Haruko Sekiguchi

Fault Geometry at the Rupture Termination of the 1995 Hyogo-ken Nanbu Earthquake

The source geometry and slip distribution at rupture termination of the 1995 Hyogo-ken Nanbu earthquake were investigated using waveform inversion on the assumption of fault branching in the northeastern part of the rupture model. Possible branching of the Okamoto fault is suggested both by the static-displacement distribution and damage extension east of Kobe (Nishinomiya area). To exclude data contaminated by the basin-edge-diffracted wave in the waveform-inversion process, we examined the spatio-temporal variation of its influence and from a comparison with a flat model, we determined windows appropriate for the data. Three subevents were identified, as was reported in previous works. The largest was in the shallow part on the Awaji side, whereas the smaller two occurred at great depths (7km) on the Kobe side. We found a smaller subevent in the deep part of the branch. Total variance reduction was larger, and the ABIC value was smaller when we assumed the branch than when we did not, which shows the superiority of the branching fault model. Resolution checks showed that the slips on proposed branched portions are physical and not caused by random-data noise or systematic errors in Greens func- tions that arise from misestimation of velocity structures. Calculation of the relative static displacement between two leveling observation stations near the branching fault also showed the greater utility of the branching fault model. The effect of slip on the branched fault has been shown in near-source ground-motion simulation using the 3D finite-difference method (Iwata et al., 1999). The characteristic distribution of ground motion in the near-source region indicated by the damage distribution is well reproduced by the modeling of both the source process and wave propagation in the realistic 3D velocity structure. Slip on the branched fault affected the ground motion in eastern Kobe (Nada and Higashi-Nada wards), Ashiya, and Nishinomiya cities, but its contribution is not dominant even in those regions; about 30 to 50% maximum velocity in the frequency range of 0.1 to 1.0 Hz. We conclude that branching rupture of the Okamoto fault during the Hyogo-ken Nanbu earthquake is the preferable interpretation on the source process of this earth- quake. This supports the idea of a geometrical barrier suggested by geological ob- servations.

Rupture Process of the 1999 Kocaeli, Turkey, Earthquake Estimated from Strong-Motion Waveforms

The multiple time-window linear waveform inversion is applied to study the source process of the 1999 Kocaeli, Turkey, earthquake using strong-motion data. Two peculiar observations characterize this event. The first one is a conflicting observation of the long surface ruptures and short source duration resulting from the observed strong-motion waveforms. The second is an anomalously short S - P time observed at a station SKR, considering its hypocentral distance. A supershear rupture propagation (Ellsworth and Celebi, 1999) and the P -wave triggering of an asperity (Anderson et al., 2000) were both proposed to explain the record at SKR. Our waveform inversion study is aimed to examine these possibilities in order to clarify the relation between the spatial distribution of the surface ruptures and coseismic slips. We divide the assumed fault plane into three parts and look for the first time-window front propagation velocity (propagation velocity of triggering front of the first time window at each subfault) in each part by performing numerous inversions, changing the first time-window front propagation velocity separately in each part, and selecting the best first time-window propagation velocities that give largest variance reduction. At the same time, appropriate smoothness of spatiotemporal slip distribution is searched for by performing inversions with different strength of smoothing constraints against the observational equations and judging the relative appropriateness of inversion results in terms of the Akaikes Bayesian Information Criterion. The best source model is characterized by an asperity that is about 35 km east of the epicenter and near a segment boundary at Sapanca Lake, triggered by the first time-window front with 5.8 km/sec, close to P -wave velocity. The time progression of the rupture suggests a scenario where a P wave arriving from the hypocenter or somewhere around the hypocenter triggered the second largest asperity near the junction of the segments at Sapanca Lake, which is a preferable interpretation. The first time-window propagation velocities west of the hypocenter and east of SKR are expected to be 3.0 km/sec. Manuscript received 20 August 2000.

