Direction Of Rupture Propagation Research Articles

Long-period ground motion simulation and pulse probability distribution characteristics of the Kumamoto MW 7.1 earthquake in Japan

The 2016 Kumamoto MW 7.1 earthquake in Japan caused severe structural damage. It produced valuable seismic records that offered an opportunity to investigate the characteristics of long-period ground motion and pulse probability distribution. This study works in three main parts. First, we use the finite difference method (FDM) to reproduce the long-period ground motions of the 2016 Kumamoto MW 7.1 earthquake. That provides a crucial foundation for the subsequent phases. We identify velocity pulses within both synthetic and observed waveforms. By rigorously analyzing these pulses, we unveiled a wealth of insights into the characteristics of velocity pulses, strengthening the influence of the rupture directivity effect. In particular, the northeast region of the rupture exhibited a significant concentration of pulsed attributes. Finally, we further focus on establishing statistical relationships between different pulse parameters. The study revealed that the pulse probability distribution model fit well with the observed pulse distribution in the parallel to the fault (FP) and normal to the fault (FN) directions. However, the model also faced challenges in the FN direction due to the mechanism of velocity pulses and altered rupture propagation direction. Crucially, the study established that velocity pulses primarily cluster within a fault distance of less than 30 km, and the relationship between pulse peak value and fault distance followed an exponential trend. The study underscores the imperative of further refining the probability distribution model of velocity pulses, making it more robust and widely applicable. Furthermore, the correlation between velocity pulses and seismic parameters requires further validation through seismic records. This study sets the stage for a deeper understanding of earthquake-induced ground motion and pulse characteristics, contributing to the ongoing efforts in seismic risk assessment.

Earthquake source impacts on the generation and propagation of seismic infrasound to the upper atmosphere

SUMMARY Earthquakes with moment magnitude (Mw) ranging from 6.5 to 7.0 have been observed to generate sufficiently strong acoustic waves (AWs) in the upper atmosphere. These AWs are detectable in Global Navigation Satellite System satellite signals-based total electron content (TEC) observations in the ionosphere at altitudes ∼250–300 km. However, the specific earthquake source parameters that influence the detectability and characteristics of AWs are not comprehensively understood. Here, we extend our approach of coupled earthquake-atmosphere dynamics modelling by combing dynamic rupture and seismic wave propagation simulations with 2-D and 3-D atmospheric numerical models, to investigate how the characteristics of earthquakes impact the generation and propagation of AWs. We developed a set of idealized dynamic rupture models varying faulting types and fault sizes, hypocentral depths and stress drops. We focus on earthquakes of Mw 6.0–6.5, which are considered the smallest detectable with TEC, and find that the resulting AWs undergo non-linear evolution and form acoustic shock N waves reaching thermosphere at ∼90–140 km. The results reveal that the magnitude of the earthquakes is not the sole or primary factor determining the amplitudes of AWs in the upper atmosphere. Instead, various earthquake source characteristics, including the direction of rupture propagation, the polarity of seismic wave imprints on the surface, earthquake mechanism, stress drop and radiated energy, significantly influence the amplitudes and periods of AWs. The simulation results are also compared with observed TEC fluctuations from AWs generated by the 2023 Mw 6.2 Suzu (Japan) earthquake, finding preliminary agreement in terms of model-predicted signal periods and amplitudes. Understanding these nuanced relationships between earthquake source parameters and AW characteristics is essential for refining our ability to detect and interpret AW signals in the ionosphere.

A modified stochastic finite-fault method for estimating strong ground motion: Validation and application

We developed a modified stochastic finite-fault method for estimating strong ground motions. An adjustment to the dynamic corner frequency was introduced, which accounted for the effect of the location of the subfault relative to the hypocenter and rupture propagation direction, to account for the influence of the rupture propagation direction on the subfault dynamic corner frequency. By comparing the peak ground acceleration (PGA), pseudo-absolute response spectra acceleration (PSA, damping ratio of 5%), and duration, the results of the modified and existing methods were compared, demonstrating that our proposed adjustment to the dynamic corner frequency can accurately reflect the rupture directivity effect. We applied our modified method to simulate near-field strong motions within 150 km of the 2008 MW7.9 Wenchuan earthquake rupture. Our modified method performed well over a broad period range, particularly at 0.04–4 s. The total deviations of the stochastic finite-fault method (EXSIM) and the modified EXSIM were 0.1676 and 0.1494, respectively. The modified method can effectively account for the influence of the rupture propagation direction and provide more realistic ground motion estimations for earthquake disaster mitigation.

