Accession Number : ADP204487

Title :   Focal Mechanism Determination and Propagation Characteristics of High-frequency S-waves on the Tibetan Plateau,

Corporate Author : CALIFORNIA INST OF TECH PASADENA

Personal Author(s) : Zhu, L. ; Helmberger, D. V.

PDF Url : ADP204487

Report Date : 14 AUG 1995

Pagination or Media Count : 9

Abstract : One of the most useful regional discriminants is the ratio of short-period S/P energy, on the basis that nuclear explosions tend to have less S energy than earthquakes at high frequencies. However, the high-frequency S waves from earthquakes can also be easily blocked or attenuated during the propagation, especially along the boundaries between different tectonic regions and within tectonic active areas. So, an essential understanding of the regional crustal structures and associated wave propagation characteristics are needed before establishing effective discrimination methods for that area. In this report, we first determined the crustal structure of the Tibetan Plateau and the focal mechanisms of regional events recorded during the 1991-1992 Tibet passive recording experiment. In particular, we improve the source mechanism estimation technique proposed by Zhao and Helmberger (1994) by introducing a distance range scaling factor. This helps to constrain the source orientation by fully utilizing the radiation patterns contained in the different portions of the whole 3-component records. By apply the method to the regional events of the Tibet Plateau, we obtalned the focal depths and mechanisms. Most events are found to be shallower than 20 km. However, we also identified 3 subcrustal events (at depth of 70-80 km) under the Himalayan Trust Belt and the Indus Zangbo suture zone. Their mechanisms are quite different from those of shallow events in the same region. We find that the radiation pattern of P and S behave quite well as a function of frequency below 1 Hz. At frequencies above 3 Hz, the amplitudes vary strongly from station to station. High attenuation of short-period S waves is found for ray paths through the north-central plateau. This is consistent with previously observed inefficient propagation of high frequency Sn in this area.

Descriptors :   *SEISMIC DETECTION, *EARTH CRUST, *SEISMIC WAVES, *ARMS CONTROL, *NUCLEAR EXPLOSION DETECTION, *SEISMIC DISCRIMINATION, *EARTHQUAKES, *SECONDARY WAVES, SYMPOSIA, VERIFICATION, MONITORING, UNDERGROUND EXPLOSIONS, WAVE PROPAGATION, TREATIES, CHINA, RADIATION PATTERNS, TECTONICS.

Subject Categories : Government and Political Science
      Geology, Geochemistry and Mineralogy
      Seismology
      Seismic Detection and Detectors
      Nuclear Weapons

Distribution Statement : APPROVED FOR PUBLIC RELEASE