Abstract
The Fukui-earthquake fault which generated the Fukui earthquake in 1948 has been investigated by measuring microtremors with periods of a few seconds to determine its accurate position and vertical offset. The fault cannot be observed in geological surface survey, being masked by thick sedimentary layers. Eight horizontal-component seismographs with a proper period of 2 sec have been set up at intervals of 200m along a profile which may cross the fault.
The amplitudes of microtremors with a period of 2 sec change rapidly around the fissure zone generated during the 1948 Fukui earthquake. The amplitudes at the western side of the fissure zone are twice as large as those at its eastern side. The predominant periods also change around the fissure zone, being 0.6 sec at its eastern side and 1.5-2.0 sec at its western side. Therefore the position of the Fukui-earthquake fault can be determined within an accuracy of 200m. The depth to the basement estimated from the predominant periods of microtremors are about 100m at the eastern side of the fissure zone and about 300m at its western side. From this difference it can be concluded that the Fukui-earthquake fault is an ‘active fault’ which has moved repeatedly.
