Geodetic observations have demonstrated that the Xianshuihe Fault Zone undergoes both creep and stick-slip deformation, which reflects spatial heterogeneity of fault slip behavior. In this study, we investigated fault outcrops from both its creeping and stick-slip segments. By integrating field observations with XRD mineralogical analyses and XRF whole-rock geochemical analyses of fault rocks, we compared the lithological features, mineral assemblages, and geochemical characteristics between these two types of segments. Our results reveal distinct differences between the creeping and stick-slip segments, providing valuable insights into the mechanisms responsible for the coexistence of creep and stick-slip behavior. The bedrock of stick-slip segments is dominated by relatively strong rocks, such as limestone and sandy dolomite. These rocks contain low clay mineral contents and tend to maintain relatively high frictional strength, which favors fault locking and unstable slip. In contrast, the bedrock of creeping segments consists primarily of weaker rocks, including muddy sandstone and carbonaceous mudstone. These rocks are rich in clay minerals and are more favorable for low-friction, velocity-strengthening stable slip and shallow creep. Fluid activity further modifies fault-zone properties. Ca and Mg-rich fluids may promote fault healing via carbonate precipitation and vein formation, whereas K-rich fluids may promote feldspar alteration and clay mineral formation, thereby reducing fault rock strength and facilitating shallow creep. Therefore, the coupled effects of lithology and fluids help explain the coexistence of large earthquakes and fault creep along the Xianshuihe Fault Zone, including both the recurrence of large earthquakes and localized shallow creep during interseismic periods. Fault creep primarily accommodates shallow strain during the interseismic period. When accumulated strain reaches a critical level, fault instability may occur.

The newly established monazite fission track (MFT), with its ultra-low closure temperature (25-45 degrees C), offers a powerful tool for investigating the latest stage of rock exhumation and the geomorphic evolution of the uppermost crust. However, the absence of a well-characterized reference material has hindered the standardization and inter-laboratory reproducibility of MFT dating. Here, we evaluated the suitability of M6 monazite, a gem-quality megacryst from Esp & iacute;rito Santo, southeast Brazil, as a reference material for MFT dating. Independent age determination conducted by three laboratories, using different microscopes and analytical procedures, yields reproducible results within analytical uncertainty. Using an initial track length of 10.6 +/- 0.19 & micro;m, we obtain a weighted mean age of 3.90 +/- 0.14 Ma (1SE; MSWD = 3.1). Furthermore, M6 also exhibits highly uniform uranium content and remarkably consistent cerium content (22.9 +/- 0.4 wt%) which makes it an excellent compositional reference for U-Th concentration analysis via LA-ICP-MS. The M6 reference material thus holds significant potential for advancing MFT annealing kinetics studies and for developing integrated U-Pb, FT, (U-Th)/He and trace element multi-dating through simultaneous LA-ICP-MS analysis.

The Haiyuan fault zone is a large-scale left-lateral strike-slip fault on the northeastern margin of the Tibetan Plateau, exhibiting intense tectonic deformation and frequent seismic activity. The Laohushan fault, located in the "seismic gap", is a potentially high-risk area for large earthquakes. Recently, geodetic and seismological studies have revealed clear shallow creep behavior on the eastern segment of the Laohushan fault. However, the fault rock components, deformation mechanisms, and transition condition between shallow creep and deep earthquake nucleation along the Laohushan fault are still poorly known. To address this scientific issue, this study focuses on the fault rocks of the Laohushan fault, conducting detailed analyses of their mineral compositions, microstructural deformation features, and mechanical properties of these rocks. The results indicate that the shallow creep segment of the Laohushan fault is made up of a wide damage zone with numerous thin fault gouge layers spread throughout it. These clay-rich gouges exhibit widely well-developed foliation with aligned clay minerals, resulting in weak mineral slip zones. These clay minerals have low friction coefficients and stable sliding behavior, serving as the principal materials for shallow creep. Low-grade metamorphic rocks and crystalline rocks representing varied depths were exposed along the Laohushan fault. These fault rocks are dominated by high-strength minerals such as quartz, feldspar, pyroxene, and olivine, with widespread brittle fractures and fluid-rock alteration reactions, which provide conditions for earthquake nucleation at focal depths. Meanwhile, fluid activity affects fault strength by promoting mineral alteration and fracture healing, which influences fault sliding behavior. Based on these findings, we develop a rheological structure model for the Laohushan fault, which includes rock physics constraints on the transition from shallow creep to earthquake nucleation at depth. This provides a scientific framework for understanding fault deformation behavior, seismogenesis, and assessing future seismic hazards along the Laohushan fault.

