Accession Number:

ADP204445

Title:

Lg Site Amplification Calibration for Isolation of Lg Propagation Effects,

Descriptive Note:

Corporate Author:

CALIFORNIA UNIV SAN DIEGO LA JOLLA

Personal Author(s):

Report Date:

1995-08-14

Pagination or Media Count:

10.0

Abstract:

We observe geographically clustered variations in LgPg amplitude ratios of regional earthquakes and NTS events, using southern California seismic network SCSN recordings. These variations can occur abruptly over length scales on the order of ten kilometers and are the cause of significant errors in discrimination of regional earthquakes from nuclear explosions. Observations from events at different distances and azimuths suggest that variations are not due to distance effects, very near receiver or source effects, or source radiation, and so we conclude that the variations are largely due to the effects of propagation along the different source-receiver paths. Our goal is determine which measurable path parameters, if any, e.g. topographic roughness, waveguide thickness, waveguide thickness variation, attenuation correlate with variations in the crustal phases. To avoid ambiguity over which phase varies, Lg or Pg, we will estimate the Lg site amplification at each station using the diffuse near receiver scattered component of teleseismic coda. Near receiver scattered coda and Lg can both be described in terms of surface waves, and measurements using a temporary array indicate that for the same passbands, Lg and diffuse teleseismic coda have similar phase velocities. The array measurements also indicate that the incoming directions of arrival of energy in diffuse coda are fairly evenly distributed. By beamforming, using all SCSN records for a given event, we make an estimate of the coherent near source scattered component of teleseismic coda, which we remove from each individual coda record. We find, in agreement with previous researchers, that the deep teleseismic coda is almost entirely composed of near receiver scattered energy while 10 to 60 of the shallow teleseismic coda may be composed of near source scattered energy.

Subject Categories:

  • Government and Political Science
  • Seismology
  • Seismic Detection and Detectors
  • Nuclear Weapons

Distribution Statement:

APPROVED FOR PUBLIC RELEASE