A METHOD TO ESTIMATE MAGNITUDE OF EARTHQUAKES FROM ENVELOPE OF INITIAL SEGMENT OF SEISMOGRAM AFTER P PHASE ARRIVALS FOR EARTHQUAKE EARLY WARNING SYSTEMS

Abstract

We developed, and sample tested a method to estimate magnitude of an earthquake from initial P wave arrivals with a 4 sec window. We determined the area under the envelope of the seismogram for a 4 second window after P arrivals for Kik-net strong motion records of 119 earthquakes in Japan with Moment magnitude (Mw) ranging from 4 to 7.5. Processing steps of detrending and bandpass filtering was carried out. Bandpass was applied for removal of noises and improving signal quality and P-phase detection. Detrending or baseline correction was carried out using a 3rd degree polynomial. The envelope was calculated using the filtered data and absolute values and passing through a Butterworth lowpass filter with cutoff 25 Hz. Scatter was large for single stations and hence we are using average of nearby stations rather than a single station. Average of nearest stations of range 50km was taken and linear relations were obtained. The results show a good correlation to area under envelope and earthquake magnitude. The equation was tested on a separate earthquake set and good correlation was observed between predicted and actual magnitude. This method is suitable for dense networks such as the Japanese Kyoshin Kiban network. Advantage of this method is that it doesn’t saturate for larger magnitude since it is directly based on ground motion.