HR: 0830h
AN: S21D-0340    [PDF]
TI: How to pick depth phases
AU: * Hsu, V
EM: vindell@aftac.gov
AF: Air Force Technical Applications Center, 1030 S. Highway A1A, Patrick AFB, FL 32925
AB: Determining accurate hypocenter depth is scientifically challenging and is important because it is a useful discriminant between nuclear explosions and earthquakes. Shallow events (depths $<$ 50 km or so) are especially difficult to determine with accuracy if only teleseismic P arrivals are used. Depth phases, pP and sP can help greatly in that aspect. The effect of using depth phases is like adding a station directly above the hypocenter. Depth phases, however, are used infrequently by institutes monitoring global seismicity such as USGS, IDC, and USNDC. The problem may be due to a lack of clear guidance for depth phase picking for the seismic analysts. The purpose of this paper is to review theoretical properties of depth phases and illustrate with examples how to pick depth phases. With Gaussian beam modeling, it is found that the focal mechanism orientation plays a major role in deciding if a station is going to see depth phases or not. Based on the relative location (azimuth and distance) of the stations to the source and focal mechanism orientation, a station may see just P, or just P and pP, or P, pP, and sP, or just P and sP. Without the focal mechanism being available at the time of picking for location, the identification of depth phases becomes tricky. Many researchers have tried special data processing, such as cepstral analysis, on single station data to identify depth phases. Our study shows that depth phases are best identified with data from an entire network data under scrutiny. The paper will also address issues of double explosions designed to mimic depth phases and multiple earthquakes separated by a very short time interval.
DE: 7215 Earthquake parameters
DE: 7219 Nuclear explosion seismology
SC: Seismology [S]
MN: 2003 Fall Meeting