Slip zone and energetics of a large earthquake from the Taiwan Chelungpu-fault Drilling Project (original) (raw)
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Tectonophysics, 2009
Continuous cores and a suite of geophysical measurements were collected in two scientific drill holes to understand physical mechanisms involved in large displacements during the 1999 Chi-Chi earthquake. Physical properties obtained from wire-line logs (including P-and S-wave sonic velocity, gamma ray, electrical resistivity, density and temperature) are primarily dependent on parameters such as lithology, depth, and fault zones. The average dip of bedding, identified from cores and FMI (or FMS) logs, is about 30 degrees towards SE. Nevertheless, local azimuthal variations and increasing or decreasing bedding dips appear across fault zones. A prominent increase of structural dip to 60°-80° below 1856 m could be due to deformation associated with propagation of the Sanyi fault. A total of twelve fault zones identified in Hole A are located in the Plio-Pleistocene Cholan Formation, Pliocene Chinshui Shale, and Miocene Kueichulin Formation. The shallowest fault zone occurs at 1111 m depth (FZA1111). It is a 1-m gouge zone including 12 cm of thick indurate black material. We interpreted this zone as the slip zone during the Chi-Chi earthquake. FZA1111 was characterized by 1) beddingparallel thrust fault with 30° dip; 2) the lowest resistivity; 3) low density, V p and V s ; 4) high V p /V s ratio and Poisson's ratio; 5) low energy and velocity anisotropy, and low permeability within the homogeneous gouge zone; 6) increasing gas (CO 2 and CH 4) emissions; and 7) richness in smectite within the primary slip zone. In situ stresses at the drill site were inferred from leak-off tests, borehole breakouts and drilling-induced tensile fractures from borehole FMS/FMI logs, and shear seismic wave anisotropy from DSI logs. The dominant fast shear wave polarization direction is in good agreement with regional maximum horizontal stress axis, particularly within the strongly anisotropic Kueichulin Formation. A drastic change in orientation of fast shear polarization across the Sanyi thrust fault at 1712 m depth reflects the change of stratigraphy, physical properties, and structural geometry.
Geophysical Research Letters, 2007
The Taiwan Chelungpu-fault Drilling Project (TCDP) drilled a 2-km-deep research borehole to investigate the structure and mechanics of the Chelungpu Fault that ruptured in the 1999 M w 7.6 Chi-Chi earthquake. Geophysical logs of the TCDP were carried out over depths of 500-1900 m, including Dipole Sonic Imager (DSI) logs and Formation Micro Imager (FMI) logs in order to identify bedding planes, fractures and shear zones. From the continuous core obtained from the borehole, a shear zone at a depth of 1110 meters is interpreted to be the Chelungpu fault, located within the Chinshui Shale, which extends from 1013 to 1300 meters depth. Stress-induced borehole breakouts were observed over nearly the entire length of the wellbore. These data show an overall stress direction ($N115°E) that is essentially parallel to the regional stress field and parallel to the convergence direction of the Philippine Sea plate with respect to the Eurasian plate. Variability in the average stress direction is seen at various depths. In particular there is a major stress orientation anomaly in the vicinity of the Chelungpu fault. Abrupt stress rotations at depths of 1000 m and 1310 m are close to the Chinshui Shale's upper and lower boundaries, suggesting the possibility that bedding plane slip occurred during the Chi-Chi earthquake.
Core-log integration studies in hole-A of Taiwan Chelungpu-fault Drilling Project
Geophysical Journal International, 2008
Taiwan Chelungpu-fault Drilling Project (TCDP) was initiated to understand the physical mechanisms involved in the large displacements of the 1999 Taiwan Chi-Chi earthquake. Continuous measurements of cores (including laboratory work) and a suite of geophysical downhole logs, including P-and S-wave sonic velocity, gamma ray, electrical resistivity, density, temperature, electrical borehole images and dipole-shear sonic imager, were acquired in Hole-A over the depth of 500-2003 m. Integrated studies of cores and logs facilitate qualitative and quantitative comparison of subsurface structures and physical properties of rocks. A total of 10 subunits were divided on the basis of geophysical characteristics. Generally, formation velocity and temperature increase with depth as a result of the overburden and thermal gradient, respectively. Gamma ray, resistivity, formation density, shear velocity anisotropy and density-derived porosity are primarily dependent on the lithology. Zones with changes of percentage of shear wave anisotropy and the fast shear polarization azimuth deduced from Dipole Shear-Imager (DSI) are associated with the appearance of fractures, steep bedding and shear zones. The fast shear wave azimuth is in good agreement with overall dip of the bedding (approximately 30 • towards SE) and maximum horizontal compressional direction, particularly in the Kueichulin Formation showing strong shear wave velocity anisotropy. Bedding-parallel fractures are prevalent within cores, whereas minor sets of high-angle, NNW-SSE trending with N-and S-dipping fractures are sporadically distributed. The fault zone at depth 1111 m (FZA1111) is the Chi-Chi earthquake slip zone and could be a fluid conduit after the earthquake. The drastic change in fast shear wave polarization direction across the underlying, non-active Sanyi thrust at depth 1710 m reflects changes in stratigraphy, physical properties and structural geometry.
