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Abstract Jurassic to Eocene sedimentary rocks in central Arabia have been deformed by faulting and jointing. We measured these structures in the field. Paleostress directions from these data demonstrate that successive extensional events occurred. Introduction The effects on platforms of distal tectonic events can be used to study the deformation in front of mountain belts (Bergerat, 1987) or between more highly deformed regions (Bergerat et al., 1992). These studies may yield clues to the directions of plate motion which may be difficult to decipher within the mountain belts due to the complexity of the deformation, or they may aid in determining the deformation process. Unlike regions previously studied in this context, the Arabian Platform is not just in front of a mountain belt such as the West European Platform (Bergerat, 1987) nor trapped between deformed zones such as the Colorado Plateau (Bergerat et al., 1992), but is the foreland of the Bitlis/Zagros/Oman deformation belt and is also near the Red Sea/Gulf of Aden which have been forming as small ocean basins simultaneously with convergence in the mountain belt. Previous workers have argued that the Red Sea/Gulf of Aden could not have formed as a "passive" rift related to the formation of the Bitlis/Zagros belt because the Arabian Platform is undeformed. In this paper, we show that the central Arabian platform has indeed been deformed and we link its deformation to tectonic events occurring on the margins of the Arabian plate supporting models for a passive origin of the Red Sea/Gulf of Aden (Gordon and Hempton, 1986; Hempton, 1987; Bohannon et al., 1989).

The direct and feedback relationships between tectonics, climate and denudation are a matter of debate.

SUMMARY We attempt at providing new insights about the earthquake behaviour in western Taiwan, based on a comparison between historical information and present-day instrumental records. We provide a consistent picture of the earthquake history during the last four centuries and draw some inferences in terms of seismic cycle. Before instrumental seismic observation in Taiwan started at the end of the 19th century, ancient written earthquake records are available from the ancient Chinese governments and the public concerning the 17th, 18th and 19th centuries. Distribution of casualties and property damage, indicating seismic intensity, can be estimated from archives. Using the Central Weather Bureau (CWB) intensity scale we calibrate the intensity–magnitude relationships from the instrumental seismicity recorded from 1995 to 2005, showing that within the range of historical uncertainties and earthquake depths in western Taiwan, 0–40 km, the depth is not critical in these relationships. With estimated intensities, the magnitudes of historical earthquakes can be evaluated based on a single average empirical relationship between ML, the local magnitude, and I0, the epicentral intensity: ML = 0.08I0 2 –0 .04I0 + 3.41. Three types of diagrams are then proposed to describe the historical seismicity. The first type involves simple representation of earthquake events according to time and magnitude. The second type involves cumulative plot of the released elastic energy with time, as calculated from reconstructed magnitudes. The third type of diagram shows the evolution of cumulative seismic strain release with time, based on a Benioff’s law indicating that the release of elastic strain related to an earthquake is proportional to the square root of the dissipated energy. These curves highlight inferences of historical seismicity analysis in terms of earthquake frequency and seismic cycle duration in the different segments of the front belt and foreland zones of western Taiwan, with large contrasts suggesting different levels in earthquake hazard.

SUMMARY Application of the interferometric method to four ERS2-SAR images acquired before and after the 1999 Chichi earthquake has allowed determination of the coseismic surface displacement in the footwall area of the Chelungpu fault. The interferometric results revealed a relative shortening in the round trip distance between the radar antenna and the ground of the footwall side of Chelungpu fault, during the earthquake. This shortening progressively increased from the west to the east and reaches the maximum amount of approximately 26 cm near the central segment of the Chelungpu fault. Our interferometric results have been precisely examined using a dense GPS network in the investigated area. We mapped the GPS coseismic measurements into the radar line of sight and implemented a forward simulation of SAR interferogram from this synthesized result to control our unwrapping performance. In this study, these two observations are compared with a 3-D dislocation model of the fault. Finally, a deformation analysis based on our interferometric result has indicated that a segment with irregular deformation behaviour can be distinguished in the footwall area of the Chelungpu fault. This segment may result from either the influence of inherited basement faults or the presence of a structural terrace that provide local opportunities for superficial deformation.

