Body wave synthesis for shallow earthquake sources : inversion for source and earth structure parameters

Citation

Langston, Charles Adam (1976) Body wave synthesis for shallow earthquake sources : inversion for source and earth structure parameters. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/Z3ZM-AC93. https://resolver.caltech.edu/CaltechETD:etd-09142006-142025

Abstract

Expressions for displacements on the surface of a layered half space due to an arbitrary oriented shear dislocation are given in terms of generalized ray expansions. Useful approximations of these expressions for shallow events as recorded at teleseismic distances for realistic earth models are presented. The results of this procedure are used to generate synthetic P, SV and SH waveforms for various assumptions of stress drop. The Thomson-Haskell layer matrix method for computing far-field body wave displacements from shear dislocations is also formulated to complement the ray theory methods when complicated earth structures are considered. An iterative generalized inverse technique is developed using analytic partial derivatives for estimating source parameters from data sets of P and S seismograms from shallow earthquakes.

With the inverse technique and ray theory methods long period P and SH waveforms are analyzed from the Koyna, India, earthquake of 10 December 1967. Using published crustal models of the Koyna region and primarily by modelling the crustal phases P, pP, and sP, the first 25 seconds of the long-period waveforms are synthesized for 17 stations and a focal mechanism obtained for the Koyna earthquake which is significantly different from previous mechanisms. The fault orientation is 67° dip to the east, -29° rake plunging to the northeast, and N16°E strike, all angles ± 6°. This is an eastward dipping, left-lateral oblique-slip fault which agrees favorably with the trend of fissures in the meizoseismal area. The source time duration is estimated to be 6.5 ± 1 5 sec, from a triangular time pulse which has a rise time of 2.5 sec and a tail-off of 3.9 sec, source depth of 4.5 ± 1.5 km and seismic moment of 3.2 ± 1.4 x 10(25) dyne-cm. Some short period complexity in the time function is indicated by modelling short-period WWSSN records but is complicated by crustal phases. The long-period P waveforms exhibit complicated behavior due to intense crustal phase interference caused by the shallow source depth and radiation pattern effects. These structure effects can explain much of the apparent multiplicity of the Koyna source. An interpretation of the Koyna dam accelerograms has yielded an S-P time. Taken together with the I. M. D. epicenter and present depth determination, it places the epicenter directly on the meizoseismal area.

Simultaneous modelling of source parameters and local layered earth structure for the April 29, 1965, Puget Sound earthquake was done using both the ray and layer matrix formulations. The source parameters obtained are: dip 70° to the east, strike 344° rake -75°, 63 km depth, average moment of 1.4 ± 0.6 x 10(26) dyne-cm and a triangular time function with a rise time of 0.5 sec and fall-off of 2.5 sec. An upper mantle and crustal model for southern Puget Sound was determined from inferred reflections from interfaces above the source. The main features of the model include a distinct 15 km thick low velocity zone with a 2.5 km/sec P wave velocity contrast lower boundary situated at approximately 56 km depth. The crustal model is less than 15 km thick with a substantial sediment section near the surface. A stacking technique using the instantaneous amplitude of the analytic signal is developed for interpreting short period teleseismic observations. The inferred reflection from the base of the low velocity zone is recovered from short period P and S waves. An apparent attenuation is also observed for pP from comparisons between the short and long period data sets. This correlates with the local surface structure of Puget Sound and yields an effective Q of approximately 65 for the crust and upper mantle.

To substantiate the unusual structure found from the Puget Sound waveform study the structure under Corvallis, Oregon, was examined using long period Ps and Sp conversions and P reverberations from teleseismic events as recorded at the WWSSN station COR. By modelling these phases in the time domain using a data set composed of six deep and intermediate depth earthquakes a similar low velocity zone structure is again inferred. The lower boundary occurs at 45 km depth and has S and P velocity contrasts of 1.3 and 1.4 km/sec, respectively. The material comprising the low velocity zone has a Poisson ratio of at least 0.33 and is constrained by the average P and S travel times determined from the converted phases.

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