HR: 14:35h
AN: S53D-04 INVITED    [Abstracts]
TI: Regional Seismic Depth-Phase Identification: Strengths and Weaknesses of Cepstral Signal Processing Techniques
AU: Reiter, D
EM: delaine@westongeophysical.com
AF: Weston Geophysical Corp., 181 Bedford St., Ste 1, Lexington, MA 02420, United States
AU: * Stroujkova, A
EM: ana@westongeophysical.com
AF: Weston Geophysical Corp., 181 Bedford St., Ste 1, Lexington, MA 02420, United States
AB: One of the primary discriminants between man-made versus natural seismicity is the depth of an event. The identification of seismic depth phases (surface reflections) is the most straightforward way to establish event focal depth. However, at regional distances the accurate detection and identification of seismic depth phases such as pPn and sPn remains an elusive goal. In this distance range the first arriving phase (Pn) is commonly believed to be a "whispering gallery mode" produced by multiple reflections from velocity gradients at the bottom of the Moho. These multiple reflections and other crustal reflections and reverberations, arriving soon after the direct wave, create a complex coda. The depth phases may be buried within this coda, but are often difficult to differentiate from other wave phenomena. We have extensively tested an array-based signal processing technique known as the Cepstral F-Statistic Method to detect regional depth phases in seismic coda. Cepstral methods exploit the periodicity that occurs in the power spectrum when echoes are present between primary and coda arrivals, such as those expected between the direct (P) and depth (pP and/or sP) phases. However, the original cepstral method does not distinguish between peaks (i.e. detections) due to depth phases and those due to other crustal and scattering wave phenomena. To address this weakness we assign significance to cepstral peaks through the calculation of an F statistic. In addition we estimate the slowness and back azimuth of the phases corresponding to peaks in the cepstral F statistic to help assess their potential as depth phases. Even with these and other improvements, the application of cepstral methods to data at regional distances from small-to-intermediate-sized events has produced inconsistent results, resulting in a high number of false detections. In this work we present a case study of depth-phase identification in the region surrounding the Korean Peninsula. This region represents the ideal proving ground for cepstral techniques at regional distances, because of its relatively simple tectonic structure, importance in nuclear monitoring, and availability of high quality short-period array data (KSAR/KSRS seismic array, Wonju, South Korea). Our results indicate that cepstral-based identification of the depth phases is problematic in more than 50% of the cases. However, we have found that while cepstral methods cannot independently and accurately identify regional depth phases, they can be effectively combined with other methods to produce good regional depth estimates.
DE: 7203 Body waves
DE: 7219 Seismic monitoring and test-ban treaty verification
SC: Seismology [S]
MN: 2007 Fall Meeting