As a result of oblique collision, the Taiwan orogen propagates southward. The Hengchun peninsula in the southern tip of the Taiwan Central Range, preserving the youngest, the least deformed and the most complete accretionary prism sequences, allows therefore better understanding of the tectonic evolution of Taiwan orogen. On the Hengchun peninsula, four main stages of paleostress can be recognized by the analysis of brittle tectonics. After recording the first two stages of paleostress, rocks of the Hengchun peninsula (the Hengchun block) have undergone both tilting and counterclockwise rotation of about 90j. The structural boundaries of this rotated Hengchun block are: the Kenting Melange zone in the southwest, the Fongkang Fault in the north, and a submarine backthrust in the east. The angle of this rotation is principally calculated by the paleomagnetic analysis data and a physical model experiment. Through a systematic back-tilting and back-rotating restoration, the original orientations of the four paleostress stages of Hengchun peninsula are recognized. They are, from the ancient to the recent, a NW-SE extension, a combination of NW-SE transtension and NE-SW transpression, a NE-SW compression, and finally a combination of NE-SW transtension and NW-SE transpression. This result can be explained by a phenomenon of stress axes permutation, instead of a complex polyphase tectonism. This stress axes permutation is caused by the horizontal compression increase accompanying the propagation of the accretionary prism. Combining the tectonic and paleomagnetic data with paleocurrent and stratigraphic data enables us to reconstruct the tectonic evolution of the Hengchun peninsula. This reconstruction corresponds to the deformation history of a continental margin basin, from its opening to its intense deformation in the accretionary prism. D 2002 Elsevier Science B.V. All rights reserved.

International audience; The Barrôt area presents an important contrast of deformation between a foliated, faulted and non-rotated Permian substratum and a folded and strongly rotated Meso-Cenozoic sedimentary cover. The Permian substratum and the sedimentary cover are separated by a décollement level of Triassic gypsum. While numerous studies have been done on the Dôme de Barrôt (i.e., the Permian substratum), we focused our work on the deformed Meso-Cenozoic sedimentary cover around the Dôme de Barrôt where we measured and analyzed both the magnetic susceptibility fabrics and the paleomagnetic directions. Our results highlight an arcuate pattern of the directions of shortening trajectories revealed by type II-III fabrics of anisotropy of magnetic susceptibility (which predates the folding) and a later syn- or post-folding anticlockwise rotation of large amplitude (64. ±. 14°) for the sedimentary cover. The arcuate pattern of the directions of shortening and the different rotations between the Permian substratum and the sedimentary cover can be partly explained after restoring the paleogeographic location of the sedimentary cover before the Oligocene Alpine compression event. We tend to interpret the remaining 30° anticlockwise rotation as a result of the high efficiency of the Triassic gypsum décollement surface, a nearby regional left-lateral shear zone (RDFZ), and a possible near-field gravitational sliding near the later exhumed Dôme de Barrôt.

Plate breakup over plumes is characterized by the development of margins showing extensive magma production both underplated at Moho level and extruded as thick piles of seaward dipping lava formations. The Disko-Svartenhuk area in west Greenland is one of the few places in the world with exposed seaward dipping basalts forming a prism whose thickness increases seaward. We present a quantitative tectonic study of this margin, which we tentatively restored in its geodynamic position during the different stages of plate separation between Greenland and North America. Our structural data are constrained with recently published 39Ar/40Ar and new and coherent 40K/40Ar geochronology in dikes of different orientations. The first-order structure is that of a tectonically-driven seaward crustal flexure linked to definitive plate breakup between Greenland and Baffin Island during the Eocene, coeval with the formation of the upper part of the exposed seaward dipping volcanic prism. This flexing is associated with a significant crustal stretching associated with arrays of continentward dipping normal faults. This across-strike structure is correlated to a fundamental along-strike segmentation with the three “segments” adopting a “zigzag” strike. There is a clear increase of extensional strain at the extremities of the segments. Eocene extension trended N060 on average in the northern and southern segments but was NW trending in the central NE trending Nugssuaq segment. We discuss the interpretation of such an extension perpendicular to the different margin segments. From a regional point of view the N060 extension is distinct in orientation from the approximately N-S trend of plate separation between North America and Greenland in the Baffin Bay during the Eocene. However, the extension recorded in the margin is nearly perpendicular to the obliquely spreading Eocene accretion axis in the Baffin Bay. This latter point suggests a mantle or ridge control of the development of the margin over regional far-field lithospheric stress models.

