This session covers the broad field of earthquake source processes, and
includes the topics of imaging the rupture kinematics and simulating
earthquake dynamics using numerical methods, to develop a deeper
understanding of earthquake source physics. We also invite presentation
that link novel laboratory experiments to earthquake dynamics, and
studies on earthquake scaling properties.

Earthquake sources are imaged using seismic data and surface deformation
measurements (e.g.GPS and InSAR) to estimate rupture properties on
faults and fault systems. Each data set and each method has its strength
and limitations in the context of the source-inversion problem, but the
uncertainties are often not well quantified and the robustness of the
source models not well known.
The session invites contributions that address the source-inversion
problem and provide new methods, innovative applications, and
thought-provoking new ideas. Contributions are welcome that make use of modern
computing paradigms and infrastructure to tackle large-scale forward
simulation of earthquake process, but also inverse modeling to retrieve
the rupture process with proper uncertainty quantification.

Earthquake source imaging, numerical modeling of rupture dynamics, and
source-scaling relations help to understand earthquake source processes.
Furthermore, new numerical modeling approaches for multi-scale
earthquake physics, including earthquake-cycle simulations, may include
fault-zone evolution and even target seismic hazard assessment. The
question that these lines of research are targeting are profound and of
first-order socio-economic relevance:

Which first-order physical processes control, at a given space-time
scale, the macroscopic evolution of dynamic rupture and its seismic
radiation? Is the physics of fault rupture the same for large and small
earthquakes? How can modern earthquake hazard assessment profit from a
deeper understanding of rupture dynamics? Which source processes need to
be considered to better understand, and then model, tsunami generation,
triggering phenomena, induced seismicity and earthquake cycles?

Within this framework our session also provides a forum to discuss case
studies of kinematic or dynamic source modeling of recent significant

Co-organized by EMRP1/NH4
Convener: P. Martin Mai | Co-conveners: Alice-Agnes GabrielECSECS, Henriette Sudhaus, Martin Vallée
| Attendance Thu, 07 May, 08:30–10:15 (CEST)

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Chat time: Thursday, 7 May 2020, 08:30–10:15

Chairperson: Martin Vallee
D1608 |
Mohammadreza Jamalreyhani, Mehdi Rezapour, Simone Cesca, Sebastian Heimann, Hannes Vasyura-Bathke, Henriette Sudhaus, Marius Paul Isken, and Torsten Dahm

The Mw 7.3 Sarpol-Zahab earthquake occurred on 12 November 2017 in the Lurestan arc of the Zagros Simply Folded Belt (ZSFB). It is estimated that 600 people were killed and 8000 were injured in this earthquake. This earthquake has been the largest instrumentally recorded earthquake in the ZSFB and its moment, as well as its mechanism, were unexpected. We present an earthquake source study on the Mw 7.3 Sarpol-Zahab earthquake, two large following earthquakes in the region in 2018 and their corresponding aftershock sequences to gain insight of seismotectonic of the Lurestan arc fold-thrust belt.

In this study, we complement previous studies on this earthquake, by non-linear probabilistic optimization of joined geodetic and seismic data using a new, efficient Bayesian bootstrap-based optimization scheme to infer the finite fault geometry and fault slip together with meaningful uncertainty estimates of the model parameters. Our optimization is based on the modeling of ascending and descending Sentinel-1 satellite data, seismological waveform from global seismic networks and the strong motion network of Iran. The posterior mean model of the Sarpol-Zahab earthquake shows that the causative fault plane is centered at is 14±2 km depth and has a low dip angle of 17°±2° and a strike of 350°±10°. The rake angle of 144°±4° points to an oblique thrust mechanism. The rupture area of the uniform-slip, rectangular model is 40±2 km long and 16±2 km width and shows 4.0±0.5 m fault slip, which results in a magnitude estimate of Mw 7.3±0.1.

Later, in August and November 2018, two large earthquakes with Mw 6.0 and Mw 6.4 occurred about 40 km east and 60 km south of the Sarpol-Zahab epicenter, respectively. These earthquakes could have been triggered by the 2017 Sarpol-Zahab earthquake. We apply the same joint inversion modeling to derive the corresponding fault plane solutions. We found strike-slip mechanisms for both events but centroid depths at 10±2 km and 16±2 km for Mw 6.0 and Mw 6.4, respectively.

The 2017 Sarpol-Zahab earthquake and the following studied 2018 earthquakes were followed by a sustained aftershock sequence, with more than 133 aftershocks exceeding Ml 4.0 until December 30, 2019. We rely on the local and regional seismic broad-band stations of Iran and Iraq permanent networks to estimate full-waveform moment tensor solutions of 70 aftershocks down to Ml 4. Most of these aftershocks have shallow centroid depths between 5 and 12 km, so that they occurred in the uppermost part of the basement and/or in the lower sedimentary cover, which is ~8 km thick in this area.

Our results suggest that the Sarpol-Zahab earthquakes activated low-angle thrust faults and shallower strike-slip structures, highlighting that both thin- and thick-skin deformation take place in the fold-thrust belts in the Lurestan arc of the Zagros. Such information on the deformation characteristics is important for the hazard and risk assessment of future large earthquakes in this region.
Additionally, we demonstrate how the joint inversion of different geophysical data can help to better resolve the fault geometry and the earthquake source parameters.

How to cite: Jamalreyhani, M., Rezapour, M., Cesca, S., Heimann, S., Vasyura-Bathke, H., Sudhaus, H., Paul Isken, M., and Dahm, T.: The 2017 November 12 Mw 7.3 Sarpol-Zahab (Iran-Iraq border region) earthquake: source model, aftershock sequence and earthquakes triggering, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-759, https://doi.org/10.5194/egusphere-egu2020-759, 2019

How to cite: Jamalreyhani, M., Rezapour, M., Cesca, S., Heimann, S., Vasyura-Bathke, H., Sudhaus, H., Paul Isken, M., and Dahm, T.: The 2017 November 12 Mw 7.3 Sarpol-Zahab (Iran-Iraq border region) earthquake: source model, aftershock sequence and earthquakes triggering, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-759, https://doi.org/10.5194/egusphere-egu2020-759, 2019

How to cite: Jamalreyhani, M., Rezapour, M., Cesca, S., Heimann, S., Vasyura-Bathke, H., Sudhaus, H., Paul Isken, M., and Dahm, T.: The 2017 November 12 Mw 7.3 Sarpol-Zahab (Iran-Iraq border region) earthquake: source model, aftershock sequence and earthquakes triggering, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-759, https://doi.org/10.5194/egusphere-egu2020-759, 2019

D1609 |
Stephen Hicks, Ryo Okuwaki, Andreas Steinberg, Catherine Rychert, Nicholas Harmon, Rachel Abercrombie, Petros Bogiaztis, David Schlaphorst, Jiri Zahradnik, J-Michael Kendall, Yugi Yagi, Kousuke Shimizu, and Henriette Sudhaus

Rupture propagation of an earthquake strongly influences potentially destructive ground shaking. Variable rupture behaviour is often caused by complex fault geometries, masking information on fundamental frictional properties. Geometrically smoother ocean transform fault (OTF) plate boundaries offer a favourable environment to study fault zone dynamics because strain is accommodated along a single, wide zone (up to 20 km width) offsetting homogeneous geology comprising altered mafic or ultramafic rocks. However, fault friction during OTF ruptures is unknown: no large (Mw>7.0) ruptures had been captured and imaged in detail. In 2016, we recorded an Mw 7.1 earthquake on the Romanche OTF in the equatorial Atlantic on nearby seafloor seismometers. We show that this rupture had two phases: (1) up and eastwards propagation towards the weaker ridge-transform intersection (RTI), then (2) unusually, back-propagation westwards at super-shear speed toward the fault’s centre. Deep slip into weak fault segments facilitated larger moment release on shallow locked zones, highlighting that even ruptures along a single distinct fault zone can be highly dynamic. The possibility of reversing ruptures is absent in rupture simulations and unaccounted for in hazard assessments.

