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 west 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 -1 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 E-W regional balanced cross-sections, based on surface geology, subsurface data, and coseismic and interseismic geodetic data. From the Coastal Plain to the east edge of the coseismic deformation area, we propose a series of three active west-dipping backthrusts: the Houchiali fault, the Napalin-Pitou backthrust, and the Lungchuan backthrust. 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 east, 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 backthrust, and the Tainan detachment were activated during or right after the earthquake. Local extension is considered as transient deformation at the west edge of the shallow main slip zone.

Structure and Deformation History of the Rapidly Growing Tainan Anticline at the Deformation Front of the Taiwan Mountain Belt

“…Seismic Lines D1 and D5, as well as D6 and rD1 (Figure a), located north of the Tainan Tableland, image a pure‐shear fault‐bend fold developing on an ~38°W dipping back thrust, interpreted as the continuation of the Houchiali fault, that roots on a 3.8‐km‐deep subhorizontal detachment within the Lower Gutingkeng mudstone. This detachment is referred to as the Tainan detachment (Le Béon et al, ). The fold appears to have vanished at the latitude of Seismic Line rD2 (Figure a).…”

“…The Holocene shortening rate that we determined accounts for 25% of the regional shortening rate, making the anticline and the Houchiali fault a significant active structure at the scale of the fold‐and‐thrust belt. In contrast, the fold inception age of 310 ± 50 ka is very young, and the cumulative shortening of 2–3 km is limited in regard of the ~3‐Ma deformation history of the foothills (Le Béon et al, ; Mouthereau et al, ).…”

“…At the regional scale, the Tainan detachment depth determined in this study is a key piece of information in order to build regional cross sections across the foothills of the mountain belt (Figure c). The Lower Gutingkeng Tainan detachment likely extends from the Tainan wedge tip to ~19 km to the east, beneath the Lungchuan triangle zone (Huang et al, ; Le Béon et al, ). In addition to the Houchiali fault, two other back thrusts root on this detachment, which accommodated limited cumulative shortening.…”

Pre-seismic strain anomalies and coseismic deformation of Meinong earthquake from continuous GPS

“…12)than that of far filed stations. It also implies that the horizontal displacement is mainly controlled by the foldand-thrust system connected with N-S trending shallow dé-collement (Le Béon et al 2017). This triggering of coseismic slip from the shallow structure was also highlighted by Lin et al (2016) in 2010 Jiashian earthquake.…”

Introduction to the special issue on the 2016 Meinong, Taiwan, earthquake

“…By integrating all these information, they concluded that multiple pre-existing blind faults were reactivated during the 2016 Meinong earthquake sequences. Based on surface and subsurface geology and geodetic observations during the 2016 Meinong earthquake, Le Béon et al (2017) built two E-W balanced cross-sections to identify the shallow structures that may have been reactivated during the earthquake. They interpreted these activated structures as a series of west-dipping back-thrust ramps connected to the westward-propagating Tainan detachment.…”