Stable continental regions (SCRs) are characterized by low strain rates and long earthquake recurrence intervals, but the patterns and drivers of their seismicity remain debated. This study investigates the rupture history of the Liuyuan fault in the low-strain Beishan region of China to determine whether SCR earthquakes are regular, clustered, or random. We integrate paleoseismic trenching, IRSL and cosmogenic 10Be dating of trench units and rockfalls, and microstructural analyses of fault-zone materials. Our results reveal two distinct earthquake clusters at 65.5-46.2 ka and 8.8-4.6 ka, possibly separated by a long period of quiescence, which remains unconfirmed due to a sedimentary record gap. The late Pleistocene cluster is independently corroborated by cosmogenic 10Be ages of rockfalls, which cluster at 46-48 ka and overlap within uncertainty. Microstructural evidence, including multiple generations of quartz veins and abundant phyllosilicates, points to a fluid-driven, fault-valve mechanism. We propose that episodic increases in deep-sourced fluid pressure, facilitated by a steep fault geometry within a transpressional regime, trigger these rupture clusters. A comparison with other SCRs globally suggests that this clustered behavior, observed in some intraplate settings, may be controlled by key factors: a weakened lithosphere that localizes strain and transient stress perturbations from fluid migration or surface processes. These findings challenge the assumption of time-independent seismicity in some SCRs and have significant implications for seismic hazard assessment, particularly for critical infrastructure in low-strain environments like the Beishan region, a proposed site for high-level radioactive waste disposal.

Serpentine minerals exist pervasively near the subduction mantle wedge. Due to their distinct physical and chemical properties, they have the potential to significantly impact the frictional behavior and stability of subduction fault zones. Here we investigated the frictional properties of an antigorite-lizardite mixture gouge under hydrothermal conditions with an effective normal stress of 30 MPa, a pore fluid pressure of 100 MPa, and temperatures ranging from 100 degrees C to 500 degrees C. To explore the velocity dependence of frictional strength, the loading rate is switched between 0.04, 0.2, and 1.0 mu m/s. The results show that the friction coefficient of the gouge decreases systematically with temperature, with values ranging from 0.75 to 0.36. The thermal weakening of the gouge friction can be attributed to enhanced intergranular pressure solution processes activated at the relatively high fluid pressure. Stable velocity-strengthening behavior was observed in most cases. However, velocity-weakening behavior, evidenced by sustained oscillation, was found at 300 degrees C and 400 degrees C at the slowest loading rate (0.04 mu m/s), with a transition to velocity-strengthening behavior at higher loading rates. Our results further suggest that, under the high pore fluid pressure and temperature conditions characteristic of the subduction zone mantle wedge, the nucleation of unstable slip at low slip rates can decelerate as fault slip accelerates into the velocity-strengthening regime, ultimately terminating as a slow-slip event. These insights contribute to a better understanding of the processes underlying slow slip events in subduction zones.

Pseudotachylytes and cataclasites record transient seismic slips within the brittle–ductile transition zone and ductile flow layers. Investigating the mechanisms of pseudotachylytes can provide the most direct geological evidence for revealing seismic fault slip and coseismic processes. We investigate the deformation and chemical composition of pseudotachylytes, cataclasites, and mylonites collected from the Anning River fault zone in this study. Three kinds of pseudotachylyte veins were found in granite gneiss and cataclasite. Microstructural analyses show that pseudotachylytes and cataclasites developed within granitic gneiss and mylonites, and EBSD analysis indicates granitic gneiss deformed at temperatures of 250–350 °C. All of the pseudotachylytes are enriched in Fe and Ca, with SiO2 content closely resembling that of the wall rock of granitic gneiss. The geochemical results indicate that pseudotachylytes originated from the in situ melting of granitic gneiss, which was produced during coseismic frictional heating. Based on the deformation and geochemical data of mylonites, cataclasites, and pseudotachylytes, a simple model of the seismogenic layer is established for rock deformation during coseismic, post-seismic relaxation, and interseismic periods. Mylonite represents the rheological flow of the brittle–ductile transition zone during interseismic periods, cataclasites display brittle fracturing during coseismic rupture, and pseudotachylytes stand for localized melting induced by coseismic frictional heating. During the post-seismic relaxation, crack healing and static recrystallization of quartz occur.