How to cite: Hicks, S., Okuwaki, R., Steinberg, A., Rychert, C., Harmon, N., Abercrombie, R., Bogiaztis, P., Schlaphorst, D., Zahradnik, J., Kendall, J.-M., Yagi, Y., Shimizu, K., and Sudhaus, H.: Back-propagating super-shear rupture in the 2016 M7.1 Romanche transform fault earthquake, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5312, https://doi.org/10.5194/egusphere-egu2020-5312, 2020

D1610 |
Saulė Simutė, Lion Krischer, Christian Boehm, Martin Vallée, and Andreas Fichtner

We present a proof-of-concept catalogue of full-waveform seismic source solutions for the Japanese Islands area. Our method is based on the Bayesian inference of source parameters and a tomographically derived heterogeneous Earth model, used to compute Green’s strain tensors. We infer the full moment tensor, location and centroid time of the seismic events in the study area.

To compute spatial derivatives of Green’s functions, we use a previously derived regional Earth model (Simutė et al., 2016). The model is radially anisotropic, visco-elastic, and fully heterogeneous. It was constructed using full waveforms in the period band of 15–80 s.

Green’s strains are computed numerically with the spectral-element solver SES3D (Gokhberg & Fichtner, 2016). We exploit reciprocity, and by treating seismic stations as virtual sources we compute and store the wavefield across the domain. This gives us a strain database for all potential source-receiver pairs. We store the wavefield for more than 50 F-net broadband stations (www.fnet.bosai.go.jp). By assuming an impulse response as the source time function, the displacements are then promptly obtained by linear combination of the pre-computed strains scaled by the moment tensor elements.

With a feasible number of model parameters and the fast forward problem we infer the unknowns in a Bayesian framework. The fully probabilistic approach allows us to obtain uncertainty information as well as inter-parameter trade-offs. The sampling is performed with a variant of the Hamiltonian Monte Carlo algorithm, which we developed previously (Fichtner and Simutė, 2017). We apply an L2 misfit on waveform data, and we work in the period band of 15–80 s.

We jointly infer three location parameters, timing and moment tensor components. We present two sets of source solutions: 1) full moment tensor solutions, where the trace is free to vary away from zero, and 2) moment tensor solutions with the isotropic part constrained to be zero. In particular, we study events with significant non-double-couple component. Preliminary results of ~Mw 5 shallow to intermediate depth events indicate that proper incorporation of 3-D Earth structure results in solutions becoming more double-couple like. We also find that improving the Global CMT solutions in terms of waveform fit requires a very good 3-D Earth model and is not trivial.

How to cite: Simutė, S., Krischer, L., Boehm, C., Vallée, M., and Fichtner, A.: Seismic source inversion using Hamiltonian Monte Carlo and a 3-D Earth model in the Japanese Islands, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9229, https://doi.org/10.5194/egusphere-egu2020-9229, 2020

D1611 |
Sebastian von Specht, Kuo-Fong Ma, Yen-Yu Lin, and Fabrice Cotton

Over the last decades, many types of slip-rate functions (SRFs) have been introduced into kinematic rupture modeling. Commonly used SRFs are the Haskell-type rectangular pulse, cosine and trapezoidal windows and the Kostrov-/Yoffe functions. All these functions and many functional shapes inferred from multiwindow inversion techniques can be well described or are even identical to the functional form of the generalized beta distribution—a widely used and well studied probability density function (pdf) in statistics. The generalized beta pdf has three parameters, where one parameter relates to the SRF duration and two describe the shape of the pulse. The shape parameters have simple analytic expressions for their estimators. Using the generalized beta pdf with free shape parameters as SRF can effectively reduce the number of required free parameters in the inversion when compared to multiwindow SRF techniques. The generalized beta pdf provides us not only with analytic solutions of the derivative (slip-rate change) and antiderivative (slip) of the slip-rate function but also analytic expressions for their Fourier spectra. We apply the beta SRF for rupture modeling of two well studied earthquakes in Taiwan—the 2016 MW 6.4 Meinong earthquake and the MW 6.3 2018 Hualien earthquake—and compare results in terms of slip distribution and model uncertainties.

How to cite: von Specht, S., Ma, K.-F., Lin, Y.-Y., and Cotton, F.: A generalized slip-rate function for kinematic modeling, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21170, https://doi.org/10.5194/egusphere-egu2020-21170, 2020

D1612 |
Navid Kheirdast, Anooshiravan Ansari, and Susana Custódio

The earthquake rupture process is often represented by a source function that defines slip in space and time. If we assume slip to occur on a planar surface, then the source function becomes a function of three independent variables: two spatial dimensions (slip down-dip and slip along-strike) and one temporal dimension (source time function at each point on the fault). Finite fault inverse problems aim at exploring this model space in order to find the source function that generates synthetic ground motion that best fits the observed data. This inverse problem is severely ill-conditioned. In order both to ensure a regular solution and to avoid over-fitting the data, both physical and mathematical constraints can be imposed.  Common methods of finite fault source inversion typically apply a one-dimensional regularization in time, which gives preference to compacted-support source-time functions, like triangular or trapezoidal functions in time, or to two-dimensional regularizations that ensure smooth variations of slip over the fault plane (Mai et. al, 2016). In this work, we propose an innovative three-dimensional regularization for kinematic source inversions in the frequency domain, which simultaneously requires smooth variations of slip over space (2D) and frequency (1D, smooth spectra) . This new three-dimensional regularization selects the spatial slip distributions that are more similar to those of neighboring frequencies, thus effectively transferring knowledge from one frequency to another. In the framework of Tikhonov regularization, having more than one regularization condition requires more than one damping factor to be inserted in the inversion misfit. Additionally, no orthogonal decomposition (like Generalized Singular Value Decomposition) exists when more than one regularization conditions are imposed. Thus, we investigate a new 3D regularization method using a Bayesian approach with a Markov Chain Monte Carlo (MCMC) simulation. The new method is tested using the SIV-inv1 benchmark exercise. The proposed method is also preliminarily applied to study the rupture process of the 2019 M5.9 Torkamanchay, Iran, earthquake.

How to cite: Kheirdast, N., Ansari, A., and Custódio, S.: A new three-dimensional regularization for finite fault source inversions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-833, https://doi.org/10.5194/egusphere-egu2020-833, 2019

D1613 |
Rishabh Dutta, Sigurjón Jónsson, and Hannes Vasyura-Bathke

Earthquake fault ruptures are typically complex and can consist of en echelon segments, have bends, large step-overs, and be curved or warped at different spatial scales. Although surface fault ruptures can be mapped using a variety of geological and geophysical techniques, the subsurface topology of faults is challenging to estimate. One of the main options is to use geodetic data (InSAR and GPS) of coseismic surface deformation to estimate the subsurface earthquake fault geometry along with the distributed slip. The general practice is to assume a planar fault surface and estimate the strike and dip of a simple rectangular fault prior to the spatially-variable slip estimation. Using such simplistic fault geometry during source fault estimations of large earthquakes rarely captures all the crustal deformation details seen in the data and can cause biased estimation results of the fault slip. Here, we show how complex non-planar fault geometry can be estimated simultaneously with spatially-variable slip from geodetic data in a Bayesian framework, where our non-planar fault geometry parametrization approach allows for various undulations of the fault surface in both the along-strike and down-dip directions.