Sensitivity of the second seismic moments resolution to determine the fault parameters of moderate earthquakes

Second-degree seismic moments provide a simple description of the spatiotemporal extent of the earthquake source. Finite source attributes such as rupture length, width, duration, velocity, and propagation direction can be estimated by computing second-degree seismic moments without the need for a predefined rupture model. This is achieved by analyzing the properties of apparent source time functions (ASTFs) obtained from seismic signals recorded at different stations after eliminating instrument responses and path effects. In this study, to define the limits of its application in the analysis of small earthquakes and to evaluate the sensitivity and reliability of the results to uncertainties due to observations and prior knowledge, we modeled a synthetic seismic source and examined how potential uncertainties in hypocentral depth, velocity model, focal mechanism, source duration, and number of recording stations can affect the inversion results. An accurate ASTF is essential to obtain robust results and our findings show that the mean values of the key source parameters, i.e., fracture size, source duration, and rupture velocity, are generally well reproduced in all sensitivity tests, with some exceptions, within the standard deviation. We also demonstrate that large uncertainties in the hypocentral depth and inaccurate velocity models introduce a significant bias, especially in rupture size and average centroid velocity, indicating the strong influence of ray path calculation in the inversion process. These resolution limits must therefore be taken into account when interpreting the results obtained with this technique.

A Reliable Procedure to Estimate the Rupture Propagation Directions from Source Directivity: The 2016–2018 Central Italy Seismic Sequence

Abstract We present a new approach to estimate the predominant direction of rupture propagation during a seismic sequence. A fast estimation of the rupture propagation direction is essential to know the azimuthal distribution of shaking around the seismic source and the associated risks for the earthquake occurrence. The main advantage of the proposed method is that it is conceptually reliable, simple, and fast (near real time). The approach uses the empirical Green’s function technique and can be applied directly to the waveforms without requiring the deconvolution of the instrumental response and without knowing a priori the attenuation model and the orientation of the activated fault system. We apply the method to the 2016–2017 Amatrice-Visso-Norcia high-energy and long-lasting earthquake series in central Italy, which affected a large area up to 80 km along strike, with more than 130,000 events of small-to-moderate magnitude recorded until the end of August 2022. Most of the selected events analyzed in this study have a magnitude greater than 4.4 and only four seismic events have a magnitude in the range of 3.3–3.7. Our results show that the complex activated normal fault system has a rupture direction mainly controlled by the pre-existing normal faults and by the orientation of the reactivated faults. In addition, the preferred direction of rupture propagation is also controlled by the presence of fluid in the pre-existing structural discontinuities. We discuss the possible role of fluids as a cause of bimaterial interface. Another important finding from our analysis is that the spatial evolution of seismicity is controlled by the directivity.

A Reliable Procedure to Estimate the Rupture Propagation Directions from Source Directivity: The 2016–2018 Central Italy Seismic Sequence

A Reliable Procedure to Estimate the Rupture Propagation Directions from Source Directivity: The 2016–2018 Central Italy Seismic Sequence

Impacts and causative fault of the 2022 magnitude (Mw) 7.0 Northwestern Luzon earthquake, Philippines

