Jacques Angelier

Shear concentration in a collision zone: kinematics of the Chihshang Fault as revealed by outcrop-scale quantification of active faulting, Longitudinal Valley, eastern Taiwan

Abstract 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).

Active fault creep variations at Chihshang, Taiwan, revealed by creep meter monitoring, 1998-2001

[1]xa0The 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.

Stress permutations: Three-dimensional distinct element analysis accounts for a common phenomenon in brittle tectonics

[1]xa0Using three-dimensional (3-D) distinct element modeling, we explored a variety of simulations to characterize and interpret the stress permutations in brittle tectonics. Stress inversions of fault slip data or earthquake focal mechanisms often revealed such permutations. The main aim of our study is to produce simple, mechanically consistent 3-D models that account for these switches between the principal stress axes σ1/σ2 or σ2/σ3. Even with simple boundary conditions the stress changes induced by variations in rheology are large enough to modify the local tectonic behavior and to produce permutations of principal stress axes. Rather than simple directional changes of stress axes, which exist but often remain limited, the relative variations in principal stress magnitudes are the major cause of permutations σ1/σ2 and σ2/σ3. In nature, permutations being left apart, the orientations of axes often remain tightly clustered. In our experiments we adopted a ratio Φ = (σ2 − σ3)/(σ1 − σ3) of 0.5, which makes permutations difficult (low Φ favors σ2/σ3 permutations, high Φ favors σ1/σ2 permutations), and we explored a variety of tectonic situations involving compression, extension, and strike slip. Our experiments indicate that the major causes of stress permutations are the heterogeneity of the brittle deformation (e.g., intact rock massifs between heavily faulted deformation zones) and the anisotropy of the mechanical properties that results from fracturing and faulting, which concur to modify the mechanical balance inside the analyzed volume and to produce stress permutations. Our models show that contrasts and anisotropy in rock properties favor stress permutations. Of major importance is the existence of relatively resistant zones at tips of the deformed zones, acting as channels where stress concentrates and switches occur. Because in nature such zones move in time and space, it is not surprising that stress permutations are so pervasive.

Continuous monitoring of an active fault in a plate suture zone: a creepmeter study of the Chihshang Fault, eastern Taiwan

Abstract 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±0.3xa0mm and 17.3±0.7xa0mm, 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 (31xa0mm/year of horizontal displacement) and the creepmeter measurement across the Chihshang Fault zone (17–19xa0mm/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.

New geomorphic data on the active Taiwan orogen: A multisource approach

A multisource and multiscale approach of Taiwan morphotectonics combines different complementary geomorphic analyses based on a new digital 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 features 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 morphologic analyses (drainage and erosional surfaces) have 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.

Quantification of Hanging-Wall Effects on Ground Motion: Some Insights from the 1999 Chi-Chi Earthquake

Accelerometric records of the Chi-Chi earthquake from sites on the hanging wall exhibit larger acceleration than those from the footwall. Based on ground accelerations recorded at 79 near-field stations (10 hanging-wall stations and 69 footwall stations, respectively) and precise mapping of fault-rupture traces, the hanging-wall/footwall effects of the Chi-Chi earthquake have been fully studied. We show that the hanging-wall effects cannot be simply accounted for by a proper choice of distance metric. The closest distance to the rupture plane ( D rup ) is then selected to develop an empirical ground-motion model by using the data collected during the mainshock of the Chi-Chi earthquake that struck Taiwan. With the exception of some sites immediately next to the rupture traces ( D rup ≤ 5 km), the acceleration residuals between this empirical model and the recorded data at the footwall stations are close to zero for stations in the distance range from 5 to 50 km. On the other hand, the average acceleration amplification on the hanging wall is equal to the natural logarithmic values of 0.64 ± 0.4 for all hanging-wall sites within 20 km of D rup . The hanging-wall/footwall effects have also been evaluated for several response spectral periods. It is observed that both the horizontal and vertical components of spectral acceleration are apparently amplified for sites on the hanging wall at a distance from 5 to 20 km for spectral periods 0.02 to 0.5 sec, whereas the vertical component has less amplification than the horizontal in all the spectral periods considered. The horizontal component of spectral acceleration at the hanging-wall sites also shows a larger value for the long-period motion, relative to the footwall, for periods larger than 1.0 sec. The hanging-wall effects are relatively constant, at low frequencies, as the distance extends to about 20 km. This observation cannot be explained by the simplified empirical model. Rather, it suggests that waves trapped in the hanging-wall wedge may have been involved.

Paleoseismic event and active faulting: from ground penetrating radar and high-resolution seismic reflection profiles across the Chihshang Fault, eastern Taiwan

Abstract Ground penetrating radar (GPR) and high-resolution shallow seismic reflection have been carried out to delineate the subsurface pattern and paleoseismic facies of the active Chihshang Fault, a segment of the Longitudinal Valley Fault, eastern Taiwan. Optimum radar results were obtained along streets in the Chinyuan village. A total of 720xa0m GPR profiles were acquired with 200xa0MHz antenna. Two seismic sections were collected using Mini-Sosie sources, providing a frame to the radar profiles. Three types of structures revealing evidence for a paleoseismic event can be observed in one GPR profile; these are: upward fault termination, colluvial wedge, and sand injection. Other profiles reveal unconformities, reverse-faults, one fault-related fold, and the presence of a colluvial wedge on the downthrown block, all indicating paleoseismic activity. The remarkable feature of the Chihshang Fault at Chinyuan is that the dips of this active fault at surface changes rapidly along strike. Combining the GPR with shallow seismic reflection allows us to reconstruct the three-dimensional fault pattern of the Chihshang Fault and resolve doubts about the abrupt changes of fault dips. These GPR and seismic studies are consistent with the surface observation and reveal the complicated pattern of antithetic subsidiary faults in the near-surface part of the major E-dipping Chihshang thrust fault.