DETERMINATION OF SEISMIC ATTENUATION STRUCTURE AND SOURCE STRENGTH BY INVERSION OF SEISMIC INTENSITY DATA

METHOD AND NUMERICAL EXPERIMENT

Abstract

Seismic intensity data, which measure the degree of ground shaking, contain information on attenuation along the path from the source to the station and earthquake source strength. With some reasonable assumptions about the seismic intensity data, a properly formulated damped least squares estimation procedure can be used to determine simultaneously both three-dimensional attenuation structure and source strength. By means of this method, sufficiently distributed earthquakes with a large amount of seismic intensity data may provide information on the crust and upper mantle Q structure in a region where a spatial variation of attenuation is large enough to affect the distribution of seismic intensity. To check the validity of the method, numerical experiments were undertaken for artificial data including errors comparable to those expected from using seismic intensity. The results obtained for a two-dimensional island arc structure model show the method to be practical. The reliability of the obtained solution for Q structure can be measured by the corresponding diagonal element of the resolution matrix. By using a large number of seismic intensity data up to about one thousand and selecting solutions with high resolution, a Q structure with an accuracy of 1.5×10^-3 in 1/Q and a source strength with an accuracy of 0.1 in magnitude scale can be obtained. This method is extendable to many regions including island arcs and continents because large amounts of intensity data are easily attainable.