At 00:43 UTC on 27 July 2022, a 15-km deep major earthquake with magnitude (Mw) 7.0 struck Northwestern Luzon, Philippines. The strongest ground shaking felt was at PHIVOLCS Earthquake Intensity Scale (PEIS) VII (destructive), equivalent to Modified Mercalli Intensity (MMI) VII, in Abra and along the coastal areas of Ilocos Sur. More than a thousand landslides, rockfalls and tension cracks were mapped, near the epicentral region, in the northwestern part of the Central Cordillera. Most of the landslides were shallow-seated, many of which were situated along road cuts. Liquefaction manifested as lateral spreads, sand boils, fissures, ground subsidence, and localized swelling was documented along the coastal areas of Ilocos Sur and river channels in Abra and Ilocos Sur. Sea level disturbance was also observed in some coastal areas of Ilocos Sur and La Union. Damages to buildings and infrastructures were documented in areas that experienced PEIS VI (very strong), equivalent to MMI VI and PEIS VII (destructive). Earthquake data, including hypocentral location, aftershock distribution, focal mechanism solutions and strong motion data, and InSAR observation indicate that the earthquake was generated by an almost north-south striking reverse left-lateral oblique fault that is gently dipping to the east. There is no clear indication of a surface rupture based on InSAR analysis and field investigation. The spatial distribution of geologic impacts, such as earthquake-induced landslides and liquefaction, is strongly controlled by the causative fault, the direction of rupture propagation and geology. Peak ground acceleration (PGA) records show a unidirectional rupture propagation and are congruent with the spatial distribution of earthquake impacts. Although earthquake parameters, deformation analysis and field data suggest that the Abra River Fault is the probable causative fault, the derived geometry and kinematics from the seismotectonic analysis challenge our existing understanding of the nature of the Abra River Fault, as well as the other segments of the Philippine Fault. The need to understand these earthquake sources in the country is needed for a better seismic hazard and risk assessment.

Dynamic simulations of coseismic slickenlines on non-planar and rough faults

SUMMARY Knowing the directions of rupture propagation of palaeo-earthquakes is a challenging, yet important task for our understanding of earthquake physics and seismic hazard, as the rupture direction significantly influences the distribution of strong ground motion. Recent studies proposed a relationship between the direction of rupture propagation and curvature of slickenlines formed during seismic slip. The relationship was established using a global catalogue of historic surface-rupturing earthquakes and dynamic models of idealized, planar faults. At the same time, some slickenlines previously documented on geometrically complex fault segments show their convexity opposite from the simple model prediction, which we refer to as ‘abnormal convexity’. To explain such observations, we perform simulations of spontaneous earthquake ruptures on non-planar and rough faults. We find that in the case of strike-slip faults, a non-planar fault model can lead to abnormal convexity of slickenlines in places where the fault dip angle changes abruptly from the average dip of the fault. Abnormal convexity of slickenlines is produced when the initial along-dip stresses are larger than, and are opposite in direction to, the dynamic stresses imparted by the mixed-mode rupture. Such results are also confirmed in our rupture simulations with a rough fault. Our results also show that the parameter space for which abnormal convexity of slickenlines occurs near Earth’s surface is narrow, especially when the fault strength and initial shear stresses increase with depth. Nevertheless, slickenlines on geometrically complex faults need to be carefully interpreted and investigation of rupture direction using curved slickenlines should focus on structurally simple parts of faults.

Seismic source mapping by surface wave time reversal: application to the great 2004 Sumatra earthquake

SUMMARY Different approaches to map seismic rupture in space and time often lead to incoherent results for the same event. Building on earlier work by our team, we ‘time-reverse’ and ‘backpropagate’ seismic surface wave recordings to study the focusing of the time-reversed field at the seismic source. Currently used source-imaging methods relying on seismic recordings neglect the information carried by surface waves, and mostly focus on the P-wave arrival alone. Our new method combines seismic time reversal approach with a surface wave ray-tracing algorithm based on a generalized spherical-harmonic parametrization of surface wave phase velocity, accounting for azimuthal anisotropy. It is applied to surface wave signal filtered within narrow-frequency bands, so that the inherently 3-D problem of simulating surface wave propagation is separated into a suite of 2-D problems, each of relatively limited computational cost. We validate our method through a number of synthetic tests, then apply it to the great 2004 Sumatra–Andaman earthquake, characterized by the extremely large extent of the ruptured fault. Many studies have estimated its rupture characteristics from seismological data (e.g. Lomax, Ni et al., Guilbert et al., Ishii et al., Krüger & Ohrnberger, Jaffe et al.) and geodetic data (e.g. Banerjee et al., Catherine et al., Vigny et al., Hashimoto et al., Bletery et al.). Applying our technique to recordings from only 89 stations of the Global Seismographic Network (GSN) and bandpass filtering the corresponding surface wave signal around 80-to-120, 50-to-110 and 40-to-90 s, we reproduce the findings of earlier studies, including in particular the northward direction of rupture propagation, its approximate spatial extent and duration, and the locations of the areas where most energy appears to be released.

