NEAR-FIELD OBSERVATION AND PRELIMINARY STUDY ON THE GEOSOUND GENERATED BY THE MAGNITUDE 3.1 EARTHQUAKE SERIES IN HUNANZHEN RESERVOIR, ZHEJIANG. ON MAY 22, 1982

The results of near-field observation and perliminary study on the geosounds associated with the magnitude 3.1 earthquake series in Hunanzhen reservoir area on May 22,1982 are presented in this paper. The observations show that every earthquake event was accompanied by geosound. The audible earthquake sounds are generated by near-vertically incident high frequency elastic waves,geosound,radiating from the related earthquakes and coupling directly into the atmosphere through local ground motion on the earth's surface. The activity of geosound shows a characteristic change by stages during the development of the earthquake series of magnitude 3.1. The; frequencies of geosounds are between 60 to 250 Hz. The dominant frequencies of the acceleration power spectrum of geosounds are between 80 to 150 Hz. The time series of dominant frequencies have been studied,from which we find that the dominant frequencies of geosounds decrease gradually during the foreshock period,and become lowest at the time of the main shock of magnitude 3.1,but during the after shock period the dominant frequencies gradually increase. The related function is given by fD=130.7(ML+2)-0.3,The study indicates that the relation of the dominant frequencies to time and earthquake magnitude mainly depends on the fracture size of the earthquake source and also on the stress state and quality factor,Q,of rock at the source. The results of observation and studies arc consistent with the experimental studies in recent years,on the fractur-ing processes and spectral characteristics of acoustic emission of rocks,and indicate that continuous observation and analysis in seismic fault zones and mine are significant for the prediction of rockbursts and large earthquakes.