Inversion of far-regional broadband P waves for the estimation of source parameters from shallow depth earthquakes

摘要

Attempts in separating the earthquake source effect from complex upper mantle wave propagation effects have been unsuccessful due to band-limited World-Wide Standardized Seismograph Network data and limitations in the computation of accurate seismograms. In this study, we examine the potential for estimating source parameters using broadband P waves observed between distances of 10° and 30°. We used the reflectivity method, which retains all of the rays, to compute synthetic Green's functions from the Earth-flattened global AK135 and regional T7 isotropic velocity models. We applied the moment tensor inversion and iterative dislocation grid search methods to estimate seismic moment, moment tensor, focal mechanism, and focal depth for three example seismic events, the 1995 (Mw 5.7) west Texas, 1999 (Mw 5.6) Scotty's Junction, Nevada, and 1999 southern Iran (Mw 4.8) earthquakes. The west Texas and Scotty's Junction earthquakes were well recorded, and they had independently established source parameters. Independent estimates from different data sets and agencies were used to validate the inversion results. We selected the southern Iran earthquake to show the utility of the method at lower magnitudes. Selected vertical and radial component seismograms were windowed about the P wave arrival, corrected for instrument response, and converted to velocity, preserving the absolute amplitudes and periods within approximately 0.5 to 100 s. Both inverse methods were successful in separating the source effect with the use of the AK135 velocity model. The inversion results using the T7 model appear slightly biased and had worse waveform fits compared to AK135. More importantly, the use of two different velocity models resulted in biases in focal depths that were less than ±2 km, thereby providing a robust inverse method for focal depth. Utilizing far-regional P waves would be most useful for remote earthquakes not adequately recorded locally and too small in magnitude to be recorded beyond epicentral distances of 30° for teleseismic analysis.