We exemplify this approach through synthetic tests considering a checkerboard-like slip pattern on a listric non-planar fault. The results show that fault slip can be over-estimated by about 50-100% when using pre-assumed planar fault geometry. In contrast, both the non-planar fault geometry and spatially-variable slip are better retrieved when using our estimation approach. We then apply this modeling approach to the 2011 MW9.1 megathrust Tohoku-Oki (Japan) earthquake. Here we use prior information like the location of the trench and earthquake hypocenters during the Bayesian estimation to reduce the extent of the model space. The resulting fault geometry shows variations in fault dip in both the along-strike and down-dip directions that compare well with Hayes’ slab1.0 model of the subduction interface. The estimated fault slip is also comparable to the results that pre-defined the fault geometry using the slab1.0 model. In the future, the proposed method could lead to more realistic source models of major earthquakes, aided by improving computational resources and spatial resolution of geodetic data.

How to cite: Dutta, R., Jónsson, S., and Vasyura-Bathke, H.: Simultaneous Bayesian Estimation of Complex Non-planar Earthquake Fault Geometry and Spatially-variable Slip from Geodetic Data, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21277, https://doi.org/10.5194/egusphere-egu2020-21277, 2020

D1614 |
Yong Zhang, Yueyi Xu, and Rongjiang Wang

The fault geometry closely controls earthquake rupture process. Previous seismic inversion of the fault geometry is to derive the multiple-point moment tensor solutions. Because of the trade-off between the moment tensor and rupture velocity, the inversion has high instabilities. In contrast, geodetic inversion has less unknowns, since there is no need to solve for rupture velocity. But from the elastic dislocation theory, the relations between the surface deformation and sub-fault parameters (i.e. strike, dip and rake) are nonlinear. In this study, we develop a linear technique to invert geodetic data for sub-fault moment tensors. From the sub-fault moment tensor solutions, the strike, dip, rake, and their spatial variations can be estimated, which provide valuable information for assessing the complexities in fault geometry. We applied this technique to several significant earthquakes, i.e., the 2008 Mw7.9 Wenchuan earthquake, the 2015 Mw7.8 Gorkha earthquake, and the 2017 Mw6.5 Jiuzhaigou earthquake. The results of the 2008 Wenchuan earthquake suggest that the strike, dip and rake are all variable from southwest to northeast, which are well consistent with the aftershock distributions and mechanisms. The dip variations of the 2015 Gorkha earthquake suggest the earthquake has ruptured a listric fault (dep decreases with depth). Particularly, a dip anomaly appears in the northeast corner of the rupture area, indicating a geometric barrier accounting for the slip gap between the mainshock and largest Mw7.3 aftershock. For the 2017 Jiuzhaigou earthquake, two right-stepping and left-lateral strike-slip segments were distinguished. Accordingly, a compressional step-over was identified between the two segments.

How to cite: Zhang, Y., Xu, Y., and Wang, R.: Geodetic inversion of complex fault geometries for strong earthquakes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12148, https://doi.org/10.5194/egusphere-egu2020-12148, 2020

D1615 |
Felipe Vera, Frederik Tilmann, and Joachim Saul

We present a back-projection method capable of being parameterized with multiples arrays. The rupture imaging is weighted to restrict uncertainties induced by non-symmetric azimuthal coverage of seismic arrays. The strategy also exploits the differences in time delays between P and depth phase (pP) waveforms by assuming them as proxies of the rupture that can be simultaneously back-projected. Surprisingly, this helps to improve the final results, even when depth phases overlap with the direct arrivals due to the rupture time exceeding the pP-P delay. Thus, the approach heightens the spatiotemporal resolvability enough to image rupture complexities. The rupture image of two large events demonstrates its robustness. The first one is the 14 November 2007 Mw 7.7 Tocopilla earthquake in northern Chile. The high-frequency rupture (0.5 - 2.0 Hz) encircles two asperities while the short-period energy radiated predominates up-dip of the coseismic slip. We propose the contribution of asperity rupture complexities and along-dip barriers to high-frequency emissions beyond the megathrust frictional structure. The second one is the Mw 7.5 Palu strike-slip earthquake, which occurred on 28 September 2018 in Sulawesi island. The back-projection reveals a prominent supershear rupture at a speed of 4.5 km/s. The result correlates with space geodesy data highlighting the successful recovery of fault structures. Finally, we discuss the potential and challenges of automating this analysis for near-real-time applications, including near-source back-projection with strong-motion data.

How to cite: Vera, F., Tilmann, F., and Saul, J.: Multi-Array Multi-Phase Back-Projection: Improving the imaging of earthquake rupture complexities, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11233, https://doi.org/10.5194/egusphere-egu2020-11233, 2020

D1616 |
Thomas Ulrich, Bo Li, and Alice-Agnes Gabriel

Back-projection uses the time-reversal property of the seismic wavefield recorded at large aperture dense seismic arrays. Seismic energy radiation is imaged by applying array beam-forming techniques. The spatio-temporal rupture complexity of large earthquakes can be imaged simply and rapidly with a limited number of assumptions, which makes back-projection techniques an important tool of modern seismology. However, back-projection analyses exhibit frequency and array dependency (e.g. Wu et al., AGU19). In addition, the method relies on station network geometry and data quality and can suffer from imaging artifacts (e.g., Fan and Shearer, 2017) and back-projection results may not be consistently interpreted.

The Mw7.5 Palu, Sulawesi earthquake that occurred on September 28, 2018, ruptured a 180 km long section of the Palu-Koro fault. The earthquake triggered a localized but powerful tsunami within Palu Bay, which swept away houses and buildings. The supershear earthquake and unexpected tsunami led to more than 4000 fatalities. Ulrich et al. (2019) propose a physics-based, coupled earthquake-tsunami scenario of the event, tightly constrained by observations. The model matches key observed earthquake characteristics, including moment magnitude, rupture duration, fault plane solution, teleseismic waveforms, and inferred horizontal ground displacements. It suggests that time-dependent earthquake-induced uplift and subsidence could have sourced the observed tsunami within Palu Bay.

Back-projection has been used to track the rupture propagation of the Palu earthquake. Bao et al. (2019) image unilateral rupture traveling at a supershear rupture speed. Their results show array dependent ruptures, from a rather relatively linear rupture using the Australian array, to a spatio-temporally more scattered image using the seismic array in Turkey. In addition, they do not resolve any portion of the rupture as traveling at sub-Rayleigh speeds, while Wei et al. (AGU19) suggest a gradually accelerating rupture.

In this study, we build upon the dynamic rupture model of Ulrich et al. (2019) to investigate the reliability of standard back-projection techniques using a realistic and perfectly known earthquake model. In particular, we investigate whether or not rupture transfers across the segmented fault system, and the effect of specific geometric features of the fault system, such as fault bends, on rupture dynamics, leave a clear signal on the inferred beam power. Also, we investigate the effect of secondary phases, such as reflections from the free-surface or from fault segment boundaries, naturally captured by dynamic rupture modeling. In addition, we study the effect of small-scale source heterogeneities on the back-projection results by including different levels of fault roughness in the dynamic rupture simulations. Finally, we investigate the array dependence of back-projection results.

Overall, this study should help to better understand which features of rupture dynamics back-projection can capture. Our results are a first step towards fundamental analysis to better understand which features can be captured by back-projection and to provide guidelines for back-projection interpretation.