Elastic Contrast, Rupture Directivity, and Damage Asymmetry in an Anisotropic Bimaterial Strike‐Slip Fault at Middle Crustal Depths

Mature faults with large cumulative slip often separate rocks with dissimilar elastic properties and show asymmetric damage distribution. Elastic contrast across such bimaterial faults can significantly modify various aspects of earthquake rupture dynamics, including normal stress variations, rupture propagation direction, distribution of ground motions, and evolution of off-fault damage. Thus, analyzing elastic contrasts of bimaterial faults is important for understanding earthquake physics and related hazard potential. The effect of elastic contrast between isotropic materials on rupture dynamics is relatively well studied. However, most fault rocks are elastically anisotropic, and little is known about how the anisotropy affects rupture dynamics. We examine microstructures of the Sandhill Corner shear zone, which separates quartzofeldspathic rock and micaceous schist with wider and narrower damage zones, respectively. This shear zone is part of the Norumbega fault system, a Paleozoic, large-displacement, seismogenic, strike-slip fault system exhumed from middle crustal depths. We calculate elastic properties and seismic wave speeds of elastically anisotropic rocks from each unit having different proportions of mica grains aligned sub-parallel to the fault. Our findings show that the horizontally polarized shear wave propagating parallel to the bimaterial fault (with fault-normal particle motion) is the slowest owing to the fault-normal compliance and therefore may be important in determining the elastic contrast that affects rupture dynamics in anisotropic media. Following results from subshear rupture propagation models in isotropic media, our results are consistent with ruptures preferentially propagated in the slip direction of the schist, which has the slower horizontal shear wave and larger fault-normal compliance.

Sequences of seismic and aseismic slip on bimaterial faults show dominant rupture asymmetry and potential for elevated seismic hazard

We perform numerical simulations of sequences of earthquake and aseismic slip on planar rate and state faults separating dissimilar material within the 2-D plane strain approximation. We resolve all stages of the earthquake cycle from aseismic slip to fast ruptures while incorporating full inertia effects during seismic event propagation. We show that bimaterial coupling results in favorable nucleation site and subsequent asymmetric rupture propagation. We demonstrate that increasing the material contrast enhances this asymmetry leading to higher slip rates and normal stress drops in the preferred rupture propagation direction. The normal stress drop, induced by the bimaterial effect, leads to strong dynamic weakening of the fault and may destabilize the creeping region on a heterogeneous rate and state fault, resulting in extended rupture propagation. Such rupture penetration into creeping patches may lead to more frequent opening of earthquake gates, causing increased seismic hazard. Furthermore, bimaterial coupling may lead to irregular seismicity pattern in terms of event length, peak slip rates, and hypocenter location, depending on the properties of the creeping patches bordering the seismogenically active part of the fault. Our results highlight robust characteristics of bimaterial interfaces that persist over long sequence of events and suggest the need for further exploration of the role of material contrast in earthquake physics and models of seismic hazard.

The Doppler effect induced by earthquakes: A case study of the Wenchuan MS8.0 earthquake

