Hao Kuo-Chen

Deep crustal structure of an arc‐continent collision: Constraints from seismic traveltimes in central Taiwan and the Philippine Sea

The collision of continental crust of the Eurasian Plate with the overriding Luzon Arc in central Taiwan has led to compression, uplift, and exhumation of rocks that were originally part of the Chinese rifted margin. Though the kinematics of the fold-thrust belt on the west side of the orogen has been described in detail, the style of deformation in the lower crust beneath Taiwan is still not well understood. In addition, the fate of the Luzon Arc and Forearc in the collision is also not clear. Compressional wave arrival times from active-source and earthquake seismic data from the Taiwan Integrated Geodynamic Research program constrain the seismic velocity structure of the lithosphere along transect T5, an east-west corridor in central Taiwan. The results of our analysis indicate that the continental crust of the Eurasian margin forms a broad crustal root beneath central Taiwan, possibly with a thickness of 55 km. Compressional seismic velocities beneath the Central Range of Taiwan are as low as 5.5 km/s at 25 km depth, whereas P wave seismic velocities in the middle crust on the eastern flank of the Taiwan mountain belt average 6.5–7.0 km/s. This suggests that the incoming sediments and upper crust of the Eurasian Plate are buried to midcrustal depth in the western flank of the orogen before they are exhumed in the Central Range. To the east, the Luzon Arc and Forearc are deformed beneath the Coastal Range of central Taiwan. Fragments of the rifted margin of the South China Sea that were accreted in the early stages of the collision form a new backstop that controls the exhumation of Eurasian strata to the west in this evolving mountain belt.

Investigating the lithospheric velocity structures beneath the Taiwan region by nonlinear joint inversion of local and teleseismic P wave data: Slab continuity and deflection

The interaction between two flipping subduction systems shapes the complicated lithospheric structures and dynamics around the Taiwan region. Whether and in what form the Eurasian Plate subducts/deforms under Taiwan Island is critical to the debate of tectonic models. Although an east dipping high-velocity anomaly down to a depth below 200 km has been reported previously, its detailed morphology remains uncertain and leads to different interpretations. With a two-step strategy of nonlinear joint inversion, the slab images of the Eurasian Plate were retrieved in a geometry that is hyperthin in the south, becoming massive and steeper in the central, and severely deformed in the north. The possible depth and dimension of a slab break were also investigated through synthetic tests of whether the slab had torn. Moreover, the slab deflection found at ~23.2°N latitude seems to correspond to where the nonvolcanic tremors and recent NW-SE striking structures have occurred in southern Taiwan.

Structural complexities in a foreland thrust belt inherited from the shelf‐slope transition: Insights from the Alishan area of Taiwan

The Alishan area of Taiwan spans the transition from the platform with full thickness of the Eurasian continental margin in the north to the thinning crust of its slope in the south. This part of the foreland thrust and fold belt includes important along-strike changes in structure, stratigraphy, and seismic velocities. In this paper we present the results of new geological mapping from which we build geological cross sections both across and along the regional structural trend. Fault contour, stratigraphic cutoff, and branch line maps provide 3-D consistency between the cross sections. Minimum shortening is estimated to be ~15 km with displacement overall to the northwest. A P wave velocity model helps constrain the structure at depth by providing insight into the possible rock units that are present there. P wave velocities of ≥ 5.2 km/s point toward the presence of basement rocks in the shallow subsurface throughout much of the southeastern part of the area, forming a basement culmination. The changes in strike of thrusts and fold axial traces, the changing elevation of thrusts and stratigraphic contacts, and the growing importance of Middle Miocene sediments that take place from north to south are interpreted to be associated with a roughly northeast striking lateral structure coincident with the northern flank of this basement culmination. These transverse structures appear to be associated with the inversion of Eocene- and Miocene-age extensional faults along what was the shelf-slope transition in the Early Oligocene, uplifting the margin sediments and their higher P wave velocity basement during Pliocene-Pleistocene thrusting.

