In the second stage, the obtained velocity source function is used for simulating radiated seismic waves and synthetic seismograms sampled by stations around the rupture zone and in the far field.

Detailed comparisons between the simulated source properties and those obtained by analyzing the synthetic seismograms demonstrate the relations between different source processes and inferred seismic parameters (potency, strain drop, directivity, rupture velocity, corner frequency, and others). One main effect shown in these simulations emphasizes the important role of rock damage and granulation process generating dynamic expansion-compaction around the process-zone. This expansion-compaction process leads to isotropic source term, while shear motion that accumulates behind the propagating front produces deviatoric deformation and shear heating behind the rupture front. Changing through our simulations, source geometries, and fault zone properties, we demonstrate that the process-zone dissipation due to the damage-breakage mechanism, and the isotropic source component, significantly affect the radiation pattern, rupture directivity, S/P energy partitioning, seismic potency and moment, and more. The results are significant for understanding better the proper usage and limitations of methods applied within the observational framework of earthquake seismology.

How to cite: Lyakhovsky, V., Kurzon, I., and Ben-Zion, Y.: Dynamic rupture and seismic radiation in a damage-breakage rheology model, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4044, https://doi.org/10.5194/egusphere-egu2020-4044, 2020.

The near source strong ground motions of the 2013 MS 7.0 Lushan, China, earthquake were simulated using empirical Green's function (EFG) method. At first, we estimated the amount and location of strong motion generation areas (SMGA) based on the character of both slip distributions from far-field seismic inversion and the envelopes of recorded acceleration from main shock, and determined the amount of subfaults on SMGAs referring to the scaling law introduced by Somerville et al.. Then, we implemented the genetic algorithm searching for the optimized source parameters. Based on the source models, we synthetized the waveforms for the 30 stations selected near the source region. Our results show that the comparison between the synthetic waveforms and the observed records agree very well with each other, especially for the part of high-frequency larger than 1.0 Hz. We found that there are two obvious SMGAs on the fault, which take the position that the asperities from far-field seismic inversion take. The combined strong motion generation areas we obtained were smaller than those values predicted by extension of the scaling law by Somerville et al..

How to cite: Zhang, W. and Yu, X.: Strong ground motion simulation of the 2013 MS 7.0 Lushan, China, earthquake, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1800, https://doi.org/10.5194/egusphere-egu2020-1800, 2020.

Rate- and state-dependent friction laws (RSF laws) are empirical laws derived from laboratory experiments related to rock friction. They have been used to quantitatively describe complex fault friction processes. With a combination of the RSF laws and the McKenzie-Brune frictional heat generation model, we have studied the effects of frictional heating processs on the fault strength variation and temporal evolution of temperature based on the spring-slider-fault system subjected to Ruina and Chester-Higgs RSF laws. The system equations are solved efficiently by Dormand-Prince method with adaptive steps. First, with a comparison to the Ruina- model in which the temperature effect due to frictional heating on frictional strength is neglected, the numerical results show that the fault will be unstable slightly earlier for the Chester-Higgs- model in which the temperature effect due to frictional heating on frictional strength is taken into consideration, which indicates that the rise of temperature caused by frictional heating can lead to a slight time advance of fault instability. Second, by contrast with Ruina- model, the frictional strength will keep a little bit higher for the Chester-Higgs- model when the fault sliding at high speed, indicating that frictional heat can strengthen faults to a certain extent. Third, the simulation results also suggest that, at the same rupture velocity, the temperature change for the Chester-Higgs- model is much smaller than that given by the Ruina- model, indicating that frictional heat can also restrain the sharp rise of temperature on fault surface. In addition, under the same parameters and initial conditions, the seismic occurrence time giving by the Chester-Higgs- model is obviously shorter than that by the Ruina- model, indicating that a significant effect of friction heating generated on entire fault temporal evolution could greatly reduce the seismic recurrence time. Correspondingly, both static stress drop and total slip resulted from the Chester-Higgs- model is also smaller than that from the Ruina- model, respectively.

How to cite: Gao, Y. and Shi, B.: Numerical investigation of frictional heating effect on the earthquake faulting based on the Ruina- and Chester-Higgs- models, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3226, https://doi.org/10.5194/egusphere-egu2020-3226, 2020.

