SM8.2

The 2020 Mw 6.8 Elazığ (Sivrice) earthquake occurred on the Pütürge segment of the East Anatolian Fault zone. This strike-slip segment is situated between strong earthquakes that happened 100–150 years ago, and, since that time, the segment remained with eight Mw5-6 events, but with no Mw 6+. We relocate the mainshock and aftershock sequence and infer basic characteristics of the event using the ISOLA multiple point source approach and backpropagation of S-waveforms from local strong-motion recordings. Together with clear secondary P wave onsets identified in the recordings, the results suggest complex rupture propagation with reversal of the rupture propagation. We apply a recently developed Bayesian dynamic source inversion with slip-weakening friction and spatially inhomogeneous stress and friction parameters to gain better insight into the rupture process. Using high-quality near field recordings in the low-frequency range (<0.3Hz), we obtain a complex dynamic rupture model explaining the weak rupture initiation followed by a cascade of at least three rupture episodes, including the rupture reversal. The dynamic model explains significant features of the recordings even in a broader frequency range interesting for seismic engineering applications (<2.5Hz), e.g., a directivity pulse associated with rupturing the event’s strongest asperity with 4 m of slip and local stress drop of 40 MPa. We show that by reducing the initial stress in the top 10 km by 10%, the rupture fails to develop into the larger event, finishing as an Mw 5.8 earthquake. Considering the latter experiment corresponds to an earlier state of the fault in the seismic cycle, we hypothesize that the interseismic M5+ events on the Pütürge segment were undeveloped rudiments of potentially large events. Thus, the fault seems to have been ready for the Mw6.8 earthquake only by the time of the earthquake occurrence in 2020. This suggests that at the time of the Elazığ earthquake initiation, the final Mw6.8 magnitude was not determined, making it a treacherous case for early warning systems.

How to cite: Gallovič, F., Zahradník, J., Plicka, V., Sokos, E., Evangelidis, C., Fountoulakis, I., and Turhan, F.: Complex rupture dynamics of the 2020 Mw 6.8 Elazığ (Sivrice) earthquake, Turkey, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14875, https://doi.org/10.5194/egusphere-egu21-14875, 2021.

We analyze the effect of earthquake source parameters on ground-motion variability based on near-field wavefield simulations for large earthquakes. We quantify residuals in simulated ground motion intensities with respect to observed records, the associated variabilities are then quantified with respect to source-to-site distance and azimuth. Additionally, we compute the variabilities due to complexities in rupture models by considering variations in hypocenter location and slip distribution that are implemented a new Pseudo-Dynamic (PD) source parameterization.

In this study, we consider two past events – the Mw 6.9 Iwate Miyagi Earthquake (2008), Japan, and the Mw 6.5 Imperial Valley Earthquake, California (1979). Assuming for each case a 1D velocity structure, we first generate ensembles of rupture models using the pseudo-dynamic approach of Guatteri et.al (2004), by assuming different hypocenter and asperities locations (Mai and Beroza, 2002, Mai et al., 2005; Thingbaijam and Mai, 2016). In order to efficiently include variations in high-frequency radiation, we adopt a PD parameterization for rupture velocity and rise time distribution in our rupture model generator. Overall, we generate a database of rupture models with 50 scenarios for each source parameterization. Synthetic near-field waveforms (0.1-2.5Hz) are computed out to Joyner-Boore distances Rjb ~ 150km using a discrete-wavenumber finite-element method (Olson et al., 1984). Our results show that ground-motion variability is most sensitive to hypocenter locations on the fault plane. We also find that locations of asperities do not alter waveforms significantly for a given hypocenter, rupture velocity and rise time distribution. We compare the scenario-event simulated ground motions with simulations that use the rupture models from the SRCMOD database (Mai and Thingbaijam, 2014), and find that the PD method is capable of reducing the ground motion variability at high frequencies. The PD models are calibrated by comparing the mean residuals with the residuals from SRCMOD models. We present the variability due to each source parameterization as a function of Joyner-Boore distance and azimuth at different natural period.

