Shallow geological structures triggered during the Mw 6.4 Meinong earthquake, southwestern Taiwan

Abstract

The Meinong earthquake generated up to ∼10 cm surface displacement located 10-35 km W of the epicenter and monitored by InSAR and GPS. In addition to coseismic deformation related to the deep earthquake source, InSAR revealed three sharp surface displacement gradients. One of them is extensional and is inconsistent with the westward interseismic shortening of ∼45 mm/yr in this region. The gradient sharpness suggests slip triggering on shallow structures, some of which were not well documented before. To characterize these shallow structures, we investigated potential surface ruptures in the field. Sets of ∼NS tension cracks distributed over 25-300 m width, with cumulative extension in the same order as InSAR observations, were found along 5.5 km distance along the extensional gradient and are interpreted as surface rupture. We build two EW regional balanced cross-sections, based on surface geology, subsurface data, and coseismic and interseismic geodetic data. From the Coastal Plain to the E edge of the coseismic deformation area, we propose a series of three active W-dipping back-thrusts: the Houchiali fault, the Napalin-Pitou backthrust, and the Lungchuan back-thrust. They all root on the 3.5-4.0 km deep Tainan detachment located near the base of the 3-km-thick Gutingkeng mudstone. Further E, the detachment would ramp down to ∼7 km depth. Coseismic surface deformation measurements suggest that, in addition to the deeper (15-20 km) main rupture plane, mostly the ramp, the Lungchuan back-thrust, and the Tainan detachment were activated during or right after the earthquake. Local extension is considered as transient deformation at the W edge of the shallow main slip zone.