Tomotaka Iwata

Source Characterization for Broadband Ground-Motion Simulation: Kinematic Heterogeneous Source Model and Strong Motion Generation Area

We estimate strong motion generation areas that reproduce near-source ground motions in a broadband frequency range (0.2-10 Hz) using the empirical Greens function technique. The strong motion generation areas are defined as extended areas with relatively large slip velocities within a total rupture area. Four M 6 class (the 1997 Kagoshima on March and May, 1997 Yamaguchi, and 1998 Iwate) and several moderate-size earthquakes in Japan were analyzed. We examine the relationship between the strong motion generation area and the area of asperities, which is characterized based on heterogeneous slip distributions estimated from low-frequency (<1 Hz) waveform inversions (Somerville et al. , 1999). We performed waveform fitting in acceleration, velocity, and displacement, then obtained the strong motion generation area occupying about a quarter of the total rupture area. The size and position of the strong motion generation area coincide with those of characterized asperities. We find self-similar scaling of seismic moment to both the size of the strong motion generation area and the rise time for a magnitude range of analyzed earthquakes. Based on our results, we propose a characterized source model for the broadband ground-motion simulation, which consists of strong motion generation areas with large slip velocities and a background slip area with a small slip velocity. Waveform modeling of the characterized source model suggests that the strong motion generation areas play important roles in simulating broadband ground motions and have the potential to be an about 10-MPa stress-release area on the fault. Manuscript received 22 August 2002.

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.

S-Wave Velocity Structure of the Taichung Basin, Taiwan, Estimated from Array and Single-Station Records of Microtremors

The objective of this study is to estimate the S-wave velocity structure of the Taichung basin in a near-fault region, which is needed for strong-motion evaluation for the 1999 Chi-Chi earthquake. We have conducted array measurements of microtremors with a total of 12 arrays at four sites and single-station measurements of microtremors at 48 sites in and around the Taichung basin. Based on the Rayleigh- wave inversion technique using phase velocities estimated from array records of microtremors, we find that a thick layer (the thickness of about 1000 to 1400 m) with an S-wave velocity of VS 1100 m/sec exists in the east-central part of the Tai- chung basin. We estimate the thicknesses of sedimentary layers above the pre- Tertiary bedrock at 48 sites to fit calculated peak and trough frequencies of horizontal-to-vertical spectral ratios of Rayleigh waves to observed peak and trough frequencies, assuming the same S-wave velocities estimated using array records. The pre-Tertiary bedrock depth was estimated to be about 5 to 6 km in this region. The estimated thickness of the layer with VS 1100 m/sec is largest in the east-central part of the basin and rapidly decreases to less than 400 m in the northeastern and western parts inside the basin. The estimated S-wave velocity structures reasonably explain arrival time of initial P and S waves of aftershock records observed by Higashi et al. (2001).

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.

Source model for strong ground motion generation in the frequency range 0.1–10 Hz during the 2011 Tohoku earthquake

The source model of the 2011 Tohoku earthquake, which is composed of four strong motion generation areas (SMGAs), is estimated based on the broadband strong ground motion simulations in the frequency range 0.1–10 Hz using the empirical Green’s function method. Two strong motion generation areas are identified in the Miyagi-oki region west of the hypocenter. Another two strong motion generation areas are located in the Fukushima-oki region southwest of the hypocenter. The strong ground motions in the frequency range 0.1–10 Hz along the Pacific coast are mainly controlled by these SMGAs. All the strong motion generation areas exist in the deeper portion of the source fault plane. The stress drops of the four SMGAs range from 6.6 to 27.8 MPa, which are similar to estimations for past M 7-class events occurring in this region. Compared with the slip models and aftershock distributions of past interplate earthquakes in the Miyagi-oki and Fukushima-oki regions since the 1930s, the SMGAs of the 2011 Tohoku earthquake spatially correspond to the asperities of M 7-class events in 1930s. In terms of broadband strong ground motions, the 2011 Tohoku earthquake is not only a tsunamigenic event with a huge coseismic slip near the trench but is also a complex event simultaneously rupturing pre-existing asperities.

