Characteristics of the potential submarine landslide in the Keelung Shelf, off northern Taiwan (original) (raw)
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Active normal faults and submarine landslides in the Keelung Shelf off NE Taiwan
Terrestrial, Atmospheric and Oceanic Sciences
The westernmost Okinawa Trough back-arc basin is located to the north of the Ryukyu islands and is situated above the northward dipping Ryukyu subducted slab. In the northern continental margin of the Okinawa Trough, the continental slope between the Keelung Valley and the Mein-Hua Submarine Canyon shows a steep angle and future slope failures are expected. The question is how slope failures will proceed? A sudden deep-seated slump or landslide would probably cause local tsunami and hit northern coast of Taiwan. To understand the probable submarine landslides, we conducted multi-channel seismic reflection, sub-bottom profilers, and multi-beam bathymetry surveys off NE Taiwan. Two general trends of shallow crustal faults are observed. The NE-SW trending faults generally follow the main structural trend of the Taiwan mountain belt. These faults are products of inversion tectonics of reverse faults from the former collisional thrust faults to post-collisional normal faults. Another trend of roughly E-W faults is consistent with the current N-S extension of the southern Okinawa Trough. The fault offsets in the eastern portion of the study area are more pronounced. No obvious basal surface of sliding is found beneath the continental margin. We conclude that the movement of the submarine landslides in the Keelung Shelf off northeastern Taiwan could be in a spread type. The submarine landslides mainly occur in the continental slope area and it is more obvious in the east than in the west of the Keelung Shelf.
Journal of Asian Earth Sciences, 2017
This study analyzes both 2D and 3D seismic images around the Palm Ridge area offshore of southwestern Taiwan to understand how the deformation front shifted westward and how tectonic activities interact with submarine canyon paths in the transition area between the active and passive margins. Palm Ridge is a submarine ridge that developed on the passive China continental margin by down-dip erosion of several tributaries of Penghu Canyon; it extends eastward across the deformation front into the submarine Taiwan accretionary wedge. The presence of proto-thrusts that are located west of the frontal thrust implies that the compressional stress field has advanced westward due to the convergence of the Philippine Sea Plate and Eurasian Plate. Since the deformation front is defined as the location of the most frontal contractional structure, no significant contractional structure should appear west of it. We thus suggest moving the location of the previously mapped deformation front farther west to where the westernmost proto-thrust lies. High-resolution seismic and bathymetric data reveal that the directions of the paleo-submarine canyons run transverse to the present slope dip, while the present submarine canyons head down slope in the study area. We propose that this might be the result of the westward migration of the deformation front that changed the paleo-bathymetry and thus the canyon path directions. The interactions of down-slope processes and active tectonics control the canyon paths in our study area.
Tectonophysics, 2016
What is the structural geometry of the southern Taiwan transition zone from the Manila subduction offshore to the Taiwan onshore collision, specifically in the western flank of the Hengchun peninsula that corresponds to the summit of the Manila subduction accretionary prism? This paper aims to decipher the onshore/offshore structures and tectonic deformation that occur west of the Hengchun Ridge through both detailed topographic analyses and interpretation of numerous old and new seismic profiles. From a geomorphic point of view, both Fangliao and Hongchai submarine canyons have different structural and landslide implications. The Fangliao Canyon is guided by a N-S elongated mud diapir (the Fangliao Ridge), intruding an inferred N010°E trending, left lateral strike-slip fault zone. Conversely, the arcuate and concave shape of the Hongchai Canyon appear to follow the crown and the northern boundary of a newly recognized Hongchai submarine landslide situated on the steep western flank of the onshore asymmetric Hengchun Anticline. Our results highlight that both Fangliao and Hengchun Faults are linear, near-vertical left-lateral strike-slip faults. They converge onshore to the Chaochou Fault. This study demonstrates that neotectonics combine with morphostructural analysis of the submarine canyon drainages lead to a better comprehension of the present deformation in the northern part of the Manila accretionary prism.
