A Real-Time Detection and Location of Aftershocks of the 1984 Western Nagano Prefecture Earthquake by using Personal Computer

Abstract

A real-time system of automatic detection and location of seismic events has been developed by using a personal computer. Since speed of computation by a personal computer is low, a simple digital band-pass filter has been developed for the real-time system. The band-pass filter needs only several times of addition and subtraction to get an output. Event dection is based on a ratio of short to long term average of outputs of the filter whose cutoff frequencies are set to decrease amplitude of long period noise owing to microtremor and amplitude of short period noise owing to culture. Arrival times of the P and S waves are determined by applying Akaike Information Criterion (AIC) to outputs of the band-pass filter with narrow band whose central frequency is set to be a value of predominant frequency of the seismic signal.
A temporary seismic observation with 8 stations for the aftershocks of the 1984 Western Nagano Prefecture Earthquake has been made by the use of radio and telephone telemetries. The real-time system of the automatic location of the seismic events was tested to demonstrate that hypocenter distribution obtained by the real-time system is nearly consistent with that determined from arrival time data which were read manually. It is shown that hypocenters of 60% among triggered events can be determined by this real-time system.