Accession Number : ADA332499

Title :   Calibration of Regional Wave Discriminants in Diverse Geological Environments: Topographic Correlations

Descriptive Note : Final rept. 1 May 94-30 Apr 97

Corporate Author : CALIFORNIA UNIV SANTA CRUZ EARTH SCIENCE BOARD

Personal Author(s) : Lay, Thorne

PDF Url : ADA332499

Report Date : 12 SEP 1997

Pagination or Media Count : 43

Abstract : This is the final report for this grant to explore waveguide effects on high frequency regional signals. Three studies are presented. The first is a theoretical investigation of the effects of thin crustal structure on Lg propagation. This study demonstrates that standard Lg in the frequency band 0.3 to 2 Hz does not develop in crustal structures less than 10 km thick due to a lack of overtone energy. This is the primary explanation for why oceanic crust blocks Lg phases in this frequency band. A second study demonstrates that surface topography variations are correlated with Pg/Lg amplitude ratios for frequencies less than 3 Hz, using broadband observations in the western U.S.. Optimal variance reduction of Pg/Lg amplitude ratio measurements (a common seismic discriminant), are achieved by using empirical relationships with propagation distance and roughness. Pn/Lg data show less sensitivity to surface topography variations, but strong distance dependence. The third study demonstrates the complexity of high frequency wavefields in tectonically active regions, using a very dense short period array deployed in the Santa Cruz Mountains area. Large apparent back-azimuth deflections, of up to 60 degrees, are found for P wave particle motions and f-k measurements, and appear to be the result of shallow dipping layers under the array. Spatial coherence of the high frequency wavefield exhibits strong decreases with frequency and sensor offset.

Descriptors :   *SEISMIC WAVES, *SEISMIC DISCRIMINATION, SURFACE ROUGHNESS, SEISMIC DATA, CORRELATION, SEISMIC VELOCITY, OCEANIC CRUST, SEISMIC ARRAYS, PHASE VELOCITY.

Subject Categories : Seismology

Distribution Statement : APPROVED FOR PUBLIC RELEASE