Fangliao Slide — a large slope failure in the upper Kaoping Slope off southwest Taiwan
Terrestrial, Atmospheric and Oceanic Sciences, 2018
Based on seismic reflection profiles and multi-beam bathymetric data, a large submarine landslide named Fangliao Slide is mapped for the first time off SW Taiwan. The Fangliao Slide occurred on the continental slope to the west of the Fangliao Canyon at water depths between 420 and 900 m. The seafloor of the Fangliao Slide has a gentle slope angle (~1-2°). The landslide covers an area of ~15 km length and ~10 km width and a volume of ~26 km 3. The headwall of the landslide has ~30 m vertical offset at the southern flank of mud diapir MD7-1, and the sidewalls are bounded by fault A in the west and faults C and D in the east. The sliding area is composed of five bathymetric terraces, indicating that the slope failures have occurred several times. The Fangliao Slide can be divided into an upper domain and a lower domain, separated at the water depth of ~600 m where the gas hydrate off SW Taiwan becomes dissociate. The initial slope failure of the Fangliao Slide was probably linked to mud diapirism of MD7-1 and the slope failure in the lower domain was probably augmented by the gas hydrate dissociation. The seafloor morphology in the lower domain is therefore more corrugated than in the upper domain.
Back analysis of an earthquake-triggered submarine landslide near the SW of Xiaoliuqiu
Terrestrial, Atmospheric and Oceanic Sciences, 2018
Occurred in the offshore of SW Taiwan on 26 December 2006 with a magnitude of 7, the Pingtung earthquake had triggered numbers of submarine landslides. This event provides an excellent opportunity to incorporate the back analysis approach to evaluate the in situ shear strength parameters. According to the chirp sonar images of the seabed near the SW Xiaoliuqiu obtained before and after the earthquake were adopted to establish the slope profile and identified the location of a circular sliding surface. Consequently, the in situ, effective strength parameters under the critical condition can be calculated by back slope stability analysis. Submarine sediment sampler was obtained via gravity sampling method and the laboratory tests were performed to determine the index properties and strength parameters. Test results indicate the cored sediment has the characteristics of normally consolidated (NC) clay. The effective friction angle (φ') is 15.3° with cohesion (c') of 19.4 kPa. The effective and total stress methods were used to perform the back analysis. The strength parameters derived from back analysis of effective and total stress methods all indicate values approach the CIU triaxial tests results. Consequently, the representativeness of the marine sediment characteristics obtained from laboratory tests is identified. The total stress approach yields an undrained strength ratio c u /σ' v0 of 0.26 which well fit the ratio used in geotechnical practice for estimating NC clay. According to the analytical approach, the landslide was applied seismic forces (seismic coefficient k h = 0.14) and generated excess pore pressure of 31 kPa at the sliding surface.
A tear fault boundary between the Taiwan orogen and the Ryukyu subduction zone
Tectonophysics, 1997
More than twenty seismic lines were analysed across the intersection of the Taiwan orogen and the Ryukyu arc-forearc system. The main results of this study concern the recognition of two superposed sedimentary basins that were previously unknown. We have named the recent one the Hoping Basin and the older, tectonized one the Suao Basin. The Suao Basin, containing sediments more than 3 km thick, records subsidence of about 3 km on its southwestern portion after being shortened in the direction of plate convergence. On the basis of seismicity, gravity, present-day plate kinematics as well as seismic reflection data, we relate the dramatic subsidence of the Suao Basin to the initiation of a WNW-trending tear fault within the Philippine Sea plate. This tear fault decouples the subducting Philippine Sea plate to the northeast and the apparent overthrusting Philippine Sea plate in the Coastal Range to the southwest. Back-arc rifting may thus occur in the westernmost section of the Okinawa trough above the sinking Philippine Sea slab while lithospheric shortening prevails south of Hualien. As a consequence, the triangular area located southwest of the tear fault, between the Taiwan coastline and the toe of the Yaeyama ridge, no longer belongs to the Ryukyu forearc. It is now passively transported with the non-subducting portion of the Philippine Sea plate. We propose an evolutionary model for the relative chronology of tectono-sedimentary events in this region over the last 5 m.y.