How to cite: Sivasubramonian, J. and Mai, P. M.: Spatial Variability of Near-field Ground Motions from Pseudo-Dynamic Rupture Simulations, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9463, https://doi.org/10.5194/egusphere-egu21-9463, 2021.

The microseismic events can often be characterized by a complex non-double couple source mechanism. Recent laboratory studies recording the acoustic emission during rock deformation help connecting the components of the seismic moment tensor with the failure process. In this complementary contribution, we offer a mathematical model which can clarify these connections. We derive the seismic moment tensor based on classical continuum mechanics and plasticity theory. The moment tensor density can be represented by the product of elastic stiffness tensor and the plastic strain tensor. This representation of seismic sources has several useful properties: i) it accounts for incipient faulting as a microseismicity source mechanism, ii) it does not require a pre-defined fracture geometry, iii) it accounts for both shear and volumetric source mechanisms, iv) it is valid for general heterogeneous and anisotropic rocks, and v) it is consistent with elasto-plastic geomechanical simulators. We illustrate the new approach using 2D numerical examples of seismicity associated with cylindrical openings, analogous to wellbore, tunnel or fluid-rich conduit, and provide a simple analytic expression of the moment density tensor. We compare our simulation results with previously published data from laboratory and field experiments.  We consider three special cases corresponding to "dry" isotropic rocks, "dry" transversely isotropic rocks and "wet" isotropic rocks. The model highlights theoretical links between stress state, geomechanical parameters and conventional representations of the moment tensor such as Hudson source type parameters. 

How to cite: Yarushina, V. and Minakov, A.: Can plasticity explain microseismic source mechanisms?, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2573, https://doi.org/10.5194/egusphere-egu21-2573, 2021.

The induced seismicity in the Groningen gas field, The Netherlands, has led to intense public concerns and comprehensive investigations. One of the main challenges for assessing future seismic hazard in the Groningen gas field is to estimate the maximum possible earthquake magnitude (Mmax) that could be induced by gas extraction. Previous methods are strongly rooted in empirical and statistical approaches that are inherently limited by the scarcity of data. Here, we combine a physics-based dynamic rupture model based on the 3D theory of fracture mechanics with field-based and lab-based constraints to estimate Mmax in the Groningen gas field. If earthquakes in the reservoir have a rupture depth extension constrained by the reservoir thickness, the largest earthquakes should develop a large aspect ratio (longer horizontally than vertically). The model is thus an extension of the 3D theoretical rupture model on long faults with uniform stress and strength developed by Weng & Ampuero (2019), in which we have incorporated spatial heterogeneities, such as along-strike variable fault width, depth-dependent initial stresses and friction properties. The essential parameters that control rupture propagation and earthquake magnitude are the stored elastic energy and the fracture energy. Our method requires estimates of the stored elastic energy on reservoir faults as a result of the stresses induced by differential reservoir compaction during depletion. The fracture energy is constrained by laboratory experiments and theoretical frictional models. Coupling physics-based rupture models with field and lab observations provides an estimate of Mmax in the Groningen gas field and serves as a practical step toward physics-based seismic hazard assessment for other gas fields in the world.

Citation:

Weng, H. and J. P. Ampuero (2019). "The Dynamics of Elongated Earthquake Ruptures." Journal of Geophysical Research: Solid Earth.

How to cite: Weng, H., Ampuero, J.-P., and Buijze, L.: Physics-Based Estimates of the Maximum Magnitude of Induced Earthquakes in the Groningen Gas Field, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6144, https://doi.org/10.5194/egusphere-egu21-6144, 2021.

