| In the first part of the thesis, the inverse problem of seismic wave attenuation is solved both by an iterative back-projection method and a generalized inversion method. The seismic wave quality factor, Q, can be estimated approximately by inverting the S-to-P amplitude ratios. In two southern California cases, a highly attenuating body is imaged at shallow crest (3 to 5 km deep) in the Coso-Indian Wells region, and a midcrust (8 to 12 km deep) attenuating anomaly is imaged south of the Brawley seismic zone in the Imperial Valley. By applying the two methods to the same data set, formal errors and resolution can be directly computed for the final model, and the objectivity of the final result can be enhanced. In the study done on Hawaii, individual magma reservoirs are imaged along the East Rift Zone at shallow depths (4-6 km), the Kilauea magma chamber is also imaged as attenuating.; In the second part of the thesis, long period seismograms recorded at Pasadena of earthquakes occurring along a profile to Imperial Valley are studied in terms of source phenomena (e.g., source mechanisms and depths) versus path effects. Some of the events have known source parameters, determined by teleseismic or near-field studies, and are used as master events in a forward modeling exercise to derive the Green's functions that describe the propagation effects along the profile. Both timing and waveforms of records are matched by synthetics calculated from 2-dimensional velocity models. The detailed nature of the transition zone in the model at the base of the crust controls the early arriving shorter periods (strong motions), while the edge of the basin controls the scattered longer period surface waves. From the waveform characteristics alone, shallow events in the basin are easily distinguished from deep events, and the amount of strike-slip versus dip-slip motion is also easily determined. |
