Seismological Studies Using Observed and Synthetic Waveforms

Citation

Butler, Rhett Giffen (1979) Seismological Studies Using Observed and Synthetic Waveforms. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/bfn4-vz23. https://resolver.caltech.edu/CaltechTHESIS:07112024-201950214

Abstract

Application of waveforms to four topics in seismology is pre­sented. Detailed waveform analyses of three earthquakes are reported in Chapter I. The Oroville, California earthquake of 8/1/75 has a north-south striking, westward dipping normal fault mechanism with a small component of left-lateral motion. A surface wave seismic moment of 1.9 x 10²⁵ dyne-cm. is a factor of 3 greater than the teleseismic body wave determination. Slow deformations on the Oroville fault may explain the enhanced excitation of the surface waves. The Tangshan, China earthquake of 7/27/76 and its principal aftershock represent a complex intraplate event sequence with strike-slip, normal, and thrust faulting. The main shock was a bilateral strike-slip event, striking N40°E, with a seismic moment of 1.8 x 10²⁷ dyne-cm. Associated thrusting occurred concurrently with the main shock. The principal aftershock was an oblique, normal double event, striking approximately perpendicular to the main event, with a seismic moment of 8 x 10²⁶ dyne-cm. The 4/26/73 Hawaii earthquake is a subcrustal, double event. The events are consistent with left-lateral strike-slip motion on en echelon southward dipping faults. Evidence of lateral heterogeneity in the Hawaii source region is suggested by incompatibility between and azimuthal amplitude anomalies associated with the P and SH data.

In Chapter II shear travel times are obtained by a waveform correla­tion technique. A total of 87 SH travel-times are measured from the 1968 Borrego Mountain, California and 1973 Hawaii earthquakes. The Bor­rego data have a trend toward faster travel times at 40°, but show an overall 6 second slow baseline with respect to the Jeffreys-Bullen Table. The Hawaii data contain large azimuthal scatter suggesting lateral heterogeneity in the near source region. The shear phase SS is modeled using a Hilbert transform to mimic distortion incurred at an internal caustic in its propagation. Significant variation is found in SS travel time residuals for paths reflected under the Cana­dian shield. A correlation of the variation with tectonic sub-pro­vince is suggested. Differential travel times of multiple ScS deter­ mined by waveform cross-correlation are shown to contain a systematic bias late with respect to conventional visual onset timing methods. The timing bias for Scs₂-ScS differential times ranges between 2.2 and 3.8 seconds late, and depends upon the average Q_β of the mantle.

In Chapter III direct body waves and fundamental surface waves are calculated for a credible, hypothetical great earthquake on the San Andreas fault. Amplitudes and durations of long period ground motion (T > 1 second) are found for a receiver in downtown Los Angeles. Calculations are carried out for various epicenters, dislocation profiles, and time functions. Ground motion from Love radiation is found to be most important with peak-to-peak amplitudes up to 14 cm. and durations up to 5 minutes.

Chapter IV presents a study of short period P wave amplitudes from nuclear explosions in the Soviet Union recorded by WWSSN stations in the United States. Thirty-four events in five test sites are analyzed. A well-defined amplitude pattern is obtained for each source region. A pattern of lateral variation of amplitude in the United States is obtained for a northern azimuth of approach. Stations in the western United States do not show systematically lower amplitudes than eastern stations, in contrast to previous studies. A preliminary data set of earthquakes in the Kurile Islands and South America indicate the amplitude pattern in the U.S. varies azimuthally.

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