Resistivity structure of the central part of the Yamasaki fault studied by the multiple electrodes resistivity method

The Yamasaki fault system is a well known major, active fault in South-Western Japan. Large historical earthquakes have occurred along it. Several resistivity models of the Yamasaki fault have been proposed as the result of intensive electromagnetic investigations during the 1970's and 1980's. In 1996, a trench study was made of the cross-section of the Yasutomi fault which is the kernel of the Yamasaki fault system. The cross-section showed the occurrence of past major paleo-earthquakes. To clarify the deeper structure of the fault, multiple electrode resistivity soundings were conducted across the fault in 1997. Although the target sounding depth of the previous EM survey is different, the resistivity measurements show the boundary structure of the central part of the fault beneath the trench survey area. They also show the extent of a shallow conductive zone across the fault.