Slip plane identification associated with the 1999 Taiwan Chi-Chi earthquake: X-ray CT image analyses of fault-related samples recovered from Taiwan Chelungpu-fault Drilling Project

We captured the X-ray computed tomography (XCT) images of three major fault zones as a series within the Chelungpu fault, on the core samples recovered from Hole B, Taiwan Chelungpu-fault Drilling Project. XCT images of the shallower fault zone showed sharp shear planes with lower attenuation of X-ray both around between the upper gray gouge and the black gouge zones and between the black gouge and the lower gray gouge zones, which were thought to be strong candidates of the slip planes during the 1999 earthquake. Microscopic examinations revealed that the upper shear plane was characterized as weak shear zone whereas the lower shear plane was done as intense shear zone accompanying preferred orientations of grains and anastmosing network structures. Both the shear planes exhibited lack of evidence indicating frictional melting. Although there is not enough evidence at present to identify the faulting mechanism during the 1999 earthquake and the slip plane or zone that moved during the event completely, present data can lead that frictional heating occurred but frictional melting did not occur in the shear planes within the shallower fault zone during the 1999 earthquake.