The 21-September-1999 Chichi earthquake (Mw=7.6) occurred near the western thrust front of the Taiwan mountain belt. The characteristics of this major earthquake bring new constraints on the deep structure of the western foothills of Taiwan. Seismotectonic information is combined with subsurface data in order to establish new balanced geological sections across the thrust zone. The structural style of the Taiwan belt front is characterized by the superimposition of multiple ‘decollements’, not only in the shallow sedimentary cover but also in the basement. Interpretations of drillings and seismic-reflection data indicate that the thrust is inherited from a normal fault, reactivated and inverted when collision reached the Foothills belt. The Chelungpu–Sani thrust observed at the surface is connected in depth (∼10 km) with a thrust ramp that dips 25° to the east in the basement. The tectonic history of this front thrust implies that a ‘decollement’ level exists, probably located at the brittle-ductile transition in the crust, and results in decoupling of the cover and shallow basement relative to the deeper basement. Such a ‘decollement’ could explain the basin inversion observed in the foreland platform. The occurrence of a thick-skinned tectonic deformation at the Taiwan belt front had been predicted by analyses of structural geology; it is now confirmed by the study of the Chichi earthquake.

Data from continuously monitored creepmeters across the active Chihshang Fault in eastern Taiwan are presented. The Chihshang Fault is an active segment of the Longitudinal Valley Fault, the main suture between the converging Philippine and Eurasian plates in Taiwan. Since the 1951 earthquake (Mw=7.0), no earthquake larger than magnitude 6.0 occurred in the Chihshang area. At least during the last 20 years, the Chihshang Fault underwent a steady creep movement, resulting in numerous fractures at the surface. Five creepmeters were installed in 1998 at two sites, Tapo and Chinyuan, within the Chihshang active fault zone. One-year results (from August 1998 to July 1999) show a horizontal shortening of 19.4±0.3 mm and 17.3±0.7 mm, at Tapo and Chinyuan, respectively. These annual shortening rates are in a good agreement with other estimates of strain rate independently obtained from geodetic measurements and geological site investigation. The creepmeter measurements were made on a daily basis, providing accurate information on the previously unknown evolution of creep during the year. The records of fault creep at the Tapo site thus revealed close seasonal correlation with average rainfall: the period of high creep rate coincides with the wet season, whereas that of low creep rate coincides with the dry season. Also, in comparison with the Tapo site, the creep behaviour as a function of time is complex at the Chinyuan site. Possible factors of irregularity are under investigation (thermal effect acting on the concrete basement of the creepmeters, earth tide effect, water table variations in a nearby rice field, and rainfall). The comparison between GPS measurements across the Longitudinal Valley (31 mm/year of horizontal displacement) and the creepmeter measurement across the Chihshang Fault zone (17–19 mm/year of horizontal displacement) suggests that there exists other shortening deformation across the active fault zone in addition to those we have measured from the creepmeters.

Juxtaposed against the remnant forearc basin sequences along thrust faults, the Lichi Melange of the Coastal Range of Taiwan is composed of exotic ophiolite and sedimentary blocks, metric to kilometric in size, and coherent turbidite beds, all embedded in a sheared scaly argillaceous matrix. The Lichi Melange is controversial in origin, being interpreted either as a subduction complex, or as an olistostrome. By separating four main deformation levels within the Lichi Melange and adjacent sedimentary rocks, we establish detailed geological maps and structural profiles in two key areas of the Lichi Melange. We reconstruct also the evolution in cross-section and calculate the approximate minimum amount of shortening that corresponds to folding and thrusting in these areas. Our field studies suggest that the Lichi Melange most likely arose from the shearing of lower forearc sequences rather than from a subduction complex or an olistostrome. This conclusion is supported by the structural analysis, the clay mineral distribution, and some interfingering sedimentary relationships between the Lichi Melange and the lower Takangkou Formation. We also undertake a comprehensive tectonic analysis of the shear surfaces in the Lichi Melange. The direction of the maximum compressional stress that we obtain is N100° ∼120°E, compatible with that of plate convergence. During the most recent stage of collision, between the Eurasian plate (eastern Central Range of Taiwan) and the Philippine Sea plate (Coastal Range), a major fault zone developed along the innately weak zone of melange, further increasing the shear deformation pattern of the Lichi Melange. This Longitudinal Valley Fault separates the Eurasian plate and the Philippine Sea plate and is one of the most active faults in Taiwan. It can be considered as the present plate boundary in the Taiwan arc–continent collision terrane. According to our reconstruction, this plate boundary of the Longitudinal Valley originated as a submarine arc–prism boundary.

