The three-dimensional Q structure down to 150 km beneath the Hidaka Mountains , Hokkaido, Japan was determined using an inverse method. Apparent Q values were determined along ray paths from 456 earthquakes to the eight stations operated by Research Center for Earthquake Prediction of Hokkaido University. The hypocenters of these earthquakes were located beneath the Hidaka Mountains, with magnitude of 3.4-4.0. This region was divided into 600 blocks and then absolute Q values of the individual

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... locks were determined from these apparent Q values by using the inverse technique with least-squares method and constraints. In order to determine the Q values, the pulse broadening method (PB method) was used. We did not use spectral methods which need a time window with, at least, a few seconds from P arrivals, because seismograms observed at the eight stations have many phases of unknown origins which appear just after P arrivals. On the other hand, the PB method requires only one pulse of the first P arrivals; the Q values obtained by the PB method are hardly affected by scattering and reflections. The Q values determined by the PB method, however, involve considerable errors because of the narrow time window. We introduced some constraints on the Q values in the inversion process. The result shows that an extremely low-Q (Q=50) region is located beneath the area from the western part to the central part of the Hidaka Mountains and it extends to the depth of 100 km, and that relatively high-Q material exists beneath the summit of the Hidaka Mountains. Comparisons of the obtained three-dimensional Q structure with a three-dimensional Vp structure and with a seismic activity map, show the low-Q zones correspond to low-Vp and high seismicity zones. It is concluded that the low-Q zone is not caused by high temperature but composed of highly fractured rocks because no volcanic activity is found and rather low heat flow was observed in this region.