能登半島南部の重力異常と基盤構造

重力異常と活断層, 地震活動との関係

抄録

Detailed gravity anomaly map over the southern part of the Noto Peninsula was constructed by using approximately 7, 750 gravity data. In the southern part of the Ouchi-depression, we found an apparent discrepancy, about 2.5km in horizontal distance, between steep horizontal gradient zone of gravity anomaly and active faults reported from geological and geomorphologic studies. The discrepancy presumably indicates that there are two sets of faults in this area: one is a reported active fault situated at the plain-hill boundary in the Ouchi-depression, and the other one is beneath the steep gradient zone of gravity anomalies in the hill area.
From gravity anomalies and deep drilling data, we estimated a 3-D basement structure in this region.
In the Ouchi depression region, boundaries of the basement thus estimated generally show good agreement with the topographic ones, which are also believed as active faults. We emphasize, however, that a large discrepancy (approx. 2.5km) exists between topographic and basement boundaries in the southwestern part of the depression.
In the southeastern flank of the Hodatsu hill, north of Himi city, a zone of steep change of the basement depth (approx. 1, 000m) is estimated. There are no reported active faults along the zone but that is a region of relatively high seismic activity. These data suggest that ambient seismogenic faults may underlie beneath this region.