Modelling volcanic processes at active volcanoes often requires a multidisciplinary approach, which adequately describes the complex and ever-dynamic nature of volcanic unrests. Campi Flegrei caldera (southern Italy) is an ideal laboratory where numerical modelling of injection-induced seismicity could be tested to match the observed seismicity. In the current study, thermal-hydraulic-mechanical (THM) effects of hot-water (fluid) injections were investigated to ascertain whether the observed seismicity (past and ongoing seismic swarms) could be quantitatively reproduced and modelled in isothermal or non-isothermal approximations. Fluid-flow modelling was carried out using a coupled TOUGHREACT-FLAC^3D approach to simulate THM effects of fluid injections in a capped reservoir, where the sealing formation serves as a geological interface between supercritical reservoir and fractured shallow layers of the caldera. Results from previous seismic, deformation, tomographic and rock physics studies were used to constrain the model for realistic volcano modelling. The results indicated that fluid injections generated overpressure beneath the caprock and subjected it to different stress regimes at its top and bottom, and this prompted deformation. Thus, caprock deformation, triggered by injection-induced basal compressional forces and top extensional fractures, is a critical factor determining the required timing for pressure build-up and fault reactivation, and magnitudes of seismicity. Higher fluid injection rates and temperature contrasts, heterogeneity due to fault and its contrast with the host rock, and caprock hydraulic properties were among the identified secondary factors modulating fault reactivation and seismicity. Simulation results revealed that seismicity can be better modelled in isothermal (HM) approximations. A comparative study of the THM-modelled seismicity and 4-month-long (August 5^th to December 5^th, 2019) seismic monitoring data recorded at the Osservatorio Vesuviano showed that our model reproduced the magnitudes and depths (~2.5 Ms within 2 km) at the onset of the ongoing unrests on October 5^th, 2019. However, the model could not adequately reproduce the highest magnitude (3.3 Ms at 2.57 km) seismicity on April 26^th, 2020 observed since 1984 major unrests.

How to cite: Akande, W. G., Gan, Q., Cornwell, D. G., and De Siena, L.: Numerical Modelling of Injection-induced Seismicity at Campi Flegrei Caldera, Southern Italy, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9064, https://doi.org/10.5194/egusphere-egu21-9064, 2021.

Subduction zones are where the largest earthquakes occur. In the past few decades, scientists have also discovered the presence of episodic aseismic slip, including slow slip events (SSEs), along most of the subduction zones. However, it is still unclear how these SSEs can influence megathrust earthquake ruptures. The Costa Rica subduction zone is a particularly interesting area because a SSE was recorded 6 months before the 2012 Mw7.6 earthquake in the Nicoya Peninsula, suggesting a potential stress transfer from the SSE to the earthquake slip zone. SSEs beneath the Nicoya Peninsula were also recorded both updip and downdip the seismogenic zone, making it a unique area to study the complex interaction between SSEs and earthquakes.

As most of the shallow SSEs were recorded around the Nicoya Peninsula, we chose to start using a 1D planar fault embedded in a homogeneous elastic half-space, with different dipping angles following several geometric profiles of the subduction fault beneath the Nicoya Peninsula section of the Costa Rica margin. This 1D modelling study allows us to better investigate the interaction between shallow and deep SSEs and megathrust earthquakes with high numerical resolution and relatively short computation time. The model provides information on the long-term seismic history by reproducing the different stages of the seismic cycle (interseismic slip, shallow and deep episodic slow slip, and coseismic slip).

We study the influence of the variation of numerical parameters and frictional properties on the recurrence interval, maximum slip velocity and cumulative slip of SSEs (both shallow and deep) and earthquakes and their interaction with each other. We then compare our results with GPS and seismic observations (i.e. cumulative slip, characteristic duration, moment rate, depth and size of the rupture, equivalent magnitude) to identify an optimal set of model parameters to understand the interaction between various modes of subduction fault deformation.

How to cite: Alalouf, L. and Liu, Y.: Modeling the Interaction between Slow Slip Events and Earthquake Ruptures in the Nicoya Peninsula, Costa Rica., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6324, https://doi.org/10.5194/egusphere-egu21-6324, 2021.

Numerical models are well-suited to overcome limited spatial-temporal observations to understand earthquake sequences, which is fundamental to ultimately better assess seismic hazard. However, high-resolution numerical models in 3D are computationally time and memory consuming. This is not optimal if the aspects of lateral or depth variations within the results are not needed to answer a particular objective. In this study we quantify and summarize the limitations and advantages for simulating earthquake sequences in all spatial dimensions.

