Broad-Band Modelling of Regional Seismograms: the Basin and Range Crustal Structure

Abstract

S U M M A R YThree-component broad-band displacement seismograms with paths sampling the Basin and Range province are studied to constrain the crustal structure. To find an average model that fits the data in both absolute time and waveform, we generate broad-band reflectivity synthetics and conduct sensitivity tests on different parts of a layered crustal model, where only a few layers are involved. Generalized rays are used to help identify the various phases. It proves useful to decompose a regional seismogram into segments so that the impact of model parameters on each segment can be clearly identified. Thus, for mid-crustal earthquakes, it is established that the top crustal layer controls the Rayleigh wave, the Airy phase, in shape over the range from 300 to 600 km, and the crustal layer just above the source depth controls its timing. The P,,, waves, the P , and P,, portion, are controlled in broad-band character by the mid-crust while the top layer contributes to their long-period motion. These crustal parameters control the tangential motion similarly. The SV wave, the segment between the P,, wave and the Rayleigh wave, is mostly controlled by the shear velocity of the lower crust. In judging the goodness of fit between the array observations and synthetic waveforms, we allow individual data segments to shift relative to the I-D synthetics by a few seconds to account for some lateral variation. The amount of time shift is found by the cross-correlation in displacement between the data segment and the synthetics. Applying these tests in a forward modelling approach, we find that a simple two-layer crustal model is effective in explaining this data set. In this model, the main crustal layer has P and S velocities of 6.1 km s-' and 3.6 km sC1, similar to those found by Langston & Helmberger (1974). A surface layer of thickness 2.5 to 3.5 km is required to fit the Rayleigh waves. The refined model can be used as a reference model for further studies in this region.

Surface Wave Path Corrections and Source Inversions in Southern California

“…There is also a tendency for slow velocities along fault zones. In general, the surface geology is apparently affecting the Rayleigh wave velocities more, as suggested in the sensitivity studies of Song et al (1996). We view these maps as a useful product of regional calibration to be used in conjunction with the reference SoCal model.…”

“…The early experiments with 1D modeling by Song et al (1996) indicate that the waveforms of both Rayleigh and Love waves are almost the same except for an overall time shift with small velocity changes in the source layer. Here we will demonstrate this feature with a 2D test.…”

Earthquake Centroid Locations Using Calibration from Ambient Seismic Noise

“…Here, we are interested in shorter paths where surface waves are less dispersive and dominated by the airy phase. Its overall period is controlled by the surface layering and its timing by the S velocity in the source layer, (Song et al, 1996). Thus, they become useful for locating events at local and near-regional distance.…”

Source Characterization of the 6 August 2007 Crandall Canyon Mine Seismic Event in Central Utah

“…Such a mechanism could be explained by an asymmetric collapse of the mine cavity due to unevenly distributed in-situ stresses, sympathetic shear on a roof fault, or between the roof and floor of the mine, and warrants further investigation. (Song et al, 1996) . Radiation patterns of potential mechanisms.…”

“…The Green's functions for a one-dimensional (1-D) velocity model of eastern California and western Nevada (Table 1; Song et al, 1996) are calculated as synthetic displacement seismograms using a frequency-wavenumber integration method (Saikia, 1994). The synthetic data is filtered with a 4-pole acausal Butterworth filter with a low-corner of 0.02 Hz and a high-corner of 0.1 Hz (10-50s period).…”