The arcuate Pakuashan anticline is located in the outermost front units of the Western Foothills of Taiwan. This oblique feature of the deformation front is investigated in terms of combined morphostructural analysis, based on imagery and digital elevation model as well as microtectonic analysis of fault slip data. A subsurface structural study based on available seismic and well data was also carried out, resulting in improved mapping of the Neogene series and associated structures. This mapping allowed construction of several along-strike cross sections. Such combined analyses revealed that the transverse Pakuashan fold is located above a major transfer fault zone. This active fault zone accommodates differential westward propagation of thrust units; its kinematic evolution is principally controlled by the geometry of the foreland Peikang High, behaving as a buttress for the west verging thrust sheets. A preliminary analytical model of the oblique thrusting at Pakuashan is based on similar cases studied by Apotria et al. [1992]. It involves quaternary transfer faulting accommodating the motion of connected thrust sheets, moving over oblique ramps linked to a preexisting major basement boundary (the hinge fault of the Peikang High). This analytical modeling accounts for the occurrence of local extension at the intersection of oblique ramps in the southern Pakuashan. Numerous complementary structural and tectonic evidences led us to establish a complete deformation model, involving local rotation in southern Pakuashan which caused differential slips in northern Pakuashan, resulting in tear faulting. These evidences include large extension at the intersection of oblique ramps, distributed extension in the transverse zone, regional wrench deformation, absence of major reorientation of local stress inside the transverse zone, along-strike variation of structural styles coupled with low to high uplift rate from the Northern to the Southern part of the Pakuashan fold. Thus a synthetic reconstruction of the Pakua Transfer Fault Zone evolution is proposed, as a typical example of active transfer faulting, evolving gradually from a primary tear fault with a slight curvature to the left-lateral tear fault or transfer fault that offsets two distinct frontal thrust-and-fold sheets.

Repeated measurements of active deformation were carried out at three sites along the active Chihshang Fault, a segment of the Longitudinal Valley Fault zone of eastern Taiwan (the present-day plate boundary between the Philippine Sea Plate and Eurasia). Reliable annual records of displacement along an active fault, were obtained based on detailed surveys of faulted concrete structures. Along the active Chihshang Fault striking N18°E, we determined average motion vectors trending N37°W with an average shortening of 2.2 cm/yr. Thus, the transverse component of motion related to westward thrusting is 1.8 cm/yr, whereas the left-lateral strike-slip component of motion is 1.3 cm/yr. The fault dips 39–45° to the east, so that the vertical displacement is 1.5–3 cm/yr and the actual oblique offset of the fault increases at a rate of 2.7–3.7 cm/yr. This is in good agreement with the results of regional geodetic and tectonic analyses in Taiwan, and consistent with the N54°W trend of convergence between the northernmost Luzon Arc and South China revealed by GPS studies. Our study provides an example of extreme shear concentration in an oblique collision zone. At Chihshang, the whole horizontal shortening of the Longitudinal Valley Fault, 2.2 cm/yr on average, occurs across a single, narrow fault zone, so that the whole reverse slip (about 2.7–3.7 cm/yr depending on fault dip) was entirely recorded by walls 20–200 m long where faults are tightly localized. This active faulting accounts for more than one fourth (27%) of the total shortening between the Luzon Arc and South China recorded through GPS analyses. Further surveys should indicate whether the decreasing shortening velocity across the fault is significant (revealing increasing earthquake risk due to stress accumulation) or not (revealing continuing fault creep and ‘weak’ behaviour of the Chihshang Fault).

