A central problem in earthquake physics and fault mechanics is understanding the coupling of fluid and solid phases during fault slip. This coupling is mostly treated as a one-way coupled problem where the pore pressure is imposed as a perturbation in effective normal stress. However, more recent work indicates that the two-way coupling of a porous fluid-filled bulk and pressure changes in the shear zone significantly alters rupture properties. Further, a qualitative analysis of this problem in a poroelastic medium reveals that pore pressure inside an mm to micron thick frictional shear zone cannot be constant as slip dynamically evolves. This analysis calls into question the practice of imposing pore pressure as a perturbation to effective normal stress at an infinitesimal interface and raises fundamental questions regarding the interpretation of the effective stress principle. Here we explore two ways to couple shear zone processes on a mm-micron scale to the meter-kilometer scale bulk processes. Efficient coupling across these scales is achieved with a spectral boundary integral representation of a poroelastic bulk. Furthermore, the boundary integral representation reduces the dimension of the computational problem that needs to be discretized by one. In other words, it allows us to simulate 3D physics by only discretizing in 2D. We develop boundary integral solutions in 2D and 3D medium that are appropriate for modeling shear zone that can undergo pressure changes, expansion/contraction, and shear localization. First, we explore an efficient approach where shear zone properties are averaged and dimensionally reduced, thus with finite shear zone effects built into the boundary conditions of the bulk in 2D and 3D. Second, we show how a shear zone can be explicitly modeled, but the coupling to the surrounding bulk is done with a boundary integral representation. Thus, offering relatively efficient modeling of processes such as shear localization, dilatancy, thermal pressurization, and how such processes interact with the bulk. We suggest that such use of boundary integrals may be applied more generally to achieve two-way fluid-solid coupling at lower computation expense.

How to cite: Heimisson, E. R. and Wang, Y.: Linking fluid flow in a shear zone to the surrounding bulk with poroelastic boundary integral solutions, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4283, https://doi.org/10.5194/egusphere-egu23-4283, 2023.

Fluid-induced earthquakes are an adverse effect of industrial operations like hydraulic fracturing (e.g., 4.7 Mw in Alberta, Canada), and enhanced geothermal systems (e.g., 5.5 Mw in Pohang, South Korea). Identifying all underlying physical processes contributing to fluid-induced seismicity presents an open challenge. Recent work reports signatures of event-event triggering or aftershocks --- common for tectonic settings --- within the context of fluid-induced seismicity. In particular, the statistical properties including the productivity relation and the Omori-Utsu relation appear to hold for fluid-induced seismicity as well. Here, we investigate the underlying potential cause of these field observations from a modelling perspective. By extending a novel conceptual model by integrating (non-)linear viscoelastic effects with a combination of fluid diffusion and invasion percolation associated with a point source, we are able to capture the essential characteristics of crustal rheology and stress interactions in a porous medium. We show that this gives rise to realistic aftershock behaviour with statistical properties indistinguishable from the case of seismicity resulting from tectonic loading. This is even true if the loading due to fluid injections occurs at time scales much faster than the tectonic loading. In our model framework, such tectonic loading can be mimicked by a spatially uniform drive replacing the point source of the fluid injection and its propagation to initiate slips and earthquakes. This indicates that the emergence of the Omori-Utsu relation is independent of how the system is loaded or driven and it is indeed only controlled by the viscoelasticity of the medium. Similarly, the scaling exponent of the productivity relation --- which quantifies how the number of aftershocks increases with the magnitude of the main shock --- is independent of how the system is driven. At the same time, the spatial footprint of fluid-induced events and its dependence on the permeability field are primarily unaltered by the presence of aftershocks. Finally, within our model framework, we systematically investigate the impact of varying fluid injection rates during the viscoelastic stress redistribution on the detection of aftershocks and event-event triggering sequences. When the injection rate is sufficiently high, the aftershock detection and recovery of the Omori-Utsu and productivity relations is only feasible when the internal stress redistribution is directly accessible. 

