Toshiaki Sato

Rupture process of the 1944 Tonankai earthquake (Ms 8.1) from the inversion of teleseismic and regional seismograms

[1]xa0We digitized teleseismic and regional records of the 1944 Tonankai earthquake. We used the multiple time window method to invert these records for the spatial and temporal distribution of slip and rake. We assume a 220 × 140 km fault with a spacing of 20 × 20 km and a maximum rupture velocity of 2.5 km/s. The inversion resolved an asperity along the accretionary wedge and under the Shima and Atsumi Peninsulas. The rupture propagated from southwest to northeast, and there was little slip near the hypocenter, consistent with the absence of uplift in the overlying submarine forearc basin. The peak slip is 2.3 m, and the total seismic moment is 2.4 × 1028 dyn cm (Mw 8.2). We compared the predicted and observed vertical geodetic displacements and inferred that the limit of rupture is consistent with the change from subsidence to uplift west of the Atsumi Peninsula. This suggested that the earthquake did not rupture the plate interface in the Tokai gap. We modeled the tsunami using sea-bottom displacements generated from this slip model. The tide gauge amplitudes and frequency content are consistent with those predicted. Resolution tests from inverting synthetic seismograms computed using a simplified slip model indicated that the combination of teleseismic and regional data sets best recovers the asperity locations and peak slips given similar station geometry and degrees of freedom. An inversion of the synthetic regional data set indicates that slip artifacts can occur early in the rupture process due to many clipped records while the inversion of synthetic teleseismic data set had less resolution and recovered only 50% of the peak slip.

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).

Three-Dimensional Finite-Difference Waveform Modeling of Strong Motions Observed in the Sendai Basin, Japan

We perform three-dimensional (3D) finite-difference (FD) waveform modeling of strong motions in the frequency range 0.2 to 1.67 Hz observed in the Sendai basin, Japan, during the Japan Meteorological Agency magnitude ( M J) 5.0 1998 Miyagiken-Nanbu earthquake. In a previous, we estimated S -wave velocity structures above the pre-Tertiary bedrock at six sites in the Sendai basin based on array records of microtremors. To interpolate these velocity structures in space we conduct single-station microtremor measurements at a total of 61 locations and estimate the S -wave velocity structure at each site by modeling the horizontal-to-vertical spectral ratio of fundamental mode Rayleigh waves. An initial 3D model of the basin is constructed using the velocity structures estimated from both array and single-station microtremor measurements, along with other information such as surface geology. This model encompasses a region 33 km long, 30 km wide, and 19 km deep. The final model is obtained through a trial-and-error process by fitting 3D FD synthetic waveforms to the bandpass-filtered (0.2 Hz to 1.67 Hz) displacement records 12 stations for the 1998 Miyagiken-Nanbu earthquake. We compute the synthetics using a fourth-order staggered-grid 3D FD method with variable grid spacing. As the 3D model is modified, the source parameters (strike, dip, rake, and seismic moment) are estimated by a grid search method using 1D site-specific models derived from the modified 3D model. The observed waveforms are reproduced well at most stations by the final 3D basin model. This agreement suggests the validity of the final 3D basin model for theoretical strong-motion prediction of large earthquakes in the frequency range from 0.2 to 1.67 Hz.nnManuscript received 7 June 2000.

S-Wave Velocity Structure of Sediments in Anchorage, Alaska, Estimated with Array Measurements of Microtremors

