Earth Planets Space, Vol. 58 (No. 11), pp. e49-e52, 2006E-LETTER
Institute of Seismology and Volcanology, Hokkaido University, North 10 West 8, Sapporo 060-0810, Japan
(Received July 21, 2006; Revised September 8, 2006; Accepted September 14, 2006; Online published November 15, 2006)
The frequency-magnitude distribution (b-value) for seismicity within the Pacific plate is not definitely homogeneous. An anomaly of b-value higher than 0.9 is detected within the descending Pacific plate in the Hokkaido corner. In the western Hokkaido, the anomaly exists at a depth of about 150 km, which is directly beneath active volcanoes. In the eastern Hokkaido, the anomalies exist at approximately 200 and 300 km depths, which is tens of kilometers sideways from active volcanoes. The results obtained in this paper are more reliable than previous studies on b-value heterogeneity within descending plates, since I used a catalog homogeneous, both in time and space, which was obtained through re-examination of all seismograms and relocation of hypocentral parameters based on arrival times from fixed seismic stations.
Key words: b-value, Pacific plate, deep seismic zone, Hokkaido, volcanoes.