How to cite: Khajehdehi, O. and Davidsen, J.: Do the statistical properties of aftershocks change in fluid-induced settings?, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10831, https://doi.org/10.5194/egusphere-egu23-10831, 2023.

Both numerical simulations and observational pieces of evidence suggest that the earthquake triggering mechanism depends non-linearly on time. The rate and state friction (RSF) demonstrate these dependencies with a changing weight of healing and weakening terms during its state's evolution. A clock advance due to a nearby rupture using the RSF models either agrees well with the Coulomb's static failure during the fault healing stage or becomes highly susceptible to velocity changes when the failure is imminent. Here we aim to formulate an analytical relation for earthquake triggering effects on nearby faults using transient signals. The dynamic mechanical weakening on the fault interface is quantified as a function of a transient oscillatory signal's peak ground velocity (PGV) and peak spectral frequency (PSF), elastic properties of the fault, and different state weakening terms. So far, the tested numerical simulations show a good agreement with our proposed analytical approach. As a case study, nearby seismic waveforms recorded during the M6.4 (04.07.2019) event that preceded the larger  M7.1 (06.07.2019) Ridgecrest earthquake are used to calculate mechanical weakening, which correlates well with the computed PGV values attenuating with distance. The results support that if inadequate instrumentation exists, those dynamic weakening effects can be approximated empirically using the source parameter of the triggering event as a function of distance and directivity. Derivation of this analytical relation with additional verifications from numerical simulations will contribute to simultaneously including dynamic and static effects. This may lead to a more realistic estimation of increased seismic risk on nearby faults after an earthquake.  

How to cite: Sopacı, E. and Özacar, A. A.: Transient signal-based quantification of earthquake triggering effects on nearby faults using rate and state friction, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11198, https://doi.org/10.5194/egusphere-egu23-11198, 2023.

We analyze frictional motion for a laboratory fault as it passes through the stability transition from stable sliding to unstable motion. We study frictional stick-slip events, which are the lab equivalent of earthquakes, via dynamical system tools in order to retrieve information on the underlying dynamics and to assess whether there are dynamical changes associated with the transition from stable to unstable motion. We find that the lab seismic cycles exhibit characteristics of a low-dimensional system with average dimension similar to that of natural slow earthquakes (<5). We also investigate local properties of the attractor and find maximum instantaneous dimension >10, indicating that some regions of the phase space require a high number of degrees of freedom (dofs). Our analysis does not preclude deterministic chaos, but the lab seismic cycle is best explained by a random attractor based on rate- and state-dependent friction whose dynamics is stochastically perturbed. We find that minimal variations of 0.05% of the shear and normal stresses applied to the experimental fault influence the large-scale dynamics and the recurrence time of labquakes. While complicated motion including period doubling is observed near the stability transition, even in the fully unstable regime we do not observe truly periodic behavior. Friction's nonlinear nature amplifies small scale perturbations, reducing the predictability of the otherwise periodic macroscopic dynamics. As applied to tectonic faults, our results imply that even small stress field fluctuations (less or about 150 kPa) can induce coefficient of variations in earthquake repeat time of a few percent. Moreover, these perturbations can drive an otherwise fast-slipping fault, close to the critical stability condition, into a mixed behavior involving slow and fast ruptures.

How to cite: Gualandi, A., Faranda, D., Marone, C., Cocco, M., and Mengaldo, G.: Deterministic and Stochastic Chaos characterise Laboratory Earthquakes, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11710, https://doi.org/10.5194/egusphere-egu23-11710, 2023.

