STRESS-FIELD IN THE CONTINENTAL PART OF CHINA DERIVED FROM TEMPORAL VARIATIONS OF SEISMIC ACTIVITY

The seismic activity in East Asia during the last 3000 years and more has systematically been analyzed. A quantitative estimate has been attempted for the seismicity during the last 500 years, employing a statistical approach. The earthquake-generating stresses in and around China have been investigated by analyzing the relationships of the temporal variations of seismicity in various regions of East Asia. Seismicities in the region from the northern part of the North-South Seismic Belt of China (NSB) (using events with M ≧ 7) to the Japan trench (using events with M ≧ 7.5) through North China (using events with M ≧ 7), South China (using events with M ≧ 6) and the Korean peninsula (using events with M ≧ 6.5) were investigated. They all show high activity from about the 16th century to the beginning of the 18th century. The activity then subsided until the beginning of the 20th century. From the beginning of the 20th century, the activity of each region has risen again and remains high now. The short term variations of seismicity since 1900 in North China, the Korean peninsula and Japan also exhibit the same pattern. Synchronous variations of seismicity in these regions imply that regional earthquake-generating stress fields between the Japan trench and the northern part of the NSB of China originate under common tectonic conditions and that there is a transmission of the tectonic force from the subduction of the Pacific Ocean plate along the Japan trench to the northern part of the NSB through the Korean peninsula and North China. The seismic activities since 1900 in South China (using events with M ≧ 5), the Taiwan (using events M ≧ 7.3) and the Ryukyu regions (using events with M ≧ 6.7) were found to be high between 1900 and 1940. This high activity was followed by a quiet period starting from around 1945. Since 1960, the active period has continued up to the present time. These results suggest that the earthquake-generating stress fields between Taiwan and South China seem to be formed under common tectonic conditions i.e., due to the transmission of the tectonic force resulting from the subduction of the Philippine Sea plate along the Ryukyu trench and the collision between the Philippine Sea plate and the Eurasian plate in the Taiwan region. During this century, the Tibetan plateau (using events with M ≧ 6.8) and the Xinjiang region (using events with M ≧ 7) were seismically active from 1900 to 1916, then from around 1931 to 1955, and lately from around 1970 to the present. These active periods correspond to those (using events with M ≧ 7.7) at the boundary regions between the Indo-Australian and the Eurasian plate. This implies that the earthquake-generating stress fields in and around West China result from the transmission of the tectonic forces originating from the collision of the Indo-Australian plate and the Eurasian plate along the Himalayan mountain range. © 1990.