EES-11 - LANL

Using Shear Wave Travel Time Correction Surfaces to Improve Regional Seismic Location in China

LA-UR-01-151
M. L. Begnaud, A. A.Velasco, and L. K. Steck
Los Alamos Seismic Research Center, Los Alamos National Laboratory

Introduction

For low magnitude seismic events, the number of seismic phases detected by stations in a sparse network may be small, making accurate event locations very difficult. For these events, secondary arrivals can provide a key role in stabilizing location. Given the complexity of the velocity structure in China, it is difficult to utilize secondary phases without taking into account propagation characteristics. Thus, we will create 2-D shear phase travel-time correction surfaces in an attempt to improve earthquake locations and depths for an area encompassing a large portion of China (20-55N, 65-115E). P phase correction surfaces have already been successfully used for this region, improving regional locations using a sparse network. While shear arrivals are commonly identified in regional waveforms, they are not systematically defined for use in location determination. When S phases are used in regional locations without travel-time corrections, the locations show a bias, even when using P phase correction surfaces. By combining 2-D S phase correction surfaces with P phase correction surfaces, we will determine if improvements are made to regional location estimates.

Utilizing a combined data set from the ISC, USGS EDR, pIDC REB catalogs, and regional P and S phase picks from digital data, we will create these travel-time surfaces using a select group of constrained events (30 defining phases, azimuthal gap less than 180°). P and S phases will be selected that have absolute travel-time residuals less than 10 sec. Correction surfaces for up to 76 stations around China will be created from travel-time residuals binned on a tesselation grid and using modified Bayesian kriging. We will test the effect of these surfaces on earthquake locations constrained using interferomeric synthetic aperture radar to determine if regional location biased is removed.

Study Region

Figure 1 - Western China study region. Stations with P phase Propagation Path Corrections (PPCs) are displayed as labeled red triangles [Steck et al., 2000]. Blue squares are nuclear test sites. Red box is the study region. Blue box is area of ground truth testing for the S phase PPCs.

Please direct comments and questions to the principal author.