Thickness detection of sedimentary basin by P-to-S-phases of earthquake recordings at Ibaraki-ken, Japan

The Kanto sedimentary basin has very complicated geological formations with various sedimentary layers. The thickness of sedimentary basins varies irregularly from zeros to over several thousand meters down to the pre- Neogene bedrock. Therefore, the understanding the thickness is the fundamental in order to study the seismic ground motions. The vertical seismic profile (VSP) method may give a high-resolution seismic image at a borehole in both of P and S wave information. However, VSP is only limited to a few areas since it is too costly to be widely used. A dozen of deep-wells down to 2000 meters with VSP in the Kanto area have been drilled over past 15 years by NIED. In order to extend the high-quality VSP results farthest, we utilize these VSP data to obtain regression functions for sedimentary depth against the travel time difference of direct P and the direct S waves. On the other hands, the travel time difference between the direct P and the converted PS waves may be detected from P-to-S converted phases by means of all-pass function deconvolved from Receiver Functions (APRF) (Izumi et.al., 1990, Satoh, 2005, Hao, et.al., 2005, 2006). Comparing the classic receiver function for detection of interfaces with abrupt velocity changes in the earth crust, this APRF method can identify P-to-S phase in the near- surface interfaces much clearly, even for a high frequency of 10Hz. We use APRF with the VSP regression functions to estimate the thickness of sedimentary basin beneath each ground motion station. The objective area in this paper is whole of Prefecture Ibaraki-ken, which is located at north-east part of Kanto basin. The northern and northwestern of Ibaraki-ken are mountain areas, and Mt. Tsukuba and Lake kasumigaura are located in the center. A total of 112 strong motion stations, in which 84 stations from Sk-net, 19 K-net, and 14 KiK-net, have been targeted to study the underground structure beneath each station. Over 3000x3 records have been visually identified for the initial P and S phases from events during 1999 to 2005 for Sk-net, and from 1996 to 2007 for K-net and KiK-net. Averagely, about 30 events, whose amplitudes ranged from 2 to 558 gal, are used at each station. The Ts-Tp travel time difference was related both of velocities of Vp and Vs, as well as depth of sedimentary. One of key problems was how to convert the Ts-Tp travel time difference to the depth since a trade-off between factors of the velocity and depth. The regression functions of sedimentary depth against the Ts-Tp gave the statistics solution with a VSP calibration at deep-wells around. Recently, we have obtained the five regression functions at NRT, CHB, EDS, FUT, and YOR in the areas (Hao and Fujiwara, 2007). By combination of the 112 Ts-Tp results and the regression functions, the sedimentary depths in Ibaraki area have been revealed. The results have been confirmed at several areas where there were other deep wells and fault investigations. Acknowledgements The dense strong-motion observations by K-net, Sk-net and KiK-net made this study possible.