How to cite: Ulrich, T., Li, B., and Gabriel, A.-A.: Synthetic analysis of seismic back-projection using 3D dynamic rupture simulations of the 2018, Palu Sulawesi earthquake , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20703, https://doi.org/10.5194/egusphere-egu2020-20703, 2020

D1617 |
Victor Tsai and Greg Hirth

A fundamental question of earthquake science is what produces damaging high-frequency ground motion, with the classic Brune-Haskell model postulating that abrupt initiation of fault slip causes it. However, even when amended with heterogeneous rupture, frictional slip models fail to explain observations of different sized repeating earthquakes, and have challenges explaining high-frequency radiation patterns as well as the dependence of stress drops on fault maturity and depth. We propose an additional cause for high-frequency earthquake spectra from elastic collisions of structures within a rupturing fault zone. The collision spectrum is set by an impact contact time that is proportional to the size of colliding structures, so that spectra depend on fundamentally different physical parameters compared with slip models. When added to standard frictional models, the collision model can reconcile the discrepant observations, since the size, shape and orientation of structures vary between different fault zones but remain constant within a given fault segment. High-frequency earthquake ground motions and damage may therefore be an outgrowth of fault-zone structure rather than sudden initiation of slip.

How to cite: Tsai, V. and Hirth, G.: Elastic Impact Consequences for High-Frequency Earthquake Ground Motions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10834, https://doi.org/10.5194/egusphere-egu2020-10834, 2020

D1618 |
Alexandre Schubnel, Samson Marty, Blandine Gardonio, Harsha Bhat, Eiichi Fukuyama, and Raùl Madariaga

Over the past decades, an increasing number of seismological observations and improvement in data quality have allowed to better detect foreshock sequences prior to earthquakes. However, due to strong spatial and temporal variations of foreshock occurrence, their underlying physical processes and their links to earthquake nucleation are still under debate. Here we address these issues by looking at precursory acoustic activity during laboratory earthquakes (stick-slip instabilities).

Here, laboratory earthquake experiments were performed on saw-cut Indian metagabbro under upper crustal stress conditions ranging from 30 to 60 MPa confining pressure. Using a high-frequency monitoring system and calibrated piezoelectric acoustic sensors we continuously record particle velocity field at 10 MHz sampling rate during the experiments. Based on a trigger logic we identify acoustic emissions (AE) within continuous data. From P-wave arrival-time data and from spectral analysis we are able to estimate the following seismological parameters for each AE: location, absolute magnitude, stress-drop and size.

First, we show that the source parameters of AE (Mw -9.0 to Mw -7.0) follow the same scaling relationship as natural earthquakes justifying the use of acoustic precursors as proxy to foreshocks. We observe that foreshock triggering is systematically related to aseismic slip and that the dynamics of foreshocks mirrors the acceleration of slip-rate preceding failure. Experimental scalings demonstrate that : i- the nucleation evolves  from an aseismic process into a cascading one, and ii) the duration and magnitude of the pre-seismic moment correlates with the magnitude of the mainshock, at least at the scale of the laboratory. Finally, using Hertz contact theory, we find a scaling law between the seismic energy released by foreshocks, the fault roughness  and the normal stress acting on the fault interface.

How to cite: Schubnel, A., Marty, S., Gardonio, B., Bhat, H., Fukuyama, E., and Madariaga, R.: Dynamics of foreshocks and pre-slip during the nucleation of laboratory earthquakes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13327, https://doi.org/10.5194/egusphere-egu2020-13327, 2020

D1619 |
Elisa Tinti, Emanuele Casarotti, Alice-Agnes Gabriel, Taufiqurrahman Taufiqurrahman, Thomas Ulrich, and Duo Li

The 2016 Central Italy sequence showed a remarkable complexity involving multiple faults. Highly heterogeneous slip distributions were inferred from kinematic finite source inversions. The coverage and quality of the geodetic and seismic data allow resolving high-resolution details of rupture kinematics of the largest event of the sequence, the Mw 6.5 30 October 2016 Norcia earthquake. Composite fault rupture models suggest that two fault planes may have slipped simultaneously. Nevertheless, kinematic modeling cannot assess the mechanic viability of such multiple fault plane models.

Using SeisSol, a software package for simulating wave propagation and dynamic rupture based on the arbitrary high-order accurate derivative discontinuous Galerkin method, we therefore try to generate spontaneous dynamic ruptures models compatible with the two fault planes constrained by kinematic inversions. To this end, we adopt a simple slip-weakening friction law with spatially variable dynamic friction and initial strength parameters along multiple faults, compatible with the slip distributions found in the literature. Although we do not to aim to explore the full parameter space, our approach allows testing the feasibility of kinematic models in conjunction with successfully generating spontaneous dynamic rupture scenarios matching seismic and geodetic observations with geological constraints. Such linking enhances and validates the physical implications of kinematic earthquake source inversion.

How to cite: Tinti, E., Casarotti, E., Gabriel, A.-A., Taufiqurrahman, T., Ulrich, T., and Li, D.: Kinematic constraining of the multi-fault rupture dynamics of the Norcia, Mw 6.5, 30 October 2016, Central Italy earthquake, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10142, https://doi.org/10.5194/egusphere-egu2020-10142, 2020

D1620 |
Jolan Liao, Kuo-Fong Ma, Sebastian Specht, and David Oglesby

The September 20, 1999 (UTC) Mw7.6 Chi-Chi earthquake in Taiwan was a devastating event of historic proportions. Although this event caused severe damage, it also provided a large data set of high-quality near-field strong motion acceleration records from the Taiwan Strong Motion Instrumentation Program. Despite ongoing advances in kinematic modeling in the last two decades, some questions remain unresolved.  One of those questions is the seismic energy partition in radiated energy and fracture energy. We address this question by investigating the dynamic rupture behavior of this event. We constructed a 3D dynamic rupture model which is constrained by the well resolved spatiotemporal slip distribution and in-situ stress measurements from fault-zone drilling. In our model, we consider the fault ruptures with both spatially uniform and non-uniform frictional behavior and perform a series of numerical experiments with different sets of input variables (e.g., slip-weakening distance, dc, and initial stress on fault plane) based on a slip-weakening friction law. We examined the parameters controlling the slip patterns as the result from kinematic modeling. For the constraints of the input variables, we first derived the constitutive relationship between slip and stress change on the subfaults from the temporal and spatial slip distribution of the kinematic models by Ji et al. (2003), and then determined the dynamic parameters (e.g., apparent slip-weakening distance, dc', and the ratio of strength excess and stress drop,  S). Our initial normal stress on the fault plane is based on the geophysical logging analysis of the Taiwan Chelungpu-fault Drilling Project. Our optimal model can simulate a rupture similar to the kinematic model  by Ji et al. (2003) and suggests that the final slip distribution is mainly controlled by the spatial distribution of the normal stress. We require a downscaling (α) of the apparent slip-weakening distance, αdc', from the derived constitutive law of the kinematic model to allow a dynamic rupture propagation with large slip velocity comparable to the observations. With the downscaled slip-weakening distance, αdc', and a heterogeneous stress distribution, the slip-weakening curves from our optimal model suggests the downscaling in radiated seismic energy and fracture energy accordingly .

