| | AbstractShort-period
regional phases play an important role in identifying low-magnitude
seismic events in the context of monitoring the Comprehensive Test Ban
Treaty. Amplitude ratios of regional phases comprised mainly of
P-wave energy (Pn, Pg) to those comprised mainly of
S-wave energy (Sn, Lg) effectively discriminate between
explosions and earthquakes in many regions, particularly at frequencies
higher than 3 Hz. At lower frequencies, discrimination is usually poor
due to large scatter that causes overlapping of event populations.
Scatter in regional discriminant measures such as Pg/Lg
ratios is caused by both source and propagation effects, and reducing
the scatter imparted by the latter is essential to improving the
discriminant performance when events do not share identical paths.
Regional phases experience distance-dependent amplitude variations due
to effects such as critical angle amplification, geometric spreading,
and attenuation. Discriminant measures are usually corrected for
empirically determined distance trends for a given region, but large
scatter persists after such corrections. This study seeks to develop
more sophisticated empirical corrections for path properties in order to
further reduce the scatter in regional discriminant measures caused by
propagation effects. Broadband seismic waveforms recorded at station
WMQ, in western China, demonstrate that regional Pg/Lg
ratios show significant distance dependence for frequencies less than 6
Hz. However, variations in crustal structure cause additional
path-specific amplitude fluctuations that are not accounted for by
regionally averaged distance corrections. Blockage of Lg phases
on paths traversing the margins of the Tibetan Plateau is one such
effect. Regression analysis demonstrates that Pg/Lg ratios
measured at WMQ display significant correlations with path-specific
properties such as mean elevation, topographic roughness, basement
depth, and crustal thickness. Multiple regressions using optimal
combinations of parameters yield corrections that reduce variance in
Pg/Lg measurements for frequencies less than 3 Hz by a
factor of 2 or more relative to standard distance corrections. This
should systematically enhance the performance of the Pg/Lg
discriminant at low frequencies. The method presented here can be used
for all regions and all short-period regional discriminants. It is
likely that the extraordinary crustal heterogeneity in western China
represents an extreme case of path-dependent effects.Return to Table of
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