The chronology of fault activity in bedrock is critical to constraining and understanding periods of active faulting, assessing seismic hazards, and mitigating the effects of earthquakes. However, because of the lack of suitable materials for dating, the temporal reconstruction of faulting in bedrock remains highly challenging for geologists. In the present study, we determine for the first time the electron spin resonance (ESR) ages of fault barite (BaSO4), which is produced by episodes of intense faulting on basalt bedrock fault surfaces. Three barite samples were obtained from a basalt fault section (27 degrees 5 ' 23 '' N, 100 degrees 25 ' 45 '' E, 1.8 km above sea level) of the Lijiang-Xiaojinhe Fault (LXF), southeastern Tibetan Plateau, for ESR measurements. Similar to marine barite, the ESR spectrum of fault barite shows an electron-type center with g = 2.0037, 2.0034, and 2.0028 attributed to SO3-. The signal intensity systematically increased with increasing gamma-ray dose. Dose rates were calculated using a model based on the location and burial depth of the barite samples, as well as their surrounding bedrock. The three barite samples yield ESR ages of 131 +/- 26, 503 +/- 61, and 1416 +/- 246 ka, respectively, which indicate that the LXF was active during the Early and Middle Pleistocene. The three ESR ages for fault barite from basalt extend the time range of activity of the LXF compared with previous carbonate ESR and radiocarbon dating results. Consequently, we propose that ESR dating of barite is valuable for reconstructing the history of bedrock fault activity. However, given that this investigation represents a preliminary application of the fault-barite ESR method, further study is needed to confirm its usefulness and the accuracy and precision of dating results.

To explore the effect of fluid (water) on fault reactivation and stability across the brittle-ductile transition, we performed frictional experiments on saw-cut samples of Carrara marble using a gas medium triaxial apparatus to simulate fresh faults. The experimental temperatures (T) were 70 similar to 500 degrees C, the confining pressure (P-c) were 60 to 130 MPa, pore fluid pressure (P-p) was constant 30 MPa, and the slip velocity (V) was switched among 0.08 mu m center dot s(-1), 0.4 mu m center dot s(-1), 2 mu m center dot s(-1) and 10 mu m center dot s(-1), respectively. The mechanical data and microstructure analysis of postmortem fault surfaces showed five types of fault behaviors: stable sliding, slow sliding, tremors, stick-slips, and fault closure. Under low effective confining pressure (30 MPa) and 70 degrees C, the fault exhibited stable sliding, tremors and slow sliding, with a transition from velocity strengthening behavior to velocity weakening behavior; at temperatures of 100 similar to 400 degrees C, the fault showed slow sliding and stick-slips, with a velocity weakening behavior; at 500 degrees C, the fault showed stable sliding, and transitioned back to velocity strengthening again. However, at high effective confining pressure(100 MPa), the closured fault did not show any sliding at temperatures of 70 similar to 300 degrees C. In this study, the Carrara marble fault experienced unstable sliding under pore fluid (water) pressure within the temperature range of 100 to 400 degrees C, which was wider than the temperature range of 200 to 300 degrees C for dry samples. This indicates that pore fluid (water) plays a significant role in the reactivation and sliding behavior of Carrara marble faults, and can promote the unstable fault sliding effectively. The primary deformation mechanisms for all samples included fracturing, dilation, and crystal plastic deformation. With increased temperature and confining pressure, the dissolution effect of pore fluid (water) and dynamic recrystallization were enhanced, especially at low effective confining pressure of 30 MPa and high temperature of 400 degrees C. Several processes, such as, recrystallized calcite and dilation caused by calcite dissolution, the lubrication of particles by pore fluid (water), as well as fault healing induced by pressure solution, contributed to the intense unstable sliding. Assuming a geothermal gradient of 25 to 30 degrees C/km, it is inferred that carbonate-dominated faults start to experience unstable sliding at a depth of approximately 2 to 4 km in the crust, followed by a transition to stable sliding at a depth range of approximately 13 to 20 km, and eventually fault closure.