Estimation of Source Rupture Process and Strong Ground Motion Simulation of the 2002 Denali, Alaska, Earthquake

A M W 7.9 inland crustal earthquake occurred in the Denali fault system, Alaska, on 3 November 2002 at 22:12 (UTC). In this study, we estimated the source process of the 2002 Denali earthquake by a multiple time-window linear kinematic waveform inversion using strong motion and Global Positioning System (gps)-measured static displacement data. The obtained source model could explain both the observed strong motion waveforms and gps-measured static displacements. Large slips on the fault plane are observed at approximately 80–90 km and 150–200 km east from the hypocenter. These features are consistent with the observed surface rupture distribution and the other inversion results obtained using teleseismic body waves. We also observed some portions of the whole fault with a local rupture propagation velocity of more than 4.0 km/sec that exceeded the shear-wave velocity of the source region. The relation between the rupture area and seismic moment of this earthquake seems to follow the bilinear L -model scaling rather than the self-similar source scaling model. The combined area of asperities is somewhat smaller than that expected from the empirical scaling relationship with seismic moments developed by compiling inverted source models. Finally, we conducted a forward ground motion simulation using the finite difference method to estimate the influence of the heterogeneous source process obtained here on the spatial distribution of strong ground motions. The calculated ground motions are relatively large above and around the large slip areas and also in the region east of the fault area because of the forward directivity effect of unilateral rupture propagation.

Source Rupture Process of the 2004 Chuetsu, Mid-Niigata Prefecture, Japan, Earthquake Inferred from Waveform Inversion with Dense Strong-Motion Data

Abstract The kinematic source rupture process of the 2004 Chuetsu, mid-Niigata prefecture, Japan, earthquake, is estimated from strong-motion data by the linear waveform inversion method. In order to develop appropriate Green’s functions, one-dimensional velocity structure models for each station are constructed by modeling the aftershock waveforms. The estimated top of the bedrock is deeper at the western side of the fault and relatively shallow at the eastern side. This tendency coincides with other investigations of seismic tomography and microtremor array observations. The obtained source model shows large slips in the vicinity of the hypocenter. The reverse slipping rupture originated from the deeper part of the fault and propagated toward the up-dip and southwest directions. The near-surface slip is small. Two tests are demonstrated to check the stability of the obtained source model. The first test examines the effects of the number of available stations on the solution, and the second test examines how the difference in the target waveform type affects the solution. The number of stations affects the variation of slip amount, and more than 12 stations appear to be sufficient to obtain a stable solution in this case. The difference in the target wave type in the data set does not significantly affect the solution if the number of stations is sufficient and the Green’s functions are well calibrated. The model obtained using calibrated velocity structure models shows clear image of high-slip area compared to the model obtained using a single velocity structure model.

Estimation of rupture propagation direction and strong motion generation area from azimuth and distance dependence of source amplitude spectra

Strong motion generation areas which reproduce ground motions in 0.2 to 10Hz were estimated using the empirical Greens function method. This strong motion generation area was somewhat smaller than the total rupture area, and coincident with the area of asperities derived from heterogeneous slip distributions estimated by waveform inversions using lower frequencies (<1Hz). We confirmed that the azimuth and distance dependence of observed source amplitude spectra in the near-source area, i.e. rupture directivity effects, were controlled by rupture propagation style and size of the strong motion generation area. We found that the source displacement spectra at stations in forward rupture propagation directions had higher corner frequencies and steeper high-frequency decays, compared with stations in sideways directions. Stations in backward directions had opposite tendencies. Different relationships between size and average corner frequencies of the strong motion generation area were proposed for unilateral and bilateral ruptures with radial propagation.

Earthquake Ground Motion in the Mygdonian Basin, Greece: The E2VP Verification and Validation of 3D Numerical Simulation up to 4 Hz

n a low‐seismicity context, the use of numerical simulations becomes essential due to the lack of representative earthquakes for empirical approaches. The goals of the EUROSEISTEST Verification and Validation Project (E2VP) are to provide (1) a quantitative analysis of accuracy of the current, most advanced numerical methods applied to realistic 3D models of sedimentary basins (verification) and (2) a quantitative comparison of the recorded ground motions with their numerical predictions (validation). The target is the EUROSEISTEST site located within the Mygdonian basin, Greece. The site is instrumented with surface and borehole accelerometers, and a 3D model of the medium is available. The simulations are performed up to 4 Hz, beyond the 0.7 Hz fundamental frequency, thus covering a frequency range at which ground motion undergoes significant amplification. The discrete representation of material heterogeneities, the attenuation model, the approximation of the free surface, and nonreflecting boundaries are identified as the main sources of differences among the numerical predictions. The predictions well reproduce some, but not all, features of the actual site effect. The differences between real and predicted ground motions have multiple origins: the accuracy of source parameters (location, hypocentral depth, and focal mechanism), the uncertainties in the description of the geological medium (damping, internal sediment layering structure, and shape of the sediment‐basement interface). Overall, the agreement reached among synthetics up to 4 Hz despite the complexity of the basin model, with code‐to‐code differences much smaller than predictions‐to‐observations differences, makes it possible to include the numerical simulations in site‐specific analysis in the 3D linear case and low‐to‐intermediate frequency range.