Reference Events for Regional Seismic Phases at IMS Stations in China

Seismic-event location within the context of monitoring the Comprehensive Nuclear-Test-Ban Treaty entails a priori knowledge of the travel time of seismic phases for a given source to stations of the International Monitoring System (IMS). Such travel-time information (or ground truth, GT) is provided empirically by seismic reference events, events that have well-determined hypocenter locations (epicenters typically known to ±5 km with high confidence) and origin times. In this study we present new reference events for the calibration of six seismic stations of the IMS in China, a region with high seismic activity. We use the Annual Bulletin of Chinese Earthquakes, which lists about 1000 earthquakes in and near China each year with consistent phase picks at regional stations, to determine precise relative earthquake locations from double-difference cluster analysis. The resulting high-resolution image of active faulting at seismogenic depths in areas of dense seismicity is correlated with the tectonic structure derived from mapped fault information at the surface to validate the absolute locations. We generated 59 reference events with M>=3.5, distributed in six clusters in central and eastern China, and recorded by at least one of the six IMS stations. The scatter in relative travel-time residuals is reduced from 1.28 sec before to 0.61 sec after relocation, consistent with the relocated positions of the events. The degree of correlation between seismicity structure and well-characterized fault data indicates that, in four clusters, the locations of the new reference events are accurate to within 5 km (GT5), and in two clusters within 10 km (GT10).