How to cite: Liao, J., Ma, K.-F., Specht, S., and Oglesby, D.: Dynamic modeling of the 1999 Chi-Chi (Mw7.6) earthquake: New insights on energy partition in large earthquakes by incorporating in-situ stress measurements into the constitutive relationship from kinematic modeling, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12423, https://doi.org/10.5194/egusphere-egu2020-12423, 2020

D1621 |
Julien Renou, Martin Vallée, and Hideo Aochi

Our knowledge of earthquake source physics, giving rise to events of very different magnitudes, requires observations of a large population of earthquakes. The development of systematic analysis tools for the global seismicity meets these expectations, and allows us to extract the generic properties of earthquakes, which can then be integrated into models of the rupture process. Following this approach, the SCARDEC method is able to retrieve source time functions of events over a large range of magnitude (Mw > 5.7). The source time function (which describes the temporal evolution of the moment rate) is suitable for the analysis of transient rupture properties which provide insights into the generation of earthquakes of various sizes. Our study aims at observing the rupture development of such earthquakes in order to add better constraints on dynamic source models. We first focus on the development of earthquakes through the analysis of the SCARDEC catalog. The phase leading to the peak of the source time function ("development phase'') is extracted to characterize its evolution. From the computation of moment accelerations at prescribed moment rates, we observe that the evolution of the moment rate during the developement phase is independent of the final magnitude. A quantitative analysis of the moment rate increase as a function of time further indicates that this phase does not respect the steady t² self-similar growth. These observations are then compared with dynamic source models. We develop heterogeneous dynamic models which take into consideration rupture physics. Heterogeneous distributions of the friction parameter and the initial stress contribute to generate highly realistic rupture scenarios. Rupture propagation is strongly influenced by these two dynamic parameters which induce a clear preferential direction of propagation together with a local variability of the rupture velocity. Variability of the kinematic parameters also tends to correlate rupture velocity and slip velocity, which is a key feature for the transient behavior of the development phase previously observed. These findings are expected to put further constraints on future realistic dynamic rupture scenarios.

How to cite: Renou, J., Vallée, M., and Aochi, H.: Observations and modeling of the rupture development based on the analysis of Source Time Functions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9557, https://doi.org/10.5194/egusphere-egu2020-9557, 2020

D1622 |
Jan Ridderbusch, Henriette Sudhaus, Andreas Steinberg, Stefanie Donner, and Abdolreza Ghods

On August in 2012 a Mw6.4 earthquake hit the region near the town Ahar in NW Iran. With only 11 minutes delay it was followed by another large and close by Mw6.3 earthquake. The 2012 Ahar earthquakes have been unexpected in their large magnitudes and the activated faults are poorly studied. A mapped east-west striking surface rupture is attributed to the first earthquake, which shows a strike-slip mechanism. The second earthquake is reported to have a thrust mechanism and a deeper hypocenter, but is much more poorly constrained than the first earthquake. The short time interval between those two earthquakes made it impossible to distinguish their effects in the available static surface displacement data based on InSAR and GNSS, and difficult in global seismological records. Any source analysis using static displacement data and/or teleseismic waveforms therefore has to rely on the corresponding cumulative surface displacements and recorded waveforms of the first earthquake, respectively. In contrast, in regional waveform data, the seismic phase arrivals of both earthquakes are well separated in time. To tackle the coupling of the earthquakes we conducted a combined-data study that solves for the individual sources of the earthquake doublet simultaneously in a non-linear probabilistic finite-fault optimisation. In our combined-data study we improve the constraints on the doublet sources, particularly the second earthquake. We use InSAR data from RADARSAT-2 acquisitions and published co-seismic displacement vectors based on GNSS data. For the InSAR data, unfortunately, only measurements of an ascending orbit are available. The seismological data are teleseismic (distance larger than 1000 km) and regional waveform recordings (distances less than 1000 km). For the modelling we use Green’s functions of a layered regional velocity model and rectangular, constant-slip rupture models.

Our non-linear, finite-fault optimisation makes use of Bayesian bootstrap data weighting, which enables a very efficient estimation of model parameter uncertainties. This method accounts for modelling and data errors and can sample non-Gaussian posterior probabilities of model parameters. Our results show that the two earthquakes activated two different faults. The first earthquake ruptured a shallow east-west striking dextral fault extending from the surface vertically down to approximately 8 km depth (6 to 14 km confidence). The second earthquake ruptured a north to north-east striking fault with a dip of about 40 degree with an oblique rupture mechanism. The fault activated by the second earthquake seems to be located below the first one, at levels deeper than 9 km and a bit shifted to the west.We verify our results with model-independent seismic multi-array backprojection of the radiated seismic energy.

We used the python-based software toolbox Pyrocko for the data processing. The included module Grond implements the Bayesian bootstrap optimisation approach. Both are open-source under the GNU General Public License and available on pyrocko.org. The RADARSAT data used in this study have been provided through the RADARSAT-2 SOAR-EU loan agreement #16736. This research is further supported by the German Research Foundation DFG through an Emmy-Noether-Grant.

How to cite: Ridderbusch, J., Sudhaus, H., Steinberg, A., Donner, S., and Ghods, A.: Simultaneous optimisation of two sources of the 2012 Ahar earthquake doublet (Mw 6.4 and 6.3, Iran) based on InSAR data, GNSS data and seismic waveforms, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10432, https://doi.org/10.5194/egusphere-egu2020-10432, 2020

D1623 |
Dimas Salomo Januarianto Sianipar, Bor-Shouh Huang, Kuo-Fong Ma, Tio Azhar Prakoso Setiadi, Ming-Che Hsieh, Haekal Azief Haridhi, and Daryono Daryono

The western extension and deformation mechanism of Flores back-arc thrust in eastern Sunda-Banda Arc (Indonesia) are poorly investigated, and, thus, poorly constrained. From late July to August 2018, a sequence of large earthquakes (M6.4+) took place in the north of Lombok Island that marked the previously westernmost termination of the continuous zone of the back-arc thrusting. The 2018 Lombok earthquake sequences that began with Mw 6.4 (28 July 2018), and followed by Mw 6.9 (5 August 2018), and Mw 6.9 (19 August 2019) with massive subsequent aftershocks claimed on more than 500 casualties, nearly 500,000 people displaced and serious damages on Lombok Island. Here we relocate the aftershocks and perform the finite fault inversions of M6.4+ earthquake sequences constrained with teleseismic body and surface waves. Both refined hypocenters of aftershocks and rupture processes of large earthquakes provide detail kinematic descriptions of the source mechanisms of the sequences. The aftershocks distributions and slip model suggest that the earthquakes occurred on south-dipping low angle thrust faulting that striking to the east while it also activated aftershocks on surrounding complex shallow faulting with distinguishing distributions. The source inversions of large earthquakes over the entire of the western part of Flores back-arc thrust resulted as simple circular rupture propagations initiated from ~15 km depth for all events except the westernmost events (Mw 6.9 on 5 August 2019) that had a more complex rupture and initiated from shallower depth, and with slip distributed cross over the former identified westernmost termination of the Flores back-arc thrust. Our study suggests the further extension of back-arc thrusting and the possible structures revealed from the subsequence aftershocks. The source characterizations revealed in this study would be important for further seismic hazard analysis in this region.

How to cite: Sianipar, D. S. J., Huang, B.-S., Ma, K.-F., Setiadi, T. A. P., Hsieh, M.-C., Haridhi, H. A., and Daryono, D.: Source Investigation of the 2018 Lombok (Indonesia) Earthquake Sequences, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18938, https://doi.org/10.5194/egusphere-egu2020-18938, 2020

D1624 |
Yongzhe Wang, Wanpeng Feng, Kun Chen, and Hailin Du

Interferometric Synthetic Aperture Radar (InSAR) data is of high spatial resolution and has been widely used in measuring surface deformation generated by earthquakes. However, the temporal resolution of InSAR data is relatively poor from an individual mode SAR sensor. A series of earthquakes hit Kumamoto, Japan in April 2016. These earthquakes were considered to be a sequence that started from two foreshocks (TFS) larger than Mw 6.0 and reached its climax for the largest earthquake of Mw7.0 only after 28 hours. To better reveal the source model characteristics of the TFS and the main shock, we firstly determined the geometrical parameters using the aftershock re-location data and the surface fault rupture data of field survey, and then applied multi-mode InSAR data only covering the TFS and the whole sequence, respectively, to carry out the joint inversion of respective source models based on the time correlation between the TFS and the main shock. The results show that both the source models determined here are well consistent with previous results constrained by seismological data. The strategy of inversion used in this study suggests that we may separate multiple seismic sources sequence from geodetic observations using the joint inversion based on the time correlation.

