Physics
Scientific paper
Feb 1997
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997pepi..104..109o&link_type=abstract
Physics of the Earth and Planetary Interiors, v. 104, p. 109-125.
Physics
2
Scientific paper
Two models of coda wave envelope formation based on single isotropic scattering in complicated inhomogeneous structures are tested to explain anomalous seismogram envelopes observed at the coda portion. One is a hybrid model with a reflector embedded in a scattering medium. The seismogram envelope composed of scattered waves is calculated from the summation of the energy density of coda waves for the real source and the mirror image source. The size of the reflector affects the duration of the anomalously large amplitude coda envelope following the reflected wave. The second model is a simulation of coda waves in a medium which is composed of many small cubic blocks. Three-dimensional variation of the attenuation and scattering intensity is represented adequately by combination of blocks. The energy density of coda waves is calculated with the scattering coefficient of the block which includes the scattering point and the attenuation factor integrated along the ray path of each scattered wave. Coda envelopes based on the two models are simulated in order to explain anomalous seismogram envelopes which are observed in Japan. In the Kanto area, central part of Japan, reflected S waves from the upper boundary of the subducting Pacific plate are observed for shallow events and generate large amplitude wave trains in the coda portion. The parallel distribution of coda decay curves following the direct S wave and the reflected S wave is explained by the large extension of the reflector. The large amplitude offset requires a two-layer structure in the distribution of the scattering and attenuation intensity. In Awaji Island, western part of Japan, the coda amplitude does not decrease for about 10 s after the arrival of the direct S wave for shallow local events. The simulation of coda waves with a two-layer strong scattering body for S waves ranging from about 20 to 30 km in depth might explain the flat portion of the observed S coda envelope. The simulation of P wave seismogram envelopes for the artificial blast suggests that there exists a strong scattering body for P waves ranging from 20 to 25 km in depth. The lower crust beneath the northern part of Awaji Island might be strongly inhomogeneous not only for S waves but also for P waves.
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