A multisource and multiscale approach of Taiwan morphotectonics combines different complementary geomorphic analyses based on a new elevation model (DEM), side-looking airborne radar (SLAR), and satellite (SPOT) imagery, aerial photographs, and control from independent field data. This analysis enables us not only to present an integrated geomorphic description of the Taiwan orogen but also to highlight some new geodynamic aspects. Well-known, major geological structures such as the Longitudinal Valley, Lishan, Pingtung, and the Foothills fault zones are of course clearly recognized, but numerous, previously unrecognized structures appear distributed within different regions of Taiwan. For instance, transfer fault zones within the Western Foothills and the Central Range are identified based on analyses of lineaments and general morphology. In many cases, the existence of geomorphic features identified in general images is supported by the results of geological field analyses carried out independently. In turn, the field analyses of structures and mechanisms at some sites provide a key for interpreting similar geomorphic featues in other areas. Examples are the conjugate pattern of strike-slip faults within the Central Range and the oblique fold-and-thrust pattern of the Coastal Range. Furthermore, neotectonic and morphological analyses (drainage and erosional surfaces) has been combined in order to obtain a more comprehensive description and interpretation of neotectonic features in Taiwan, such as for the Longitudinal Valley Fault. Next, at a more general scale, numerical processing of digital elevation models, resulting in average topography, summit level or base level maps, allows identification of major features related to the dynamics of uplift and erosion and estimates of erosion balance. Finally, a preliminary morphotectonic sketch map of Taiwan, combining information from all the sources listed above, is presented.

International audience; A tectonic analysis of brittle deformaton was performed in the Rhine-Saône transform in order to decipher and understand the deformation processes in a typical continental hansfer zone. Cenozoic major and minor fault patterns are described, and brittle structures are interpreted in terms of paleostress orientations. The Eocene N-S compression related to Africa-Eurasia convergence is marked mainly by strike-slip faults. It was followed by a major phase of crustal extension, mainly Oligocene in age, related to the west European rifting. This extensional phase is expressed by normal faults, tension gashes, and extensional strike-slip faults which are especially abundant in the transform zone. These two events are found to be quite different in terms of distribution of horizontal shess trajectories. The trend of ol axes is very homogeneous for the N-S compression, whereas o3 orientations associated with the major extension vary from E-W in most of the Rift system to I.&V-SE within the transform zone. Finally, the Mio-Pliocene tectonic emplacement of the Jura fold-and-thrust belt is marked in the field by strike-slip faults associated with a well-defined trend of ol oriented W-IIW-E-SE to NW-SE. The preexisting basement fracture pattern between the grabens was also considered in order to estimate which faults were likely to have been reactivated during the Oligocene transform kinematics, and the theoretical shear motion on these inherited fault surfaces, when submitted to E-W extension, was calculated. Left-lateraUextensional reactivation of a preexisting 60"N trending fault system in the brittle upper crust is likely to have accommodated the shear strain arising between the rift segments in response to crustal stretching. Two independent numerical modelings, based on distinct element and finite element analyses, respectively, enabled us to verify and to refine the hypothesis of transform kinematics related to E-W extension and inducing regional perturbations of extensional stress trajectories. The overall mechanism and the transtensional kinematics of the Rhine-Saône transform zone are thus tightly conshained. The inherited crustal anisotropy, the large-scale perturbations of extensional stress trajectories, and the development of distributed brittle deformation within the sedimentary cover are found to play a major role in the tectonic evolution of continental rift-transform systems.

