En Chao Yeh

An earthquake slip zone is a magnetic recorder

During an earthquake, the physical and the chemical transformations along a slip zone lead to an intense deformation within the gouge layer of a mature fault zone. Because the gouge contains ferromagnetic minerals, it has the capacity to behave as a magnetic recorder during an earthquake. This constitutes a conceivable way to identify earthquakes slip zones. In this paper, we investigate the magnetic record of the Chelungpu fault gouge that hosts the principal slip zone of the Chi-Chi earthquake (Mw 7.6, 1999, Taiwan) using Taiwan Chelungpu-fault Drilling Project core samples. Rock magnetic investigation pinpoints the location of the Chi-Chi mm-thick principal slip zone within the 16-cm thick gouge at ~1 km depth. A modern magnetic dipole of Earth magnetic field is recovered throughout this gouge but not in the wall rocks nor in the two other adjacent fault zones. This magnetic record resides essentially in two magnetic minerals; magnetite in the principal slip zone, and neoformed goethite elsewhere in the gouge. We propose a model where magnetic record: 1) is preserved during inter-seismic time, 2) is erased during co-seismic time and 3) is imprinted during post-seismic time when fluids cooled down. We suggest that the identification of a stable magnetic record carried by neoformed goethite may be a signature of friction-heating process in seismic slip zone.

Fault mirrors in seismically active fault zones: A fossil of small earthquakes at shallow depths

Fault mirrors (FMs) are naturally polished and glossy fault slip surfaces that can record seismic deformation at shallow depths. They are important for investigating the processes controlling dynamic fault slip. We characterize FMs in borehole samples from the hanging wall damage zone of the active Hsiaotungshi reverse fault, Taiwan. Here we report the first documented occurrence of the combination of silica gel and melt patches coating FMs, with the silica gel resembling those observed on experimentally formed FMs that were cataclastically generated. In addition, the melt patches, which are unambiguous indicators of coseismic slip, suggest that the natural FMs were produced at seismic rates, presumably resulting from flash heating at asperities on the slip surfaces. Since flash heating is efficient at small slip, we propose that these natural FMs represent fossils of small earthquakes, formed in either coseismic faulting and folding or aftershock deformation in the active Taiwan fold-and-thrust belt.

Brushlines in fault pseudotachylytes: A new criterion for coseismic slip direction

Striations along fault planes, such as slickensides, generally indicate slip direction. These linear structures typically form through friction between two solids. Here we describe brushlines, a new class of fault striation in pseudotachylytes, formed during coseismic slip at the interface between host rock and frictional melt. Unlike slickensides, brushlines primarily lack longitudinal step-like asymmetry and display a hemicylindrical morphology. This new class of striation forms while the frictional melt has sufficiently low viscosity to be brushed along the direction of coseismic displacement by small asperities on the host-rock block. In the Hoping River pseudotachylytes of eastern Taiwan, the long axis of brushlines is parallel to the earthquake slip direction determined using the offset of piercing points across the fault. This study shows that the pseudotachylyte–host rock interface may host previously overlooked, invaluable coseismic kinematic information.

Experimental and numerical investigation of the temperature response to stress changes of rocks

The temperature response to stress changes of rocks is key to understanding temperature anomalies in geoscience phenomena such as earthquakes. We developed a new hydrostatic compression system in which the rock specimen center can achieve adiabatic conditions during the first ~10 s following rapid loading or unloading, and systematically measured several representative sedimentary, igneous and metamorphic rocks sampled from two seismogenic zones [the Longmenshan Fault Zone in Sichuan, and the Chelungpu Fault Zone (TCDP Hole-A) in Taiwan], and several quarries worldwide. We built a finite element model of heat conduction to confirm the measured results of temperature response to stress changes of rocks. The results show that: (1) the adiabatic pressure derivative of the temperature (β) for most crustal rocks is ~1.5 mK/MPa to 6.2 mK/MPa, (2) the temperature response to stress of sedimentary rocks (~3.5-6.2 mK/MPa) is larger than that of igneous and metamorphic rocks (~2.5-3.2 mK/MPa), and (3) there is good linear correlation between β (in mK/MPa) and the bulk modulus K (in GPa): β=(-0.068·K + 5.69) ± 0.4, R2 = 0.85. This empirical equation will be very useful for estimating the distribution of β in the crust, because K can be calculated when profiles of crustal density (ρ) and elastic wave velocities (Vp, Vs) are obtained from gravity surveys and seismic exploration

Changes in paleostress and its magnitude related to seismic cycles in the Chelung‐pu Fault, Taiwan

Paleostress analysis was conducted through a multiple stress inversion method using slip data recoded for the core samples from the Taiwan Chelung-pu Fault Drilling Project (TCDP). Two stress fields were obtained; one of these had horizontally plunging σ1, and the other has horizontally plunging σ2 or σ3 in the compressional stress direction of the Chi-Chi earthquake. Stress magnitude for both the stress fields was constrained by stress polygons, which indicated larger SHmax for horizontally plunging σ1 than that in the case of horizontally plunging σ2 or σ3. These differences in stress orientations and stress magnitude suggest that the change in stress filed can be caused by stress drop and stress buildup associated with seismic cycles. The seismic cycles recoded in the core samples from TCDP could include many events at geological timescale and not only the 1999 Chi-Chi earthquake.

Quantitative modeling of the newly formed magnetic minerals in the fault gouge of 1999 Chi‐Chi earthquake (Mw 7.6), Taiwan

When an earthquake occurs, magnetic minerals are formed in the gouge under the combined action of frictional heating and fluid. Generally, the gouge is altered, and a detailed analysis of neoformed minerals is difficult. The Taiwan Chelungpu fault Drilling Project provided continuous records of the active Chelungpu fault gouges. By analyzing the magnetic parameters along the 16 cm thick gouge which houses the principal slip zone of the 1999 Chi-Chi earthquake (Mw 7.6), we observed a 4 cm shift between the maximum of magnetic susceptibility and the maximum of remanence. The maximum magnetic susceptibility is localized along the principal slip zone. The main identified magnetic minerals are magnetite and goethite, which have very different magnetic parameters. We propose that the maximum of the concentration of magnetite and goethite corresponds to the maximum of magnetic susceptibility and remanence, respectively. By modeling the concentration of these two magnetic minerals, we explain satisfactorily the profiles of magnetic susceptibility and remanence. This quantitative modeling indicates that ~200 ppmv of magnetite formed in the principal slip zone and its main contact area. Similarly, ~1% of goethite is formed in the center of the gouge, where the fluids are more enriched in iron. We propose that the magnetite and goethite are formed and altered during seismic cycles. Key Points Investigate the magnetic mineral assemblage in the Chi-Chi fault gougeClarify the distribution of goethite and magnetite in the gougePropose a model for goethite and magnetite distributions within the gouge

Determination of Orientation of Horizontal Stress and Localized Rotations of the Orientation Around Faults and Fractures from Breakouts in a Scientific Drilling Borehole

To understand stress perturbations associated with minor faults and fractures, relationships between the faults, fractures, lithologic boundaries and stress changes in hole B of Taiwan Chelungpu-fault Drilling Project (TCDP) were investigated. Here, we reported four patterns of stress changes in the vicinity of faults, fractures and lithologic boundaries found in TCDP hole B: (i) the stress orientation (breakout azimuth) rotates abruptly (discontinuously) in the vicinity of the faults or fractures; (ii) the orientation rotates gradually; (iii) breakouts are suppressed at faults, fractures, or lithologic boundaries; and (iv) the orientation does not change across faults or fractures.