The rise of observations from Distributed Acoustic Sensing (e.g., Zhan 2020) and high-rate GNSS networks (e.g., Madariaga et al., 2019) highlight the potential of dense ground motion observations in the near-field of large earthquakes. Here, spectral analysis of >100,000 synthetic near-field strong motion waveforms (up to 2 Hz) is presented in terms of directivity, corner frequency, fall-off rate, moment estimates and static displacements.

The waveforms are generated in 3‐D large-scale dynamic rupture simulations which incorporate the interplay of complex fault geometry, topography, 3‐D rheology and viscoelastic attenuation (Wollherr et al., 2019). A preferred scenario accounts for off-fault deformation and reproduces a broad range of observations, including final slip distribution, shallow slip deficits, and spontaneous rupture termination and transfers between fault segments. We examine the effects of variations in modeling parameterization within a suite of scenarios including purely elastic setups and models neglecting viscoelastic attenuation. 

First, near-field corner frequency mapping implementing a novel spectral seismological misfit criterion reveals rays of elevated corner frequencies radiating from each slipping fault at 45 degree to rupture forward direction. The azimuthal spectral variations are specifically dominant in the vertical components indicating we map rays of direct P-waves prevailing (Hanks, 1980). The spatial variation in corner frequencies carries information on co-seismic fault segmentation, slip distribution, focal mechanisms and stress drop. Second, spectral fall-off rates are variably inferred during picking the associated corner frequencies to identify the crossover from near-field to far-field spectral behaviour in dependence on distance and azimuth. Third, we determine static displacements with the help of near-field seismic spectra.

Our findings highlight the future potential of spectral analysis of spatially dense (low frequency) ground motion observations for inferring earthquake kinematics and understanding earthquake physics directly from near-field data; while synthetic studies are crucial to identify "what to look for" in the vast amount of data generated.

References:

Hanks, T.C., 1980. The corner frequency shift, earthquake source models and Q.

Madariaga, R., Ruiz, S., Rivera, E., Leyton, F. and Baez, J.C., 2019. Near-field spectra of large earthquakes. Pure and Applied Geophysics, 176(3), pp.983-1001.

Wollherr, S., Gabriel, A.-A. and Mai, P.M., 2019.  Landers 1992 “reloaded”: Integrative dynamic earthquake rupture modeling. Journal of Geophysical Research: Solid Earth, 124(7), pp.6666-6702.

Zhan, Z., 2020. Distributed Acoustic Sensing Turns Fiber‐Optic Cables into Sensitive Seismic Antennas. Seismological Research Letters, 91(1), pp.1-15.

How to cite: Schliwa, N. and Gabriel, A.-A.: Near-field spectral analysis of data-integrative dynamic rupture earthquake simulations of the 1992 Landers earthquake , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22673, https://doi.org/10.5194/egusphere-egu2020-22673, 2020.

An M 5.4 earthquake occurred in the southeastern part of the Korean Peninsula in 2017. It is an oblique thrust event that occurred at a relatively shallow depth (~ 5 km) although it did not create coseismic surface rupture. A coseismic slip model was successfully obtained by inverting the ground displacement field extracted by the InSAR data (Song and Lee, 2019). In this study, we performed spontaneous dynamic rupture modeling using the slip weakening friction law. The static stress drop distribution obtained by the coseismic slip model was used as an input stress field. We adopted high performance computing (HPC) using the parallelized dynamic rupture modeling code (SORD, Support Operator Rupture Dynamics). Although our target event is moderate-sized one, we can successfully produce a spontaneous dynamic rupture model using a relatively small initial nucleation patch (radius ~ 1 km) with a relatively small slip weakening distance (~ 5 cm). Our preliminary results show that the rupture creates an asperity near the initial nucleation zone with approximately 4 MPa stress drop, then propagates obliquely upward both in the northeast and southwest directions. Although we assumed a single planar fault plane in our current rupture modeling, it seems worthwhile to dynamically model the rupture process, including complex fault geometry in following studies. Dynamic rupture modeling for a natural earthquake provides an opportunity to understand the dynamic rupture characteristics of the earthquake, including both stress drop and fracture energy.

How to cite: Song, S. G., Cho, C. S., and Ely, G.: Spontaneous dynamic rupture modeling of 2017 M 5.4 Pohang, South Korea, earthquake, using the slip-weakening friction law, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12490, https://doi.org/10.5194/egusphere-egu2020-12490, 2020.

