55 巻 (2002-2003) 2 号 p. 143-151
As a result of 1995 Hyogo-ken Nanbu Earthquake (also called the Kobe Earthquake), a distinctive fault ruptures appeared along the northwestern coast of Awaji Island, which are called the Hokudan Fault System. This system consists of two fault strands, i.e., the Nojima Fault and the Ogura Fault. The southern portion of the Nojima Fault, which is about 1.5 km long from the branch point (NSM), shows a little displacement. On the other hand, the Ogura Fault and northern portion of the Nojima Fault show a distinctive displacement.
In order to delineate shallow resistivity structure around the southern portion of the Nojima Fault, 2-D resistivity surveys were made along the three transects across the fault. Our further aim was to reveal relationship between resistivity structure and difference of fault activity.
Clear resistivity contrast was detected when crossing the surface trace of the fault. Resistivity value of larger than 500 Ωm was observed for granitic rocks, whereas the value of less than 200 Ωm were measured for Tertiary or Quaternary sediments.
Surface positions of all large resistivity boundaries are consistent with surface trace of the fault. Furthermore, dip angles of two resistivity boundaries are in fair agreement with other geological evidence reported from the survey area.
A distinctive low resistive zone which has been previously observed along the Ogura Fault and the northern part of the Nojima Fault, is not detected along the southern portion of the Nojima Fault. This variation of the shallow resistivity structure around the different fault zone may indicate a difference in fault activity. The southern part of the Nojima Fault is reported to be less active during the latter half of the Quaternary, on the other hand the Ogura Fault and the northern Nojima Fault have been active until present. Our result thus shows that a conductivity structure around a fault zone can be used as indicator for recent fault activities along faults.