Microstructural Inhomogeneity and Mechanical Anisotropy Associated with Bedding in Rothbach Sandstone

This study present the result of conventional triaxial tests conducted on samples of Rothbach sandstone cored parallel, oblique (at 45 degrees) and perpendicular to the bedding at effective pressures ranging from 5 to 250 MPa. Mechanical and microstructural data were used to determine the role of the bedding on mechanical strength and failure mode. We find that samples cored at 45 degrees to the bedding yield at intermediate level of differential stress between the ones for parallel and perpendicular samples at all effective pressures. Strain localization at high confining pressure (i.e., in the compactive domain) is observed in samples perpendicular and oblique to the bedding but not in samples cored parallel to the bedding. However, porosity reduction is comparable whether compactive shear bands, compaction bands or homogeneous cataclastic flow develop. Microstructural data suggest that (1) mechanical anisotropy is controlled by a preferred intergranular contact alignment parallel to the bedding and that (2) localization of compaction is controlled by bedding laminations and grain scale heterogeneity, which both prevent the development of well localized compaction features.     

台湾车笼埔断层深井钻探计划(TCDP)概述

台湾车笼埔断层深井钻探计划(TCDP)是台湾车笼埔断层上进行的科学钻探工程。该计划于2004年开始实施,拟通过深达2 000 M的钻探,获取台湾1999年集集MW7.7地震深部断层带的岩芯样本和流体观测资料,并通过一系列的深井探测和观测研究,分析车笼埔断裂产生剧烈错动的原因。本文介绍了台湾车笼埔断层深井钻探计划(TCDP)的基本情况,论述了TCDP项目的科学假设、工作计划、合作项目以及近期的进展。全面了解TCDP相关内容,对我国大陆进行地震研究和深井钻探工作具有重要借鉴意义。     

Characteristics of Soil Response in Near-Fault Zones During the 1999 Chi-Chi, Taiwan, Earthquake

Distribution of parameters characterizing soil response during the 1999 Chi-Chi, Taiwan, earthquake (M _w = 7.6) around the fault plane is studied. The results of stochastic finite-fault simulations performed in Pavlenko and Wen (2008) and constructed models of soil behavior at 31 soil sites were used for the estimation of amplification of seismic waves in soil layers, average stresses, strains, and shear moduli reduction in the upper 30 m of soil, as well as nonlinear components of soil response during the Chi-Chi earthquake. Amplification factors were found to increase with increasing distance from the fault (or, with decreasing the level of “input” motion to soil layers), whereas average stresses and strains, shear moduli reduction, and nonlinear components of soil response decrease with distance as ~ r ^?1 . The area of strong nonlinearity, where soil behavior is substantially nonlinear (the content of nonlinear components in soil response is more than ~40–50% of the intensity of the response), and spectra of oscillations on the surface take the smoothed form close to E(f) ~ f ^?n , is located within ~20–25 km from the fault plane (~ 1/4 of its length). Nonlinearity decreases with increasing distance from the fault, and at ~40–50 km from the fault (~ 1/2 of the fault length), soil response becomes virtually linear. Comparing soil behavior in near-fault zones during the 1999 Chi-Chi, the 1995 Kobe (M _w = 6.8), and the 2000 Tottori (Japan) (M _w = 6.7) earthquakes, we found similarity in the behavior of similar soils and predominance of the hard type of soil behavior. Resonant phenomena in upper soil layers were observed at many studied sites; however, during the Chi-Chi earthquake they involved deeper layers (down to ~ 40–60 m) than during lesser-magnitude Kobe and Tottori earthquakes.     

Spatio-temporal Slip, and Stress Level on the Faults within the Western Foothills of Taiwan: Implications for Fault Frictional Properties