Geophysical Journal International, vol. 177, n°2, pp. 609 - 623, 2009; International audience; The 2003 Chengkung earthquake (Mw 6.8) occurred on the east-dipping Chihshang fault of eastern Taiwan, a listric fault with dominant reverse motion. A joint inversion taking into account geodetic (GPS) and seismological (accelerometric and teleseismic) data is performed. This modelling highlights the rupture process behaviour on the fault plane at depth. The coseismic rupture developed essentially between 12 and 26 km depth, expanding laterally by about 30 km towards the SSW. Two slip patches dominated, with slip values exceeding 1.5 m. During the 30 s of the whole process, the rupture propagation decelerated with time, starting at 3.4 ± 0.3 km s–1 in the two main slip patches and ending with velocities below 1.6 ± 0.1 km s–1. The 48 hr of aftershocks that followed the main shock are distributed essentially inside the coseismic slip patches on the fault surface, except in the deepest, southern part of the fault where slip is less resolved. The results of our modelling illustrate the particular behaviour of the 2003 Chengkung earthquake where the coseismic slip rapidly diminished towards the surface, in good agreement with the observation of fast post-seismic slip followed by interseismic creep along the Chihshang fault.

The aftershock sequence of the 1999 Chi-Chi earthquake (Mw = 7.6) provided a large number of focal mechanisms recorded with BATS Centroide Moment Tensors (CMT) which allowed us to perform reliable stress tensor inversions in North Central Taiwan. These inversions were done for three time-periods corresponding to the post-seismic phase of the 1999 Chi-Chi earthquake (Mw 7.6). In the last period the 2003 Chengkung earthquake (Mw = 6.5) occurred on the vicinity of our study area and its influence is also examined. For comparison between seismotectonic regimes of the pre- and post- Chi-Chi periods, we also carried out stress inversion using published focal mech of the pre Chi-Chi period. For each period, we delineated seismogenic domains with relatively homogeneous stress regimes in agreement with the local structures. The boundaries of these domains vary during the whole post Chi-Chi period as seismic activity decays. A 25° clockwise rotation of the main compressive stress σ1 is observed between the pre and the post Chi-Chi periods in the domain located near the northern termination of the Chi-Chi rupture. A stress perturbation related to the Chengkung earthquake could be detected in the eastern part of the Central Range. We present an interpretation of the determined stress regimes in terms of changes in mechanical behaviour of the Sanyi–Puli transfer zone.

Presents four case examples of contribution on InSAR to active tectonics issues in Taiwan Island that is one of the most seismically active regions in the world. For these studies, differential InSAR technique is used in a 2-pass approach, the resulting interferograms combine images of the ERS-1/2 satellites from 1993 to 2001. The results illustrate the different tectonic processes that InSAR can investigate in Taiwan: (1) regarding the 1999 Chi-Chi earthquake event, InSAR allows to capture coseismic displacements, and to detect displacements at nearby faults triggered by the earthquake, (2) in the Tainan area (SW Taiwan), InSAR can measure the interseismic crustal deformation field (uplift of an anticline) over eight years, (3) for the Fengshan fault and Longitudinal Valley fault, InSAR is able to monitor fault creep. This study makes it possible to consider InSAR as a tool (in complement to the GPS network) for monitoring several active faults in Taiwan that have the potential to produce earthquakes.

The purpose of this study is to develop a distance learning program based on virtual reality (VR). The stream erosion (SE) in Taiwan is chosen as a case study of interactive VR distance learning (DL). This study integrates various sources to develop an interactive VR distance learning program. Taiwan is one of the areas with a high stream erosion rate. This high erosion rate has attracted worldwide attention and is regarded as a good example for geosciences education. This research is aimed to help motivate learners with practical information that is perceptually easy to process. Also, in the VR distance learning environment unlike the traditional classroom, each student can learn individually with the computer according to his/her own time, location, and pace.

[1] The daily creep meter data recorded at Chihshang in 1998–2001 are presented. The Chihshang creep meter experiment was set up across the Chihshang thrust fault, the most active segment of the Longitudinal Valley Fault, which is the present-day plate suture between the Eurasian and the Philippine Sea plates in eastern Taiwan. Near-continuous data recording at two sites revealed different surface fault motions yet similar annual shortening rates: 16.2 mm at the Tapo site (comprising two connected creep meters) and 15.0 mm at the Chinyuan site (three creep meters straddling parallel fault branches). Four of the five creep meters showed a seasonal variation, with the fault moving steadily during the rainy season from April to October, and remaining quiescent during the rest of the year. The only exception was recorded by the creep meter located on a melange-composed hillslope, where local gravitational landsliding played an additional role other than tectonic faulting. Through comparison with daily precipitation data, we inferred that moderate rainfall suffices to trigger or facilitate slippage on the surface fault, during the transition period of the dry/wet season. During the observation period from 1998 to 2001, the subsurface seismicity exhibited clusters of microearthquakes on the Chihshang Fault at depths of 10–25 km. Recurrent earthquakes occurred regardless of whether the season was wet or dry, indicating that the stress relaxation associated with seismicity in the seismogenic zone did not transfer immediately up to the surface. The accumulated strain on the Chihshang Fault at shallow surface levels was released through creep during the wet season. In addition to these short-term seasonal variations, an apparent decrease in the annual slipping rate on the Chihshang Fault during the last few years deserves further investigation in order to mitigate against seismic hazard.

