Earth Planets Space, Vol. 57 (No. 5), pp. 403-409, 2005
Anshu Jin and Keiiti Aki
Association for the Development of Earthquake Prediction and National Research Institute for Earth Sciences and Disaster Prevention
(Received January 18, 2005; Revised March 16, 2005; Accepted March 18, 2005)
Using seismograms recorded at 582 Hi-net stations for earthquakes located within 30 km from each station, we measured coda Q for frequency bands of 1-2, 2-4, 4-8, 8-16, and 16-32 Hz, respectively. Then coda Q maps are constructed with average station spacing of 20 km over Japan, except the Hokkaido Island. The most striking feature of the obtained maps is the significant spatial variation within Japan, up to a factor of 3 for the lower frequency bands, as well as its strong frequency dependence. Such high spatial resolution was not possible to achieve without the high density and sensitivity of the Hi-net. We found several low coda Q regions for frequency band of 1-2 Hz including the southwestern Shikoku, eastern Shimane-western Tottori along the Japan Sea coast and the disjointed spots along the Pacific coast from the Kanto-Tokai region to southern edge of the Kii-peninsula. However, the most conspicuous low coda Q zone is a narrow belt from Niigata towards south-west to the Biwa lake along the Japan Sea coast. This low Q zone appears at frequency bands of both 1-2 and 2-4 Hz, and it coincides with the zone of high deformation rate revealed from the GPS data. For frequency bands 4-16 Hz (2-4 Hz in Kyushu), the low coda Q areas agree with volcanic and geothermal areas. For frequency band of 16-32 Hz, the coda Q is distributed nearly uniformly throughout the study area.
Key words: Hi-net, coda Q, brittle zone, ductile zone.