Asymmetrical seismic pulses before a large earthquake

This study is concerned with the behavior of seismic noise in the period range of a few minutes based on records made by broadband IRIS stations before the catastrophic earthquake of March 11, 2011 in Japan. A formalized technique was used to identify discrete asymmetrical pulses whose amplitudes were an order of magnitude higher than the noise level and that were spaced at intervals longer than 30 minutes. The frequency of pulses at the MAJO station on Honshu 400 km of the earthquake epicenter increased on January 30, 2011 and remained high until March 2, 2011. No such phenomena have been observed at MAJO in the same periods during the preceding 15 years, when the station was in regular operation. The records of other similar stations farther than 1700 km from the epicenter were not found to contain increases in the rate of pulses. We believe that these asymmetrical pulses were due to dilatancy in shallow crustal layers beneath the MAJO station.