Earth Planets Space, Vol. 63 (No. 7), pp. 709-712, 2011
Institute of Seismology and Volcanology, Hokkaido University, Sapporo 060-0810, Japan
(Received April 7, 2011; Revised June 15, 2011; Accepted June 22, 2011; Online published September 27, 2011)
I find that a long-term seismic quiescence started 23.4 years before the 2011 off the Pacific coast of Tohoku Earthquake (M = 9.0). An earthquake catalog compiled by the Japan Meteorological Agency (JMA) is analyzed. The catalog includes 5770 earthquakes shallower than 60 km with M ≥ 4.5. A detailed analysis of the earthquake catalog between 1965 and 2010 using the gridding technique ZMAP shows that the 2011 Tohoku earthquake is preceded by a seismic quiescence anomaly that began in November 1987. The quiescence-anomaly area is located around the deeper edge of the asperity ruptured by the main shock, and the Z-value is +4.9 for a time window of Tw = 15 years, using a sample size of N = 150 earthquakes. It is suggested that a seismic quiescence which starts more than 20 years before the main shock is common to giant earthquakes (M ∼ 9.0) in subduction zones.
Key words: Seismic quiescence, the 2011 Tohoku earthquake, JMA earthquake catalog, ZMAP, Z-value.