Geomechanical appraisal of seismogenic faults in northeast Taiwan

Although the seismicity of Taiwan is induced by the convergent motion between the Eurasian plate and the Philippine Sea plate, the shallow earthquakes are diffusely distributed and not clearly related to plate interactions. Since shallow earthquakes are attributed to the upper crustal deformation, their occurrences are controlled not only by plate interactions but also by the mechanical behavior of the colliding lithospheres. In order to better understand the origin and distribution of shallow earthquakes, the mechanisms of crustal deformation in northeast Taiwan were investigated and used to surmise potential seismogeinc faults. Northeast Taiwan, situated at the junction of the Taiwan orogen and the Ryukyu Arc, is subjected to the compressional force exerted by the indentation of Luzon Arc and the tensile force by the suction at the Ryukyu Trench. Manipulated by these two forces, the stress regime rapidly changes over from compressional in the Taiwan collision zone to extensional in the Ryukyu subduction zone. In the compressional regime, NW-SE compression overwhelms the gravity and drives crustal contraction. Active thrusting occurs in the frontal and rear sides of the mountain belt where the gravitational force is the least and the deviatoric stress the greatest. In the extensional regime, trench suction induces lithospheric stretching, which resulted in the extensional collapse of the collision orogen. Gravity dictates the maximum stress, while the minimum stress lies horizontal and orthogonal to the trench. The central part of the orogen, where the crustal extension is concentrated, collapsed to a rift valley with intense normal faulting and seismicity. In the transitional regime between the two opposite stress regimes, the horizontal stress is nearly equal to the vertical stress, and the crustal deformation is dominated by distributed shear which shows up as strike-slip faults.