We use preseismic, coseismic, and postseismic GPS data of the 1999 Chi-Chi earthquake to infer spatio-temporal variation of fault slip and frictional behavior on the Chelungpu fault. The geodetic data shows that coseismic slip during the Chi-Chi earthquake occurred within a patch that was locked in the period preceding the earthquake, and that afterslip occurred dominantly downdip from the ruptured area. To first-order, the observed pattern and the temporal evolution of afterslip is consistent with models of the seismic cycle based on rate-and-state friction. Comparison with the distribution of temperature on the fault derived from thermo-kinematic modeling shows that aseismic slip becomes dominant where temperature is estimated to exceed 200° at depth. This inference is consistent with the temperature induced transition from velocity-weakening to velocity-strengthening friction that is observed in laboratory experiments on quartzo-feldspathic rocks. The time evolution of afterslip is consistent with afterslip being governed by velocity-strengthening frictional sliding. The dependency of friction, μ, on the sliding velocity, V, is estimated to be ${{\partial \mu }/{\partial \, {\rm ln}\, V}} = 8 \times 10^{ - 3}$ . We report an azimuthal difference of about 10–20° between preseismic and postseismic GPS velocities, which we interpret to reflect the very low shear stress on the creeping portion of the décollement beneath the Central Range, of the order of 1–3 MPa, implying a very low friction of about 0.01. This study highlights the importance of temperature and pore pressure in determining fault frictional sliding.     

High magnetic susceptibility produced by thermal decomposition of core samples from the Chelungpu fault in Taiwan

We carried out thermomagnetic susceptibility analyses of fault rocks from core samples from Hole B of the Taiwan Chelungpu Fault Drilling Project (TCDP) to investigate the cause of high magnetic susceptibilities in the fault core. Test samples were thermally and mechanically treated by heating to different maximum temperatures of up to 900 °C and by high-velocity frictional tests before magnetic analyses. Thermomagnetic susceptibility analyses of natural fault rocks revealed that magnetization increased at maximum heating temperatures above 400 °C in the heating cycle, and showed three step increases, at 600 to 550 °C and at 300 °C during the cooling cycle. These behaviors are consistent with the presence of pyrite, siderite and chlorite, suggesting that TCDP gouge originally included these minerals, which contributed to the generation the magnetic susceptibility by thermomechanical reactions. The change in magnetic susceptibility due to heating of siderite was 20 times that obtained by heating pyrite and chlorite, so that only a small fraction of siderite decomposition is enough to cause the slight increase of the susceptibility observed in the fault core. Color measurement results indicate that thermal decomposition by frictional heating took place under low-oxygen conditions at depth, which prevented the minerals from oxidizing to reddish hematite. This finding supports the inference that a mechanically driven chemical reaction partly accounts for the high magnetic susceptibility. A kinetic model analysis confirmed that frictional heating can cause thermal decomposition of siderite and pyrite. Our results show that decomposition of pyrite to pyrrhotite, siderite and, to some extent, chlorite to magnetite is the probable mechanism explaining the magnetic anomaly within the Chelungpu fault zone.     

Clayey injection veins and pseudotachylyte from two boreholes penetrating the Chelungpu Fault, Taiwan: Their implications for the contrastive seismic slip behaviors during the 1999 Chi-Chi earthquake

Abstract   The seismic slip that occurred during the 1999 Chi-Chi earthquake in Taiwan showed contrastive behaviors in different regions along the Chelungpu Fault: A large and smooth slip occurred in the north, while a relatively small slip associated with high-frequency seismic wave radiation occurred in the south. The core samples from shallow boreholes at northern (Fengyuan) and southern (Nantou) sites penetrating the seismic Chelungpu Fault were analyzed. The fault zones at the northern site are characterized by soft clayey material associated with clayey injection veins. This suggests that the fault zones were pressurized during ancient seismic slip events, and hence the elastohydrodynamic lubrication occurred effectively. In contrast, the fault rock from the southern site is old pseudotachylyte that has been shattered by repeated ancient seismic slip events. Statistical analysis of many pseudotachylyte fragments reveals that the degree of frictional melting tended to be low. In this case, the seismic slip is restrained by the mechanical barrier of a highly viscous melt layer. These contrastive fault rocks were produced by repeated ancient seismic slip events, but the two corresponding mechanisms of friction are likely to have also occurred during the 1999 Chi-Chi earthquake, thus causing the contrastive slip behaviors in the north and south.     

