The discrimination filter to avoid EEW alerting triggered by teleseismic earthquakes using geometric distribution of triggered stations.

Abstract:

In operation of Earthquake Early Warning System(EEWS), the alerting criteria is one of the most important parameters in optimizing acceptable warning system. During early stage of testing EEW systems from 2011 to 2013, we applied a very simple criteria for event alerting with the combination of the numbers of station and magnitude. As a result of the testing we found out that the real-time test result of Earthquake Early Warning (EEW) system in Korea showed that most of false alarms with big magnitude over 5.0 were related to the teleseismic events. This teleseismic-relevant false alarm is caused by logical co-relation during association procedure and the corresponding geometric distribution of associated stations is crescent-shaped. Seismic stations are not deployed uniformly, so the expected bias ratio varies with evaluated epicentral location. This ratio is calculated in advance and stored into database, called as TrigDB, for the discrimination of teleseismic-origin false alarm. We tested the change of the geometrical distribution density and directivity of triggered stations in the two ways based on the nearest station or the grid point to the epicenter. Considering together with conventional spatial density of triggered station, TrigDB works as a very efficient filter to quickly reject teleseismic-relevant false alarms except some moderate or big regional distance earthquakes outside of seismic network.