Earthquakes due to either natural or anthropogenic sources cause important human and material damage. In both cases, the presence of pore fluid influence the triggering of seismic instabilities. Preliminary results, done in the context of the European Research Council CoQuake’s project (www.coquake.eu), show that the earthquake instability could be avoided by active control of the fluid pressure [Stefanou, (2019)].
In this contribution, we propose to study the ability of Fast Boundary Element Methods (Fast BEMs) [Chaillat and Bonnet (2013)] to provide a multi-physic large-scale robust model required for modeling earthquake processes, pore-fluid-induced seismicity and their control.
The main challenges concern:

•  the modelling of a realistic on-fault behaviour as well as hydro-mechanical couplings;
• the extension of Fast Boundary Element methods to fault mechanic problems incorporating the effect of fluid injection of the on-fault behaviour;
• the simulation of both small and large time scales corresponding to earthquakes and fluid diffusion respectively by using a single advance in time algorithm.

The main methods used for numerical modeling of earthquake ruptures at a planar interface between two elastic half-spaces are spectral BEMs as in [Lapusta and al. (2000)]. As a first step, we consider this method for a simple problem in crustal faulting. A rate-and-state friction law is considered and different adaptive time stepping algorithms inspired from the literature are tested to take into account both small and large time scales with the correct resolution in time. These solving methods are compared on different benchmarks and convergence studies are conducted on each of them.
Then, poroelastodynamic effects are considered. To this aim, a dimensional analysis of generic poroelastodynamic equations [Schanz (2009)] is performed. It allows determining which of the poroelastodynamics effects are predominant depending on the observation time of the fault. The obtained equations corroborate and justify simplified multiphysics models from the literature, for example [Heimisson and al. (2021)]. A first multi-physics test using Fast BEMs to solve a simplified crustal faulting problem with fluid injection is considered. The objective of this project is to provide a viable efficient tool to explore the advantages and limitations of novel strategies of earthquake control using fluid injection to drive the fault from an unstable state of high potential energy to a stable state of lower potential energy.

References:

S. Chaillat, M. Bonnet. Recent advances on the fast multipole accelerated boundary element method for 3D time-harmonic elastodynamics, Wave Motion, 1090-1104, 2013
E. R. Heimisson, J. Rudnicki, N. Lapusta. Dilatancy and Compaction of a Rate-and-State Fault in a Poroelastic Medium: Linearized Stability Analysis., Journal of Geophysical Research: Solid Earth, 126(8), 2021
N. Lapusta, J. Rice and al.. Elastodynamic analysis for slow tectonic loading with spontaneous rupture episodes on faults with rate- and state-dependent friction, Journal of Geophysical Research: Solid Earth, 23765-23789, 2000.
M. Schanz. Poroelastodynamics: Linear Models, Analytical Solutions, and Numerical Methods., Applied Mechanics Reviews, 62(3)., 2009.
I. Stefanou. Controlling Anthropogenic and Natural Seismicity: Insights From Active Stabilization of the Spring‐Slider Model, Journal of Geophysical Research: Solid Earth, 8786-8802, 2019.

How to cite: Bagur, L., Chaillat, S., Semblat, J.-F., Stefanou, I., and Romanet, P.: Fast Boundary Element Methods for fault mechanics and earthquake control, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14727, https://doi.org/10.5194/egusphere-egu23-14727, 2023.