Soil layer shear wave velocity is a key parameter in numerical simulation models of ground motion of various sites. For three-dimensional models, there is a high cost to measure the shear wave velocity. It is a common method to estimate the shear wave velocity by a empirical relationship of depth and velocity depends on several drilling data. This paper studies the depth-shear wave velocity empirical relationships of various soil layers in Yuxi, Qingdao, and Fuzhou. It is found that the correlation degree between depth and shear wave velocity is higher in the soil layers with obvious grain characteristics, such as breccia layer, round gravel layer, gravel layer and fine sand layer, and the error of the empirical relationship is lower. Conversely, the correlation degree is lower and the error of the empirical relationship is high in clay layers. The possible reason for this phenomenon is: the layer description in the drilled histogram cannot represent the clay layers with different properties effectively.

For soil layers with obvious particle characteristics, the shear wave velocity has a significant positive correlation with the particle size. The size of the sediment particles is related to the carrying capacity of the surface water. A larger the water flow and faster flow velocity lead to a larger sediment particles. Therefore, this paper considers that the shear wave velocity of the soil layer in the study area is related to the hydrodynamic deposition environment. Smaller sediments carry longer distances in the water stream, resulting in lower sedimentary layer wave velocity; larger sediment particles carry shorter distances in the water stream, resulting in higher sedimentary layer wave velocity. Further analysis shows that the shear wave velocity of the clay layer has a certain relationship with the particle characteristics of the other soil layers in the same drill. In the environment where the sedimentary soil layer with larger particles is formed, the shear wave velocity of the clay layer is also higher. This article discusses this phenomenon and further analyzes the influence of the porosity ratio of the clay layer on its depth-shear wave velocity empirical relationship in the Yuxi area. It is found that the void ratio of the clay layer has a negative correlation with its shear wave velocity. The depth-shear wave velocity empirical relationship of the clay layer in Yuxi area was modified to improve accuracy.

The study of the relationship between the sedimentary characteristics, particle characteristics of the soil layer and the shear wave velocity, a key factor in the site conditions, is an attempt to improve the accuracy of geophysical model parameters using geological data. In the research of numerical simulation of site ground motion, it is possible to use abundant geological data to supplement models using few geophysical exploration data, or areas where it is difficult to carry out geophysical exploration, and it has certain application value.

How to cite: Li, T., Chen, X., and Li, Z.: Study on the Relationship Between Void Ratio and Empirical Relationship of Depth and S-WAVE Velocity in Clay, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12157, https://doi.org/10.5194/egusphere-egu2020-12157, 2020.

Dynamic source inversions of individual earthquakes provide constraints on stress and frictional parameters, which are inherent to the studied event. However, general characteristics of both kinematic and dynamic rupture parameters are not well known, especially in terms of their variability. Here we constrain them by creating and analyzing a synthetic event database of dynamic rupture models that generate waveforms compatible with strong ground motions in a statistical sense.

We employ a framework that is similar to the Bayesian dynamic source inversion by Gallovič et al. (2019). Instead of waveforms of a single event, the data are represented by Ground Motion Prediction Equations (GMPEs), namely NGA-West2  (Boore et al., 2014). The Markov chain Monte Carlo technique produces samples of the dynamic source parameters with heterogeneous distribution on a fault. For all simulations, we assume a vertical 36x20km strike-slip fault, which limits our maximum magnitude to Mw<7. For dynamic rupture calculations, we employ upgraded finite-difference code FD3D_TSN (Premus et al., 2020) with linear slip-weakening friction law. Seismograms are calculated on a regular grid of phantom stations assuming a 1D velocity model using precalculated full wavefield Green's functions. The procedure results in a database with those dynamic rupture models that generate ground motions compatible with the GMPEs (acceleration response spectra in period band 0.5-5s) in terms of both median and variability.

The events exhibit various magnitudes and degrees of complexity (e.g. one or more asperities). We inspect seismologically determinable parameters, such as duration, moment rate spectrum, stress drop, size of the ruptured area, and energy budget, including their variabilities.  Comparison with empirically derived values and scaling relations suggests that the events are compatible with real earthquakes (Brune, 1970, Kanamori and Brodsky, 2004). Moreover, we investigate the stress and frictional parameters in terms of their scaling, power spectral densities, and possible correlations. The inferred statistical properties of the dynamic source parameters can be used for physics-based strong-motion modeling in seismic hazard assessment.

How to cite: Gallovic, F. and Valentova, L.: Rupture parameters of dynamic source models compatible with NGA-West2 GMPEs, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-16610, https://doi.org/10.5194/egusphere-egu2020-16610, 2020.

