VARIATIONS IN MICROSEISM BEFORE AND AFTER THE TANGSHAN EARTHQUAKE

The principle and the main characters of selected band-pass microseismometer are described briefly. The data of microseism, wind speed and air pressure are processed by using the low-pass and band-pass digital filters. The result obtained show that the correlations between microseisms and local meteorological factors are poor, the amplitude of short period microseism recorded at Chanlei (昌黎) station (△= 80km) decreased sharply about several days before the occurrence of the Tangshan earthquake. However, the level of microseism shows a tendency to decrease about several months before some large aftershocks (M ≥ 6.0), but there is no clear change of microseism just before them.The variations of microseism is discussed together with the abnormal increase of ground temperature (80 cm depth) near epicentral region before the Tangshan earthquake. It is suggested, that such phenomena are probably related to the preparatory process of the Tangshan earthquake.