The Neo-Tectonic Structure of the Southwestern Tip of the Okinawa Trough
Terrestrial, Atmospheric and Oceanic Sciences, 2009
The Ilan Plain is located at the southwestern tip of the Okinawa Trough backarc basin, which propagates westward into the Taiwan orogen. From three moderate earthquakes which occurred near the coastline of the Ilan Plain, one of magnitude 6.2 on May 15, 2002 and the other two of magnitudes 5.51 and 5.49 on March 5, 2005, we attempted to understand the relationship between the earthquake mechanisms and the geological context. Seven seismic reflection profiles collected roughly parallel to the coastline of the Ilan Plain were used in this work. A structural fault, with a significant normal faulting component trending approximately ENE-WSW, has been identified to the east of the doublet earthquakes of March 5, 2005. Because this fault follows the ENE-WSW trend of the aftershock seismicity, we named it the Ilan Shelf Fault, which might extend to the east beyond the Ilan continental shelf spur. However, the centroid seismic moment tensor solutions for the doublet earthquakes of March 5, 2005 are of a left-lateral strike-slip faulting type which are consistent with onland GPS observations, such that the Ilan Shelf Fault may have a left-lateral strike-slip component. Based on GPS data, we suggest that the northern Central Range block, bounded in the west by the Lishan Fault and in the north by the Ilan Shelf Fault, rotates clockwise. This rotation is ascribed to the northwestward collision of the Luzon Arc against Taiwan. Thus, there is a left-lateral strike-slip and an extensional motion along the Ilan Shelf Fault. The northern Central Range block is tilting northward which may also induce the normal faulting component of the Ilan Shelf Fault. In short, the Ilan Shelf Fault could be a major tectonic and structural feature bounding the southern end of the Okinawa Trough backarc basin.
Marine Geophysical Researches, 2004
Swath bathymetry data and seismic reflection profiles have been used to investigate details of the deformation pattern in the area offshore southwestern Taiwan where the Luzon subduction complex encroaches on the passive Chinese continental margin. Distinctive fold-and-thrust structures of the convergent zone and horst-and-graben structures of the passive margin are separated by a deformation front that extends NNW-ward from the eastern edge of the Manila Trench to the foot of the continental slope. This deformation front gradually turns into a NNE-SSW trending direction across the continental slope and the Kaoping Shelf, and connects to the frontal thrusts of the mountain belt on land Taiwan. However, the complex Penghu submarine canyon system blurs the exact location of the deformation front and nature of many morphotectonic features offshore SW Taiwan. We suggest that the deformation front offshore SW Taiwan does not appear as a simple structural line, but is characterized by a series of N-S trending folds and thrusts that terminate sequentially in an en-echelon pattern across the passive Chinese continental slope. A number of NE-SW trending lineaments cut across the fold-and-thrust structures of the frontal accretionary wedge and exhibit prominent dextral displacement indicative of the lateral expulsion of SW Taiwan. One of the prominent lineaments, named the Yung-An lineament, forms the southeastern boundary of the upper part of the Penghu submarine canyon, and has conspicuous influence over the drainage pattern of the canyon.
Geophysical Journal International, 2009
Northern Taiwan underwent mountain building in the early stage of the Taiwan orogeny but is currently subjected to post-collisional crustal extension. It may be related to gravitational collapse or to the rifting of the Okinawa Trough, which lies offshore northeastern Taiwan. The Ilan Plain, northeastern Taiwan, which is bounded by the normal fault systems and filled up with thick Pliocene-Pleistocene sedimentary sequences, formed under such an extension environment. Over there on 2005 March 5 two earthquakes with about the same magnitude (M L = 5.9) occurred within 68 s and produced intense aftershocks activity according to the records of Central Weather Bureau Seismic Network of Taiwan. We relocated the earthquake sequence by the three-dimension earthquake location algorithm with the newly published 3-D Vp and Vp/Vs velocity model, and determined the first-polarity focal mechanisms of the earthquake doublet. One major cluster of aftershocks which trends E-W and dips steeply to the south can be identified and picked up as a potential fault plane. The focal mechanisms of the two main shocks are both classified as normal type by first-polarity but strike-slip by centroid moment tensor inversion; however two methods both yield consistent E-W strike. Static coseismic deformation was additionally determined from Global Positioning System (GPS) daily solutions at a set of continuous GPS stations and from strong-motion seismographs. These data show NW-SE extension at high angle to the fault plane, which cannot be explained from a simple strike-slip double-couple mechanism. On the other hand, the small vertical displacements and steep fault plane cannot be explained from a simple normal event as well. We present from elastic dislocation modelling that the geodetic data are best explained by significant component of tensile source with centimetre-scale of opening on a 15-km-long fault extending from 1 to 13 km depth. We therefore interpret the crisis as the result of dyke intrusion at the very tip of the Okinawa Trough, which is reasonably driven by backarc spreading.
Tectonics, 2014
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.