Some characteristics of the stress field of the 1995 Hyogo-ken Nanbu (Kobe) earthquake

We investigate the characteristics of the stress field associated with the 1995 Hyogo-ken Nanbu (Kobe) earthquake. We use for the study a tomographic model of the fault slip inferred from the numerous near-field recordings, and we calculate the space and time evolution of shear stress on the fault during the earthquake. We show that the spatial distribution of stress drop is very heterogeneous. The apparent strength of the fault at the onset of the earthquake is low (of the order of 1 MPa or less), which indicates that the pre-earthquake tectonic shear stress was close to the static friction over most of the fault. At many locations the stress drop rotates significantly during sliding. As was originally proposed by Spudich [1992], we use the requirement of colinearity between the directions of maximum shear stress and instantaneous slip to determine the initial stress on the fault at the onset of the earthquake. The pre-earthquake tectonic stress varies greatly over the fault and ranges from about 1 to 10 MPa. Average values of the initial and final shear stresses over the fault are 3.3 and 1.6 MPa, respectively, indicating that about half of the pre-earthquake tectonic stress was released during the earthquake. There is a relatively strong correlation between the initial and final stress distributions, which suggests that intrinsic fault properties, not modified by the earthquake, control the spatial distribution of tectonic stress over the fault. We show that on average over the fault the dynamic friction coefficient is equal to about 40% of the static friction coefficient. Diagrams depicting the evolution of shear stress as a function of slip are consistent with slip-weakening behavior, but their interpretation is questionable because of the poor resolution of the data at high frequency and because of the constraints imposed on the model to perform the inversion.

Ground motion and rupture process of the 2004 Mid Niigata Prefecture earthquake obtained from strong motion data of K-NET and KiK-net

The 2004 Mid Niigata Prefecture earthquake (37.289°N, 138.870°E, 13.1 km, MJMA 6.8; JMA), also known as the 2004 Niigata Prefecture Chuetsu earthquake, was a thrust type earthquake that occurred on October 23, 2004 at 17:56 (JST). Strong ground motions of PGA 800-1700 cm/s2 and PGV 60-130 cm/s were observed at stations located immediately above the source region. We deduced the rupture process of this earthquake with a multi-time-window linear waveform inversion procedure. We used near-fault strong ground motion data observed at nine K-NET and KiK-net stations within 50 km from the epicenter. In order to obtain appropriate Green’s functions for the waveform inversion, we constructed two velocity structure models for stations on the hanging wall and one structure model for stations on the footwall. The estimated total slip distribution contains three asperities: (a) around the hypocenter, (b) in the upper-middle section of the fault plane, and (c) southwest of the hypocenter. The maximum slip is 3.8 m at the hypocenter and the total seismic moment is 1.2 × 1019 Nm, which corresponds to Mw=6.7. The moment rate functions in asperities (a) and (c) have a short rise time, while those in asperity (b) have a longer rise time.

The Earthquake‐Source Inversion Validation (SIV) Project

Finite-fault earthquake source inversions infer the (time-dependent) displacement on the rupture surface from geophysical data. The resulting earthquake source models document the complexity of the rupture process. However, multiple source models for the same earthquake, obtained by different research teams, often exhibit remarkable dissimilarities. To address the uncertainties in earthquake-source inversion methods and to understand strengths and weaknesses of the various approaches used, the Source Inversion Validation (SIV) project conducts a set of forward-modeling exercises and inversion benchmarks. In this article, we describe the SIV strategy, the initial benchmarks, and current SIV results. Furthermore, we apply statistical tools for quantitative waveform comparison and for investigating source-model (dis)similarities that enable us to rank the solutions, and to identify particularly promising source inversion approaches. All SIV exercises (with related data and descriptions) and statistical comparison tools are available via an online collaboration platform, and we encourage source modelers to use the SIV benchmarks for developing and testing new methods. We envision that the SIV efforts will lead to new developments for tackling the earthquake-source imaging problem.

Source process of the 2007 Niigata-ken Chuetsu-oki earthquake derived from near-fault strong motion data

The 2007 Niigata-ken Chuetsu-oki earthquake generated strong ground motions in Kashiwazaki and Kariwa, where the world largest nuclear power plant was in operation. Due to the complexity of the aftershock distribution, activation of the northwest-dipping fault and/or the southeast-dipping fault is proposed. To explore the fault geometry and source process of the earthquake, we performed multi-time window linear waveform inversions for both the fault planes from near-fault strong motion data. A fault plane model of 30 km in length by 24 km in width was set to cover the region of aftershock distribution within 24 h of the mainshock. Both inverted slip models provided moment magnitudes of 6.7 with a small asperity near the rupture starting point, and a large asperity approximately 10 km southwest of the rupture initiation, which is located in the region of relatively sparse aftershock distribution. Both the small and large asperities are located near the intersection between the two conjugate fault plane models, and the asperities of both models have similar radiation patterns. Therefore, the difference of the residuals between the observed and synthetic waveforms for both models was not significant, indicating that it is difficult to conclude which fault is the rupture.

