Accession Number : ADA337812

Title :   Characterization of Seismic Sources Using Empirical Green's Functions

Descriptive Note : Final rept. 1 Jul 96-30 Jun 97

Corporate Author : MASSACHUSETTS INST OF TECH CAMBRIDGE EARTH RESOURCES LAB

Personal Author(s) : Rieven, Shirley A. ; Rodi, William L.

PDF Url : ADA337812

Report Date : 21 NOV 1997

Pagination or Media Count : 67

Abstract : Our work on the 1993 micro earthquake swarm at the Larderello geothermal field suggests that the most significant obstacle to the practical application of correlation techniques, such as EGFs or relative event location for seismic events recorded at regional distances, is due to the degree of waveform complexity that is related solely to scattering effects along the path. In this study, we showed that despite similar focal mechanisms and a regional scale that is on the order of 1 to 2 wavelengths, the complexity of the waveforms was severe, particularly for S waves. We showed that the event waveform types exhibited at least two distinctly different characters, thus forcing the use of more than one master event. We show that the practical difficulties were partially mitigated by a very careful and detailed analysis of correlation coefficients between master and slave events and multiple pass culling of the rms residuals from the inversion results. This effort clearly reduces the efficiency with which these techniques can be applied to large clusters and presents some new challenges in developing practical algorithms. However, in the case of the Larderello 1993 cluster, we are encouraged that the final result so dearly agreed with known geologic structures. This suggests that refinements of the correlation techniques, in ways that reduce the effects of scattering, may further improve the results. One possible direction may be to introduce more thorough modeling of the scattered wavefield and removal of the non essential phases from the waveform. This may improve the correlation coefficients by increasing the usable window length.

Descriptors :   *SEISMIC WAVES, *SEISMIC DISCRIMINATION, *EARTHQUAKES, *GREENS FUNCTIONS, ALGORITHMS, WAVEFORMS, UNDERGROUND EXPLOSIONS, SEISMIC DATA, CORRELATION TECHNIQUES, STRUCTURAL GEOLOGY, SECONDARY WAVES, GEOTHERMY.

Subject Categories : Seismology

Distribution Statement : APPROVED FOR PUBLIC RELEASE