We analyze broadband seismograms for magnitude 5 to 7 earthquakes which occurred in and around Japan to develop a technique to determine moment magnitude from first-arrival P waveforms. We theoretically consider displacements of the P-wave portion on broadband seismograph record as an approximate source time function and integrate them to obtain scalar moments at each station. We show that a combination of multiple records will give a good estimation of moment magnitude without correcting for the source mechanism. This technique is quite simple and robust, so it will be effective for a rapid evaluation of the tsunami potential of a large earthquake.

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