Simulation of long-period (5-10s) ground motions in the Oita basin during the 2000 Tottori-ken Seibu earthquake
Yasuhisa Nagawa
The long-period ground motions inside basins have been vigorously investigated since the 1985 Mexico earthquake. Studies of such ground motions showed amplification and elongation of duration by secondary surface waves excited at interfaces between sedimentary basin and bedrock.
During the 2000 Tottori-ken Seibu earthquake, ground motions at stations in basins as well as on bedrocks were recorded by dense strong-motion seismograph network (K-NET, KiK-net). The maximum velocity distribution maps for radial, transverse and vertical components in the period range of 5 to 10 s clearly show the strong excitation of SH and/or Love waves in azimuths expected from its source mechanism. Besides, there are several high amplitude stations placed in some sedimentary basins such as Osaka, Nobi, Oita and so on. The maximum velocity amplitude is 3 to 10 times larger than stations outside the basins.
This study was conducted to examine the excitation and propagation of surface waves in the Oita basin by analyzing seismograms from the 2000 Tottori-ken Seibu earthquake.
A very large later phase was found in the seismogram at OIT010 (K-NET station inside the Oita basin). In contrast, such phase did not clearly appear at the surrounding K-NET stations. Semblance analysis using observed seismograms at 3 stations in the Oita basin (BRI, JMA, K-NET) showed a very large later phase propagating dispersively from N-NNW to S-SSE. This direction is orthogonal to particle motion diagram at each station, although it is different from the epicenter direction (N037E). These facts imply that the later phase is the surface wave (Love-wave) excited at the basin edge.
Waveform simulation was performed to explain this seismic wave propagation phenomena using the 3D FDM (Pitarka, 1999). The 3D velocity structure model covering an area of 300km(EW) * 300km(NS) * 50km(Depth) was constructed. A point source was putted on location which corresponds to the main slip area from source inversion result by Iwata and Sekiguchi (2002). Horizontally multi-layered crustal structure model was assumed outside the Oita basin. The Oita basin structure model is assumed from the basement depth model estimated by Kusumoto et al. (1997) and gravitational anomaly map. Inside the basin, I assumed the velocity structure is composed of 4-layers and the shapes of the interfaces are similar to the basement shape. The assumed basin size is approximately 100km long and 30km wide, with 4km deep at the deepest point.
Synthetic seismograms fit well to observed ones at stations from source region to the Oita basin. At stations inside the Oita basin, the large later phases of the synthetic seismograms propagate dispersively to the direction different from direct S-wave, and the directions are orthogonal to particle motion diagrams. I conclude that these characteristics of the synthetic later phases coinside with ones of observed later phase qualitatively.