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Constraining Event Depths and Crustal Velocities Using Regional Depth Phases

Weston Geophysical Corporation, 181 Bedford Street, Suite 1, Lexington, Massachusetts 02420 ana_s{at}juno.com

A method for determining average crustal seismic velocities above a cluster of shallow seismic events using depth phases observed at regional and teleseismic arrays has been developed. The method uses travel-time differences between the direct and the surface-reflected phases to simultaneously find the event depths and the best-fitting seismic velocities in the hypocentral area. Simplicity of the method makes it suitable for rapid data analysis as it requires a relatively small amount of data and may provide useful information about the seismic velocities in remote regions with shallow seismicity. However, the method requires accurate depth phase identification, which is often a difficult task.

The technique has been applied to find above-source seismic velocities and depths of 14 clustered events from the Xiuyan (1999–2000) sequence in China. The estimated depths of the events are between 5 and 8 km. The obtained velocities are and , which corresponds to a slightly elevated V_P/V_S ratio of 1.86±0.12 as compared to the value predicted from IASP91 (V_P/V_S=1.72; see Kennett and Engdahl, 1991).