Seismologists have found that the first arrival frequencies of P waves at different seismic stations have different widths, that is, different periods or frequencies, and they think that this phenomenon can be used to identify whether a Doppler effect is induced by earthquakes. However, the fault rupture process of a real earthquake is so complex that it is difficult to identify a frequency shift similar to the Doppler effect. A method to identify whether a Doppler effect is induced by an earthquake is proposed here. If a seismic station is in the direction of fault rupture propagation, this station could observe a Doppler effect induced by the earthquake. The Doppler effect causes the frequency of the seismic wave to shift from low frequency to high frequency, and the high frequency amplitudes become mutually superimposed. Under the combined influences of the absorption effect, geometric spreading effect and Doppler effect, the high frequency amplitude of the seismic wave will gradually become higher than the low frequency amplitude with increasing epicentral distance. If we find that the high frequency amplitude is higher than the low frequency amplitude with increasing epicentral distance in the direction of fault rupture propagation, then there is a Doppler effect. The fault that generated the Wenchuan earthquake is a reverse fault, and its horizontal rupture propagation velocity was low. To link fault rupture propagation velocity with the Doppler effect and identify the Doppler effect more easily, we decompose three-component records into two directions: the direction of fault rupture propagation and the direction perpendicular to the fault rupture propagation along the fault plane. The initial components of the two directions are processed by wavelet transform. Several seismic stations in the direction of fault rupture propagation of the Wenchuan earthquake were selected, and it was found that with increasing epicentral distance, the high frequency amplitudes of the wavelet spectra become obviously higher than the low frequency amplitudes. It can be concluded that due to the existence of the Doppler effect, high frequency amplitudes can overcome the influences of the absorption and geometric spreading effects on seismic waves in the fault rupture propagation process.

Relationship Between Asperities and Velocity Pulse Generation Mechanism

Near-fault ground motion records often capture instances of pulse-like behavior, where a burst of energy is expressed as large wave amplitude that occur over short time. The pulse-like ground motion can cause serious damage to long-period structures. Using numerical simulations of near-fault ground motions, we analyze the mechanisms involved in the generation of velocity pulses in the 1994 Northridge Earthquake and the 1979 Imperial Valley Earthquake. The degree to which the asperities affect the pulse generation process is investigated by identifying individual velocity pulses from the superposition process of sub-fault ground motions. Pulse indicators Ep and PGVp represent pulse characteristics in the ground motions at the stations located near the epicenter (near-epicenter stations) and the stations located along the forward rupture propagation direction of the asperity (rupture-direction stations), respectively. To observe the effects of the asperities and the spatial relationship between the pulse-like ground motion stations and the asperities, we determine the contribution of the sub-fault motions to the pulse amplitude. Furthermore, we analyze the pulse indicators and the frequency components using simulated ground motions from two different slip distributions. The near-epicenter station ground motions, produced by homogeneous slip distribution, exhibit higher pulse amplitude and more concentrated low-frequency energy than those generated by the inhomogeneous slip distribution. The rupture-direction station ground motions, produced by inhomogeneous slip distribution, present higher pulse amplitude and more concentrated low-frequency energy than those generated by the homogeneous slip distribution. Our analysis reveals that during the fault rupture process, the pulse energy and the pulse amplitude are influenced by both the slip distribution on the fault plane and the spatial relationship between the seismic station and the asperity.

The effect of rupture propagation on predominant direction of pulse-like ground motions and landslides

Based on coupled analysis and energy approach, this study investigates the relationship between the direction of rupture propagation and the predominant direction of pulse-like ground motions and landslides during Chi-Chi earthquake and Hualien earthquake. 46 pulse-like ground motions are selected to study. The results show that pulse ground motions containing high-energy pulse tend to be perpendicular to the direction of fault rupture propagation in thrust events. In contrast, it occurred along the direction of fault rupture propagation in strike-slip events. The predominant direction of landslides is stronger directionality of rupture propagation in thrust events. It can provide research foundation for the risk zoning of regional earthquake landslides.

Hypocenter Hotspots Illuminated Using a New Cross‐Correlation‐Based Hypocenter and Centroid Relocation Method