Physical property anisotropy of foliated fault rocks: Study from the Nobeoka Thrust,Shimanto Belt,southwest Japan

To investigate the physical property anisotropies of foliated fault rocks in subduction zones, the hanging wall phyllites and footwall cataclasites exhumed along the Nobeoka Thrust, a fossilized out‐of‐sequence‐thrust in the Shimanto Belt, Japan, was focused. Discrete physical property (electric resistivity, P‐ and S‐wave velocities, and porosity) measurements were conducted employing geologic coordinates (depth‐parallel direction, strike direction, and maximum dip direction of foliation), using the core samples obtained from the Nobeoka Thrust Drilling Project and compared the data to borehole geophysical logs. A higher sample P‐wave velocity (Vp), lower S‐wave velocity (Vs), higher Vp/Vs, and lower sample porosity and resistivity compared to the logs, are inferred to have been caused by the larger sampling scale of the logs and lower fluid saturation of the borehole. The phyllites and cataclasites exhibited substantial vertical and horizontal anisotropy of Vp (0.4–17.3 % and 2.7–13.8 %, respectively), Vs (0.5–56 % and 7.7–43 %, respectively), and resistivity (0.9–119 % and 2.0–65.9 %, respectively). The physical property anisotropies are primarily affected by the dip angles of foliation. The fault rocks that have gentler dip angles exhibit a higher Vp in the strike and maximum dip direction and a lower Vp in the depth‐parallel direction. In contrast, the fault rocks that have steeply dipping structures show a higher Vp in the strike and depth‐parallel directions with a lower velocity in the maximum dip direction. Resistivity anisotropy show a trend opposite to that of the Vp in relation to the dip angles. Our results show lower Vp anisotropy than those obtained in previous studies, which measured wave speeds perpendicular or parallel to foliation under confining pressure. This study highlights the significance of dip angles on vertical properties in geophysical surveys across foliated fault rocks.     

Anomalous electrical resistivity anisotropy in clean reservoir sandstones

We report novel laboratory measurements of the full electrical resistivity tensor in reservoir analogue quartzose sandstones with clay contents less than 1.5%. We show that clean, homogeneous, visually uniform sandstone samples typically display between 15% and 25% resistivity anisotropy with minimum resistivity normal to the bedding plane. Thin‐section petrography, analysis of fabric anisotropy, and comparison to finite‐element simulations of grain pack compaction show that the observed anisotropy symmetries and magnitudes can be explained by syn‐depositional and post‐depositional compaction processes. Our findings suggest that: electrical resistivity anisotropy is likely to be present in most clastic rocks as a consequence of ballistic deposition and compaction; compaction may be deduced from measurements of electrical anisotropy; and the anisotropy observed at larger scales in well logging and controlled‐source electromagnetic data, with maximum resistivity normal to bedding, is most likely the result of meso‐scale (10^?1 m–10¹ m) periodic layering of electrically dissimilar lithologies.     

Experimental studies on gas and water permeability of fault rocks from the rupture of the 2008 Wenchuan earthquake,China

The permeabilities of fault rocks from the rupture of Wenchuan earthquake were measured by using nitrogen gas and distilled water as pore fluids under the confining pressure ranging from 20 to 180 MPa at room temperature. Experimental results indicate that both gas and water permeabilities decrease with increasing confining pressure, described by power law relationship, i.e., b = 0.2×10–3kl–0.557. The water permeability is about one order less than gas permeability and also half order smaller than the permeability corrected by the Klinkenberg effect, so-called intrinsic permeability. The differences in the permeabilies imply that the reduction of effective pore size caused by the adhesion of water molecules to clay particle surface and water-swelling of expandable clay minerals contributes to lessening the water permeability besides the Klinkenberg effect. Hence, the liquid permeability of fault rocks cannot be deduced by gas permeability by the Klinkenberg correction reliably and accurately, and it is necessary to use liquid as pore media to measure their transport property directly.     

台湾车笼埔断层带上的黏土矿物变化和摩擦温度(英文)

The Chi-Chi earthquake occurred in September 21, 1999, causing disastrous damages over wide areas in north-western part of Taiwan. Surface ruptures and seismic records showed particular and different motions in northern and southern part of Chelungpu fault.     

Effect of near-fault earthquake on bridges: lessons learned from Chi-Chi earthquake

The objective of this paper is to describe the lessons learned and actions that have been taken related to the seismic design of bridge structures after the Chi-Chi, Taiwan earthquake. Much variable near-fault ground motion data was collected from the rupture of Chelungpu fault during the Chi-Chi earthquake, allowing the seismic response of bridge structures subjected to these near-fault ground motions to be carefully examined. To study the near-fault ground motion effect on bridge seismic design codes, a two-level seismic design of bridge structures was developed and implemented. This design code reflects the near-fault factors in the seismic design forces. Finally, a risk assessment methodology, based on bridge vulnerability, is also developed to assist in decisions for reducing seismic risk due to failure of bridges. Director of Center for Research on Earthquake Engineering. Supported by: the Science Council, Chinese Taipei, under grant no. SC 90-2211-E-002-028.     

