Abstract
Abstract 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.
Highlights
Coda waves of local earthquakes have been studied extensively since Aki (1969), and the properties of coda waves were summarized by Sato and Fehler (1998) in a systematic mannered book
The high-resolution coda Q map shown in Fig. 2 could never be constructed without benefits of the high density and high sensitivity of the Hi-net stations. 3.1 The low coda Q regions at lower frequency bands
According to the “brittle-ductile interaction hypothesis” of earthquake loading processes by plate-driving forces proposed by Aki (2004), the coda Q−1 represents the density of the fractures in the ductile part of lithosphere, and N (Mc) represents the response of the brittle part to the stress induced by the ductile fracture
Summary
Coda waves of local earthquakes have been studied extensively since Aki (1969), and the properties of coda waves were summarized by Sato and Fehler (1998) in a systematic mannered book. The spatial resolution of the coda Q maps from earlier studies was low because of the large station spacing as well as the use of relatively distant earthquakes.
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