AbstractAccurate earthquake locations are important for understanding earthquake processes. Recent advances in earthquake relocation methods have resulted in relative centroid locations with smaller uncertainties using cross‐correlation‐based methods. However, relative hypocenter locations, which are traditionally determined using the onset times of event waveforms, generally possess larger uncertainties than that of the centroids. Here, we develop a new cross‐correlation‐based relative hypocenter relocation method based on the network correlation coefficient method, whereby we incorporate an additional step to search for a suitable window to cross‐correlate the event waveform onsets. We then perform a joint inversion on the obtained relative hypocenter and centroid locations to directly compare their relative locations under a unified coordinate. We apply this method to three regions where repeating earthquakes have been detected, and reveal both the repetitive rupture of similar centroids by these repeating events and newly illuminated hypocenter hotspots that serve as the onset region of earthquakes that grow into different but selective magnitudes. Our joint analysis of hypocenter and centroid locations enables quick estimations of rupture propagation directions without the need to perform slip inversion analyses. We estimate the location uncertainties via bootstrapping, with the 95% confidence intervals yielding sub‐100 m horizontal uncertainties in both the centroid and hypocenter locations with the best station coverage and event availability. These results point to limited but existing correspondence between the hypocenter and centroid locations and the earthquake magnitudes, as well as complexity in the hypocenter distributions, even within repeating earthquake sequences whose rupture areas largely overlap.

Rupture Directivity in 3D Inferred From Acoustic Emissions Events in a Mine-Scale Hydraulic Fracturing Experiment

Rupture directivity, implying a predominant earthquake rupture propagation direction, is typically inferred upon the identification of 2D azimuthal patterns of seismic observations for weak to large earthquakes using surface-monitoring networks. However, the recent increase of 3D monitoring networks deployed in the shallow subsurface and underground laboratories toward the monitoring of microseismicity allows to extend the directivity analysis to 3D modeling, beyond the usual range of magnitudes. The high-quality full waveforms recorded for the largest, decimeter-scale acoustic emission (AE) events during a meter-scale hydraulic fracturing experiment in granites at ∼410 m depth allow us to resolve the apparent durations observed at each AE sensor to analyze 3D-directivity effects. Unilateral and (asymmetric) bilateral ruptures are then characterized by the introduction of a parameter κ, representing the angle between the directivity vector and the station vector. While the cloud of AE activity indicates the planes of the hydrofractures, the resolved directivity vectors show off-plane orientations, indicating that rupture planes of microfractures on a scale of centimeters have different geometries. Our results reveal a general alignment of the rupture directivity with the orientation of the minimum horizontal stress, implying that not only the slip direction but also the fracture growth produced by the fluid injections is controlled by the local stress conditions.

Simulation of near-fault ground strains and rotations from actual strike-slip earthquakes: case studies of the 2004Mw 6.0 Parkfield, the 1979Mw 6.5 Imperial Valley and the 1999Mw 7.5 Izmit earthquakes

SUMMARYPrevious studies have demonstrated that finite-fault simulations of actual or hypothetical earthquakes using deterministic, physics-based simulation techniques constitute an effective tool for characterizing near-fault ground strains and rotations in the low-frequency range. The characteristics of these motions are further investigated in this study by performing forward ground-motion simulations of three well-documented strike-slip earthquakes (i.e. 2004 Mw 6.0 Parkfield, 1979 Mw 6.5 Imperial Valley and 1999 Mw 7.5 Izmit) using models of the seismic source and crustal structure available in the literature. Time histories of ground strains and rotations are numerically generated at near-fault stations and at a dense grid of observation points extending over the causative fault. This is achieved by finite differencing translational motions simulated at very closely spaced stations using a kinematic modelling approach. The simulation results show that the three strike-slip earthquakes produce large-amplitude pulse-like shear strain and torsion in the forward direction of rupture propagation. The time histories of specific components of displacement gradient, strain and rotation at near-fault stations can be estimated from those of ground velocities using a phase velocity, whereas peak ground torsions in the near-fault region can be reasonably estimated from peak horizontal ground velocities using a scaling factor. However, both the phase velocity and the scaling factor exhibit significant variability in the near-fault region of the considered earthquakes. The concept of isochrones is also utilized to associate fault rupture characteristics with near-fault ground strains and rotations. The results indicate that the seismic energy radiated from the high-isochrone-velocity region of the fault—which encompasses areas of large slip locally driven by high stress drop—arrives at a near-fault station in a short time interval that coincides with the time window of the large-amplitude pulse-like shear strain and torsion.