The differential SAR interferometry technique (interferometric synthetic aperture radar, InSAR) is applied on the Chelungpu fault surface rupture zone of the September 20, 1999, Taiwan, Chi-Chi earthquake using six ERS-2 images covering the period from February 1999 to January 2000. As compared with available geodetic data, InSAR measurements result in more extensive analysis because of high spatial sampling and centimetric accuracy. However, coseismic displacements can be evaluated only on the footwall of the fault. The analysis of interferograms shows the existence of a linear trend in phase difference mainly caused by orbital errors, which we removed from interferograms using GPS data. The corrected interferograms provide a precise map of the InSAR component of the coseismic displacement, showing a continuous decrease over the footwall from a maximum of 36.7 cm at the fault east of Taichung city to a value of about 5 cm at the coastline 30 km further west. The map analysis reveals that the Changhua fault (whose surface trace is located about 20 km west of the Chelungpu one) and the Tuntzuchio fault influence the displacement field. We interpret this in terms of minor reactivation of these faults triggered by the earthquake. A 1.7 cm uncertainty, estimated from the GPS data, is proposed to quantify the precision of the map. Beyond this single value, we highlight the interest of having several coseismic interferograms to evaluate the reliability of the map in a more comprehensive way. Comparisons with displacements inferred from models of slip distribution inverted without InSAR data highlight the advantage of carrying out a joint inversion including our results as new constraints.

In Taiwan area, the instrumental observation of earthquakes has been initiated since 1898, and up to 2000, about 180,000 earthquakes have been accumulated in the synthetic catalog organized by the Central Weather Bureau of Taiwan (abbr. CWB). In this study, we aim to have a systematical comprehension from preliminary statistical analysis of seismicity in terms of Taiwan’s seismotectonics. We firstly applied a completeness test for the CWB seismicity catalog for determining the reliable records which can be considered to correspond with Taiwan’s tectonic activity. Afterward, b-value and z-value have been realized after such reliable records. The b-value analysis has extracted the relationship between the seismic behaviors and tectonic setting. The application of the z-test demonstrated a series of rate changes before and after the Chi-Chi earthquake.

International audience

International audience; Structural and paleostress analyses provide new insights into the Quaternary kinematics of the outermost fold-and-thrust units of southwestern Taiwan Foothills. The frontal folds are interpreted as fault-related folds, and their tectonic evolution through space and time is tightly constrained. Fold development is correlated with reef building on top of the anticlines. Moreover, we provide field evidence that NW–SE fault zones oblique to the structural grain of the belt probably acted as transfer fault zones during the the Quaternary fold-thrust emplacement. Two successive Quaternary stress regimes are evidenced in southwestern Taiwan: A NW–SE compression, followed by a recent nearly E–W compression. The latter shows an along-strike change from pure E–W contraction to the north to perpendicular N–S extension in the south. This southward decrease in N–S confinement probably represents the on-land signature of the incipient Quaternary tectonic escape predicted by analogue and numerical modelling and evidenced at present-day by Global Positioning System data.