Changes of parameters of magnetic anisotropy of foliated rocks under pressure

Summary The changes of magnetic anisotropy under pressure were studied on a set of rocks with marked macroscopic stratification, collected from the Rjvíz borehole. The anisotropy parameters are relatively very stable under directional pressure, acting parallel with the rocks' bedding, and, on the contrary, significantly unstable under pressure acting perpendicular to the bedding. Systematically different magnitudes of stress sensitivity coefficients, , for parallel directional susceptibility were observed in both cases in the same types of rocks. The main cause is probably the different capability of transferring external stress to ferrimagnetics via the stratified non-magnetic matrix. With regard to magnetomechanical phenomena, therefore, a singel value of the stress sensitivity coefficient is insufficient to characterize rocks with a markedly anisotropic matrix.     

Determining fault slip distribution of the Chi-Chi Taiwan earthquake with GPS and InSAR data using triangular dislocation elements

The reactivation of the Chelungpu fault triggered the 20 September 1999 Chi-Chi Taiwan earthquake (M_w = 7.6) which caused a 100-km long surface rupture that trends north–south. We reconstruct the fault geometry using 1068 planar triangular dislocation elements that approximate more realistically the curved three-dimensional fault surface. The fault slip distribution is then determined with the observed GPS coseismic displacements as well as interferometric synthetic aperture radar (InSAR) data. The results show that our smooth 3D fault slip model has improved the fit to the geodetic data by 44% compared with the previously published inversions. The slip distribution obtained both by inversion of GPS data only and by joint inversion of GPS and InSAR data indicates that notable slips occur on the sub-horizontal décollement at the depth of 6.1–8.9 km.     

Detecting changes in long-period site responses after the <Emphasis Type="Italic">M</Emphasis><Subscript>w</Subscript> 7.6 Chi-Chi earthquake,Taiwan, using strong motion records

Temporal changes in site effects are obtained using the HVSR(horizontal-to-vertical spectral ratio) method and strong motion records after the M w 7.6 Chi-Chi earthquake, Taiwan. Seismic data recorded between 1995 and 2010 are used, comprising 3,708 data from 15 stations adjacent to the Chelungpu fault. Temporal fl uctuations are determined by analyzing the site effect variation using a time–frequency variation(TFV) diagram based on these seismic data. Stations adjacent to the fault show signifi cant disturbances in the resonance frequency at 16–26 Hz. Station TCU129 shows a 40% drop in fundamental frequency after the main shock, and a gradual return to the original state over nine years. For stations located farther from the fault zone, sudden changes in tectonic stress play a dominant role in temporal changes to the HVSR. An impact analysis of the directional factor confi rms our fi nding that the proximity of the fault to seismic stations has the most infl uence on data.     

Forced oscillation measurements of seismic attenuation in fluid saturated sandstone

Adopting the method of forced oscillation, attenuation was studied in Fontainebleau sandstone (porosity 10%, permeability 10 mD) at seismic frequencies (1–100 Hz). Confining pressures of 5, 10, and 15 MPa were chosen to simulate reservoir conditions. First, the strain effect on attenuation was investigated in the dry sample for 11 different strains across the range 1 × 10^?6–8 × 10^?6, at the confining pressure of 5 MPa. The comparison showed that a strain of at least 5 × 10^?6 is necessary to obtain a good signal to noise ratio. These results also indicate that nonlinear effects are absent for strains up to 8 × 10^?6. For all the confining pressures, attenuation in the dry rock was low, while partial (90%) and full (100%) saturation with water yielded a higher magnitude and frequency dependence of attenuation. The observed high and frequency dependent attenuation was interpreted as being caused by squirt flow.     

