ON STRESS FIELD IN EPICENTRAL AREAS OF 1976 TANGSHAN EARTHQUAKE AND 1679 SANHE-PINGGU EARTHQUAKE

This paper discusses the characteristics of the stress field in the epicentral areasof two large earthquakes -- Tangshan of 1976 and Sanhe-Pinggu of 1679. Stressmeasurements in drill holes give lower values for the points close to the epicentral areas while higher values at points away from them. Meanwhile, a series of stress measurements at two points, Fenghuangshan in Tangshan City and Wuxiongsi of the Shunyi County, nearby the Sanhe-Pinggu epicenter in a period of several years show that the values of principal stress do not vary appreciably with time but their directions are changed obviously. Finally, a preliminary analysis on the cause of low stress values in the epicentral areas has been made and the significance relating to prediction of earthquakes mentioned.