How to cite: Wang, Y., Feng, W., Chen, K., and Du, H.: Slip models of the 2016 Mw7.0 Kumamoto, Japan mainshock and its two foreshocks constrained by multi-mode InSAR data, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12106, https://doi.org/10.5194/egusphere-egu2020-12106, 2020

D1625 |
Chieh-Chen Lee, Tai-Lin Tseng, and Pei-Ru Jian

  Taiwan region is a seismically active region formed by the oblique convergence between Philippine Sea Plate and Eurasia Plate. Focal mechanisms of most small-moderate sized earthquakes can be well constrained by the local seismic array, except for those occurred offshore Taiwan where azimuthal coverage is limited. To better understand the tectonic structures, it is desirable to improve the focal mechanisms using better located hypocenters, reasonable velocity models, and the best available stations. In this study we focus on the shallow earthquakes in Taiwan Strait and the intermediate-depth earthquakes in southernmost Ryukyu. Both regions are less explored but large historic events had been reported.

  For earthquakes in Taiwan Strait, we systematically studied earthquakes from 1996 to 2019, including the Mw5.7 Taiwan Shoal sequence happened on 2018/11/25. A total of 22 new moment tensors (MTs) were resolved in the passive margin by combining Fujian and Taiwan seismic networks from either side of the strait. For events closer to Fujian, China, the velocity model with Moho depth of 35 km yields overall lower compensated linear vector dipole (CLVD) and acceptable misfit values; while as a 40 km thick crust is better for events closer to or on the shore of Taiwan. This Moho variation under the Taiwan Strait, although subtle, agrees well with the velocity structure constrained independently by previous studies. Earthquakes in the middle of the strait are dominant in strike-slip and normal slip within 30 km depth. Shallow thrusting events are found only in the Miaoli offshore area of Taiwan. As for the 2018 Taiwan Shoal earthquake sequence, it is located right on the region absence of known fault-plane solutions, therefore offers important new constraints. All events of the sequence show high angle strike-slips and shallow centroid depth of 11-21 km, more consistent with seismicity determined by Fujian seismic center. This event is far away from the M8 1604 Quanzhou earthquake, and is also clearly unrelated to the structure of 1994 Mw 6.7 normal-faulting event in Tainan Basin. The 2018 sequence is probably the reactivation of a pre-existing normal fault that was created by rifting during the Cenozoic.

  For future work, we will re-evaluate the MTs of M>5.5 intermediate-depth earthquakes of the Ryukyu subduction zone by including waveforms of stations YNG and IGK from Japan network in the inversion. We will also test different upper mantle velocities in the model for the computation of Green’s functions. We anticipate that our work can provide a set of parameters more suitable for the MT inversion, and the MT results can delineate the Ryukyu subduction zone properties better.


keywords : Taiwan Strait, focal mechanisms, moment tensor inversion

How to cite: Lee, C.-C., Tseng, T.-L., and Jian, P.-R.: Improving Focal Mechanisms for Earthquakes in Taiwan Strait and Ryukyu Subduction Zone with Broadband Waveforms of Combined Networks, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6855, https://doi.org/10.5194/egusphere-egu2020-6855, 2020

D1626 |
Wenzheng Gong and Xiaofei Chen

Spectra analysis is helpful to understand earthquake rupture processes and estimate source parameters like stress drop. Obtaining real source spectra and source time function isn’t easy, because the station recordings contain path effect and we usually can’t get precise path information. Empirical Green’s function (EGF) method is a popular way to cancel out the path effect, main two of which are the stacking spectra method (Prieto et al, 2006) and the spectral ratio method (Viegas et al, 2010; Imanishi et al, 2006). In our study, we apply the latter with multitaper spectral analysis method (Prieto et al, 2009) to calculate relative source spectra and relative source time function. Target event and EGFs must have similar focal mechanism and be collocated, so we combine correlation coefficient of wave at all stations and focal mechanism similarity to select proper EGFs.

The Bucaramanga nest has very high seismicity, so it’s suitable to calculate source spectra by using EGF method. We calculate the source spectra and source time function of about 1540 earthquakes (3-5.7ml, 135-160km depth) at Bucaramanga nest in Colombia. Simultaneously we also estimate corner frequency by fitting spectral source model (Brune, 1970; Boatwright, 1980) and stress drop using simple model (Eshelby, 1957) of earthquakes with multiple station recordings or EGFs. We obtain about 30000 events data with 12 stations from National Seismological Network of Colombia (RSNC).

The result show that the source spectra of most earthquakes fitted well by omega-square model are smooth, and the source spectra of some have obvious ‘holes’ near corner frequency, and the source time function of a few earthquakes appear two separate peeks. The first kind of earthquakes are style of self-arresting ruptures (Xu et al. 2015), which can be autonomously arrested by itself without any outside interference. Abercrombie (2014) and Wen et al. (2018) both researched the second kind of earthquakes and Wen think that this kind of earthquakes are style of the runaway ruptures including subshear and supershear ruptures. The last kind of earthquakes maybe be caused by simultaneous slip on two close rupture zone. Stress drop appear to slightly increase with depth and are very high (assuming rupture velocity/s wave velocity is 0.9). We also investigate the high-frequency falloff n, usually 2, of Brune model and Boatwright model by fitting all spectra, and find that the best value of n for Boatwright model is 2 and for Brune model is 3.5.

How to cite: Gong, W. and Chen, X.: The characteristic of source spectra and stress drop of earthquakes in the Bucaramanga nest, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3601, https://doi.org/10.5194/egusphere-egu2020-3601, 2020

D1627 |
Jonas Folesky, Joern Kummerow, and Serge A. Shapiro

The Northern Chilean subduction zone has been monitored by the IPOC network for more than ten years. During this time period two very large earthquakes occurred, the 2007 MW7.7  Tocopilla earthquake and the 2014 MW8.1 Iquique earthquake. Over the entire subduction zone a vast amount of seismic activity has been recorded and a huge catalog was compiled including over 100000 events (Sippl et al. 2018). With this exceptional data base we attempt a systematic analysis of the stress drops of as many events from the catalog as possible. We apply different estimation techniques, namely the spectral ratio type, the spectral stacking approach, and the lower bound method. A goal of our research is a comparison and possibly a combination of the techniques to obtain reliable and well constrained results.

The data set covers events at the interface, within the subducting plate, crustal events, and intermediate depth events. It therefore bears a great potential to better understand the stress drop distribution within a subduction zone. Also, the long observation interval allows to analyze temporal variations according to pre-, inter-, and post-seismic phases of megathrust earthquakes.   

We present preliminary results where a subset of 730 events with a magnitude range of ML2.7 - ML4.8  was used for analysis with the spectral ratio technique. For these events we show maps of spatial stress drop variation, and we analyze the time dependent stress drop variance. 

