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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.

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.

[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.

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.