Rupture model of the 1999 Düzce, Turkey, earthquake deduced from high and low frequency strong motion data

(1) We investigated the rupture process of the 1999 Duzce Earthquake by applying the multi time-window linear waveform inversion method to low-frequency ground motions (0.5 Hz) in the near source area. Then, we performed theforwardmodellingofhigherfrequencyground motions (0.3-10.0 Hz) by the Empirical Greens Function (EGF) method to image the strong motion generation area (SMGA). We also examined the source model to check whether the supershear phenomenon occurred inside the asperities or not. Slip distribution model from the inversion suggests 2 asperities. The maximum slip is obtained as 5 m near the hypocenter. The high first time window front propagation velocity (FTWFPV) and consequently, high apparent rupture velocity found from the investigation of rupture progression indicated that eastern propagation partially showed supershear behaviour. The EGF simulation, which identifies an average rupture velocity (Vr) in SMGA, supports this finding. INDEX TERMS: 7212 Seismology: Earthquake ground motions and engineering; 7215 Seismology: Earthquake parameters; 7223 Seismology: Seismic hazard assessment and prediction. Citation: Birgoren, G., H. Sekiguchi, and K. Irikura (2004), Rupture model of the 1999 Duzce, Turkey, earthquake deduced from high and low frequency strong motion data, Geophys. Res. Lett., 31, L05610, doi:10.1029/ 2003GL019194.

Determination of the location of faulting beneath Kobe During the 1995 Hyogo‐Ken Nanbu, Japan, Earthquake from near‐source particle motion

We constrained the location of faulting beneath Kobe during the 1995 Hyogo-ken Nanbu (Kobe), Japan, earthquake by investigating the particle motion of strong motion records from sites close to the causative fault. The intersection of the earths surface and the extension of buried fault plane is consistent with known active fault lines but not with the severely damaged zone. The intersection line also agrees with geodetic data.

Rupture process of the 2005 West Off Fukuoka Prefecture earthquake obtained from strong motion data of K-NET and KiK-net

We have investigated the rupture process of the 2005 West Off Fukuoka Prefecture earthquake by the multitime- window linear waveform inversion method using the strong ground motion data recorded at 11 K-NET and KiK-net stations. From the waveforms of the P-wave portion, it is indicated that the energy release in the first few seconds was markedly lower than the subsequent part, and this causes difficulty in identifying onset of the S-wave. To decide an appropriate time window for the waveform inversion, we estimate the S-wave onset using aftershock records. The inverted slip distribution shows a single asperity of 8 km × 6 km and its center located 8 km to the southeast and 6 km above the hypocenter. The asperity explains most of the large-amplitude signals in the observed waveforms. The turning point from the initial low-energy-release rupture to the main high-energyrelease rupture is estimated from the spatial variation of the observed initial rupture phase. It is found 3.3 s after the initiation of the rupture at about 4 km to the southeast of the hypocenter. Stress drop during the initial rupture is estimated to be in the same order of those of moderate size aftershocks, which indicates that the initial rupture is an ordinary dynamic rupture.

Source Process of the 2011 Off the Pacific Coast of Tohoku Earthquake

The kinematic source process of the 2011 Off the Pacific Coast of Tohoku earthquake is studied using strong motion data both in low- and high-frequency ranges. The slip distribution is estimated by the waveform inversion analysis using velocity waveforms in the frequency range from 0.01 to 0.125 Hz at strong motion stations along the Pacific coast. The strong motion generation area (SMGA) is estimated by the strong ground motion simulation in 0.1–10 Hz using the empirical Green’s function method. The slip distribution is characterized by a large asperity with peak slip of 48 m which is imaged in the shallower portion of the source fault near the Japan Trench. Four SMGAs are identified in the deeper portion of the source fault. Unlike the past M7–8 subduction-zone plate-boundary events, the SMGAs and the asperity seem to be complementary in space. But the rupture time of each SMGA matches the timing of slip in each area. The total size of SMGAs is much smaller than the asperity area. This event coincides with empirical scaling relationships between total rupture area, asperity area, SMGA, and its seismic moment proposed for subduction-zone plate-boundary earthquakes by previous papers although the asperity abstracted for this event may have different nature from those of past smaller earthquakes.