Earthquake Science, 2011
This paper reports the internal structures of the Beichuan fault zone of Longmenshan fault system that caused the 2008 Wenchuan earthquake, at an outcrop in Hongkou, Sichuan province, China. Present work is a part of comprehensive project of Institute of Geology, China Earthquake Administration, trying to understand deformation processes in Longmenshan fault zones and eventually to reproduce Wenchuan earthquake by modeling based on measured mechanical and transport properties. Outcrop studies could be integrated with those performed on samples recovered from fault zone drilling, during the Wenchuan Earthquake Fault Scientific Drilling (WFSD) Project, to understand along-fault and depth variation of fault zone properties. The hanging wall side of the fault zone consists of weakly-foliated, clayey fault gouge of about 1 m in width and of several fault breccia zones of 30-40 m in total width. We could not find any pseudotachylite at this outcrop. Displacement during the Wenchuan earthquake is highly localized within the fault gouge layer along narrower slipping-zones of about 10 to 20 mm in width. This is an important constraint for analyzing thermal pressurization, an important dynamic weakening mechanism of faults. Overlapping patterns of striations on slickenside surface suggest that seismic slip at a given time occurred in even narrower zone of a few to several millimeters, so that localization of deformation must have occurred within a slipping zone during coseismic fault motion. Fault breccia zones are bounded by thin black gouge layers containing amorphous carbon. Fault gouge contains illite and chlorite minerals, but not smectite. Clayey fault gouge next to coseismic slipping zone also contains amorphous carbon and small amounts of graphite. The structural observations and mineralogical data obtained from outcrop exposures of the fault zone of the Wenchuan earthquake can be compared with those obtained from the WFSD-1 and WFSD-2 boreholes, which have been drilled very close to the Hongkou outcrop. The presence of carbon and graphite, observed next to the slipping-zone, may affect the mechanical properties of the fault and also provide useful information about coseismic chemical changes.
2001
We mapped and analyzed two vertical exposures-exposed on the walls of a 3-to 5-m-deep, 70-m-long excavation and a smaller 3-m-deep, 10-m-long excavation -across the 1999 rupture of the Chelungpu fault. The primary exposure revealed a broad anticlinal fold with a 2.5-m-high west-facing principal thrust scarp contained in fluvial cobbly gravel beds and overlying fine-grained overbank deposits. Sequential restoration of the principal rupture requires initial failure on the basal, east-dipping thrust plane, followed by wedge thrusting and pop-up of an overlying symmetrical anticline between two opposing secondary thrust faults. Net vertical offset is about 2.2 m across the principal fault zone. From line-length changes, we estimate about 3.3 m of horizontal shortening normal to fault strike. The ratio of these values yields a total slip of 4.0 m and an estimate of about 34° for the dip of the fault plane below the excavation. This value is nearly the same as the 35° average dip of the fault plane from the surface to the hypocenter. Restoration of the exposed gravelly strata and adjacent overbank sediments deposited prior to the 1999 event around the principal rupture suggests the possible existence of a prior event. A buried 30-m-wide anticlinal warp beneath the uplifted crest of the 1999 event is associated with three buried reverse faults that we interpret as evidence for an earlier episode of folding and faulting in the site. The prior event is also recorded in the smaller excavation, which is located 40 m south and is oriented parallel to the larger excavation. Radiocarbon dating of samples within the exposed section did not place tight constraints on the date of the previous event. Available data are interpreted as indicating that the previous event occurred before the deposition of the less than 200 14 C yr B.P. overbank sands and after the deposition of the much older fluvial gravels. We interpret the previous event as the penultimate event relative to the 1999 Chi-Chi earthquake. We estimated the long-term slip rate of the Chelungpu fault to be 10-15 mm/yr during the last 1 Ma, based on previously published retrodeformable cross sections. This rate is, however, significantly higher than geodetic rates of shortening across the Chelungpu thrust where two pairs of permanent Global Positioning System stations suggest 7-10 mm/yr of shortening across the fault. Given the 4 meters of average slip, the long-term slip rate yields an interseismic interval of between 267 and 400 yr for the Chelungpu fault.
