Use of GSETT-3 gamma data in the Slowness-Azimuth Calibration of IMS primary arrays at regional distances

For the purpose of verifying compliance with the CTBT seismic monitoring is one of the four techniques used by the IDC. In order to improve the accuracy of the automatic and the reviewed bulletin epicenter locations the IDC uses SASC for the IMS seismic stations. SASC determination is a straightforward calculation done by comparing for selected events the azimuth and slowness from the waveform processing using array techniques to the theoretical values based on the event locations and the velocity model.The main problem, however, is to build a set of reference events, whose locations are accurate enough and not based on information from the stations to be calibrated. A reference event list assumed to meet this requirement is the Gamma bulletin, which was collected since 1993 and was compiled in the framework of the GSETT-3. In this work calculation of SASC for regional to teleseismic distances (up to 30 degrees) was performed for 11 IMS primary arrays. The calculation was done using Pg, Pn, P, Sg, Sn, and S phases based on the detection list obtained from the pIDC and the Gamma bulletin for 6 years (1993–1999). The number of Gamma events varies from several hundreds for some arrays (BRAR) to several tens of thousands for others (i.e. ARCES, ILAR). Due to the fact that the Gamma bulletin is purely voluntary, the coverage is non-uniform both in time and in space and the location accuracy is non-uniform. This drawback can be overcome by encouraging signatory states to submit quality Gamma bulletin data to the IDC. The work presented here can be used as a routine procedure for improving IMS array performance, especially at regional distances.