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Recent estimates of the rupture scenarios of moderate-to-large earthquakes highlights the need of incorporating segmented fault surfaces into seismic hazard models rather than individual, planar ones. The 2014 Northern Nagano earthquake sequence provides an opportunity to rethink the fault system geometrical complexities of the Itoigawa-Shizuoka tectonic line, one of the most inland active faults in Japan, and their impact on the fault rupture nucleation and development. For this study we used continuous records of 37 seismic stations to detect and locate 2500 events that occurred from 1st October 2014 to 31 December 2014. By refining the automatic arrival time picks, based on the cross-correlation and the hierarchical clustering, we relocated the earthquake hypocenters with the double-difference locationand a three-dimensional (3D) velocity model. We obtained a high-quality earthquake location catalog and the composite focal mechanisms which served for constraining the 3D geometry of the segmented fault system. We found that the early aftershocks are distributed along two sub-parallel, NNE-SSW striking, en-echelon, left-stepping fault segments and analyses on the spatio-temporal evolution of fault activation suggest that the rupture propagates in a clockwise direction, involving first the shallow northern portion of the segmented fault system, then the deeper northern portion of it, and finally the deeper and the shallower parts of the southern portion. Results suggest that a relationship exists between the rupture mechanism and the 3D segmented fault geometry. This relationship between initial structure of the segmented fault controls the earthquake rupture becomes crucial to assess reliable scenarios in seismic hazard., The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

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 margin 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 margin shelf and its necking zone. ©2017. American Geophysical Union., 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.

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 to test the hypothesis of whether or not these regional-scale features of the margin are contributing to along-strike changes in structural style, 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 involvement 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 Eurasian Margin is obliquely colliding with the Luzon Arc to form the Taiwan orogen. This configuration is particularly apparent in south-central part of the island providing a case example to investigate the effects of structural inheritance in the development of the thrust and fold belt. The Eurasian Margin evolved from a pre-Cenozoic continental basement that underwent rifting in the Early Eocene and subsequent sea-floor spreading to form the South China Sea during the late Early Oligocene. The margin underwent localized extension in the Middle Miocene, before the initiation of collision with the Luzon Arc by the Early Miocene. The important along-strike changes in structure and topography of south-central Taiwan thrust and fold belt are evidenced in the detailed geological map and 3 balanced geological cross sections. A 3D tomography model is integrated in this study to help constrain the structure at depth. Major along-strike changes seem to be related to structures oriented at a high angle to the thrust system. These include changes in strike of thrusts and fold traces, the changing elevation of thrusts and stratigraphic contacts, and the growing importance of Middle Miocene sediments within the thrust system that take place from north to south. Horizontal slices of the tomography model illustrate that N-S changes in velocity have the orientation of the inherited structural grain of the Eurasian margin. In particular, the inherited location of the Mesozoic margin’s shelf-slope transition affects the distribution of seismicity and the location of lateral stratigraphic and structural changes. Also, it appears to be associated with the inversion of Eocene- and Miocene-age extensional faults, deeply rooted in the pre-Cenozoic basement that trend oblique to the thrust belt. The inversion of inherited structures affects the uplift of Miocene syn-extensional and syn-tectonic Plio-Pleistocene foreland basin sediments, and of the pre-Cenozoic basement. Section A shows imbricated thrusts merging into a flat, gently west-dipping detachment at the base of PlioPleistocene syn-tectonic sediments in the foothills. While in the area of the Hsuehshan Range, the involvement anduplift of pre-Cenozoic basement is associated to higher topography and to the occurrence of Eocene-Oligocene sediments at the surface. Section B shows thick Miocene stratigraphy and pre-Cenozoic basement rocks involved in the imbricated thrusts. Basement involvement is here also associated with the increased topography observed in the Alishan Range. The Choshui lateral structure that separates sections A and B is associated with a hangingwall culmination wall. It is evidenced in the map by changes in orientation of contacts and fold axis. This lateral structure is also constrained by larger Vp values in the tomography, corresponding to the Alishan basement involvement area. In section C to the south, the imbricated thrusts involve thick Miocene and Plio-Pleistocene sedimentary strata and the westward dipping sole thrust is above the pre-Cenozoic basement. These sedimentary sequences are interpreted to be offscraped above inherited extensional faults that seem to be associated to significant seismic activity at depths of 5-10 km below the sole thrust.

