Experiments in Refining Seismic Magnitude Estimates for Seismic Events.
Abstract
Two experiments were conducted to investigate the possibility of reducing the observed scatter in seismic magnitude estimates for seismological events. The first experiment sought to remove the effect of dispersion on calculated seismic magnitude by empirically estimating a log (square root of (dU/dT)) term at each station for an event. Subtracting these terms from the seismic magnitude values did not produce any improvement in the scatter, and it is concluded that other terms predominate in causing the scatter. It was also noted that measured dU/dT was not stable for nearly identical source-receiver paths and therefore that this quantity cannot be reliably measured in a routine visual manner. The second experiment involved tracing surface-wave rays by laws of geometrical optics over the globe as represented by a grid of phase-velocity values for 20-second Rayleigh waves. The ray-tracing indicated large zones of intense focusing and defocusing, refracted paths, and multipath propagation to stations. The square root of intensity calculated by ray tracing correlated significantly in most cases with observed amplitudes corrected for other effects. However, the full benefit of ray tracing can only be realized with an improved velocity grid, an addition of a Q grid, and corrections for transmission losses at first-order velocity contrasts.
Document Details
- Document Type
- Technical Report
- Publication Date
- Nov 11, 1975
- Accession Number
- ADA032074
Entities
People
- D. H. Von Seggern
- D. W. Rivers
- P. A. Sobel