STATISTICAL-ANALYSIS OF IRREGULAR WAVE-GUIDE INFLUENCES ON REGIONAL SEISMIC DISCRIMINANTS IN CHINA

Authors
Citation
Gw. Fan et T. Lay, STATISTICAL-ANALYSIS OF IRREGULAR WAVE-GUIDE INFLUENCES ON REGIONAL SEISMIC DISCRIMINANTS IN CHINA, Bulletin of the Seismological Society of America, 88(1), 1998, pp. 74-88
Citations number
33
Categorie Soggetti
Geochemitry & Geophysics
ISSN journal
00371106
Volume
88
Issue
1
Year of publication
1998
Pages
74 - 88
Database
ISI
SICI code
0037-1106(1998)88:1<74:SOIWIO>2.0.ZU;2-L
Abstract
Short-period regional phases play an important role in identifying low -magnitude seismic events in the context of monitoring the Comprehensi ve Test Ban Treaty. Amplitude ratios of regional phases comprised main ly of P-wave energy (Pn, Pg) to those comprised mainly of S-wave energ y (Sn, Lg) effectively discriminate between explosions and earthquakes in many regions, particularly at frequencies higher than 3 Hz. At low er frequencies, discrimination is usually poor due to large scatter th at causes overlapping of event populations. Scatter in regional discri minant measures such as Pg/Lg ratios is caused by both source and prop agation effects, and reducing the scatter imparted by the latter is es sential to improving the discriminant performance when events do not s hare identical paths. Regional phases experience distance-dependent am plitude variations due to effects such as critical angle amplification , geometric spreading, and attenuation. Discriminant measures are usua lly corrected for empirically determined distance trends for a given r egion, but large scatter persists after such corrections. This study s eeks to develop more sophisticated empirical corrections for path prop erties in order to further reduce the scatter in regional discriminant measures caused by propagation effects. Broadband seismic waveforms r ecorded at station WMQ, in western China, demonstrate that regional Pg /Lg ratios show significant distance dependence for frequencies less t han 6 Hz. However, variations in crustal structure cause additional pa th-specific amplitude fluctuations that are not accounted for by regio nally averaged distance corrections. Blockage of Lg phases on paths tr aversing the margins of the Tibetan Plateau is one such effect. Regres sion analysis demonstrates that Pg/Lg ratios measured at WMQ display s ignificant correlations with path-specific properties such as mean ele vation, topographic roughness, basement depth, and crustal thickness. Multiple regressions using optimal combinations of parameters yield co rrections that reduce variance in Pg/Lg measurements for frequencies l ess than 3 Hz by a factor of 2 or more relative to standard distance c orrections, This should systematically enhance the performance of the Pg/Lg discriminant at low frequencies. The method presented here can b e used for all regions and all short-period regional discriminants. It is likely that the extraordinary crustal heterogeneity in western Chi na represents an extreme case of path-dependent effects.