Abstract

The tracking of guided-wave trains by raytracing in 3-D structures can provide a means of interpreting complex seismograms at regional distances. The method relies on the interpretation of Lg as the constructive interference of multiple S reflections within the crust, and strong scattering can be simulated by the inclusion of secondary sources.

This approach has been applied to Californian events observed in the southwestern United States whose records at regional distances frequently exhibit an extended and complex Lg coda with significant late energy arriving with group velocities of 2.5 km/sec or less. The general character of this energy precludes an interpretation as the result of stochastic scattering processes or dispersion in low velocity surface sediments; therefore some alternative explanation needs to be sought.

The observed wavetrains of events from two sites in southwestern California (Coalinga and North Baja) at stations LAC, MNV, and ELK have been compared with the predictions from tracking guided-wave patterns. Multipathing of energy by major changes in the thickness of the crustal wave guide is consistent with the extended and often “pulse-like” nature of the Lg coda. Examples of plausible multipathing mechanisms include reflection from the ocean-continent transition, scattering associated with topographic features such as the Sierra Nevada, and resonance within the narrow corridor of Baja California.

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