Development of Regional Discriminants.

Abstract

Abstract : With the installation of broadband, high dynamic range instruments, it has become possible to compare the regional waveforms of earthquakes and explosions at magnitudes 3 to 6. These waveforms are similar for event sequences in many situations and can be inverted for source mechanisms. We find that flat-layered models are sufficient for inverting seismograms at periods greater than a few seconds. This paper presents three studies aimed at determining crustal models, source finiteness and modeling complex structure near a receiver. We have conducted a set of sensitivity tests on the parameters of 1-D models to compare their impact on different segments of regional seismograms. We found that P sub nl waves (extended P-waves) are controlled in broadband character by the mid-crust while the top layer contributes to the long-period motions. The SV wave is mostly controlled by the shear wave velocity of the lower crust, especially the crustal layer just below the source depth. The top crustal layer controls the shape of the surface waves at ranges from 300 to 600 km, and the upper crust, especially the crustal layer just above the source depth, controls their timing. Applying these tests in modeling three earthquakes in the Basin-and-Range province, we found that a simple two-layer crustal model could effectively explain the data both in timing and in shape. The main crustal layer has P and S velocities of 6.1 km/sec and 3.6 km/sec, similar to those found by Langston and Helmberger (1974). A surface layer of thickness 2.5 to 3.5 km is required to fit the Rayleigh waves. Fast estimation of point-source parameters for earthquakes has witnessed much progress in recent years due to the development of broadband seismic networks.