Tsunami Efficiency Due to Very Slow Earthquakes

AbstractOften, tsunami “sources” have been treated as a quasistatic problem. Initial studies have demonstrated that, for earthquake rupture velocities in the span of 1.5–3 km/s, the kinematic and static part of the tsunami can be treated separately. However, very slow earthquake rupture velocities in the span of 0.1–1 km/s have not been included in tsunami analytical or numerical modeling. Here, we calculated the tsunami efficiency, extending Kajiura’s definition for different models. We demonstrated that rupture velocity cannot be neglected for very slow events, that is, rupture velocities slower than 0.5 km/s. We also examined the relation of magnitude, earthquake rupture velocity, and tsunami amplitude to the efficiency of very slow tsunamigenic earthquakes. Hypothetical megathrust earthquakes (Mw>8.5) with very slow rupture velocities amplify energy from 10 to 60 times larger than moderate to large earthquakes (7.0<Mw<8.5) in the direction of rupture propagation.

Coseismic slickenlines record the emergence of multiple rupture fronts during a surface-breaking earthquake

Coseismic changes in slip direction recorded by curved slickenlines on fault surfaces are commonly observed following surface-breaking earthquakes. Such observations represent a dynamic record of seismic slip and may provide a new set of constraints on the evolution of propagating rupture and hence earthquake dynamics. We test this hypothesis by conducting dynamic rupture simulations of the 2011 Mw 6.6 Fukushima-Hamadori Earthquake (Japan). These simulations aim to reproduce the well-documented field observations of curved slickenlines that formed during coseismic fault displacement at the ground surface. We consider relatively simple dynamic rupture models with a dipping fault embedded into a homogeneous or layered elastic halfspace. Among a wide range of model parameters tested, we find a model with shallow (<1.5km) low-velocity layers derived from a regional 3D velocity model combined with a depth-dependent prestress result in curved slip trajectories that closely match slickenline observations. This same model also generates a slip distribution and rupture propagation direction consistent with published inversions constrained by seismological and geodetic data. Furthermore, the characteristics of slickenline curvature are consistent with theoretically predicted earthquake rupture direction. Unlike previous theoretical studies, coseismic changes in rake angle occur due to the emergence of multiple slip fronts within the shallow low-velocity medium. Our results indicate that on-fault geological observations can supplement seismological studies of earthquake rupture evolution beyond traditional datasets.

Seismic velocity structure at the southern termination of the 2016 Kumamoto Earthquake rupture, Japan

The earthquake rupture termination mechanism and size of the ruptured area are crucial parameters for earthquake magnitude estimations and seismic hazard assessments. The 2016 Mw 7.0 Kumamoto Earthquake, central Kyushu, Japan, ruptured a 34-km-long area along previously recognized active faults, eastern part of the Futagawa fault zone and northernmost part of the Hinagu fault zone. Many researchers have suggested that a magma chamber under Aso Volcano terminated the eastward rupture. However, the termination mechanism of the southward rupture has remained unclear. Here, we conduct a local seismic tomographic inversion using a dense temporary seismic network to detail the seismic velocity structure around the southern termination of the rupture. The compressional-wave velocity (Vp) results and compressional- to shear-wave velocity (Vp/Vs) structure indicate several E–W- and ENE–WSW-trending zonal anomalies in the upper to middle crust. These zonal anomalies may reflect regional geological structures that follow the same trends as the Oita–Kumamoto Tectonic Line and Usuki–Yatsushiro Tectonic Line. While the 2016 Kumamoto Earthquake rupture mainly propagated through a low-Vp/Vs area (1.62–1.74) along the Hinagu fault zone, the southern termination of the earthquake at the focal depth of the mainshock is adjacent to a 3-km-diameter high-Vp/Vs body. There is a rapid 5-km step in the depth of the seismogenic layer across the E–W-trending velocity boundary between the low- and high-Vp/Vs areas that corresponds well with the Rokkoku Tectonic Line; this geological boundary is the likely cause of the dislocation of the seismogenic layer because it is intruded by serpentinite veins. A possible factor in the southern rupture termination of the 2016 Kumamoto Earthquake is the existence of a high-Vp/Vs body in the direction of southern rupture propagation. The provided details of this inhomogeneous barrier, which are inferred from the seismic velocity structures, may improve future seismic hazard assessments for a complex fault system composed of multiple segments.