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Theunissen T.,Montpellier University | Font Y.,Geoazur | Lallemand S.,Montpellier University | Lallemand S.,LIA Associated International Laboratory | Liang W.-T.,Academia Sinica, Taiwan
Geophysical Journal International

The Ryukyu subduction is known to generate very few earthquakes in its central segment contrarily to its two extremities. We focus in this study on the southernmost part of the Ryukyu subduction zone offshore east Taiwan. Our first task was to build a homogeneous earthquake catalogue for the period 1900-2007. The new catalogue provides homogenized M'W magnitudes and shows that several M'W≥ 7.0 earthquakes occurred offshore Hualien and Suao cities. We then focused on the 1920 June 5 earthquake (reported surface wave magnitude 8.1) previously located beneath the accretionary prism. The revised moment magnitude has been estimated in our catalogue at 7.7 ± 0.2. It is the biggest earthquake ever recorded in the Taiwan area but the fault that has produced this earthquake has not yet been identified with confidence. We relocated this event using regional phases (seismological bulletins archived at the Central Weather Bureau of Taiwan) about 50 km NNE and shallower of its former location, that is, within the Ryukyu Arc basement. According to earthquake bulletin information, revised magnitude, new hypocentral determination and known regional faults, we propose four potential active faults as candidates for the slip associated to this event: (1) the interplate seismogenic zone (ISZ), (2) an out-of-sequence thrust cutting through the forearc and branching on the ISZ, (3) a NS strike-slip fault cutting through the Ryukyu arc and (4) a N-S, westward dipping thrust fault, affecting the Philippine Sea Plate east of the Luzon Arc. The best compromise is to consider a rupture along the ISZ with a shallow nucleation possibly along a splay-fault followed by a downward and lateral propagation of the rupture that would explain the lack of significant seafloor motion and subsequent tsunami. We also estimate the maximum seismic coupling of the ISZ in the region east of Taiwan to about 0.4. In parallel, the evidences of aseismic slip occurring along the ISZ allow us to conclude that this region should only be affected by M < 8 earthquakes. © 2010 The Authors Geophysical Journal International © 2010 RAS. Source

Kang C.-C.,National Central University | Chang C.-P.,National Central University | Chang C.-P.,LIA Associated International Laboratory | Chang C.-P.,Institute National Des Science Of Lunivers | And 8 more authors.
Journal of Asian Earth Sciences

Taiwan's mountain belt is an ideal location to address major questions regarding mechanisms of lithospheric deformation in convergent settings, mountain building processes from oceanic subduction to continental subduction, and post orogenic extension. In the northeast of this belt, the Ilan Plain is a triangular, deltaic plain characterized by a flat topography close to the sea level, and surrounded by the high mountains of the Hsuehshan Range to the northwest, and the Central Range to the southeast. Its eastern coast faces the western tip of the Okinawa Trough, the back-arc basin of the Ryukyu subduction zone. In this study, we analyzed the present-day surface deformation of the Ilan Plain, aiming at deciphering its relationships with basement faults and the regional geodynamic setting. Our approach is mainly based on surface vertical displacements revealed by Persistent Scatterer Interferometry Synthetic Aperture Radar (PSI), which indicate that there is an area of active subsidence (~18. mm/yr) located in the southern part of the plain in probable connection with active basement faults and in agreement with previous geodetic measurements and existing geophysical data. Our PSI results also suggest that the subsidence occurring in the Ilan Plain has moved from north to south during Quaternary in relation with extrusion of the belt due to the westward propagation of the Okinawa Trough through the Taiwan Mountains. © 2015 Elsevier Ltd. Source

Theunissen T.,Montpellier University | Lallemand S.,Montpellier University | Lallemand S.,LIA Associated International Laboratory | Font Y.,University of Nice Sophia Antipolis | And 8 more authors.