During the last decades, many numerical models have been developed to explore the conditions for seismic and aseismic slip. Those models explore the behavior of frictional faults, embedded in either elastic or inelastic mediums, and submitted to a far field loading (seismic cycle models), or initial stresses (single dynamic rupture models). Those initial conditions impact both fault and off-fault dynamics. Because of the sparsity of direct measurements of fault stresses, modelers have to make assumptions about the initial conditions. To these days, Anderson theory is the only framework that can be used to link fault generation and reactivation to the three-dimensional stress field.  In this study, we focus on the initial stresses in 2D plane strain models developed to compute off-fault deformation. It has been demonstrated that initial conditions, in particular the angle between fault and the greatest compressive stress, is of crucial importance for the localization and intensity of off-fault inelastic deformation. However, because those models are performed on a 2D plane, the importance of the out-of-plane stress have never been investigated. We show that it can lead to set up a stress field that is not in agreement with Anderson theory (i.e., modelling a strike-slip fault in a three-dimensional stress field appropriate for reverse faulting). We investigate the influence of initial stresses by comparing equivalent models with “correct” and “incorrect” initial stress fields, keeping constant rupture-related parameters (stress drop, seismic ratio), angle between fault and greatest principal stress, and depth. We first use purely elastic models to study the influence of initial stresses on the assessment of two plastic criteria (Drucker-Prager and Coulomb stress change). We show that setting up the incorrect initial stress field can lead to underestimating the different yield criteria. The error is of the order of magnitude of the dynamic stress drop. Moreover, setting up the incorrect pre-stresses leads to errors in the estimation of potential off-fault failure modes. Then, we explore the influence of pre-stresses conditions on off-fault inelastic deformation. Using two different modelling strategies (a plastic deformation model and a micromechanics model computing dynamic damage), we show that setting up the incorrect stress field can lead to underestimate the size of the damage zone by a factor of 3 to 6 for the studied cases.  Moreover, because of the interactions between fault slip and off-fault deformation, we show that initial stress field influences the rupture propagation. Setting up the correct stress field can significantly slow the rupture, because of the more important quantity of damage induced.

How to cite: Thomas, M., Jeandet, L., and Bhat, H.: On the importance of 3D stress state in 2D earthquake rupture simulations with off-fault deformation, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6708, https://doi.org/10.5194/egusphere-egu23-6708, 2023.

Earthquake stress drop values estimated from ground-motion spectra commonly vary by several orders of magnitude, particularly for small earthquakes (~M < 3). Stress-drop values have been found to vary with faulting style, faulting type (intraplate, interplate), depth, and to exhibit differences between natural and induced earthquakes. Nevertheless, distinguishing uncertainties from real trends across data sets is challenging, in part due to the variation in methodological approaches and observational constraints. However, the proliferation of high-quality, dense seismic data in recent years has shown that at least some of the variability in stress drop values almost certainly reflects diversity in fault strength and geological conditions. Coupling well-constrained observations to a variety of modeling approaches will help uncover what controls earthquake rupture processes, but deconvolving observational constraints from real variation in rupture behavior is key.

We present our stress drop estimates from data sets representing a wide range of fault loading conditions and geological environments, from interplate, intraslab and forearc subduction faults, to volcanic, intraplate, and human induced events. Stress-drop values range primarily between 1 – 100 MPa for events that meet the criteria for spectral-ratio analysis.  We present correlations of low relative stress drop values in areas of high seismic attenuation indicative of lower rock strength, and a slight correlation with depth that corresponds to modeled deviatoric stress values. We also show one notable subset of induced events near active injection wells that exhibit stress drop values of ~0.1 MPa and have distinctive low-frequency content. Their spatial distribution, waveform, and source spectral characteristics suggest either slower rupture, lower stress drop values, or a combination of both, and may represent part of the transition between aseismic and seismic slip. We show using a Large-n array that while stress drop values are roughly constant (within 2 orders of magnitude), estimates can vary by roughly 25% when station coverage is limited to 15 stations or less with a maximum azimuthal gap of 90°.  Our findings highlight the importance of using modeling approaches to explore relative influence of fault strength and methodological approaches in stress drop variation. In particular, models that incorporate both frictional and thermoelastic approaches may provide clues to the variability of conditions that can activate faults, both within stable sliding and seismic rupture conditions.

How to cite: Harrington, R. M., Liu, Y., Yu, H., Verdecchia, A., Kemna, K. B., Bocchini, G. M., Dielforder, A., Roth, M. P., Kirkpatrick, J., Cochran, E. S., Chang, H., and Abercrombie, R. E.: Deciphering earthquake source observations to motivate questions for physics-based models of earthquake simulation, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7207, https://doi.org/10.5194/egusphere-egu23-7207, 2023.

We computed moment (M_w) and local magnitude (M_L) of about 250,000 earthquakes occurred in Italy from 2009 to 2018 and recorded at seismic stations of the Italian National Network managed by INGV.

