Surface wave amplitude anomalies: is reciprocity valid?

Details Surface wave amplitude anomalies: is reciprocity valid? Surface wave amplitude anomalies: is reciprocity valid?

Aug 1987

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1987pepi...47..267t&link_type=abstract

Physics of the Earth and Planetary Interiors, Volume 47, p. 267-287.

Physics

3

Scientific paper

Surface wave amplitude anomalies are studied using Global Digital Seismograph Network (GDSN) records of some recent circum-Pacific events (e.g., 1983 Costa Rica, 1983 Akita-oki, 1982 Ometepec and 1983 Peru-Ecuador). Some event-station pairs can be effectively used to interchange the source and receiver on a given path, and thus provide information on an approximately reciprocal path. At certain stations, amplitude asymmetries of R3 to R2 are significant. Band-pass filtered records show that such asymmetries are frequency dependent. As these anomalies are primarily caused by focusing and defocusing effects, we have done ray tracing experiments using the phase velocity model of Tanimoto in an attempt to account for the observed anomalies. While anomalies such as those at TATO and MAJO for the Costa Rica event or at COL for the Akita-oki event are well predicted by the ray tracing, others are not. At station HON, where the ray tracing experiment failed to predict any asymmetry, we studied surface wave propagation on nearly reciprocal paths in some detail using records of smaller events. The results indicate that actual propagation in the heterogeneous Earth considerably varies between closely spaced, but not exactly reciprocal paths. Such sensitivity to source and receiver placement is not predicted by smooth Earth structures. This suggests that there must be substantial small scale structural variations. The length scale of these variations is presumably on the order of the wavelengths of 100-250 s surface waves (400-1000 km). There is an unexpectedly large vertical component associated with the overtone phase X3 at some stations. In the radial component of these records we identify two group arrivals around the X3 phase; the first one directly propagated from the source and the second one, which has a prominent vertical component, may be due to modal conversion through the path.

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