The southernmost part of the Ryukyu subduction, where the Philippine Sea Plate is subducting under the Eurasian Plate, is known to be a very seismically active region of transition from a north-dipping subduction along the Ryukyu subduction to an ~SE-NW collision along the Taiwanese orogenic wedge. In this paper, we will focus on the Ryukyu forearc area close to Taiwan where the deformation is paroxysmal. In order to decipher the nature of the seismic deformation in this region, a three month passive experiment, combining 22 Ocean Bottom Seismometers and 51 onland stations, has been led. Starting from an a-priori heterogeneous model, we have obtained 801 well-located earthquake hypocenters, a precise P-wave tomography model and 14 focal mechanisms. The seismicity along the Ryukyu forearc is mainly located not only in the vicinity of the Interplate Seismogenic Zone (ISZ) but also within both the subducting PSP and the overriding plate. Seismicity within the upper-plate is essentially localized east of Nanao basin where NW-SE extension occurs, and northwest of the Hoping basin where strike-slip dominates. As revealed by both the P-wave velocity structure and the newly derived seismicity, we argue that a sub-vertical step offsetting the subducting PSP around 10. km may support the presence of a trench-parallel tear. The PSP also undergoes extension in its upper part that is probably caused by buckling and slab pull. The P-wave velocity structure reveals three other major features: (1) a continuity between the Central Range and the Ryukyu Arc with a shallower Moho (~30 km depth) between ~122.3°N and ~122.5°N along the Ryukyu Arc, (2) high P-wave velocities along the eastern side of the Central Range and, (3) two bodies with similar high crustal velocities (6.5-7.0. km/s) at 12-18. km depths, embedded within the Ryukyu arc basement, just north of Hoping Basin and north of the Nanao Basin. © 2012 Elsevier B.V. Source

Siame L.L.,Aix - Marseille University | Siame L.L.,LIA Associated International Laboratory | Siame L.L.,Institute National Des Science Of Lunivers | Chen R.-F.,Academia Sinica, Taiwan | And 24 more authors.
Journal of Asian Earth Sciences

To tackle the history of active thrusts, it is necessary to open the observation window on time scales on the order of 10 4-10 5 years by studying the surface morphologies resulting from their activities. Because fluvial systems are particularly sensitive to recent environmental changes, geomorphic features such as alluvial terraces are frequently used as markers to gauge tectonic deformation. Together with the measurement of cumulative displacements, the chronological framework of emplacement and abandonment of these geomorphic markers is thus fundamental to determine long-term fault slip-rates. In Taiwan, the geomorphic features associated with fault activity have been studied in detail with a high level of resolution; however, the use of deformed and partially preserved alluvial terraces is often hampered by the absence of well-documented ages. The purpose of this paper is two-fold. First, we take the opportunity to review the chronological constraints that have been published in Taiwan so far. Second, we present how the cosmogenic dating method (in situ-produced 10Be) can be used to constraint the chronological framework of alluvial deposits over a Pleistocene time scale. Thanks to a comparison of our cosmogenic-derived ages with existing data, we present a consistent regional chronological framework for the Pakua-Tadu area along the Changhua Fault, one of the most active frontal thrusts in the Western Foothills of the Taiwan mountain belt. We also discuss its relationships with global eustatism and its tectonic implications for the timing of propagation of the deformation front during the last 450kyr. © 2012 Elsevier Ltd. Source

Siame L.L.,Aix - Marseille University | Siame L.L.,LIA Associated International Laboratory | Angelier J.,British Petroleum | Angelier J.,LIA Associated International Laboratory | And 13 more authors.
Quaternary Geochronology

The direct and feedback relationships between tectonics, climate and denudation are a matter of debate. A better understanding of these relationships requires quantifying rates of denudation in a wide range of climate and tectonic settings, as well as at various time and space scales. Because of an ongoing active collision implying high uplift rates and a climate prone to extreme rainfall events and frequent tropical typhoons facilitating strong erosion dominated by mass movements and high degree of fluvial transport, the Taiwan environment is highly dynamic. In this work, spatially-averaged denudation rates determined along the network of one of the major rivers in Taiwan (Lanyang River) from in situ-produced cosmogenic nuclides (10Be) measured in river-borne quartz minerals are compared to the erosion rates determined from the statistical analysis of modern sediment load data. Integrated over the last several hundreds of years, the denudation rate derived from in situ-produced 10Be concentrations averages 2 ± 1 mm/yr within the Lanyang watershed. Integrated over the last 50 years, the erosion rate given by modern sediment load data is 5-7 mm/yr within the same catchment area. The studied catchment being characterized by a relatively low-level of human activity, the discrepancy between the two rates is most probably due different sensibilities to high-frequency, stochastic erosional events (typhoons and earthquakes). The cosmogenic-derived denudation rates can thus be regarded as more representatives for quantifying erosion processes on the short-time scale, and be strictly compared to the long-term exhumation rates derived from low-temperature chronological data. © 2010 Elsevier B.V. Source

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