For moment magnitude computation, we start from raw velocity waveforms and invert the displacement spectra of more than 2,000,000 S-waves manually picked. We use the probabilistic method of Supino et al. [2019] to estimate the a-posteriori joint probability density function of the source parameters: seismic moment M₀, corner frequency f_c and high-frequency decay γ. M_w is obtained from M₀ using the Kanamori [1977] equation.

We start from the same waveforms to compute local magnitude using two designed on purpose codes, PyAmp and PyML [Di Stefano et al., 2023], and an attenuation law specific for the Italian region, Di Bona et al. [2016], obtaining M_L values characterized by quality and homogeneity.

Both magnitude catalogs can be reproduced due to the availability in open databases of all the input and output parameters used for processing.

We observe a self-similar scaling between f_c and M₀ for M_w larger than ~2.0. For smaller magnitudes, S-wave spectra show an almost constant corner frequency (~10 Hz), which does not scale with the earthquake source (seismic moment). We interpret this as the constant cut-off frequency of the anelastic attenuation, which acts as a low-pass filter and produces an apparent corner frequency. The latter is lower than expected, and corresponds to an apparent larger source duration.

Because of the conservation of total displacement integral after a low-pass filtering, signals must exhibit a maximum amplitude lower than expected to “compensate” the apparent larger source duration. M_L values are therefore expected to be underestimated while moment magnitudes, by definition, are not affected by this as they are proportional to the displacement integral.

Coherently, the comparison of our M_w and M_L estimates shows the systematic underestimation of M_L with respect to M_w for small magnitude events. The deviation from a 1:1 scaling relationship between M_L and M_w overlaps the magnitude range where the constant apparent corner frequency arises in the M₀-f_c scaling (M_L <~ 2).

Regarding the upcoming of a new generation of earthquake catalogs characterized by very low completeness magnitudes (M_C << 2), our results suggest that a robust analysis of the statistical features of these catalogs (e.g., event size distribution) should consider the use of a precise magnitude estimate such as M_w instead of M_L.

How to cite: Supino, M., Chiaraluce, L., Di Stefano, R., Castello, B., and Michele, M.: Moment vs local magnitude scaling of small-to-moderate earthquakes from seismic moment estimation of 10 years (2009-2018) of Italian seismicity, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11928, https://doi.org/10.5194/egusphere-egu23-11928, 2023.

Rupture directivity and its potential frequency dependence is an open issue in the seismological community, especially for small-to-moderate earthquake. Directivity itself is the focusing of the radiated seismic wave energy due to the rupture propagation along the direction of the fault.

In this research, we calibrate a non-ergodic ground motion model for the ordinates of the 5% acceleration response spectra (computation interval 0.04-2 sec) and we analyse, earthquake by earthquake, the azimuthal dependence of the aleatory component, i.e. the residual terms corrected for systematic source, site and path contributions. The final aim is the calibration of a prediction model including directivity effects that can be used for engineering purposes such as seismic hazard assessment and shaking scenarios generation.

The study area is the Central Italy, which was affected by several seismic sequences in the last 20 years, occurred on normal fault systems. The dataset we used is composed by almost 300,000 seismic recordings of 456 earthquakes in the magnitude range from 3.4 to 6.5 within the time frame 2008-2018. We find that about one-third of the analysed events are directive, characterized by unilateral ruptures along the Apennine faults direction.

Directivity effects occur over a wide frequency band and can be described by spectral curves peaked in different frequency ranges according to the event magnitude: the stronger the earthquake, the lower the frequency at which these effects are visible. Vice versa, we find no correlation between the amplitude of such peaks and the events magnitude. When normalized to the peak, the directivity curves can be grouped into families characterized by similar amplification trends variable with frequency, with the exception of 16 events, which we classify as "super-directive", that differ markedly from the others generating broadband amplifications.

Preliminary results suggest that is possible to obtain similar shapes of directivity curves for defined frequency families and that they can consequently be modeled for non-ergodic ground motion model and predictive shaking scenarios.