Probabilistic seismic hazard assessment (PSHA) is widely used to generate national seismic hazard maps, design building codes for earthquake resilient structures, determine earthquake insurance rates, and in general for the management of seismic risk. However, standard PSHA is generally based on empirical, time-independent assumptions that are simplified and not based on earthquake physics. Physics-based numerical models such as dynamic rupture simulations account for the non-linear coupling of source, path and site effects, which can be significant in their respective contributions depending on the generally complex geological environment (e.g., Wollherr et al., 2019), and could potentially complement standard PSHA. In this study we demonstrate the benefits of such an approach by modeling various rupture scenarios in the complex Húsavík–Flatey fault zone (HFFZ), Northern Iceland. The HFFZ consists of multiple right-lateral strike slip segments distributed across ~100 km. The moment accumulated on the HFF since the last major earthquake in 1872 can result in an earthquake of magnitude 6.8 to 7 (Metzger and Jonsson, 2014) posing a high risk to Húsavík’s community, flourishing tourism and heavy industry.

We perform high-resolution 3D dynamic rupture simulations using the open-source software SeisSol (www.seissol.org), which can efficiently model spontaneous earthquake rupture across complex fault networks and seismic wave propagation with high order accuracy in space and time. Our models incorporate regional topography, bathymetry, 3D subsurface structure and varying models of the complex fault network while accounting for off-fault damage.

Synthetic ground motions suggest highly heterogenous radiation patterns and intense localization of shaking in the vicinity of geometric complexities, such as fault bends or rupture transition between segments. In our models, the hypocenter location does not affect the plausible moment magnitude of large events. However, changes in rupture directivity affect the spatial distribution of ground motion significantly.  We run hundreds of dynamic rupture scenarios to generate a physics-based dynamic earthquake catalog of mechanically plausible events. Based on this, we identify a possible maximum magnitude earthquake and generate model-based ground motion prediction equations to complement standard empirical ground motion models. In addition, we use the open-source python code SHERIFs (Chartier et al., 2019) to estimate the likelihood of each rupture event, which is mainly constrained by the fault slip rate estimated and fault-to-fault (f2f) rupture scenarios that are determined by the dynamic simulations. Finally, combining the fault seismic rates and the f2f probabilities with dynamic rupture scenarios and the OpenQuake framework allows us to perform physics-based PSHA for the HFFZ, the largest strike-slip fault in Iceland.

How to cite: Li, B., Gabriel, A.-A., Wirp, S. A., Chartier, T., Ulrich, T., and Halldórsson, B.: Physics-based constraints for probabilistic seismic hazard assessment in Húsavík–Flatey fault zone, Northern Iceland, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19007, https://doi.org/10.5194/egusphere-egu2020-19007, 2020.

Earthquakes are a multi-scale, multi-physics problem. For the last decades, earthquakes have been modeled as a sudden displacement discontinuity across a simplified (potentially heterogeneous) surface of infinitesimal thickness in the framework of linear elastodynamics. Thus, earthquake models are commonly forced to distinguish artificially between on-fault frictional failure and the off-fault response of rock. 

While complex volumetric failure patterns of fault networks are observed from well-recorded large earthquakes (e.g., the 2016 M_w7.8 Kaikōura event, Klinger et al. 2018) and small earthquakes (e.g., events in the San Jacinto Fault Zone, Cheng et al. 2018) as well as in laboratory experiments (e.g., in high-velocity friction experiments, Passelègue et al., 2016) inelastic deformation within a larger volume around the fault is generally neglected when studying kinematics, dynamics and the energy budget of earthquakes. Fault behaviour is then dominantly controlled by lab-derived friction on a surface. Recent 2D collapsing of material properties, stresses, geometry, and strength conditions from seismo-thermo-mechanical models to elastodynamic frictional interfaces illustrated resulting earthquake complexity and modeling challenges (van Zelst et al., 2019).

To understand the mechanics of slip in extended fault zones the ERC project TEAR (https://www.tear-erc.eu) aims to solve the governing equations of earthquake sources based on the conservation of mass, momentum and energy and rheological models for generalized visco-elasto-plastic materials. We here present (i) 2D numerical experiments of rupture dynamics and displacement decoupling under loading for varying fault zone properties resembling observations from the San Jacinto Fault Zone in a weak discontinuity approach  sing a diffuse fault representation (adapted stress-glut approach, Madariaga et al., 1998) within a PETSc spectral element discretisation of the seismic wave equation; (ii) Verification of modeling rupture dynamics using a novel diffuse interface approach using ExaHyPE (www.exahype.eu, Reinarz et al. 2019) that allows spontaneous, finite crack formation (Tavelli et al., in prep.) and adaptive mesh refinement (AMR) zooming into the process zone at the rupture tip.

