Abstract

Using data from 385 events recorded at Lanzhou in central China, I compare two empirical methods for predicting and correcting for path effects on regional-phase amplitude ratios. The first method interpolates source- and distance-corrected seismic data to create geographical, path-correction surfaces. The second, or wave-guide, method employs correlations with path-averaged physical data such as topography to correct the seismic data. Path corrections can vary over an order of magnitude, and their use will sharpen differences in regional-phase ratios arising from different types of natural and man-made sources of seismic energy and, thus, will be important for monitoring a Comprehensive Test Ban Treaty.

For Pg/Lg phase ratios in the band 0.5 to 1 Hz, interpolated path corrections reduced variance best using kriging (62%) and nearly as well using a radius 250 km, moving-window-mean smoother (56%). Wave-guide methods based on mean and rms topography reduced variance by 43%. Variance reduction is given relative to the standard correction for distance alone. Wave-guide methods were further tested by attempting to find correlations with topographic data that had been rotated geographically. For conservatively chosen sets of wave-guide parameters, the true orientations yielded the best variance reduction, adding confidence to the use of this method. Wave-guide methods complement interpolation methods because they extrapolate behavior beyond active seismic areas. However, I find broad regions that are fit poorly by wave-guide predictions, especially where the path effect changes rapidly, such as along the eastern edge of the Tibetan plateau where Pg/Lg ratios are underpredicted. The two methods can be combined using Bayesian kriging, forcing predictions to fit the data in seismically active areas and allowing more powerful extrapolation beyond.

First Page Preview

First page PDF preview
You do not currently have access to this article.