How to cite: Colavitti, L., Lanzano, G., Sgobba, S., Pacor, F., and Gallovič, F.: Evidence of frequency-dependent directivity effects from non-ergodic ground motion modelling of Spectral Acceleration in Central Italy, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13574, https://doi.org/10.5194/egusphere-egu23-13574, 2023.

The existence of slow P wave, in addition to fast P wave and S wave, makes it tricky for grid-based numerical simulation methods to conduct poroelastic wave modeling. The grid spacing has to be fine enough to capture the slow P wave since the velocity of slow P wave is much smaller than that of the other two waves. Dense space and time steps significantly increase the computation cost. In this study, we propose a poroelastic finite-difference simulation method that combines discontinuous curvilinear collocated-grid and non-uniform time step Runge-Kutta scheme. Only the space and time steps for the areas near interfaces, where the contribution of slow P wave is non-negligible, are refined in an effort to speed up the computation. The refined space step is determined by the velocity of slow P wave, while the coarse space step is determined by the velocity of shear wave. The coarse and refined time steps are set according to the non-uniform time step Runge-Kutta scheme, which is derived with Taylor expansion and avoids interpolation or extrapolation for communication between different time levels. This scheme helps maintain fourth-order accuracy in the whole domain. The accuracy and efficiency of the proposed method are verified by numerical tests. Compared with the conventional curvilinear collocated-grid finite-difference method that uses a uniform space grid as well as a uniform time step, the computation efficiency is improved significantly and the computation time can be saved by more than 80%.

How to cite: Zhang, H., Ren, H., Sun, Y.-C., Li, M., Wang, T., and Fang, C.: An efficient poroelastic wave simulation based on discontinuous grid and nonuniform time step, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4960, https://doi.org/10.5194/egusphere-egu23-4960, 2023.

Ground motion prediction equations (GMPE) are traditionally used in site specific seismic hazard analysis to obtain design response spectra. These equations are obtained by regression analysis on the available strong motion data in a given tectonic and geological region. Assuming ergodicity regional GMPE are routinely used in site-specific probabilistic seismic hazard analysis. Since these empirical equations are region specific, However the obtained seismic hazard curves are not specific to the site. Due to lack of data for all possible combinations of magnitude and distances, development of site-specific GMPE is not possible in the near future. The only way to develop a site-specific GMPE is through numerical models. Given a 3D velocity structure, topography and source information these models can simulate site-specific ground motion. Once calibrated with the recorded strong motion data, numerical models can be used to simulate ensemble of ground motions by including the uncertainty in the slip models. In regions lacking strong motion data, these models have an additional advantage compared to GMPE. In the present study, an broad band simulation model is developed for a typical site in peninsular India. Spectral finite element method (SPECFEM) is used to simulate the low frequency ground motion by incorporating the 3D velocity structure in the medium. The high frequency ground motion is simulated from the stochastic seismological model (Otarola and Ruiz, 2016). Statistical kinematic rupture model is used to represent the earthquake source (Dhanya and Raghukanth 2018). The rupture length, width and correlation lengths of the random field are estimated from magnitude. Assuming the phase as random, a total of 30 rupture models are simulated for each magnitude. An ensemble of ground motions is simulated at the site for various possible combination of faults and magnitudes in a region around 500 km from the site. The simulated low-frequency and high-frequency ground motions are combined in the frequency domain to obtained broad band ground motions (0-100 Hz). The mean and standard deviation of the response spectra are estimated from these simulated motions for all possible combinations of magnitudes and distances at the given site. Further, probabilistic seismic hazard analysis is carried out using the simulated data to obtain hazard curves for spectral accelerations at various natural periods. Uniform hazard response spectra (UHRS) for 475yr and 2475 yr is obtained from the hazard analysis. A comparison with traditional hazard analysis using region specific GMPE is also presented. It is observed GMPE based UHRS show a smooth trend compared with site-specific UHRS obtained from broad band models. The PGA values obtained from physics based model are slightly higher than that obtained from GMPE based PSHA.