Plagioclase feldspar is a major mineral in mafic crustal rocks. To better understand the deformation mechanism of plagioclase feldspar during frictional faulting, we conducted shearing experiments on simulated plagioclase gouge in a wide range of effective normal stress of 100-300 MPa, pore-water pressure of 30-100 MPa, and temperatures ranging from 100 degrees C to 600 degrees C. The coefficient of friction is found to range from 0.65 to 0.74 across the entire temperature range, showing no significant thermal weakening process. Except for a case at 200 degrees C with an effective normal stress of 300 MPa, the frictional sliding is velocity weakening over the whole temperature range, showing a steady-state rate dependence (a-b) ranging from -0.5 x 10(-3) to -8.6 x 10(-3). This property facilitates nucleation of unstable slips in frictional faulting. Above 200 degrees C, the direct rate effect parameter (a) and the evolution effect parameter (b) of friction increase with temperature up to a threshold of 400 degrees C or 500 degrees C, depending on the effective normal stress. This thermal enhancement suggests thermally activated creep at contact junctions governed by intergranular pressure solution, as evidenced by microstructural signatures indicating the prevalence of very fine precipitates formed at the surfaces of gouge particles as a result of pressure solution. In frictional sliding of plagioclase, a low effective normal stress of 100 MPa corresponds to a higher degree of velocity weakening and tends to facilitate seismic slip rather than slow slips, whereas the high effective normal stress of 300 MPa corresponds to a minor velocity weakening which may cause slow-slip events in faults of limited size. Plagioclase is a feldspar that is abundant in the crust. To better understand the deformation of this feldspar during shearing processes of faults, we ran experiments on plagioclase gouge under effective normal pressure of 100-300 MPa, pore-water pressure of 30-100 MPa, and temperatures from 100 to 600 degrees C. The friction coefficient was found to be 0.65-0.74 across the whole tested temperature range, with no appreciable thermal weakening. Except at 200 degrees C with an effective normal pressure of 300 MPa, the friction stress decreases at a higher slip velocity over the whole temperature range. This property promotes unstable slips on natural faults. For temperatures above 200 degrees C, the instantaneous response to a slip-rate change and its maximum memory are stronger with an increase in temperature up to a temperature limit of 400-500 degrees C. This temperature-enhanced phenomenon implies contact creep in the gouge controlled by dissolution-precipitation processes, as evidenced by the ubiquity of fine precipitates formed in sheared samples. In frictional sliding of plagioclase, a low effective normal pressure of 100 MPa favors earthquakes over gradual slips, whereas the high effective normal pressure of 300 MPa corresponds to a minor instability which may cause gradual slips on faults of limited size.

Shallow creep has been observed in the Laohushan segment which lies within a seismic gap between the coseismic ruptures of the 1920 Haiyuan M8.5 earthquake and the 1927 Gulang M8.0 earthquake. However, there has been a lack of geological and mechanical evidence to explain the mechanism behind this shallow creep. In this study, we carried out detailed fieldwork along the Laohushan fault, analyzed the mineral compositions and microstructures of the fault rocks, and performed frictional experiments on fault gouges under various temperature and pressure conditions in the laboratory. Our findings revealed that clay minerals are prevalent in the fault rocks along the Laohushan creep segment, especially in the fault gouge at the fault core. Microstructural observations showed that extensive development of clay foliation in the fault gouges, along with carbonate veins healing cracks and minerals dissolution in the fault rocks. The frictional experiments indicated that the clay-rich gouges showed low frictional coefficients of 0.46-0.58, with positive a-b values of 0.0035-0.0052, reducing to 0-0.0015 at 200 degrees C. The experimental samples exhibited microstructural evolution of pervasive well-developed clay foliation and fewer clast fractures, which controls the velocity-strengthening sliding behavior of fault gouges. Our results suggest that the enrichment of weak clay minerals in the fault zone generates a well-foliated microfabric, which, as demonstrated by frictional experiments, controls the Laohushan shallow creep along the Haiyuan fault at depths of less than 8 km. This finding is consistent with the shallow creep inferred from geodetic data and repeating earthquakes.

