Development of a seismic network in the Western Pacific

该项目的主要目的是在帕劳群岛安装一个宽带地震站，我们已经成功地完成了该项目。该站位于科罗尔岛中部。我们修复了一个长度超过40米的旧防空洞，并在防空洞的后室制作了一个地震仪台。地震仪室与避难所的其他部分由三扇门隔开，距离地面约1米深。因此，地震计室内的温度变化很小。记录系统可以通过电话线访问，并有磁带驱动器用于数据备份。我们还在PCC内设置了一台计算机，通过电话线检索数据并存储在MO磁盘驱动器上，分析了西太平洋地区大地震的震源过程。通过波形反演分析，揭示了1994年三陆-遥贺-冲地震和1995年科比地震的破裂历史细节。1994年三陆-晴香-冲地震的破裂过程表明，大洋-大陆岩石圈板块界面的不均匀性影响破裂速度、滑动分布和地震序列特征。本文综述了我们在西太平洋地区通过高分辨率初至时反演得到的全地幔P波速度结构的研究。它揭示了岩石圈俯冲板块的命运。通过对P波和PcP波到时资料的分析，得到了核幔边界的速度结构。这些结果已经或即将在国际期刊上发表，本研究项目所获得的数据由东京大学地震研究所海洋半球中心数据中心提供。

The main purpose of this project is to install a broad-band seismic station in the Palau Islands and we have completed it successfully. The station lies at the middle part of Koror Island. We repaired an old air-raid shelter whose length is longer than 40 m, and made a seismometer table in a back room of the shelter. The seismometer room is divided from the other part of the shelter by three doors, and lies about 1 m deep from the land surface. Therefore, the temperature change is quite small in the seismometer room The recording system can be accessed using telephone line and has magnetic tape drives for data backup. We also setup a computer in PCC for retrieving the data by telephone line and storing on MO disk drive.We analyzed source processes of large earthquakes occurred in the Western Pacific region. The details of rupture history of the 1994 Sanriku-Haruka-Oki earthquake and the 1995 Kobe earthquake are revealed by waveform inversion analyzes. The rupture process of the 1994 Sanriku-Haruka-Oki earthquake shows that the heterogeneity of the plate interface between the oceanic and the continental lithospheres affect rupture velocity, slip distribution, and the characteristics of earthquake sequences. We review our study for the P-wave velocity structure of the whole mantle, that are obtained by inverting first arrival times with better resolution under the Western Pacific. It reveals the fate of subducting slabs of lithosphere. The P-wave and PcP-wave arrival time data are also analyzed to get a velocity structure of the core-mantle boundary. These results are already published or will be published soon in the international journals.The data obtained in this research project are supplied by the Data Center of the Ocean Hemisphere Center, Earthquake Research Institute, University of Tokyo.