Automatic Determination of Secondary Seismic Phase Arrival Times Using Wavelet Transforms

Abstract

For regional nuclear test monitoring applications, determining accurate locations for small-to-intermediate events (2.5 m b P -wave observations will be available for hypocenter location, secondary seismic phases, such as Lg and Pg, must be considered to improve location accuracy to the level required for nuclear test monitoring. For these events, single array or single station locations may be used to determine an epicenter for an event, based on estimates of back azimuth and epicentral distance. Because Lg propagates at a consistent group velocity of approximately 3.5±0.2 km/s, the distance estimates using the travel-time difference between this arrival and primary phases ( e.g., Pn, Pg ) are often quite reliable. However, Lg is a very complicated phase and identifying its onset time on seismograms remains one of the most difficult problems encountered in regional seismology. We have tested an improved version of the Lg wavelet phase-picker (Wavelet 1.0), based on a method developed by Tibuleac and Herrin (1999, 2001), and two other phase-picking methods: CUMSQ (Inclan and Tiao, 1994; Der and Shumway, 1999) and AR (Taylor et al., 1992). We tested the three methods on Lg and Pg (as secondary arrivals) from a data set of 97 shallow ( m b