We simulate earthquake sequences on a strike-slip fault with rate-and-state friction from 0D to 3D using both quasi-dynamic and fully dynamic approaches. This cross-dimensional comparison is facilitated by our newly developed, flexible code library Garnet, which adopts a finite difference method with a fully staggered grid. We have validated our models using problems BP1-QD & FD and BP4-QD & FD of the SEAS (Sequences of Earthquakes and Aseismic Slip) benchmarks from the Southern California Earthquake Center.

Our results demonstrate that lower-dimensional/quasi-dynamic models are qualitatively similar in terms of earthquake cycle characteristics to their higher-dimensional/fully-dynamic counterparts, while they could be hundreds to millions times faster at the same time. Quantitatively, we observe that certain earthquake parameters, such as stress drop and fracture energy release, can be accurately reproduced in each of these simpler models as well. However, higher dimensional models generally produce lower maximum slip velocities and hence longer coseismic durations. This is mainly due to lower rupture speeds, which result from increased energy consumption along added rupture front directions. In the long term, higher dimensional models produce shorter recurrence interval and hence smaller total slip, which is mainly caused by the higher interseismic stress loading rate inside the nucleation zone. The same trend is also observed when comparing quasi-dynamic models to fully dynamic ones. We extend a theoretical calculation that to first order approximates the aforementioned physical observables in 3D to all other dimensions. These theoretical considerations confirm the same trend as what is observed for stress drop, recurrence interval and total slip across dimensions. These findings on similarities and differences of different dimensional models and a corresponding quantification of computational efficiency can guide model design and facilitate result interpretation in future studies.

How to cite: Li, M., Pranger, C., and van Dinther, Y.: Characteristics of earthquake sequences: comparison from 0D to 3D, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8833, https://doi.org/10.5194/egusphere-egu21-8833, 2021.

The 30 October 2020 Samos Earthquake (Mw=7.0) ruptured a north-dipping offshore normal fault, north of the Samos Island with an extensional mechanism. Aftershocks mainly occurred at the western and eastern ends of the rupture plane in agreement with the Coulomb static stress changes. Mechanism of aftershocks located west of the rupture area supported activation of the neighboring strike-slip fault almost instantly. In addition, a seismic cluster including events with magnitudes reaching close to 4 has emerged fifty hours later at the SE side of Samos Island. This off-plane cluster displays a clear example of delayed seismic triggering that produced small magnitude earthquakes at nearby active faults. In this study, numerical simulations are conducted using rate-and-state friction dependent quasi-static&full-dynamic spring slider model with shear-normal stress coupling to mimic the instant and delayed seismic triggering observed after this event. Coulomb static stress changes and seismic waveforms recorded at nearby strong-motion stations are used as static and dynamic triggers during simulations. According to our results, earthquakes with Mw<3.5 can be triggered almost instantly at the rupture edge and failure time of earthquakes with Mw>3.5 advances for both strike-slip and normal faults which may explain the delayed triggering observed SE of Samos Island. Moreover, simulations revealed that the shear-normal stress coupling increases the triggering potential.

How to cite: Sopaci, E. and Özacar, A. A.: Simulation of Instant and Delayed Seismic Triggering Observed After the 30 October 2020 Samos Earthquake at Nearby Faults, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13090, https://doi.org/10.5194/egusphere-egu21-13090, 2021.

Faults in earthquake rupture dynamic simulations are typically treated as infinitesimally thin planes with distinct on- versus off-fault rheologies. These faults are prescribed and can be explicitly accounted for with hexahedral or unstructured tetrahedral meshing approaches.  
We present a diffuse interface alternative to dynamic rupture modelling on non-mesh aligned faults and, by design, permits modelling of non-planar faults and time-dependent fault geometries. We use se2dr, a spectral finite element (continuous Galerkin) method with a non-mesh aligned embedded diffuse discontinuity for dynamic rupture simulations.