The array measurements of microtremors were carried out at nine sites to estimate the subsurface S -wave velocity ( β ) structures of the sedimentary deposits beneath the metropolitan area of Anchorage, Alaska. The data were recorded by ten three-component accelerometers arranged in a triangular manner for three different array sizes. The phase velocities ( C ( f )) were estimated at each site from the vertical component of the recorded microtremor data by using the frequency-wavenumber technique. The C ( f ) data from different arrays were combined after checking their consistency for a series of overlapped frequency bands from different arrays and were inverted using a stochastic least-squares inversion technique to estimate the 1D β -structure underneath each site. The inversion results show that the engineering basement ( β >750 m/sec) lies at a relatively shallower depth (∼40 m) in the eastern part of the basin along the foothills of the Chugach Mountains (cm) and at deeper depths toward the southcentral (∼100 m) and western (∼150 m) parts of the basin in accord with the general dip of the basin. Below the engineering basement, a well- developed low-velocity zone (lvz) with β -values in the range of 900–1040 m/sec is found to be present in the eastern as well as along the Knik Arm side in the western part of the basin at a depth of 200 m and 900 m, respectively. Moreover, the central part of the basin is associated with a weakly developed lvz below the engineering basement depth. In the rest of the basin, the β -value increases gradually with depth. The spatial variations of β in the basin at different depths from 20 to 500 m are represented by using 2D interpolation of the β -structures obtained from the inversion of C ( f ) data. The depth to the crystalline basement of the basin, however, could not be ascertained and it seems to be much deeper than the maximum depth (2000 m) resolved by the data gathered in this study. To validate the results of inversion, the spectral ratio between the horizontal and vertical components (h/v) of the recorded microtremor data at each site has been compared with the computed h/v of Rayleigh waves based on the respective β -structure. The results showed good agreement in the frequency range of about 0.4–6.0 Hz. In this frequency range, the h/v peaks are due to the overall effect of the velocity contrasts between layers representing the subsurface β -structure.

A multiple time window rupture model for the 1999 Chi-Chi earthquake from a combined inversion of teleseismic, surface wave, strong motion, and GPS data

[1]xa0We present the results of a comprehensive analysis of the rupture process of the 1999 Chi-Chi, Taiwan, earthquake using a broad array of seismic as well as geodetic data, spanning a very wide frequency band. Our results indicate that the rupture was quite smooth, with the slip concentrated in a wide arcuate region north of the epicenter with very little slip to the east or south. We found a moment of 3.3 × 1027 dyn cm, which is consistent with the long-period determinations for this earthquake. The rupture velocity is approximately 2.25 km/s and is quite uniform across the fault. We have tested both a single fault plane model as well as a composite fault model with a second E-W striking plane to the north of the rupture to improve the fit of the geodetic data. Overall, the differences in slip distribution between the two models are negligible, but the fit to the GPS data is significantly better for the composite model, which is thus our preferred solution. This model is similar to other rupture models as far as the large-scale features are concerned but differs for the smaller asperities that are present in other models but not as much in ours. These are usually poorly constrained, and with the use of the geodetic data we believe that there is little evidence for a significant amount of slip away from our main slip concentration.

Rupture process of the 1948 Fukui earthquake (M 7.1) from the joint inversion of seismic waveform and geodetic data

[1]xa0We used teleseismic body waves, regional low-gain strong motion seismograms, and horizontal geodetic displacements from triangulation surveys to invert for the slip distribution of the 1948 Fukui, Japan, earthquake using the multiple time window method. The earthquake ruptured unilaterally from a depth of 10 km to the north with very little slip (0–0.5 m) evenly distributed across a fault of 24 by 18 km over a period of about 10 s. At 9 s after the initiation of rupture, an adjacent fault along the same strike ruptured to the south from the hypocenter to about 20–30 km toward Fukui city mostly along a shallow asperity between 0 and 12 km depth. The large amount of shallow slip (1–1.5 m) below and within the sedimentary basin and strong rupture directivity likely contributed to the high amount of damage in Fukui city. We obtain a total seismic moment of 1.6–2.2 × 1026 dyn cm (Mw 6.74–6.84) which is similar to results by Kanamori (1973), Sagiya (1999), and Kikuchi et al. (1999) using portions of the same regional strong motion and geodetic data sets. We needed to use very low rupture velocities (1.5 km/s) and a hypocenter 10–20 km north of the original location to fit the combined geodetic and seismic data sets with a single fault model. We developed an alternative fault model with two segments allowing for different rupture dynamics for each segment. This resulted in a slightly better fit to the data and allows for a more reasonable rupture velocity up to 2.4 km/s.