By this means, we start exploring scalable software for modelling shear rupture across extended, spontaneously developing fault systems for testing the hypothesis, that earthquake dynamics in fault zones can be jointly captured based on the theory of generalized visco-elasto-plastic materials.

References:

• Cheng, Y. et al. Diverse volumetric faulting patterns in the San Jacinto fault zone. JGR: Solid Earth, 123.6, 5068-5081 (2018). https://doi.org/10.1029/2017JB015408
• Klinger, Y. et al. Earthquake damage patterns resolve complex rupture processes. GRL, 45, 10,279– 10,287 (2018). https://doi.org/10.1029/2018GL078842
• Madariaga, R. et al. Modeling dynamic rupture in a 3D earthquake fault model. BSSA, 88.5 (1998): 1182-1197.
• Passelègue, F. X. et al. Frictional evolution, acoustic emissions activity, and off‐fault damage in simulated faults sheared at seismic slip rates. JGR: Solid Earth, 121(10), 7490-7513 (2016). doi:10.1002/2016JB012988
• Reinarz, A. et al. ExaHyPE: An Engine for Parallel Dynamically Adaptive Simulations of Wave Problems. arXiv preprint (2019), arXiv:1905.07987.
• Tavelli, M. et al. Space-time adaptive ADER discontinuous Galerkin schemes for nonlinear hyperelasticity with material failure, in prep.
• Van Zelst, I. et al. Modeling Megathrust Earthquakes Across Scales: One-way Coupling From Geodynamics and Seismic Cycles to Dynamic Rupture. JGR: Solid Earth, 124, 11414–11446 (2019). https://doi.org/10.1029/2019JB017539

How to cite: Hayek Valencia, J. N., Li, D., May, D. A., and Gabriel, A.-A.: Modeling earthquake rupture dynamics across diffuse deforming fault zones, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20600, https://doi.org/10.5194/egusphere-egu2020-20600, 2020.

Induced seismicity as a result of natural gas production is a major challenge from both an industrial and a societal perspective. The compaction caused by gas production leads to changes of the effective pressure fields in the reservoir and stress redistributions occur particularly in the surrounding faults. In addition, the strong coupling between fluid flow and solid rock deformations and the role of fluid flow regarding the frictional properties of the faults necessitate a coupled and comprehensive modeling framework. A general and fully coupled thermo-hydro-mechanical finite difference formulation is developed herein and the results are verified against numerical benchmarks. A visco-elasto-plastic rheological behavior is assumed for the bulk material and a return-mapping algorithm is implemented for accurate simulation of the stress evolution. The geometrical features of the faults are incorporated into a regularized continuum framework, while the response of the fault zone is governed by a rate-and-state-dependent friction model. Numerical simulations are provided for large-scale problems and their efficiency is assured through the evaluation of the consistently linearized systems of equations along with the use of advanced numerical solvers and parallel computing. Although the proposed framework is a step towards the modeling of earthquake sequences for induced seismicity applications, the features of the numerical model are highlighted for other applications, including seismic events in subduction settings where the role of fluid flow inside the faults is considerable. Another application of the present, fully coupled hydro-thermo-mechanical formulation is the prediction of the fluid pressurization phenomena, where the frictional heating increases the magnitude of the pore fluid pressure inside the faults, and the resultant degradation of dynamic frictional strength is naturally captured. 

How to cite: Goudarzi, M., van Dinther, Y., Li, M., de Borst, R., Pranger, C., Gerya, T., Petrini, C., and Vossepoel, F.: Towards coupling fluid flow and rate-and-state friction in compacting visco-poro-elasto-plastic reservoirs, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11757, https://doi.org/10.5194/egusphere-egu2020-11757, 2020.