How to cite: Stg, R.: Physics based ground motion model in seismic hazard assessment, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12317, https://doi.org/10.5194/egusphere-egu23-12317, 2023.

Tehran urban area serves as the main hub for economic and social activities in Iran. The city is located on a sedimentary basin including faults and folds, and thus it is vulnerable to large site effects. Analysis of earthquakes recorded by a temporary seismological network has approved a large amplification of seismic ground motion (about 4 to 8) over a broad frequency range.

In order to better understand and predict the effects of the geometry and mechanical properties on surface ground motions, we developed a 3D shear-wave velocity model of Tehran by integrating extensive geophysical surveys including almost 600 single station measurements and 33 ambient vibrations arrays, with geotechnical and geological data. This 3D model shows that the bedrock depth varies between 100 and 900 meters with a general increasing depth from N-NE toward the S-SW. Also, there are two main velocity layers in the basin. A surface layer, which drops from 950 m/s to 600 m/s from NE to SW and a deeper layer with Vs up to 1300 m/s.

We then used the open-source spectral-element code, EfiSpec3D (DiMartin et al., 2011), to simulate ground motion by this new sedimentary basin model at the defined 50*50 kilometers tilted square simulation block up to the maximum target frequency of 2 Hz. The source time function is a 2-Hz lowpass filtered Dirac impulse injected from the defined z-plane at 5 km depth.

The results reveal a good correlation between real and simulated earthquake ground motion by the comparison between experimental and synthetic standard spectral ratios (SSR). The results also reproduced the experimental H/V frequency peaks over the basin relatively well and suggest that 3D geometry always should be considered for an accurate estimation of realistic basin response.

How to cite: Soltani, S., Cornou, C., Guillier, B., and Haghshenas, E.: Simulations of ground motion in the Tehran basin based on newly developed 3D velocity model, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13098, https://doi.org/10.5194/egusphere-egu23-13098, 2023.

The 1934 Bihar-Nepal earthquake, one of the most catastrophic events ever to occur in the Himalayas, inflicted extensive devastation with reported MMI of IX-VI in the Kathmandu valley and the Indo-Gangetic (IG) basin. The earthquake triggered significant ground liquefaction and landslides as it occurred in the proximity of densely populated river basins causing a huge economic loss and over 15700 fatalities. However, it is unfortunate that there are no ground motion data available for the event, as it remained unrecorded due to a lack of instrumentation. Therefore, simulating ground motions for the 1934 Bihar-Nepal earthquake would provide new insights into the influence of regional characteristics on Himalayan earthquakes. However, incorporating the Himalayan topography and the IG basin in the ground motion simulation is very challenging. In contrast, proper validation of modeling of ground motions is difficult due to the unavailability of recorded data. To circumvent these challenges, we simulated broadband ground motions for the 2015 Nepal earthquake, another significant catastrophe that occurred in the same seismo-tectonic region in the Himalayas which provides a well-recorded database. For the 2015 Nepal earthquake, a thorough comparison of the recorded and simulated ground motion spectra reveals that the simulated ground motions are consistent with the recorded data in terms of amplitude, strong motion duration, and spectral ordinates. Therefore, we considered the same medium characteristics to simulate broadband seismograms for the 1934 Bihar-Nepal earthquake by combining deterministically generated low-frequency (LF) and stochastically simulated high-frequency (HF) ground motions. The HF accelerograms are generated by considering incident and azimuthal angles obtained from rays of P and S waves traced from the finite fault slip model to the station, passing through the regional layered stratified velocity model, free surface factors and energy partition factors (Otarola and Ruiz, 2016). For deterministic simulation, a 3D computational model (Sreejaya et al., 2022) for the study region of approximately 9°×7° (between 80°–89°E longitude and 23°-30°N latitude), incorporated with basin geometry, material properties, and topography of the region is embedded with the finite fault rupture model of the event to generate LF ground motions. For the finite fault source model, five samples with various spatial variability of the slip on the rupture plane are simulated as a random field (Mai and Beroza, 2000; 2002) using the seismic moment and fault dimensions provided by Pettanati et al. (2017). Ultimately, the broadband (0.01–25 Hz) ground motions are obtained at 6461 hypothetically gridded stations with a 0.1°×0.1° spacing by combining the suitably filtered LF and HF ground motions in the frequency domain with the target frequency of 0.3 Hz with a bandwidth up to 0.05 Hz. A systematic comparison of estimated MMI values (Iyengar and Raghukanth, 2003) and the observed MMI values at 459 sites revealed that the PGA between 0.25-0.6g is significant within 200 km of the epicentral distance. Thus, the results can be used for addressing the ground failure and liquefaction caused due to the earthquake and also find applications in seismic hazard assessment of the cities in the basin.