Bulletin of the Seismological Society of America, 2003
Bulletin of the Seismological Society of America, 2013
On 1 April 2006, the Taitung earthquake (M w 6.1) occurred in Taiwan at the boundary between the Philippine Sea and Eurasian plates, where high convergence rates contributed to the development of Plio-Pleistocene orogeny in the region. From the joint inversion of seismic and geodetic data, we identified the event's fault geometry and reconstructed the distribution of coseismic fault slip. We modeled fault geometries with increasing complexity and selected the model that best reproduced all datasets, simultaneously. Even though the earthquake magnitude was moderate, rupturing occurred in two steps. The initial rupture was generated on a listric, northsouth-trending fault (for which dip decreases with increasing depth), and was immediately followed by movement along a perpendicular structure that cross-cuts the main fault at 5 km south of the earthquake hypocenter. The average slip along the rupture was 30 cm, with a maximum of 87 cm. Oblique-reverse fault movement was characterized by a predominant left-lateral component. The amount of slip is well constrained for offsets of more than 5 cm, with an associated uncertainty of 32%. For slip amounts greater than 5 cm, uncertainties on rake and rupture time are 11°and 0.54 s, respectively. The rupture propagated from the hypocenter bilaterally, moving slightly faster toward the south (2:5 0:4 km=s) than to the north (1:7 0:1 km=s). To the south, the rupture was rapidly transmitted upward at the junction with the crosscutting east-west segment, whereas in the north, the rupture remained confined to the lower segment of the main fault. From Global Positioning Systems (GPS) and seismic data (time window < 1 min), we infer that the cross-cutting segment was activated following coseismic rupture on the main north-south fault, yet close enough in time to be associated with coseismic movement acquired by GPS (daily solutions).
Slip distribution and tectonic implication of the 1999 Chi-Chi, Taiwan, Earthquake
Geophysical Research Letters, 2001
We report on the fault complexity of the large (M•o = 7.6) Chi-Chi earthquake obtained by inverting densely and well-distributed static measurements consisting of 119 GPS and 23 doubly integrated strong motion records. We show that the slip of the Chi-Chi earthquake was concentrated on the surface of a "wedge shaped" block. The inferred geometric complexity explains the difference between the strike of the fault plane determined by long period seismic data and surface break observations. When combined with other geophysical and geological observations, the result provides a unique snapshot of tectonic deformation taking place in the form of very large (>10m) displacements of a massive wedge-shaped crustal block which may relate •to the changeover from over-thrusting to subducting motion between the Philippine Sea and the Eurasian plates. Introduction Located at the "corner" of convergence between the Philippine Sea and Eurasian plates (Fig.l), Taiwan results from the east-west (E-W) collision during the last 5 Ma [Teng, 1990]. Currently, southern Taiwan is still under intense collision due to the eastward subducting Eurasian plate underneath the Philippine Sea plate, but the tectonic style in northeast Taiwan reflects the northward subduction of the Philippine Sea plate beneath the Eurasian plate (e.g., Teng et al. [2000]). We expect that central Taiwan, where the 1999, Chi-Chi earthquake occurred, would contain a transfer zone where the convergence of subduction changes from E-W to N-S. The location of this transfer zone has not previously been resolved, presumably because of the complex surface geology in the Taiwan orogenic belt. The Chi-Chi earthquake initiated at a depth of 10 km and ruptured the Chelungpu fault (CLPF) producing a 80 km long complex surface rupture pattern (Kao and Chen [2000], Kao et al. [2000]). The fault surface of[
Frictional strength of fault gouge in Taiwan Chelungpu fault obtained from TCDP Hole B
Tectonophysics, 2008
Three fault zones (FZB1136, FZB1194 and FZB1243) are found in core samples in Hole B borehole penetrating through the Chelungpu fault, central Taiwan that activated at the 1999 Taiwan Chi-Chi earthquake. Each fault zone is composed of a black gouge zone (BGZ) and a gray gouge zone (GGZ) surrounded by a fault breccia zone (BZ) and/or fracture-damaged zone (FZ). We conducted tri-axial friction experiments on samples collected from subzones of the above three fault zones under the 1 km depth ambient condition with axial shortening rates of 0.1 to 10 µm/s to investigate frictional strength and its velocity dependence. We found that the coefficient of friction is 0.3 for the BGZ at FZB1136 (1136BGZ), which is lower than those at other fault zones. This result indicates that fault slip at the Chi-Chi earthquake occurred at the 1136BGZ. The estimated frictional strength profile across FZB1136 was that the frictional strength was the lowest (the coefficient of friction was 0.3) for the BGZ and increased up to 0.5 for undeformed host rock with increasing distance from the BGZ. Presence of the intermediate frictional strength zone consisting of the GGZ and the BZ suggests a development of off-fault damage zone where deformation occurred not only in the BGZ but also in the intermediate zone. This feature should be important to estimate total fracture energy released during earthquakes. We also suggest that coefficients of friction and the velocity dependences of the samples are closely related to localization of shear deformation and clay content, respectively.