How to cite: Folesky, J., Kummerow, J., and Shapiro, S. A.: Stress Drop Mapping in the Northern Chilean Subduction Zone, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5019, https://doi.org/10.5194/egusphere-egu2020-5019, 2020

D1628 |
Daniele Spallarossa, Paola Morasca, Dino Bindi, Matteo Picozzi, and Kevin Mayeda

Aim of this study is to investigate the relationship between moment magnitude (Mw) and source duration (i.e. corner frequency) for moderate to small magnitude earthquakes recorded in Central Apennines, Italy, including the 2016-2017 Amatrice-Norcia-Visso sequence. A data-set of ~ 6000 events in the magnitude range ~1 and  6.5 was used to retrieve a reference data set of source parameters by applying spectral decomposition approach (Generalized Inversion Techniques). The large population of analyzed earthquakes allowed us to investigate the scaling of the source parameters with the earthquake size, their variability with hypocentral depth and to characterize the scaling between local and moment magnitudes in the magnitude range from 1 to 6.5 (Deichmann 2017). Analyzing the same data-set and taking advantage of the available high quality data for small events recorded in the area, we focus on the scaling properties of clustered events in the magnitude range between ~1 and  3.5. By applying different methodologies, relying on cross-correlation analysis, we detect a preliminary set of clusters. Then, events within 2 km from the geographic location of each cluster were extracted from a very large (more than 500000 events) high-resolution earthquake parametric catalog. New cross-correlation analyses were carried out on stations within 50 km from the centroid of each previously identified clusters to pad each ones with low magnitude events (below 2). This multi-steps procedure allowed us to identified 2933 events belonging to 45 clusters. For an in-deep analysis of source properties, we focus on three clusters selected on the basis of the number of events and different hypocentral depth distributions. For each cluster, the P-waves pulse duration (equivalent to corner frequency) of the events were compared each other on different stations. Results clearly show that below Ml ~ 2 the pulses duration remains nearly constant also for stations with low kappa values, showing a saturation effects. For a comparison with the GIT and cross-correlation results we also evaluate source parameters using a method based on coda-envelope amplitude measurements (Mayeda et al. 2003) applying site and path parameters previously calibrated for Central Apennines by Morasca et al. 2019. This comparison from independent and completely different methodologies applied on the same clusters well agrees with the saturation observed in pulse duration, strengthen the results and allowed us to define, for the given network geometry and earthquake distribution, the magnitude threshold below which we believe it is not possible to estimate source parameters. Moreover, our analysis of two clusters co-located on map but with different depth highlights a variation in stress drop with depth;

How to cite: Spallarossa, D., Morasca, P., Bindi, D., Picozzi, M., and Mayeda, K.: Analysis of Source Parameters relationships for clusters of similar events recorded in Central Apennines, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17019, https://doi.org/10.5194/egusphere-egu2020-17019, 2020

D1629 |
Sukanta Sarkar, Namrata Jaiswal, Chandrani Singh, Arun Kumar Dubey, and Arun Singh

The tectonic structure of western Tibet is complex and formed of several blocks, which are separated by distinct suture zones. This complexity makes the region very crucial for understanding the local tectonic settings. Here, we investigate the spectral characteristics of Lg wave from 420 waveforms recorded at 26 seismic stations located across Karakoram Fault (KKF) in western Tibet. We subdivide the study region into two parts across KKF. A frequency  dependent QLg is observed in both sides of KKF with strong attenuation in the crust. The moment magnitude of each earthquake is computed using displacement spectra and subsequently compared with the reported local magnitude. Variations of the corner frequency with magnitude and distance
are also studied, which show a decreasing nature due to the path dependency.

How to cite: Sarkar, S., Jaiswal, N., Singh, C., Dubey, A. K., and Singh, A.: Source Spectral studies using Lg wave in western Tibet, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4915, https://doi.org/10.5194/egusphere-egu2020-4915, 2020

D1630 |
Hailin Du, Xu Zhang, and Yongzhe Wang

The standard method to image the source rupture process of a large earthquake is finite fault inversion, which uses the low-frequency signal to invert the slip distribution of the fault. However, in different stages of the source rupture process of a large earthquake, the seismic waves radiated by the source have different dominant frequencies, such as high frequency seismic waves excited by the rupture front. If we can analyze seismic waves in different frequency bands, it is expected to obtain a more detailed source rupture process of large earthquakes. Therefore, we respectively adopted the high frequency signal back-projection imaging method and the low frequency signal finite fault inversion method, and took the 2016 Kaikoura MW7.8 earthquake as an example to obtain the history of rupture propagation and fault slip distribution.The calculated results show that the high-frequency energy radiation of the earthquake can be divided into three stages, and the low-frequency energy radiation can be divided into two stages. The energy release process in different frequency bands is complementary in time and space. The rupture process of the whole source can be explained by the asperity model and the barrier model.

How to cite: Du, H., Zhang, X., and Wang, Y.: Multi-band imaging of seismic source rupture process, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7075, https://doi.org/10.5194/egusphere-egu2020-7075, 2020

D1631 |
Ioannis Fountoulakis, Christos Evangelidis, and Olga-Joan Ktenidou

The seismic source spatio-temporal rupture processes of events in Japan, Greece and Turkey are imaged by backprojection of strong-motion waveforms. Normalized high-frequency (> 2Hz) S-waveforms from recordings on dense strong-motion networks are used to scan a predefined 3D source volume over time. 

Backprojection is an alternative novel approach to image the spatio-temporal earthquake rupture. The method was first applied for large earthquakes at teleseismic distances, but is nowadays also used at local distances and over higher frequencies. The greatest advantage of the method is that processing is done without any a-priori constraints on the geometry, or size of the source. Thus, the spatio-temporal imaging of the rupture is feasible at higher frequencies (> 1Hz) than conventional source inversion studies, even when the examined fault geometry is complex. This high-frequency energy emitted during an earthquake is of great importance in seismic hazard assessment for certain critical infrastructures. The actual challenge in using high-frequency local recordings is to distinguish the local site effects from the true earthquake source content - otherwise, mapping the former incorrectly onto the latter limits the resolvability of the method. It is not straightforward to remove the site effect component or even to distinguish good reference stations from amid hard-soil and rock sites. In this study, the advantages and limitations of the method are explored using waveform data from well-recorded events in Japan (Kumamoto Mw7.1, 2016), Turkey (Marmara Mw6.4, 2019) and Greece (Antikythera Mw6.1, 2019). For each event and seismic array the resolution limits of the applied method are explored by performing various synthetic tests.

How to cite: Fountoulakis, I., Evangelidis, C., and Ktenidou, O.-J.: Imaging the rupture process of recent earthquakes using backprojection of local high frequency records, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13283, https://doi.org/10.5194/egusphere-egu2020-13283, 2020

D1632 |
Cheng-Feng Wu, Ting-Li Lin, and Ying-Chi Chen

In the past decade, there have been several disaster earthquakes occurred in Taiwan.
From the observed data of the disaster earthquakes, the stations located in the source
rupture direction have obvious directivity pulses, and the distribution of the earthquake
disaster is related to the peak ground velocity. Therefore, how to use a large and high-
dense seismic database to develop a near-real-time detection system on the earthquake
rupture directivity, which is a very important task in Taiwan. In this study, we determine
the earthquake rupture directivity using near-field velocity data from 1991 to 2018, which
were collected under the Taiwan Strong Motion Instrument Program (TSMIP). The used
method is mainly constructed in the interpolation of the peak-ground-velocity map and
the directional attenuation regression analysis. Through the analysis of moderate-to-large
magnitude (M L > 5.5) seismic events, the source rupture directivity can be detected
effectively and quickly by the applied method. The detection results are also comparable
with those from the previous source studies. We also find out a linear relationship between
the directivity effect and earthquake magnitude. Since the TSMIP station may provide
real-time services in the future, the detection system proposed by this research can quickly
provide disaster prediction information, which is of great importance for earthquake
emergency response and hazard mitigation.