集集地震人员死亡率研究

发生在台湾省车笼埔断层上的集集地震造成了大量人员伤亡。本文搜集了该地震高烈度区的村里级行政单元的死亡率数据,并分析了城市型、乡镇型和农村型居民地的死亡率与地震烈度的关系。表明城市型和乡镇型平均死亡率差别不大,农村型则明显较高。对死亡率与地表破裂带、高烈度分布区和局部场地条件的关系等进行了分析,在此基础上,显示了死亡率随距离分布在断层两侧的不对称性。上述结果加深了对集集地震死亡率分布及其成因的认识,对评估未来地震人员死亡有重要意义。     

库仑应力变化与余震对应关系的初步探讨——以集集地震为例

本文以集集地震为例,讨论了在多个小断层的位错模型,分层地质结构这种更加接近实际的情况下的库仑应力变化,计算结果表明有必要考虑这些符合实际的模型.本文分别讨论了走滑断层上库仑应力变化与走滑地震、逆冲断层上库仑应力变化与逆冲地震空间分布的对应关系,计算结果表明集集地震所引起的库仑应力变化确实有利于本地区余震的发生.     

Effect of frictional heating and thermal advection on pre-seismic sliding: a numerical simulation using a rate-, state- and temperature-dependent friction law

Laboratory experiments on simulated faults in rocks clearly show the temperature dependence of dynamic rock friction. Since rocks surrounding faults are permeable, we have developed a numerical method to describe the thermo-mechanical evolution of the pre-seismic sliding phase which takes into account both the rate-, state- and temperature-dependent friction law and the heat advection term in the energy equation. We consider a laminar fluid motion perpendicular to a vertical fault plane and assume that fluids move away from the fault plane. A semi-analytical temperature solution which accounts for the variability of slip velocity and stress on the fault has been found. This solution has been generalized to the case of a time varying fluid velocity and then was used to include the thermal pressurization effect. After discretizing the temperature solution, the evolution of the system is obtained by the solution of a system of first order differential equations which allows us to determine the evolution of slip, slip rate, friction coefficient, effective normal stress, temperature and fluid velocity. The numerical solutions are found using a Runge-Kutta method with an adaptative stepsize control in time. When the thermal pressurization effects can be neglected, the heat advection effect gives rise to a delay, with respect to the purely conductive case, of the earthquake occurrence time. This delay increases with increasing permeability H of the system. When the thermal pressurization effects are taken into account the situation is opposite, i.e. the onset of instability tends to precede that of the purely conductive case. The advance in the time of occurrence of instability increases with increasing coefficient of thermal pressurization. In the small permeability range (H ≤ 10^?18 m²), the seismic moment and nucleation length of the pre-seismic phase are significantly smaller than those predicted by the purely conductive model.     

Delineation of Subtrappean Mesozoic Sediments in Deccan Syneclise, India, Using Travel-Time Inversion of Seismic Refraction and Wide-Angle Reflection Data

2-D shallow velocity structure is derived by travel-time inversion of the first arrival seismic refraction and wide-angle reflection data along the E–W trending Narayanpur–Nandurbar and N–S Kothar–Sakri profiles, located in the Narmada–Tapti region of the Deccan syneclise. Deccan volcanic (Trap) rocks are exposed along the two profiles. Inversion of seismic data reveals two layered velocity structures above the basement along the two profiles. The first layer with a P-wave velocity of 5.15–5.25 km s^?1 and thickness varying from 0.7–1.5 km represents the Deccan Trap formation along the Narayanpur–Nandurbar profile. The Trap layer velocity ranges from 4.5 to 5.20 km s^?1 and the thickness varies from 0.95 to 1.5 km along the Kothar–Sakri profile. The second layer represents the low velocity Mesozoic sediments with a P-wave velocity of 3.5 km s^?1 and thickness ranging from about 0.70 to 1.6 km and 0.55 to 1.1 km along the E–W and N–S profiles, respectively. Presence of a low-velocity zone (LVZ) below the volcanic rocks in the study area is inferred from the travel-time ‘skip’ and amplitude decay of the first arrival refraction data together with the prominent wide-angle reflection phase immediately after the first arrivals from the Deccan Trap formation. The basement with a P-wave velocity of 5.8–6.05 km s^?1 lies at a depth ranging from 1.5 to 2.45 km along the profiles. The velocity models of the profiles are similar to each other at the intersection point. The results indicate the existence of a Mesozoic basin in the Narmada–Tapti region of the Deccan syneclise.