abstract

The propagation of seismic phases Pn, Pg, Sn, and Lg from earthquake and explosion sources, as recorded at regional distances, is studied by using short-period records from the WWSSN stations. The time-distance plots from explosions, shallow earthquakes, and intermediate depth-of-focus earthquakes show that the depth or nature of the seismic source has no significant effect on the velocity of propagation of any of the regional phases. The phases Pn, Sn, and Lg travel with velocities about the same as in the Eastern United States but Pg has a velocity considerably lower than that reported for the Western or the Eastern United States. Propagation paths of Lg and Sn from earthquakes in Western Russia, as recorded on the three-component, short-period records from Kabul (KBL) and Meshed (MSH), have been mapped. Paths within the region approximately bounded by azimuths N30°W to N60°E of KBL and northeast to east of MSH have been found to be consistently efficient for Lg and mostly efficient for Sn. This efficiency of propagation appears to be due to relatively thick crust and a relatively flat moho rather than the absence of large topographic features or geological faults. Data from the short-period vertical component records indicate that for transmission paths lying within the region of efficient propagation of Lg, the maximum Lg amplitude is generally large than the maximum P-wave amplitude for earthquakes but generally smaller than that for explosions. This difference in the level of Lg excitation appears to be a useful discriminant for events in Western Russia.

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