How to cite: Wu, C.-F., Lin, T.-L., and Chen, Y.-C.: Rapid Detection of Earthquake Rupture Directivity Using Strong Ground Motion Data in Taiwan, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22441, https://doi.org/10.5194/egusphere-egu2020-22441, 2020

D1633 |
Yu Jiang and Pablo González

Phase unwrapping is the process of recovering the absolute phase from unambiguous wrapped phase values that are measured modulo 2pi rad. From a mathematical point of view, phase unwrapping is an inverse problem, however, it is ill-posed and notoriously difficult to solve in the presence of noise. Meanwhile, phase unwrapping errors severely impact the estimation of earthquake and volcano source parameters using interferometric observations, therefore avoiding phase unwrapping completely is desirable. 

A potential solution to avoid the unwrapping error issue completely, is to carry out a geophysical inversion directly on the wrapped phase observations. To overcome the need for phase unwrapping, we propose a novel approach that we can invert directly the interferometric wrapped phase, circumventing the ill-posed phase unwrapping processing step. This approach includes (1) a downsampling algorithm, (2) a method to estimate the covariance function of the wrapped phase, (3) an appropriate misfit function between the observed and the simulated wrapped phase. We also assess the uncertainties of source parameters within a Bayesian approach, and finally we test the robustness of the inversion methodology in multiple simulations including variable decorrelation and atmospheric noise simulations.

We demonstrate the proposed methodology on synthetic cases with variable noise and one real earthquake case. We show that the method is robust in challenging noise scenarios. We also show an improvement with the Bayesian approach in performance with respect to similar previous methods, resulting in avoiding any influence of seed starting models, and escaping local minima. We study the impact of a small percentage of incorrectly unwrapped phase observations in current state-of-the-art methods, and show that the presence of a small fraction of unwrapping errors affect strongly the estimation process. We conclude that in the cases where phase unwrapping is difficult or even impossible, the proposed inversion methodology with wrapped phase will provide an alternative approach to assess earthquake and volcano source model parameters.

How to cite: Jiang, Y. and González, P.: Bayesian Inversion of Wrapped Satellite Interferometric Phase to Estimate Fault and Volcano Surface Ground Deformation Models, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5604, https://doi.org/10.5194/egusphere-egu2020-5604, 2020

D1634 |
Satoshi Ide and Hideo Aochi

Earthquakes are multiscale phenomena with several scale invariant parameters, such as stress drop, apparent stress, and rupture propagation velocity, and dynamic rupture grows almost self-similarly. While the rupture process is always complex, the location of sources is not completely random, but nearly predetermined, especially along a well-developed fault system like a plate interface. To explain such behavior, some hierarchical structure is required, and one candidate is the hierarchical circular patch (fractal asperity) model suggested by Ide and Aochi (JGR, 2005) and Aochi and Ide (JGR, 2009), in which fracture energy inside a patch is proportional to the patch radius. In this paper, we review the characteristics of the model and show some observational evidence, which has been recently discovered mainly for subduction-type earthquakes in the Tohoku-Oki, Japan, region. Some hierarchical patch-like structure has been identified for several repeating earthquakes of M~5 (Uchida et al., GRL, 2012; Okuda and Ide, EPS, 2018). Identical onsets of seismic waves were observed for many pairs of large (M>4.5) and small (M<4.0) earthquakes (Okuda and Ide, Nature Communications, 2018; Ide, Nature, 2019). We can also observe long-term increase of seismicity before the rupture of system-size events (Okuda et al., Zishin, 2018). These lines of evidence suggest the qualitative validity of the hierarchical model and will be useful to improve the quantitative aspects of the model, such as patch density and the slip-weakening rate, to numerically simulate realistic earthquakes and seismicity (e.g., Aochi and Ide, EPS, 2011; Ide and Aochi, Tectonophysics, 2013; Aochi and Twardzik, Pageoph, 2019).

How to cite: Ide, S. and Aochi, H.: Hierarchical seismic sources model and recent observational evidence, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11916, https://doi.org/10.5194/egusphere-egu2020-11916, 2020

D1635 |
Jan Premus and Frantisek Gallovic

Dynamic rupture modeling coupled with strong motion data fitting (dynamic source inversion) offers an insight into the rupture physics, constraining and enriching information gained from standard kinematic slip inversions. We utilize the Bayesian Monte Carlo dynamic source inversion method introduced recently by Gallovič et al. (2019), which, in addition to finding a best-fitting model, allows assessing uncertainties of the inferred parameters by sampling the posterior probability density function. The Monte Carlo approach requires running a large number (millions) of dynamic simulations due to the nonlinearity of the inverse problem. It is achieved by using GPU accelerated dynamic rupture simulation code FD3D_TSN (Premus et al., submitted) as a forward solver. We apply the inversion to the 2014 Mw6 South Napa, California, earthquake, employing strong motion data (up to 0.5 Hz) from the 10 closest stations. As an output, we obtain samples of the spatial distributions of dynamic parameters (prestress and parameters of the slip-weakening friction law). Regarding the rupture geometry, we consider two, presently ambiguous, fault planes (Pollitz et al., 2019), showing considerable differences in fitting seismograms in very close vicinity of the fault. We investigate properties of the rupture, especially in the region close to the free surface, and the viability of the model samples to explain the observed data in a broader frequency range (up to 5Hz).

How to cite: Premus, J. and Gallovic, F.: Dynamic source inversion of the 2014 Mw6 South Napa, California, earthquake, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18422, https://doi.org/10.5194/egusphere-egu2020-18422, 2020

D1636 |
Taufiq Taufiqurrahman, Alice-Agnes Gabriel, Frantisek Gallovic, and Lubica Valentova

The complex evolution of earthquake rupture during the 2016 Central Italy sequence and the uniquely dense seismological observations provide an opportunity to better understand the processes controlling earthquake dynamics, strong ground motion, and earthquake interaction. 

We here use fault initial stress and friction conditions constrained by a novel Bayesian dynamic source inversion as a starting point for high-resolution dynamic rupture scenarios. The best-fitting forward models are chosen out of ~106 highly efficient simulations restricted to a simple planar dipping fault. Such constrained, highly heterogeneous dynamic models fit strong motion data well. Utilizing the open-source SeisSol software (www.seissol.org), we then take into account non-planar (e.g., listric) fault geometries, inelastic off-fault damage rheology, free surface effects and topography which cannot be accounted for in the highly efficient dynamic source inversion. SeisSol is based on the discontinuous Galerkin method on unstructured tetrahedral meshes optimized for modern supercomputers. 

We investigate the effects of including the realistic modeling ingredients on rupture dynamics and strong ground motions up to 5 Hz. Synthetic PGV mapping reveals that specifically fault listricity decreases ground motion amplitudes by  ~50 percent in the extreme near field on the foot-wall. On the hanging-wall shaking is increased by ~150 percent as a consequence of wave-focusing effects within 10 km away from the fault. Dynamic modeling thus suggests that geometrical fault complexity is important for seismic hazard assessment adjacent to dipping faults but difficult to identify by kinematic source inversions.

How to cite: Taufiqurrahman, T., Gabriel, A.-A., Gallovic, F., and Valentova, L.: Dynamic Rupture and Ground Motion Modeling of the 2016 M6.2 Amatrice and M6.5 Norcia, Central Italy, Earthquakes Constrained by Bayesian Dynamic Source Inversion, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20584, https://doi.org/10.5194/egusphere-egu2020-20584, 2020