Slowness corrections - One way to improve IDC products

The first step to identify and locate a seismic event is the association of observed onsets with common seismic sources. This is especially important in the context of monitoring the Comprehensive Nuclear-Test-Ban Treaty (CTBT) at the International Data Center (IDC) being developed in Vienna. Austria. Well-defined slowness measurements are very useful for associating seismic phases to presumed seismic events. Shortly after installation of the first seismic arrays, systematic discrepancies between measured and theoretically predicted slowness values were observed, and therefore slowness measurements of seismic stations should be calibrated. The observed slownesses measured with small aperture arrays, some of which will be included in the International Monitoring System (IMS) now being implemented for verifying compliance with the CTBT, show large scatter;nd deviations from theoretically expected values. However, in this study a method is presented, by which mean slowness corrections can be derived, which show relatively stable patterns specific to each array. The correction of measured slowness values of these arrays clearly improved the single array location capabilities. Applying slowness corrections witt seismic phases observed by ARCES, FINES, GERES, and NORES, and associated to seismic events in the bulletins of the prototype International Data Center (pIDC) in Arlington, VG, also clearly demonstrates the advantages of these corrections. For arrays with large slowness deviations that are due to the influence of a dipping layer, the corrections were modeled with a sine function depending on the measured azimuth. In addition. the measured values can be weighted with the corresponding uncertainties known from the process of deriving the mean corrections.