Interpretation of high-frequency coda at large distances: stochastic modelling and method of inversion

Abstract

Based on previous work, a stochastic convolution model is proposed for the Lg coda observed at large epicentral distances (A > 200 km). Coda at these distances differ from local coda in several respects. Effects of various physical processes, such as dispersion, scattering, and mode conversion on coda at large distances are discussed in detail. A spectral ratio method is developed for inversion of Lg coda Q, in which the large variance associated with inversion is greatly reduced. The method is also extended to the inversion of seismic source parameters by jointly using Lg and its coda. We have tested the model and method exhaustively using data from two GDSN stations and found the results to be superior to other methods when those methods used only a small number of stations.

Yield and Discrimination Studies in Stable Continental Regions

“…Observations indicate that Q for these waves varies regionally, being relatively high in old stable regions and relatively low in younger, tectonically active regions (Nuttli, 1973;Singh and Herrmann, 1983;Cong and Mitchell, 1988). Q values for both Lg (QLg) and its coda (Q) at frequencies near 1 Hz exhibit similar values, being within 10-15% of one another for all regions in the world where both have been determined (Nuttli, 1988). The regional variations of QLg and Qc occur in patterns which vary in the same way as regional variations in crustal shear wave Q (Qp) in the upper crust obtained from surface wave attenuation studies (Mitchell, 1975;.…”

“…The regional variations of QLg and Qc occur in patterns which vary in the same way as regional variations in crustal shear wave Q (Qp) in the upper crust obtained from surface wave attenuation studies (Mitchell, 1975;. The frequency dependences of QLg and Qc also vary regionally, but in the opposite sense as the Q values; they are lower in stable regions and higher in tectonically active regions (Nuttli, 1988).…”

“…In the present study, synthetic seismograms for the Lg phase were computed at several distances for velocity and Qp models for the crust and upper mantle of the Basin-and-Range and the attenuation of Lg was determined using the stacked spectral ratio (SSR) method of Xie and Nuttli (1988); predicted Lg Q values were thus determined using the same method that was used to obtain observed Q values. The SSR method was applied to synthetic seismograms of Lg computed by mode summation (e.g., .…”

“…The seismograms were computed for along an azimuth of 450 from a vertical strike-slip fault at a depth of 5 km and oriented in a north-south direction. Seismograms were computed at 20 distances and stacked to obtain stacked spectral ratios using the method of Xie and Nuttli (1988). The time scale is reduced at each distance R using a reduction velocity of 5.0 km/s.…”

“…Their determinations utilized a stacked spectral ratio (SSR) method (Xie and Nuttli, 1988) which provides stable determinations of Q with variances which are much smaller than those associated with previously used methods. Singh and…”