Deep electromagnetic images of seismogenic zone of the chi-chi (Taiwan) earthquake

A localized shear stress concentration, provoked by the tectonic escape process, in the source region of the Chi-Chi (Taiwan) Earthquake has been used to explain the mechanism of the M_L=7.3 devastating earthquake. This study, by employing the geophysical magnetotellurics (MT) soundings technique, mapped the deep electrical structures beneath the seismogenic zone of the Chi-Chi Earthquake and shows prominent effects. About fifty MT soundings were conducted and analyzed to recover 3D resistivity distributions. Results show a distinct low resistivity anomaly, less than 10-ohm-m, with depth ranging to greater than 10 km, which extends laterally no less than 10 km beneath the hypocenter of the earthquake. Correlations of the resistivity model with the seismogenic zone of the Chi-Chi Earthquake, raises the important possibility that the fluids released though crust dehydration may have been one of the most important causes of the Chi-Chi Earthquake. The MT model also suggests that the potential sources of these fluids may be: (1) fluids trapped in pore spaces at shallow depths, less than 8 km, and (2) the accumulation of fluids in the crust, greater than 12 km, contributed by the subducted ocean crust on the top of the slab.