Study on crustal earthquakes along cinvergent plate boundary

中国四川省位于两个大陆板块汇聚的板块汇聚带，地震活动非常强烈。日本北部也位于板块汇聚带，地震活动非常强烈，但大陆板块和海洋板块正在那里汇聚。为了加深对中国四川和日本东北两类板块汇聚带典型板内浅壳地震孕育过程的认识，对这两类板块汇聚带进行了对比研究。东北大学研制的地震自动探测、定位和记录系统已安装在四川省地震局遥测地震台网上。实际上是在成都和西昌两个录制中心。同时，还建立了临时地震观测系统，以提高现有台网的观测密度。这两套系统经过中方的运行和维护，运行良好，积累了大量高质量的地震波形资料，利用这些资料，我们获得了四川省下迄今为止尚不清楚的地震构造。根据浅源地震和爆炸的走时资料，估计了该区地壳的P、S波速度结构。与两个板块汇聚带的不同类型所预期的一样，与日本东北部相比，估计的地壳结构具有较厚的地壳。根据这一地壳结构的震源分布表明，地震沿龙门山断裂带的主断裂沿着分布，孕震区局限在20 km以上，与日本东北部相似。根据积累的资料估算了尾波Q和S波分裂，并讨论了它们与地震构造的关系。

Sichuan Province in China is located at a plate convergent zone where two continental plates are converging, and hence seismic activity is very high. Northestern Japan is alsolocated at the plate convergent zone, having a very high seismic activity, but a continental plate and an oceanic plate are converging there. A comparative study has been made between these two types of plate convergent zones, Sichuan province of China and northeastern Japan, in order for deeper understanding of generation process of intraplate shallow crustal earthquakes typical to the plate convergent zone.An automatic event detection, location and recording system, developed by Tohoku University, has been installed in the telemetering seismic network of Seismological Burean of Sichuan Province, actually at the two recording centers : Chengdu and Xichang. A temporary seismic observation system is also constructed to improve the observation station density of the existing network. These two systems have worked well by the effort of operation and maintenance done by the Chinese group, and have accumulated many high quality seismic waveform data.By using accumulated data, we have obtained the seismic structure beneath Sichuan Province which was not clear so far. P and S wave velocity structure of the crust beneath this region has been estimated from travel time data of shallow earthquakes and explosions. The crustal structure estimated has a thick crust compared with that beneath northeastern Japan, as expected from the different types of the two plate convergent zones. Hypocenter distribution based on this crustal structure shows that earthquakes concentrate along the main fault of the Rongmenshang Fault Zone, and that the seismogenic zone is confined to the upper 20km depth which is similar to northeastern Japan. Coda Q and S wave splitting are also estimated from the accumulated data, and their relations to seismotectonics are discussed.