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Temporal fault‐zone observations are important to better understand the evolution of fault structure and stress configuration. However, long‐term monitoring in the fault zone is rare after a large earthquake. Here, we use seismic data in the fault zone at 1‐km depth from the Taiwan Chelungpu‐fault Drilling Project to study long‐term anisotropy after the 1999 Mw7.6 Chi‐Chi earthquake. The direct S‐wave splitting measurements resolve the overall weak anisotropy in the shallow crust. In order to resolve fault damage zone anisotropy, we perform coda cross‐correlation technique for 794 microearthquakes between 2007 and 2013. We estimate the temporal change in background shear‐wave velocity, fast shear‐wave polarization direction (FSP), and strength of anisotropy (Aani) in the fault damage zone. We show the average FSP direction is N93°E with a significant Aani of about 12%, likely due to the pervasive vertical microcracks created after the earthquake. Temporal variations of anisotropy exhibit seasonal variation with periodicity every 9–12 months that correlates with rainfall events. Furthermore, long‐term anisotropy shows a gradual rotation of FSP direction of about 15° during the first 4 years of observation. At the same time, the strength of anisotropy reduced from 17% to 10% and shear‐wave velocity increased, suggesting the fault healed after the earthquake. This study reports in‐situ evidence for two key observations: (a) long‐term, fault‐zone healing after a major earthquake and (b) modulation of 1‐km deep fault‐zone properties by surficial hydrologic processes. These observations may provide constraints on the response of the fault damage zone in the interseismic period. Plain Language Summary: Earthquakes are generated when rocks slip quickly along a fault. Damage produced by fault rupture is often observed in an area with enhanced fracture networks called the fault damage zone. Fractures inside the damage zone can open or close in response to the background stress. Therefore, observing fault fractures may provide useful information on how the background stresses evolve in the fault. Here, we observe the time‐dependent change of fractures in the fault damage zone through seismic anisotropy analysis. We measure the velocity difference of a seismic shear‐wave in different azimuths to determine the anisotropy. The azimuth of the fastest shear‐wave indicates the dominant fracture orientation inside the fault damage zone. We use borehole seismometers installed within the Chelungpu fault in Taiwan to study the long‐term anisotropy after a major earthquake, and found the fault‐zone is highly damaged with fractures aligning with the tectonic stress direction. Anisotropy varies seasonally due to fluid pressure changes in fault zone fractures during and after rainfall events. In addition, we observe a long‐term anisotropy change that might indicate fracture closure. This study presents a unique observational dataset which allows us to better understand fault zone evolution after a large earthquake. Key Points: A 7‐year (2007–2013) fault‐zone seismic anisotropy is presented from the Taiwan Chelungpu‐fault Drilling Project borehole seismic arrayAverage fast shear‐wave direction of N93°E and anisotropy strength of 12% are observed after the Chi‐Chi earthquakeAnisotropy varies seasonally with rainfalls. Long‐term anisotropy from 17% to 10% with increasing velocity suggests fault healing [ABSTRACT FROM AUTHOR]

A decrease in Q values within the hanging wall of the ruptured Chelungpu fault two years following the 1999 Chi-Chi earthquake was revealed by Q tomography images and an analysis of single-path Q. The synthetic and sensitivity tests of the Q determination were carried out accordingly to justify the temporal variation. A Q value within the hanging wall above the hypocenter was determined to be 157 ± 18 two years following the Chi-Chi earthquake, which is significantly lower than the Q tomography values of 238 ± 17 and 289 ± 13 prior to and two years after the main shock, respectively. Similar values using a signal-path Q analysis from events within the ruptured fault zone to stations along the fault were obtained. The corresponding Q values were 247 ± 85 prior to the Chi-Chi earthquake. After the earthquake we obtained Q values of 158 ± 75 and 318 ± 80 for 2 years following and 2 years after the earthquake, respectively. Considering the two independent methods in determination of Q, the reduction in Q by 89 two years following the Chi-Chi earthquake in both methods is significant. Along with 1 % V reduction revealed by the analysis of repeating earthquakes our studies suggested possible reduction both in V and Q values within the fault zone after the Chi-Chi earthquake. Temporal changes in Q after the Chi-Chi earthquake imply variations of pore-fluid saturation in the ruptured fault zone. The reduction in Q two years following the Chi-Chi earthquake indicates high pore-fluid saturation within the fractured fault zone due to the postseismic fluid redistribution. [ABSTRACT FROM AUTHOR]

Abstract: Continuous cores and a suit of geophysical measurements were collected in two scientific drill holes to understand physical mechanisms involved in the 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 both cores and FMI (or FMS) logs, is about 30° 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 12 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 (FZ1111). It is a 1 m gouge zone including 12 cm of thick indurate black material. We interpreted this zone as the slip zone during Chi-Chi earthquake. FZ1111 is characterized by: 1) bedding-parallel thrust fault with 30-degree 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 1 m gouge zone; 6) increasing gas (CO2 and CH4) emissions, and 7) appearance of 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 conjugate set of secondary directions are parallel to microcrack orientations. A drastic change of orientation of fast shear-wave polarization across the Sanyi thrust fault at the depth of 1712 m reflects the change of stratigraphy, physical properties and structural geometry. [Copyright &y& Elsevier]

Abstract: We investigated the fault geometry effects and the corresponding coseismic slip distribution using various proposed earthquake fault models for the Chi-Chi earthquake of 21 September 1999. The types of fault geometries are threefold: a simple planar fault plane, a two segmented planar fault plane and a three dimensional (3D) curved fault surface rupture propagation model. We derived the estimated spatial slip distribution from an inversion analysis of GPS coseismic displacement data and show that the 3D fault model is the preferred solution. The simple and segmented fault models lead to significant artificial slip distributions associated with the pre-defined fault geometry and the spatial distribution of GPS stations. The spatial distribution of coseismic slip deduced from the 3D fault model has three observable features: (1) the overall slip is concentrated at depth of less than 12 km, which may well correspond to a shallow-dipping detachment; (2) the maximum slip of about 10 m is located 45 km to the north of the epicenter; and (3) the slip vector is dominated by the dip-slip component. In addition, the results from the inversion of GPS data are consistent with those from the inversion analysis of teleseismic broadband data. A resolution analysis, further, demonstrates that the results are highly correlated with field GPS data studies when we used synthetic test data. The inversion of spatially distributed GPS data is highly sensitive to fault geometry. We conclude that the use of the 3D fault model is not only necessary but also certainly competent enough to well explain the inferred slip style and the observed static coseismic displacements. [Copyright &y& Elsevier]

Abstract: This paper presents a brief summary of several types of precursors in the lithosphere, atmosphere and ionosphere that have been positively identified in relation to the 20 September 1999 M w 7.6 Chi-Chi earthquake and its aftershocks under the iSTEP Program. [Copyright &y& Elsevier]