In 1999, Chi-Chi earthquake hit Taiwan and caused severe damage to the infrastructures along the Chelungpu fault because of overburden deformation. Previous study excavated several trenches near the Chelungpu fault to study the fault characteristics and the fault deformation zone. The most important trench, Chushan site, records the Chi-Chi earthquake with 1.7m vertical offset and other four large paleoseismic events. This fault trench was now retained in the Chelungpu Fault Preservation Park, Taiwan that greatly contributes to observing the deformation pattern of overburden layer induced by repeated thrust faulting. For the north wall of the Chushan trench, the east-dipping basal thrust with a dip angle of 24° splits into two branches and the sedimentary layer, which consists of silt layer and gravel layer, is deformed into an asymmetric anticline fold. This observation indicates that the overburden layer in natural is the composite strata and the presence of gravel layer in the composite strata could be an indicator for the coseismic deformation.

In this study, three-dimensional DEM simulations are conducted to identify the deformation pattern of composite strata under repeated thrust faulting. The numerical model was constructed based on the Chushan trench. Silt layers are made by balls and the gravel layer is compose of balls and ellipsoid particles. Results show that a fault-propagation fold forms during the initial stage of the deformation, and an asymmetric anticline fold with one limb slightly overturned forms in the Chi-Chi earthquake. The rotation of ellipsoid particles in the numerical model indicates the evolution of folding, which conduces to understand the deformation progress in the full faulting process.

How to cite: Hung, C.-H., Lin, C.-H., and Lin, M.-L.: Discrete Element Modeling on Deformation Pattern of Composite Strata Induced by Repeated Thrust Faulting: Case Study of Chushan Site, Central Taiwan, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6377, https://doi.org/10.5194/egusphere-egu2020-6377, 2020.

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 take advantage of new numerical algorithms for dynamic modeling of earthquake rupture to confirm and document this opening effect. 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. Through a systematic analysis of case studies, we investigate the effect of fault geometry, friction parameters and rupture behavior on the deformation pattern. Fault opening is observed in all simulations, growing dramatically as the rupture reaches the surface. Evolution of slip, fault-normal displacement and velocities, and of the predicted surface displacements and velocities are documented for each simulation. These predictions will serve as synthetic data when comparing with recorded surface deformation from real-case earthquakes.

How to cite: Bruhat, L., Rougier, E., Okubo, K., and Bhat, H. S.: Fault opening related to free surface interaction on reverse faults: insights from numerical modeling , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9691, https://doi.org/10.5194/egusphere-egu2020-9691, 2020.

We present combined 3D dynamic rupture scenarios of the 2019 M_w6.4 Searles Valley and M_w7.1 Ridgecrest earthquakes closely constrained by observations, incorporating complex subsurface material properties, high-resolution topography and off-fault plastic deformation empowered by supercomputing. A detailed 3D non-vertical fault model of the active quasi-orthogonal intersecting fault network is built by integrating relocated aftershocks and surface ruptures constrained by space geodesy and field observations. All faults are exposed to a 3D SCEC community stress model as well as long- and short-term static and dynamic stress transfers, which impact rupture dynamics, particularly in the vicinity of complexities in fault geometry.

By assuming apparently weak faults due to the effect of rapid velocity-weakening friction and elevated fluid pressure, we determine initial stresses and fault strength. Multi-fault rupture directivity and velocity of both events are constrained by aftershock calibrated back-projection. In the presented scenario two conjugate faults simultaneously rupture in the M_w6.4 event, while only the SW-segment breaks the surface. The M_w7.1 event experiences the full final state of stress (dynamic plus static effects) of the Searles Valley scenario, leading to complex rupture including re-activation of the conjugate M_w6.4 segment, mixed crack and pulse-like propagation, tunneling beneath the fault intersection and choosing one Southern branch only. Both events exhibit a high dynamic stress drop reflecting the immature fault system. The foreshock induces a considerable Coulomb stress change in the M_w7.1 hypocentral region; however, not enough to trigger rupture across the stress-shadowed main fault. Both scenarios match key observations including magnitude, rupture speed, directivity, off-fault damage, slip distribution from kinematic inversion, teleseismic waveforms, GPS, and InSAR ground deformation; while shedding light on geometric, strength and stress factors governing the complex rupture evolution and interaction of the Ridgecrest sequence.

How to cite: Gabriel, A.-A., Taufiqurrahman, T., Carena, S., Verdecchia, A., Li, B., Li, D., Ulrich, T., Gallovic, F., and Wirp, S. A.: Untangling the dynamics of the 2019 Ridgecrest sequence by integrated dynamic rupture and Coulomb stress modeling across an immature 3D conjugate fault network, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15268, https://doi.org/10.5194/egusphere-egu2020-15268, 2020.