Magnetotelluric Soundings of the Source Area of the 1999 Chi-Chi Earthquake in Taiwan: Evidence of Fluids at the Hypocenter

Chow-Son Chen,Chien-Chih Chen

doi:10.3319/tao.2000.11.3.679(cce)

Abstract

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 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.

Highlights

On 21September,1999, at 01:47 local time, e i. ., at 1747:15.79 UT on 20September, the largest earthquake of the century (ML 7 .3) occurred "suddenly and without foreshock activ ity" in central Taiwan
The most significant feature obtained by means of MT is the existence of a distinct low resistivity anomaly (LRA), less than 10 ohm-m, with depths ranging about 10-15 km that extends laterally 5 km beneath the hypocenter of the Chi-Chi earthquake
A variety of factors could result in high electrical conductivity in the crust, e.g., fluids, carbon/graphite, partial melt, high temperature etc. (Jones 1987)

Summary

Introduction

On 21September,1999, at 01:47 local time, e i. ., at 1747:15.79 UT on 20September, the largest earthquake of the century (ML 7 .3) occurred "suddenly and without foreshock activ ity" in central Taiwan. On 21September,1999, at 01:47 local time, e i. ., at 1747:15.79 UT on 20September, the largest earthquake of the century (ML 7 .3) occurred "suddenly and without foreshock activ ity" in central Taiwan. Its epicenter (23.85° N, 120.82° E) was located near the town of Chi Chi from which the earthquake received its name. TAO, Vol 11, No 3, September 2000 of about 9.8 meters is among the largest fault movements far measured for earthquakes in recent times (Ma et al 1999). The Taiwan Earthquake Early WarningSystem (Wu et al 1999) operated by the Central Weather BureauSeismological Center reported the earthquake's location, magnitude and intensity distribution within two minutes; this quick broadcast of information enabled a fast emergency response by the government

Methods

Results

Discussion

Conclusion