Exhumation of serpentinized peridotite in the northern Manila subduction zone inferred from forward gravity modeling

The Taiwan Integrated Geodynamic Research program (TAIGER) collected two wide-angle and reflection seismic transects across the northern Manila subduction zone that provide constraints on the seismic velocity structure of the crust. Two-dimensional gravity modeling along these two transects shows a significant, relatively high density (3.12 and 3.02 g/cm3) in the fore-arc region, at the interface between the subducting Eurasian Plate and the accretionary prism in front of the Luzon arc on the overriding Philippine Sea Plate. The anomalous density in this zone is higher than that in the fore-arc crust and the accretionary prism but lower than that in mantle. Numerous geophysical and geological data, together with numerical models, have indicated that serpentinization of the fore-arc mantle is both expected and observed. Serpentinization of mantle rocks can dramatically reduce their seismic velocity and therefore their seismic velocity in a density to velocity conversion. Therefore, the source of the high-density material could be serpentinized fore-arc mantle, with serpentinization caused by the dehydration of the subducting Eurasian Plate. We interpret that positive buoyancy combined with weak plate coupling forces in the northern Manila subduction zone is resulting in this serpentinized fore-arc mantle peridotite being exhumed.

3D ambient noise tomography across the Taiwan Strait: the structure of a magma‐poor rifted margin

Rifting along southeastern Eurasia in the Late Cenozoic led to the formation of a magma-poor rifted margin facing the South China Sea to the southeast and the Philippine Sea to the east. Further rifting along the outer part of the margin during the Middle to Late Miocene was accompanied by an extensive episode of intraplate flood volcanism that formed the Penghu Archipelago. Previous geophysical studies in the area of the Strait have focused primarily on the shallow structures of the rift basins and the depth to the Moho. In this study we present the regional-scale 3D S-wave structure of the Taiwan Strait that is derived from a joint Chinese and Taiwanese 3D ambient noise tomography study. The S-wave model shows a thinning of the crust beneath the rift basins where, locally, thin high-velocity layers suggest the presence of intrusive bodies. The rift basin and the foreland basin along the west coast of Taiwan are imaged as low velocity zones with thicknesses between 5 and 10 km, and extending eastward beneath the Taiwan mountain belt. In the upper 10 km of the crust, the basaltic rocks of the Penghu Archipelago are imaged as a high velocity zone that, with depth, becomes a relatively low velocity zone. We interpret this low velocity zone in the lower crust and upper mantle beneath the Penghu Archipelago to image a thermal anomaly related to the still cooling magma feeding system and the melt reservoir area that fed the flood basalts at the surface.

How the structural architecture of the Eurasian continental margin affects the structure, seismicity, and topography of the south central Taiwan fold‐and‐thrust belt

Studies of mountain belts worldwide show that along-strike changes are common in their foreland fold-and-thrust belts. These are typically caused by processes related to fault reactivation and/or fault focusing along changes in sedimentary sequences. The study of active orogens, like Taiwan, can also provide insights into how these processes influence transient features such as seismicity and topography. In this paper, we trace regional-scale features from the Eurasian continental margin in the Taiwan Strait into the south-central Taiwan fold-and-thrust belt. We then present newly mapped surface geology, P-wave velocity maps and sections, seismicity, and topography data to test the hypothesis of whether or not these regional-scale features of the margin are contributing to along-strike changes in structural style, and the distribution of seismicity and topography in this part of the Taiwan fold-and-thrust belt. These data show that the most important along-strike change takes place at the eastward prolongation of the upper part of the margins necking zone, where there is a causal link between fault reactivation, involvement of basement in the thrusting, concentration of seismicity, and the formation of high topography. On the area correlated with the necking zone, the strike-slip reactivation of east-northeast striking extensional faults is causing sigmoidal offset of structures and topography along two main zones. Here, basement is not involved in the thrusting, there is weak focusing of seismicity, and localized development of topography. We also show that there are important differences in structure, seismicity, and topography between the margins shelf and its necking zone.

The Structure of Southwest Taiwan: The Development of a Fold-and-Thrust Belt on a Margins Outer Shelf and Slope

D. B., J. A-M., and C. B. acknowledge funding provided by the Spanish Ministerio de Economia y Competividad grant CGL2013-43877-P. H. K-C. acknowledges funding by MOST 104-2628-M-008-005-MY3.

Aftershock Sequence of the 2018 Mw 6.4 Hualien Earthquake in Eastern Taiwan from a Dense Seismic Array Data Set

The authors thank Wen‐Yen Chang of National Dong Hua University, Yu‐Ting Kuo and Mr. Chin‐Shang Ku of the Insitute of Earth Sciences (IES), Academia Sinica, and Hsuan‐Yu Kuo of the National Central University for the assistance of the field deployment. H. K.‐C. is supported by MOST (MOST 104‐2628‐M‐008‐005‐MY3 and MOST 107‐2628‐M‐008 ‐004 ‐MY3).

