Magnetotelluric soundings of the source area of the 1999 Chi-Chi earthquake in Taiwan: Evidence of fluids at the hypocenter

Magnetotelluric (MT) Soundings were used as part of an integrated project to map the earthquake-bearing structures in the Chi-Chi earthquake source area as soon as possible after the 21 September 1999, magnitude 7.3, Chi-Chi earthquake in Taiwan. Seventeen MT soundings were conducted and analyzed to map 2D resistivity distributions. Results show a low resistivity anomaly, less than 10 ohm-m, with depth ranging about 10-15 km below the hypocenter of the earthquake. Based on the temperature estimates from the MT resistivities, it is inferred that the main cause of this low resistivity zone is not related to high temperature, but rather to the presence of fluids in the crust. Although a localized shear stress concentration provoked by the tectonic escape process in the region might be used to explain the mechanism of the Chi-Chi earthquake, this study, which correlates the resistivity model with aftershock seismicity, raises the important possibility that the fluids released though metamorphic dehydration may have been one of the most important causes of the Chi-Chi earthquake.