How to cite: Basu, J., kp, S., and stg, R.: The 1934 Bihar-Nepal Earthquake – Simulation of Broadband Ground Motions and Estimation of Site Amplification, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10800, https://doi.org/10.5194/egusphere-egu23-10800, 2023.

How to cite: Nunes, S., Mohammadigheymasi, H., Tavakolizadeh, N., and Garcia, N.: A synthetic ambient-noise data set fortime-lapsed monitoring, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1678, https://doi.org/10.5194/egusphere-egu23-1678, 2023.

Computational seismology encountered a dramatic advance during the past decades with the development of SEM codes that use the simultaneous increase of the available computational power. Meanwhile, the use of teleseismic events for regional seismic tomography is suggested with the application of the box-tomography methodology (Masson and Romanowicz, 2017). In this work we use these advances in order to suggest a package for box-tomography, using AxiSEM for 1-D global wavefield simulations (Nissen-Meyer et al., 2014) and SPECFEM3D for 3-D regional seismic simulations (Komatitsch and Tromp, 1999). These codes have been previously used and validated for such hybrid simulations (Monteiller et al., 2021), however with the limitation on the dimensions of the examined region, where 3-D full waveform topography is applied, due to the Cartesian setting that does not honour the curvature of the Earth. Although recent advances solved this limitation for SPECFEM3D Global, by permitting the use of a small Earth chunk, the Cartesian description of the regional model allows computing the injection of the 1-D computed wavefield from the global model to the regional box. Therefore, we developed and present comparative results of a package that transforms the geometry of the Cartesian simulation in a "spherical Earth" setting and allows the performance of hybrid simulations for box tomography in regions larger than a couple of degrees. The code changes the shape of a Cartesian rectangular mesh into a curved one and through a series of interpolations adjusts the geometry of any given structure model, the topography of the surface and the interfaces, and the position of the receivers. The simulations are tested against real data, as we perform our computations on a dynamically interesting area, with the presence of a subduction slab in the central Mediterranean. We test the methodology on seismological inverse models for the local structure (Rappisi et al., 2021).

References:

Komatitsch, D. and Tromp, J., 1999. Introduction to the spectral element method for three-dimensional seismic wave propagation. Geophysical journal international, 139(3), pp.806-822.

Masson, Y. and Romanowicz, B., 2017. Box tomography: localized imaging of remote targets buried in an unknown medium, a step forward for understanding key structures in the deep Earth. Geophysical Journal International, 211(1), pp.141-163.

Monteiller, V., Beller, S., Plazolles, B. and Chevrot, S., 2021. On the validity of the planar wave approximation to compute synthetic seismograms of teleseismic body waves in a 3-D regional model. Geophysical Journal International, 224(3), pp.2060-2076.

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How to cite: Karakostas, F., Morelli, A., Molinari, I., VanderBeek, B., and Faccenda, M.: SPHY3D: A hybrid seismic computational framework for box-tomography of spherical Earth, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6471, https://doi.org/10.5194/egusphere-egu23-6471, 2023.