We investigate the geophysical signature within the south-central Taiwan fold and thrust belt of the reactivation of pre-existing structures developed on the Eurasian margin. Seismic tomography (P-wave) and earthquake hypocenters are combined to trace structures mapped on the margin offshore western Taiwan into the fold and thrust belt. The extensional tectonic history of the margin began in the Early Eocene and culminated in the Late Eocene to Early Oligocene with sea-floor spreading and the opening of the South China Sea. Several NE trending basins developed during the rifting of a pre-Cenozoic basement and these were filled with Eocene sediments. Further extension on the outer margin took place during the Middle to Late Miocene, forming basins that are now involved in the Taiwan deformation. Finally, the margin’s transition from the platform to the slope takes place across south-central Taiwan and is oriented at a high angle to the active deformation front. We define the basement as pre-Eocene rocks and use a P-wave velocity (Vp) of 5.2 km/s as a proxy for the interface between them and their younger cover. This Vp interface is characterized by highs and lows that can be interpreted to image basement topography whose possible causes we investigate here. In the Hsuehshan Range there is a pronounced shallowing of the 5.2 km/s surface across the Shuilikeng fault. It is accompanied by an east-dipping cluster of seismicity down to more than 25 km depth, and forming what appears to be a crustal ramp across which the Eocene-age Hsuehshan Basin is being inverted. Westward, the 5.2 km/s interface forms a high called Paikang basement high, the southern flank of which is the on land projection of the Mesozoic basement shelf break. Southward, there is an increase in seismicity and topography that is associated to a NE-SW oriented lateral structure in the fold and thrust belt. South of this lateral structure, beneath the Alishan Range, a shallowing of the 5.2 km/s interface marks a change in the structural grain of the fold and thrust belt. This, together with the increase in predominatedly scattered seismicity that reaches greater than 20 km depth, can be interpreted to indicate basement involvement in the deformation. To the south of the Alishan Range a zone of higher velocity whose orientation is roughly parallel to that of the Mesozoic basement shelf break is interpreted as a basement high. Here, there appears to be SW-dipping clusters of hypocenters associated with the shallowing of the 5.2 km/s surface. We interpret this shallowing of the 5.2 km/s surface to be related to an extensional fault block (or blocks) on the upper slope area of the basement. The seismicity clusters are possibly imaging the reactivation of the extensional faults that bound it.

Studies of mountain belts worldwide have shown that the structural, mechanical, and kinematic evolution of their foreland fold and thrust belts are strongly influenced by the structure of the continental margins that are involved in the deformation. The area on and around the island of Taiwan provides an unparalleled opportunity to investigate this because the entire profile of the Eurasian margin, from the shelf in the north to the slope and continent-ocean transition in the south and the offshore, is currently involved in the collision. Taiwan, then, can provide key insights into how such features as rift basins on the shelf, the extensional faults that form the shelf-slope break in the basement, or the structure of the extended crust and morphology of the sedimentary carapace of the slope can be directly reflected in the location and pattern of its seismicity, in its topography, and in its structural architecture, among other things. The continental margin of the Eurasian Plate that is currently involved in the Taiwan orogeny is thought to have evolved from a sub-continental subduction system in the Late Cretaceous to a rifting margin by the Early Eocene and, during the late Early Oligocene, to sea-floor spreading and the formation of the South China Sea, followed by localized extension in the Middle Miocene and, finally, collision with the Luzon Arc by the Early Miocene. Imaging features of the margin’s structure in the Taiwan orogen is possible with seismic tomography, which shows, for example, that there are notable changes in velocity that can be directly attributed to structures in the basement. For example, there is a marked increase in Vp beneath the Hsuehshan Range which can be interpreted to be related to the uplift of higher velocity basement rocks by basin inversion. This is accompanied by significant seismicity that reaches a depth of more than 30 km’s, and by surface uplift to form the highest topography in Taiwan. Furthermore, beginning at 8 km depth, but becoming especially prominent at 12 km and 16 km depth, there is an embayment of relatively high Vp that can be interpreted as the onshore projection of a basement high that occurs between the Mesozoic basement shelf break and the Taihsi Basin. Southward, there is a notable northeast-southwest-oriented increase in seismicity across the on land projection of the Mesozoic basement shelf break, with seismicity predominantly located beneath the Alishan Ranges, the highest topography in this part of the mountain belt. The topography also shows a pronounced re-entrant that coincides with the orientation and onshore projection of the Mesozoic basement shelf break that extends northeastward across the Western Foothills and into the Hsuehshan Range. The shallow structure of south-central Taiwan contains a number of features, such as changes in structural grain, or basement involvement in the deformation, that can be attributed to the presence of pre-existing structures in the Eurasian continental margin as it enters into the deformation of the Taiwan fold and thrust belt.

