Millennial slip rate of the Longitudinal Valley fault from river terraces: Implications for convergence across the active suture of eastern Taiwan

Abstract

The Longitudinal Valley fault is a key element in the active tectonics of Taiwan. It is the principal structure accommodating convergence across one of the two active sutures of the Taiwan orogeny. To understand more precisely its role in the suturing process, we analyzed fluvial terraces along the Hsiukuluan River, which cuts across the Coastal Range in eastern Taiwan in the fault's hanging wall block. This allowed us to determine both its subsurface geometry and its long‐term slip rate. The uplift pattern of the terraces is consistent with a fault‐bend fold model. Our analysis yields a listric geometry, with dips decreasing downdip from about 50° to about 30° in the shallowest 2.5 km. The Holocene rate of dip slip of the fault is about 22.7 mm/yr. This rate is less than the 40 mm/yr rate of shortening across the Longitudinal Valley derived from GPS measurements. The discrepancy may reflect an actual difference in millennial and decadal rates of convergence. An alternative explanation is that the discrepancy is accommodated by a combination of slip on the Central Range fault and subsidence of the Longitudinal Valley floor. The shallow, listric geometry of the Longitudinal Valley fault at the Hsiukuluan River valley differs markedly from the deep listric geometry illuminated by earthquake hypocenters near Chihshang, 45 km to the south. We hypothesize that this fundamental along‐strike difference in geometry of the fault is a manifestation of the northward maturation of the suturing of the Luzon volcanic arc to the Central Range continental sliver.

A New On-Land Seismogenic Structure Source Database from the Taiwan Earthquake Model (TEM) Project for Seismic Hazard Analysis of Taiwan

“…The seismicity of this fault also illuminated that it has a listric shape, dipping at 75° to a depth of 5.0 km, 60° between 5.0 -15.0 km deep, and finally 45° to a depth of 20.0 km. Several previous studies reported that the slip rate of this fault is between 20.5 -32 mm yr -1 (Chen 2006(Chen , 2010Shyu et al 2006bShyu et al , 2008. However, this fault is also known for its aseismic creeping (e.g., Angelier et al 1997;Lee et al 2001).…”

Near-field strain observations of the October 2013 Ruisui, Taiwan, earthquake: source parameters and limits of very short-term strain detection

“…The 60°east-dipping LVF, which represents the main active structure of the LV, is characterized by high rates of oblique slip on its southern segment and by primary left-lateral strike-slip on its northern segment (Shyu et al 2005;Yu and Kuo 2001). The CRF, dipping 50°-60°westward underneath the eastern flank of the Central Range, is associated with the fast uplift of the Central Range (Shyu et al 2006). The Ruisui earthquake is believed to rupture a 30-35-km-long segment of this northeast-southwest trending CRF with a primary thrust mechanism, in agreement with the tectonic stress regime in this region.…”

A New Velocity Field from a Dense GPS Array in the Southernmost Longitudinal Valley, Southeastern Taiwan

“…The Luyeh Fault, which has been treated as a nearly pure thrust fault, runs along the west side of the Peinanshan massif (Lee et al 1998;Hu et al 2001). On the one hand, the Luyeh Fault seems to terminate gradually in the north (Shyu et al 2002) and was speculated to bend dramatically to E-W trending before joining the Lichi Fault in the south (Chen et al 2012). This is one of the main points we intend to address in this paper.…”

“…The river systems, which carry the sediments into the valley, often produce alluvial fans at the foot of the two ranges in the valley. In the study area of the southernmost Longitudinal Valley, along the Luyeh River there exist several (5 -7) levels of Holocene river terraces (Shyu et al 2002). The uppermost two terraces are the laterite gravels layers, which are widely distributed in the highlands of the Peinan tableland and the Kaotai tableland with a 14 C age of about 3500 yr (Shyu et al 2008).…”

“…This is one of the main points we intend to address in this paper. As for the west-dipping Central Range Fault, which has recently been ruptured during the 2006 M 6.1 Peinan earthquake, it is still under debate regarding the geological context whether it is a major geological boundary between the Central Range and the Coastal Range (Shyu et al 2002;Chen et al 2009). Furthermore, the inter-seismic surface deformation and slip behaviors of the fault remain unclear.…”