Not until magnetotellurics (MT) were conducted, was very much known about the deep electrical structures beneath Taiwan. Finally, the first MT was used as part of an integrated project to map 3-D structures; subsequently, nineteen MT soundings, uniformly covering the entire Taiwan area, were conducted and analyzed to map 3-D resistivity distributions.
Results show the existence of a distinct low resistivity zone (less than 160 ohm-m) in the uppercrust with depths ranging about 10-2 0 km and about 30° northward dipping near the latitude of 2 4.s<'N beneath Taiwan. This finding is undoubtedly the most significant feature obtained by means other than from seismicity, which provides evidence of the Philippine Sea plate subduction northward beneath Taiwan. Based on the temperature estimates from the MT results, it is inferred that the main cause of this low resistivity zone is most probably the fluids released from dehydration reactions. The Moho discontinuity defined by velocity contrast at depth about 40-50 km does not seem to correspond with any sharp boundary which could be traced in the resistivity profiles.