Fault modeling of the 2012 Wutai, Taiwan earthquake and its tectonic implications

• NW–SE trending seismogenic faults exist in a NW–SE compressional stress regime.
• The low friction is inferred from the stress drop of the Wutai mainshock.
• A rheology model is proposed using the transition of fault type and friction.

The Mw 5.9 Wutai earthquake of 26 February 2012 occurred at a depth of 26 km in southern Taiwan, where the rupture is not related to any known geologic structures. To illustrate the rupture source of the mainshock, we employ an elastic half-space model and GPS coseismic displacements to invert for optimal fault geometry and coseismic slip distribution. With observations of both coseismic horizontal and vertical displacements less than 10 mm, our preferred fault model strikes 312° and dips 30° to the northeast and exhibits a reverse slip of 28–112 mm and left-lateral slip of 9–45 mm. Estimated geodetic moment of the Wutai earthquake is 1.3 × 1018 N-m, equivalent to an Mw 6.0 earthquake. The Wutai epicentral area is characterized by a NE–SW compression as evidenced by the slaty cleavage orientations and the interpretation of stress tensor inversion of earthquake focal mechanisms. Using the stress drops of the Wutai and the nearby 2010 Mw 6.4 Jiashian earthquakes, we obtain a lower bound of ~ 0.002 for the coefficient of friction on the fault. On the other hand, studying the crustal thickness contrast in southern Taiwan provides an upper bound of the average horizontal compressive force of 1.67 × 1012 N/m transmitted through the Taiwan mountain belt and gives an estimate of the maximum friction coefficient for 0.03. The deviation of an order of magnitude difference between the upper and lower bounds for the coefficient of friction suggests that other fault systems may support substantial differential stress in the lithosphere as well.