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
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.