A REVISIT TO P-WAVE TRAVEL-TIME STATICS AT TELESEISMIC STATIONS

To account for local deviations from the one-dimensional reference velocity model, the station correction formulation of Dziewonski and Anderson (1983) is extended to include the travel time variation with respect to the ray takeoff angle. The station statics are obtained through an iterative retrieval of the lower-degree spherical harmonics normalized over the data takeoff angle range on station residual spheres, and similar event statics are introduced to decouple the effects of near-station and near-event heterogeneities. Except the zero degree constant term, all higher-degree static terms are obtained on a regional rather than an individual station/event location. The P wave static corections up to degree and order 2 are derived for 1343 stations using nearly 3 million International Seismological Center travel time residuals with respect to the iasp91 model and with event relocation and summary say sorting. The resulting station statics account for nearly 27% of the variance of the P wave travel time residuals. Patterns of the zero-degree statics correlate well with surface tectonic features and with previous results. At most stations over subduction zones the slowest direction of the second-order statics, or the second-degree in azimuth, is perpendicular to the trench.