Natural fault systems are characterised by fault zone complexity, e.g. the frictional strength and spatio-temporal slip localisation may change drastically from the outer damage zone to the fault core. Complex volumetric failure patterns are observed in well-recorded large complex earthquakes (e.g., the 2016 Mw7.8 Kaikōura event, Klinger et al. 2018), small events (e.g.,  in the San Jacinto Fault Zone, Cheng et al. 2018), and laboratory-scale experiments (e.g., in high-velocity friction experiments, Passelègue et al., 2016).

We develop a diffuse description of fault slip to better understand complex volumetric failure patterns and the mechanics of slip in diffuse fault zones. The fault is defined via a signed distance function (s(x)), which is in turn used to define a fault indicator function with compact support H. If s(x) > H the material behaves as a pure elastic solid - otherwise the tangential stress is governed by a frictional sliding law.
Our approach is implemented on a structured hexahedral mesh using a spectral finite element (continuous Galerkin) method for wave propagation using PETSc. Our diffuse fault SEM method is inspired by the stress-glut method of Andrews, 1999.  A non-mesh aligned embedded diffusive discontinuity allows for complex dynamic rupture simulations. We present 2D numerical experiments of kinematically driven rupture and spontaneous dynamic rupture on non-planar and non-mesh aligned complex fault geometries. The method can be used to model earthquake rupture dynamics on specifically complex and evolving fault faults such as the San Jacinto, CA, fault, or shallowly dipping megathrusts and splay faulting structures in subduction zones.

How to cite: Hayek Valencia, J. N., May, D. A., and Gabriel, A.-A.: Non-planar dynamic rupture modelling across diffuse, deforming fault zones using a spectral finite element method with a non-mesh aligned embedded diffuse discontinuity, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15686, https://doi.org/10.5194/egusphere-egu21-15686, 2021.

What is the effect of crustal melt accumulation on the seismic wavefield? Can we reproduce the dispersion, scattering and associated stress-anisotropy with modeling tools? By performing numerical experiments of seismic wave propagation in a synthetic and geodynamically-consistent volcanic system we can test our ability to model the seismic wavefield and to reconstruct the target “magma chamber”.

We built a synthetic volcano based on recent seismic observations at the Oldoinyo Lengai volcanic complex. The velocity model  is based on a geodynamic model that provides shear modulus, Poisson's ratio, and density. The isotropic P- and S-wave velocities can be computed directly from these parameters. To test a more realistic depth dependence, we introduced a reference 1D velocity model for Northern Tanzania and expanded this to 3D. Then, we inserted variations in the rock parameters mimicking a magma chamber and resolved it using the Fast Marching Travel Time tomography code.

To further our understanding, we also added  3D anisotropy and random velocity fluctuations to the system, acting both as synthetic input for future applications and testing of seismic techniques (e.g., shear wave splitting analysis) and as noise for the travel time tomography. For the waveform modeling we used the velocity-deviatoric stress-isotropic pressure equations together with perfectly matched layers. Also, we encoded the boundary condition between solid and air in this formulation. The 25 receivers with their real geographic locations were placed for inversion sensitivity analysis. In particular, the ability to reconstruct the magma chamber and the effect of anisotropy and velocity fluctuations at frequencies up to 5 Hz are evaluated. The results are compared with a parallel forward modeling and inversion of synthetic MT data. To confirm our results and as an additional test, we also employ adjoint tomography based on spectral element method to implement a forward waveform modeling and inversion using the tools provided in the SPECFEM3D_Cartesian package.

The results present a better idea of how to construct a realistic synthetic volcano in the future. By combining multiple seismic forward models and inversion approaches, this study yields insights into the sensitivity of the seismic wavefield to geodynamically-consistent volcanic structures.

How to cite: komeazi, A., Zhang, Y., de Siena, L., rümpker, G., Kaus, B., Reiss, M., Castro, C., Spang, A., Hering, P., Junge, A., and Baumann, T.: Synthetic seismic modeling and inversion for the Oldoinyo Lengai volcanic complex., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16028, https://doi.org/10.5194/egusphere-egu21-16028, 2021.