Plagioclase, one of the most common rock‐forming minerals, can provide useful information on the crystallisation environment, magmatic evolution and thermal history of host rocks. Owing to matrix effects, trace element and Sr isotope in situ measurements in plagioclase have been hindered by a shortage of reference materials for quality control and method validation. A natural plagioclase sample from Madagascar was evaluated for its feasibility as a reference material for Sr isotope and element determination. Measurement results from EPMA, LA‐ICP‐MS and LA‐MC‐ICP‐MS indicate that MGP‐1 plagioclase is homogeneous at the μm–mm scale with respect to major elements (RSD < 4%), most trace elements (RSD < 15%), and 87Sr/86Sr ratio (2s = 0.00013). Major and trace elements were also determined using XRF and solution ICP‐MS, showing good agreement within 10% from in situ measurement results. The 87Sr/86Sr ratio obtained by TIMS ranged from 0.703441 to 0.703477, with a mean value of 0.703459 (2s = 0.000028), consistent with the LA‐MC‐ICP‐MS mean value. Reference values and uncertainties for major and trace elements and 87Sr/86Sr ratio are presented in this study. Consequently, the MGP‐1 plagioclase can be used as a reference material for in situ measurement of trace elements and 87Sr/86Sr ratios, contributing to research on the formation and evolution of micro‐zoned plagioclase and host rocks.

SKS wave splitting measurements revealed that the distribution of fast wave direction in the Songliao Basin is complex. The formation of seismic anisotropy is restricted by the lack of study in lithospheric mantle deformation. To better understand lithospheric deformation and upper mantle anisotropy observed in this region, we performed studies on petrology, geochemistry, microstructures and seismic anisotropy of peridotite xenoliths collected from Shuangliao, southern Songliao Basin. The results show that equilibrium temperature of peridotite xenoliths in Shuangliao is 893 similar to 1152 degrees C, which suggest that peridotite xenoliths were derived from the lithospheric mantle. The mantle peridotite xenoliths show three types crystallographic preferred orientation (CPO) of olivine, which are A-type, D-type and AG-type CPO. The AG-type and D-type olivine CPO were probably formed by lithospheric deformation caused by the subduction and retreat of western Pacific Plate. Moreover, the AG-type olivine CPO may be formed by deformation in the presence of melt. The VP anisotropy (AV(P)) and maximum shear wave anisotropy (AV(Smax)) of peridotite xenoliths calculated from the fabric of xenoliths are 4.79% similar to 11.80% and 3.13% similar to 7.93%, respectively. Based on the seismic properties of peridotite xenoliths and geophysical measurements, it is inferred that the complex SKS wave splitting is attributed to the lithospheric mantle in the southern Songliao Basin with a vertical structural frame.

Graphite is considered as a material that promotes fault weakening and electrical conductivity ( σ ) enhancement at fault zones. We studied how shear deformation may affect the evolution of friction and electrical conductivity of synthetic quartz (Qz)‐graphite (Gr) mixtures and, more importantly, whether the σ of the mixtures present visible changes at the beginning of the simulated fault slip. Long‐displacement friction experiments were performed on 1.2–2.3 mm‐thick gouge specimens of varied Gr volume fraction ( X Gr = 0–100 vol.%) under identical normal stress (2 or 5 MPa), slip rate (∼1.0 mm/s), and N 2 ‐flushing conditions. The experimental results suggested that the σ of the specimens with ≥4.6 vol.% X Gr abruptly increased under limited shear displacement. With continued shear, the steady‐state electrical conductivity ( σ ss ) increased by more than seven orders of magnitude when X Gr > 3.4 vol.%, while the steady‐state frictional coefficient remained high (0.54–0.80) except for the specimens with X Gr > 13.6 vol.%. The post‐mortem microstructures revealed that the high σ ss observed in the intermediate Gr content specimens (3.4–13.6 vol.%) is associated with an ad‐hoc fabric (graphite–cortex clasts) present in the principal slip zone. For high Gr content, excess Gr flakes fill the pores and help develop mechanically lubricated surfaces. We propose that low Gr content (i.e., as low as 3.4 vol.%) can cause high conductivity anomalies in natural shear zones. Overall, the findings suggest that the initiation of slips within carbonaceous shear zones can be detected by identifying unusual temporal signals using electromagnetic stations.