This article is subject to a CC Attribution 3.0 License., The Alishan area of Taiwan includes a sector of the fold-and-thrust belt that 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 fold-and-thrust belt with the highest elevations includes important along-strike changes in structure, stratigraphy, and seismic velocities. Here 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 cut-off, 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 south-eastern 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, deeply rooted in the pre Cenozoic basement that were along what was the shelf-slope transition in the Early Oligocene. Inversion causes uplift of the margin sediments and their higher P wave velocity basement during Pliocene-Pleistocene thrusting.

The Taiwan mountain belt is generally thought to develop above a shallow, through-going basal detachment confined to within the sedimentary cover of the Eurasian continental margin. Recently acquired surface geology, when combined with seismic tomography, earthquake hypocenter, and gravity data suggest, however, that basement is also involved in the deformation in south-central Taiwan. Since basement does not crop out in south central Taiwan, we adopt a P-wave velocity of 5.2 km/s has a proxy for the basement-cover interface and use it as a reference horizon. This interpretation is then tested as a density contrast by 3D modeling of the Bouguer anomaly data. The good fit between the two suggests that this surface provides a reasonable proxy as an interpretation for the top of the basement in what follows. The data suggests that, in the west, beneath the Coastal Plain and the Western Foothills, the mountain belt is evolving above a southward deepening level of detachment that is illuminated by sub-horizontal clusters of earthquake hypocenters located near the basement-cover interface. In much of the study area, this detachment appears to be below the depth of the basal thrust determined from geological constraints provided by the thrust system mapped at the surface. Furthermore, when viewed in a northwest-southeast orientation, the hypocenters cluster that defines the detachment appears to mimic the rugose extensional fault architecture imaged in the off shore to the west of Taiwan. This suggests that pre-existing faults on the margin, although oriented at a high angle to the convergence direction, are playing an active role in the development of thrust belt while still not being completely inverted. Eastward, the detachment at the basement-cover interface joins with an east-dipping hypocenter cluster that is interpreted to form a ramp that extends to greater than 20 km depth, into the middle crust. Above this ramp, basement rocks (P-wave > 5.2 km/s) are uplifted to near the surface, forming a basement culmination beneath the Hsuehshan and Central ranges. The uplift of these basement rocks, together with the juxtaposition of higher metamorphic grade rocks across the Shuilikeng, Lishan and Chaochou faults suggests that these faults form a linked fault system that extends downward into the middle crust at the location of the ramp.

[spa] La estructura de la cordillera de Taiwán se considera constituida por un sistema de cabalgamientos y pliegues desarrollados sobre un despegue basal con suave inclinación, situado en la cobertera sedimentara de la margen continental Euroasiática. Una cantidad creciente de datos de sismicidad y de geología de superficie, sin embargo, indican la existencia de actividad generalizada de fracturas en la corteza media e inferior y sugieren que los niveles de la corteza por debajo de la ubicación del despegue basal también están actualmente involucrados en la deformación. En esta tesis, nuevos datos de geología de superficie se combinaron con varios conjuntos de datos geofísicos disponibles para encontrar un modelo para la estructura de la parte sur-central de la cordillera de Taiwán. Los resultados de este trabajo indican que debajo de la parte interna de la área de estudio el desarrollo estructural de la cordillera de Taiwán esta controlado por fallas mayores con alta inclinación, que penetran hasta partes profundas de la corteza, y que están reactivando fallas preexistentes heredadas de la margen continental Euroasiática. Rocas de basamento están elevadas a lo largo de estas fallas y forman una culminación por debajo de las partes internas de la cordillera. Por debajo de el Coastal Plain y de las Western Foothills en la parte externa de la área de estudio, sin embargo, la mayor parte de la deformación parece estar teniendo lugar cerca de la interfaz basamento-cobertera, que está actuando como un amplio nivel de despegue basal y aún conserva la geometría extensional heredada de la margen continental Euroasiática., [eng] The Taiwan thrust belt is generally thought to develop above a shallow, through-going basal detachment confined to within the sedimentary cover of the Eurasian continental margin. A number of data sets, however, such as surface geology, earthquake hypocentre, and seismic tomography data among others, suggest that crustal levels below the interpreted location of the detachment are also currently being involved in the deformation. In this thesis, new surface geology data were combined with several available geophysical data sets to find a model for the structure of the south-central part of the thrust belt that takes into account deformation taking place at depth. Results of this thesis indicate that beneath the internal Hsuehshan and Central ranges the structural development of the south-central Taiwan thrust belt is controlled by steeply dipping and deep-penetrating faults that are currently inverting pre-existing basement faults inherited from the Eurasian continental margin. Basement rocks are uplifted along these faults to form a basement culmination in the interior of the thrust belt. Beneath the more external Coastal Plain and Western Foothills, however, most of the deformation appears to be taking place near the basement-cover interface, which is acting as an extensive level of detachment and still preserves the extensional geometry inherited from the Eurasian margin.