Observation of Local Large Earthquakes by Broadband Seismographs: Implications for the Initial Rupture Mechanism of the 1989 November 1 Sanriku-Oki Earthquake.

During the earthquake swarm-like activity of Sanriku-Oki (offshore Iwate Prefecture, Japan) from October to November 1989, a relatively large earthquake (M 7.1 JMA) occurred on November 1. Streckeisen broadband seismometers installed in the Tsukuba Seismological Observatory in September 1989 successfully recorded the entire sequence of this activity, including the November 1 event. Due to the broadband characteristics of the seismometer and the high dynamic range 24-bit A/D converter, the recorded seismograms show various interesting features that might provide useful implications for both source and structure, even if seismograms from only one station are available. In the present paper we compare the broadband seismograms of the November 1 event recorded at Tsukuba to those of the second-largest event, which occurred on October 29 (M 6.5 JMA). We show that the first several seconds of both seismograms coincide with each other. This might imply that the November 1 event started as a much smaller event, similar to the October 29 event, but developed into a larger one.