Meng-Long Hsieh

Slip vectors of the surface rupture of the 1999 Chi-Chi earthquake, western Taiwan

About 100 slip vectors were surveyed along the 1999 Chi-Chi earthquake rupture (100 km long), western Taiwan, to understand the coseismic reverse-faulting processes. The surveyed slip azimuths vary locally (over 80°) where the rupture is irregular or associated with flexure. Even where the rupture is straight and has consistent slip azimuths, the fault-scarp heights, horizontal slips and fault dip-angles oscillate in a distance of hundreds of meters along the rupture. Despite these local variations, the net slip (up to 11.5 m), vertical and horizontal slips are significant greater in the north, on which maximum shortening (6 m; by pure thrust) and strike slip (8 m; by left-lateral fault) occur in association with the bend of the rupture. The slip azimuths also progressively rotate from north/northwest in the north to west/southwest in the south. These patterns generally agree with those revealed by GPS surveys, except that the slips by GPS in the south move to the west/northwest and have values unexpectedly smaller than our measurements. Our obtained SW-directed slips resulted in dominantly right-lateral faulting (up to 2.4 m) where the rupture ends to the southeast. This, however, does not correspond to focal mechanisms of any main shocks or aftershocks.

Middle-late Holocene river terraces in the Erhjen River Basin, southwestern Taiwan-implications of river response to climate change and active tectonic uplift

Abstract We reconstruct the Holocene river history of the Erhjen River (area: 140 km 2 ) by correlating river terraces aided by 28 radiocarbon dates. Multiple terraces developed in the lower Erhjen River since the middle Holocene; they converge downstream to the Coastal Plain. The rates of channel incision into bedrock calculated from these terraces at Yuehshihchieh are 7–8 mm/year during ca. 5.7–2.5 ka, 5 cm/year during 1.5–1.3 ka, and 1 cm/year since 1 ka; the last is close to the average incision rate since middle Holocene. Meanwhile, only a single but wide middle–late Holocene paleo-floodplain was developed in the upper Erhjen River; it was completely abandoned only after 0.8 ka, likely following an episode of base-level fall starting from the Coastal Plain. Based on the apparent downstream and upstream convergence of these dated terraces, we identify a doming structure or anticline within the basin, which results in a tilt rate of 10 −6 to 10 −7 per year in the lower Erhjen River valley. The major terraces here had different initial long profiles, which implies that a critical (graded) long profile may not be a prerequisite for formation of a wide erosional terrace surface. Instead, we propose that these terraces were initiated by a series of catastrophic rainfall events, probably climatic-related, which brought a large amount of bedload from hillslopes to prevent the channel from incising when valley widening was facilitated by high-discharge runoff. We find that not only terrace-surface formation but also channel incision can be strongly controlled by climatic-driven discharge and bedload conditions, as suggested by the contrast of bedrock incision rates we observe at Yuehshihchieh. Apparently, the climate and its effects on the landscape cannot be regarded as constant during the Holocene even in a humid tropical area like Taiwan. However, such a fluctuation of climate could only be recorded in a setting where rivers have a high tendency to incise so that multiple terraces can be created. The lower Erhjen River that is characterized by active tectonic tilting is an example of this setting.

Structures Associated with the Northern End of the 1999 Chi-Chi Earthquake Rupture, Central Taiwan: Implications for Seismic-Hazard Assessment

Abstract The surface rupture of the 1999 Chi-Chi earthquake ( M w 7.6) trendsmore than 100 km in a north–south direction. Surface deformation at the northernend stops abruptly at an area between the Tachia River and the Taan River where abroad pop-up structure with east to northeast strike can be found that has a trenddifferent from the north–south-striking main thrust. We combine the absolute ele-vation data before and after the Chi-Chi earthquake to obtain the regional verticaldisplacement and the magnitude of the pop-up structure. The greatest uplift couldreach as high as 15–16 m. Using deformation magnitude and the area-balancingmethod, we measure the depth of the detachment to show the subsurface geometryof the Chelungpu fault at its northern end. This shows that the geometry of theChelungpu fault controls termination of the surface rupture and the depth of thedetachment controls the amount of deformation.Introduction Perhaps the most unexpected geological feature asso-ciated with the Chi-Chi earthquake (

Marine deposits of late glacial times exposed by tectonic uplift on the east coast of Taiwan

Abstract Marine sediments deposited some 14,000 years ago, when the sea level was between 80 and 100 m below the present sea level, have been found exposed at about 17 m in altitude near Tu-lan, on the east coast of Taiwan. This implies a minimum average uplift rate of 7.6 ± 0.9 mm/yr; similar values of tectonic uplift are not uncommon on the eastern Coastal Range of Taiwan. Such rates of tectonic uplift, among the highest reported in the literature, are of the same order as present-day maximum glacio-isostatic uplift rates.