Coseismic displacements and slip distribution from GPS and leveling observations for the 2006 Peinan earthquake (Mw 6.1) in southeastern Taiwan

Abstract

Since 2001, we have set up a dense geodetic network with 52 campaign-mode GPS sites and seven continuously recording GPS stations as well as six leveling routes in the Taitung area, Taiwan. Our aim was to better characterize near-fault crustal deformation of active faults at the plate suture of the Philippine Sea plate and Eurasia in southeastern Taiwan. On 1 April 2006, a moderate shallow earthquake (M w 6.1, depth 10.8 km) occurred within this network. This earthquake resulted from rupturing of a geologically unknown or suspected fault (called the Y fault) located underneath the eastern margin of the Central Range. After removing the impacts of secular motions and postseismic slip, we estimated the coseismic displacements of the Peinan earthquake from the GPS and leveling measurements before and after the main shock. Three deformation types with distinct slip behaviors were revealed in three different regions: (1) near the epicenter-around 45 mm movement in the S-SSW direction with +20 to −20 mm vertical motion, in the northern part of the Y fault; (2) south of the epicenter across the southern part of the Y fault-approximately 35 mm in a westward movement with −60 mm subsidence (footwall side) and 40 mm in a SSW movement with at least 50 mm uplift (hanging-wall side), in the southern part of the Y fault; (3) northeast away from the epicenter-about 10 mm in a northward displacement with +15 to −10 mm vertical motions, in the Longitudinal Valley and on the western flank of the Coastal Range. This unique coseismic deformation pattern sheds new light on the characteristics of the suture zone between the Eurasian and Philippine Sea plates at the southernmost Longitudinal Valley. We used GPS and leveling measurements to invert for the fault geometry and the coseismic slip distribution. The optimal modeled fault is an 80• west-dipping fault at a depth of 0.5-20 km. The highest slip of about 0.33 m is located to the south of the hypocenter at a depth of 9-16 km. The total geodetic moment in our optimal model is 2.3 × 10 18 Nt-m, which is equivalent to an earthquake of M w 6.2. The surface coseismic displacements as well as the inferred coseismic slip distribution indicate a drastic change of slip behaviors in the middle of the Y fault. The left-lateral slippage near the hypocenter turned dramatically to reverse faulting with left-lateral component as rupturing propagated to the southern portion of the fault, suggesting that a possible right-lateral faulting occurred that coseismically cross cut the northern middle Peinanshan massif in the Longitudinal Valley.

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

“…All caused significant co-and post-seismic displacements (Chen et al , 2009Lee et al 2006;Mozziconacci et al 2013 It is thus necessary to remove the effects of these four earthquakes in order to estimate the inter-seismic velocity. The model proposed by Nikolaidis (2002) was applied to analyze the CORS position time series data sets as the following.…”

“…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.…”

“…However, the Central Range Fault also plays a role, although to a lesser degree, on the active tectonics in the southernmost Longitudinal Valley area. Indeed the 2006 M 6.1 Peinan earthquake ruptured this fault although the coseismic fault slip did not reach the surface (Wu et al 2006;Chen et al 2009;Mozziconacci et al 2013). Regarding the vertical deformation, our recent analysis of vertical velocity field combining leveling and GPS data (Chen et al 2012) showed that the faulting of the two branches of the LVF dominated the surface inter-seismic vertical deformation in the southernmost Longitudinal Valley: (a) the hanging wall of the Lichi Fault (i.e., the Coastal Range) is uplifting at a rapid rate, (b) the hanging wall of the Luyeh Fault (i.e., the Kaotai and Peinanshan tablelands) reveals a variation of moderate uplift or subsidence, (c) the footwall of the LVF (i.e., the flat area of the valley) shows significant subsidence, and (d) the Central Range exhibits a slight to moderate subsidence.…”

Determining Fault Geometry from the Distribution of Coseismic Fault Slip Related to the 2006 Taitung Earthquake, Eastern Taiwan

“…However, no surface rupture confirms this association. Chen et al (2009) suspected the existence of a perpendicular, secondary structure, south of the epicenter.…”

“…1, right side) give the same strike-slip focal mechanism for the Taitung earthquake with a north-south striking nodal plane that dips steeply to the west, whereas the other plane is oriented east-west and dips almost vertically southward. Because no surface rupture was observed for this event (Chen et al, 2009), a comparison of nodal plane geometries with local geological structures would help to identify the fault plane.…”