The geometrical description of fault patterns in a given rock mass can be made through an estimation of three sets of quantitative parameters

We describe the electronic supplement for our article, “Stress Tensor Inversion for the Chi-Chi Earthquake Sequence and its Implications on Regional Collision.” The archived data files are given on the attached CDROM, under the directory of \KaoH: (1) readme.txt: an ASCII text file describing the contents and formats of the data files (this short note and additional explanations). (2) Source\_parameters.rtf: a Rich-Text-Format file of Table 1 as shown in our article. It contains all the source parameters of events used in our study. (3) Source\_parameters.pdf: a PDF file of Table 1 (as shown …

The diversity of the research in earth sciences leads to a multisource approach to investigate the surroundings of Taiwan. Various data sets combined by a Geographical Information System (GIS) enable us to propose an integrated neotectonic map of onshore-offshore Taiwan. Several technical problems arise during the compilation of this map, such as the homogeneity of scales, projections and geodetic systems used, the validity and precision of each data set (line and pixel) and document, and the integration of qualitative and quantitative documents. Various types of information are taken into account in this approach, such as topography and bathymetry, geology (lithology, structure), geophysics (gravimetry, magnetism, etc.), geodesy (levelling, GPS, etc.), remote sensing and field works. This multisource approach has been applied to data sets both onland and offshore Taiwan, and has resulted in a preliminary neotectonic map of onshore-offshore Taiwan. This map provides a better comprehension of the geodynamic phenomena that affect Taiwan, and contributes significantly to the relations of the offshore structures and their corresponding reactivated extensional structures onshore (for instance, the Tainan, Taihsi, Okinawa, and Huatung basins) This document should benefit both academic research (structural and active fault maps) as well as the applied geological implications (such as natural hazards mapping and evaluation of geotechnic works).

Neotectonic field studies and detailed analyses of Neogene and Quaternary fault mechanisms in southwestern Anatolia enable us to recognize a succession of compressional and extensional events, and to characterize the direction of corresponding regional stresses. The three most important compressive phases occurred during the Miocene, and a much smaller one near the Plio-Quaternary boundary. The last one or two interrupted a widespread extension of much greater duration and amplitude. The whole tectonic evolution resembles that of the Aegean. The large extension by normal faulting is consistent with a minimum stress along a NNE-SSW average direction. It appears that this direction was N-S during the Pliocene and changed to NE-SW sometime during the Quaternary. This dominant NNE-SSW extension, which began during late Miocene or earliest Pliocene, was related to the development of the southwestern Anatolian graben system.

A field analysis of faults and fractures in the Ras Gharib-Ras Gemsa region of the Gulf of Suez shows that the main Late Cenozoic extension occurred perpendicular to the rift axis. Three main types of dip-slip normal faults successively developed as the tilt of blocks bounded by antithetic normal faults increased. Determinations of the amount of extension from structural data are compatible with estimates made using subsidence data through a simplified model of lithospheric stretching. The uplift of rift shoulders is related in chronology and volume to the subsidence of the rift. The geometry of fault patterns and directions of extension suggests that the Late Cenozoic total movement corresponds to a counterclockwise rotation of 4–5° of Sinai relative to Africa, with a pole close to Cairo.

Analyses of successive structures in the Aegean lead to a model of the distribution of stresses and deformations that occurred since the late Miocene. This model is consistent with Plio—Quaternary subduction. The Aegean region expanded towards the Eastern Mediterranean and the curvature of the arc was thus accentuated.

New methods of determination of stress axes, from field measurements of the orientations of faults and slickenslides, are discussed. Results show that the approximation which assumes that relative motion along any fault is controlled by a single common mean stress tensor is valid.

Upper Pleistocene and Holocene tectonic movements in the Aegean region are analyzed by geological means (deformation of shorelines, faults in Quaternary deposits, historical seismicity). Examples from Crete, Karpathos, Milos, Chios and Samos are presented. While subduction, indicated by geophysical data, occurs beneath the Hellenic Arc, extensional tectonics (i.e., normal faulting) takes place within and behind the arc, resulting in a slight expansion of the Aegean region towards the Eastern Mediterranean.

Using examples from the Gulf of Suez and southeastern France, the distribution of tension tectonic joints as well as shear tectonic joints in continuous profiles is shown to fit a Gamma distribution. The parameters of this distribution are determined using the maximum likelihood method. Apparent fit with negative exponential distribution in photointerpretation analysis results from lack of resolution. The average joint spacing is directly proportional to bed thickness; it decreases when the degree of rock consolidation increases.