Resolving the 1906 Mw 7.1 Meishan, Taiwan, Earthquake from Historical Seismic Records

The 1906Mw 7.1 Meishan earthquake, named after the village near where it occurred, was one of the most damaging earthquakes in Taiwan in the early twentieth century. The historical literature and former studies claimed to show that this earthquake was related to the Meishan fault, which had about 12.5 km right-lateral surface rupture orientated in an east–west direction. However, the surface rupture is short with respect to the magnitude of the earthquake. The north–south pattern of damage is not consistent with the strike of the surface rupture. These apparent inconsistencies raised severe doubts regarding the exact mechanism of the earthquake. Using the original Omori records of the 1906 Meishan earthquake from the historical Taipei, Taichung, and Tainan stations, we carried out an effective waveform simulation to evaluate several different geological fault models. Synthetic Omori records from these geological fault models clearly show a discrepancy in the first motions of P and S waves at the Tainan station, which suggest that the focal mechanism may not be a pure strike slip. We used observed Pand S-wave first motions in the historical Omori records as a constraint and performed a grid search to find possible focal mechanisms. The preferred focal mechanism is an oblique thrust fault oriented in the northeast–southwest direction, with a small right-lateral component. Such a focal mechanism not only reproduces the general feature shown in the observed intensity map, it is also more consistent with the spatial distribution of significant aftershocks, which are predominantly located toward the north and south of the epicenter. This result highlights the importance of historical recordings in resolving the earthquake mechanism in a complex fault system. Electronic Supplement: Figures of Pand S-wave polarities, simulated intensity maps, and simulated and observed seismograms, and table of parameters of the instrument responses from Omori’s and Gray–Milne seismographs for the ancient stations. INTRODUCTION The 1906 Meishan earthquake, one of the most damaging earthquakes in Taiwan’s history, occurred in the southwestern Taiwan at local time 6:43 a.m. on 17 March. It is located at 23.550° latitude, 120.450° longitude, and 6 km depth (Yeh et al., 1998; Fig. 1). The magnitude of this earthquake was MH 7.1, first determined by Hsu (1980a,b), and M s 6.8, subsequently determined by Abe and Noguchi (1983). This earthquake caused 1258 deaths, 2385 injuries, and leveled 6769 houses. According to the Omori (1907) field investigation and the historical literature (Taiwan Governor-General Office of Civil Affairs, 1907; Kondo, 1906), a right-lateral surface rupture with a length of about 12.5 km, identified as the Meishan fault, and liquefactions were observed along the east– west direction (Fig. 2). Thus, the 1906 Meishan earthquake had long been considered as a significant event from the Meishan fault with an east–west strike-slip focal mechanism. However, the length of the fault is disproportionally short compared to the damage caused by this Mw 7.1 earthquake. The intensity maps compiled by government agencies (Fig. 3a,b,d) show a general north–south pattern, rather than the east–west intensity pattern reported by Omori (1907; see also Fig. 3c). Significant aftershocks were also located to the north and south of the epicenter (Fig. 1). These inconsistent observations had raised long-standing debates regarding the understanding of this devastating historical earthquake. In 1906, Taiwan was administered as colony by the Japanese government which installed four seismic stations (Fig. 1) over the main island of Taiwan (TAP, TCU, and TAN) and a small island of Penghu (PNG) with the Omori instrument and Gray–Milne seismograph. It was fortunate that valuable seismograms recorded by these stations were discovered in the earthquake data archive project conducted by the Earthquake Research Institute of Tokyo University (Noguchi et al., 2001). These well-preserved historical waveforms allow us to use doi: 10.1785/0220170285 Seismological Research Letters Volume XX, Number XX – 2018 1 SRL Early Edition

A Stress Condition in Aquifer Rock for Detecting Anomalous Radon Decline Precursory to an Earthquake

Recurrent groundwater radon anomalous declines were observed from well measurements in the Antung hot spring area (eastern Taiwan) prior to five of six earthquakes that occurred between 2003 and 2011 (Mw range 5.0–6.8). The relationship between the detectability of radon anomalies and the first motions of P-waves was investigated. Based on the first motions of P-waves recorded near the investigated well, a precursory decrease in groundwater radon can be detected only when the first motion is compression. No precursory change in groundwater radon concentration was observed for the downward first motion of P-waves.