An automatic monitoring system for nonvolcanic tremors in southwest Japan

The recent development of a system for exchanging and distributing seismic waveform data over high-speed networks enables seismic events to be monitored in real time throughout Japan. In the present study, we have developed an automatic real-time monitoring system for deep nonvolcanic tremors in southwest Japan. The system automatically detects the occurrence of nonvolcanic tremors and determines their hypocenters in real time. In addition, the system creates image files for the detected tremor activities and makes them accessible via the World Wide Web. To detect tremors we carry out a two-step numerical hypothesis test, in which the first step is to test whether two given envelope seismograms are correlated and the second step is to test whether an event occurs using the results of the first test. This two-step test is applied to real-time data every 2 min. Once an event is detected, we can regard time lags that generate maximum cross correlations as the travel time differences, which we use for hypocenter determination. Although this procedure detects nontremor signals, most of them can be rejected using several criteria. Since the start of monitoring in 2006, the system has worked well for detecting a wide variety of tremor activities. Results from the present system will contribute to understanding the stress relaxation process in the transition zone between the locked and stable slip zones of the subducting Philippine Sea plate interface.