Triaxial compression experiments on intact antigorite were performed under undrained conditions with a confining pressure of 100-400 MPa, temperature of 25-700 degrees C and strain rate of-1.5 x 10(-6)-10(-5) s(-1). We report a sequential transition in mechanical behavior from faulting to slow stick-slip behavior and then to stable sliding with increasing temperature. Syndeformational dehydration of antigorite at 100 MPa and 550-650 degrees C produces slow stick-slip, and the slip velocity and shear stress drop generally decrease with increasing temperature. At 100 MPa and 650 degrees C, the slip velocity and shear stress drop of slow stick-slip gradually decrease, implying a trend toward stable sliding, which might be related to a continuous increase in fluid pressure induced by the accumulation of fluid over time in the process of antigorite dehydration. Our results indicate that dehydration makes the sample weaker but more stable and does not cause embrittlement; therefore, an intermediate-depth earthquake in a subduction zone may be triggered by brittle deformation in strong peridotite surrounding dehydrating antigorite. Similar to the antigorite fault gouge previously studied, intact antigorite can also induce slow stick-slip under partial antigorite dehydration, indicating that slow earthquakes could be triggered in the mantle wedge of a hot subduction zone.

Abstract The distribution of earthquakes at intermediate depths corresponding to pressures <2 GPa in several hot subduction zones (such as Cascadia and southwestern Japan) coincides with the breakdown of antigorite to forsterite and talc; thus, this reaction may have triggered these earthquakes. However, previous studies have overlooked the potential significance of this reaction. Here, we performed a series of time-dependent dehydration experiments on antigorite at a pressure of 200 MPa and a temperature range of 500–650 °C. The results show that dehydration is controlled by a heterogeneous nucleation and growth mechanism and has an activation energy of 354 ± 24 kJ/mol. The formation of fine-grained forsterite and large talc crystals is consistent with kinetic results indicating Avrami exponents n = ~1.4–1.1 and ~2.7, respectively. Fluid production rates at 600 and 650 °C are 2.54 × 10−6 and 4.69 × 10−5 m fluid  3   m r o c k − 3   s − 1 $\mathrm{m}_{\text {fluid }}^3 \mathrm{~m}_{\mathrm{rock}}^{-3} \mathrm{~s}^{-1}$respectively, which are much faster than those of mantle deformation, causing high fluid pressure in hot subducting mantle but not necessarily embrittlement. We emphasize the role of kinetic mechanisms in controlling the grain sizes of reaction products, which likely determine the mechanical behavior of serpentinized fault zones. Superplasticity or velocity weakening of fine-grained forsterite and velocity weakening of antigorite by water and/or talc may be responsible for earthquake nucleation and propagation in a heterogeneous system, which can be either dehydration products within a serpentinized fault zone or the mixture of antigorite fault and surrounding peridotite in hot subduction zones (<2 GPa).

The P-wave velocities of typical rocks in the Anninghe fault zone under pressures of 50–600 MPa were systematically measured. The P-wave velocities of the felsic, intermediate, and mafic rock types under atmospheric pressure were 5.86, 6.06, and 6.50 km/s, respectively, with pressure coefficients of 2.19 × 10–, 3.80 × 10–4, and 4.03 × 10–4 km/s/MPa, respectively. The results were combined with deep-imaging seismic data to establish crustal rock composition models at different depths in the study area. The composition of the Anninghe crust is very different in the horizontal and vertical directions. The most notable feature in the vertical direction is that the lithologic compositional change is gradual rather than abrupt with increasing depth. In the middle and upper crust, shallower than 25 km, the lithologic difference between the southern and northern sections of the Anninghe fault zone is primarily that the rocks in the southern section (Xichang) are more felsic than those in the northern section (Shimian).

Pseudotachylyte was found in graintic gneiss-cataclastic rocks at the western side of Anninghe fault. In this study, we performed frictional experiments of pseudotachylyte gouges under hydrothermal and dry conditions. Velocity-stepping experiments were performed at effective normal stress of 250 MPa, pore fluid pressure of 30 MPa, temperature range of 100 similar to 500 degrees C, and shear velocities between 0. 04, 0. 2 and 1 mu m . s(-1) Experimental results show that at 100 similar to 500 degrees C, the friction coefficient of pseudotachylyte gouges increase with the increasing of temperature, and friction coefficient for dry samples is lower than that of hydrothermal conditions. The slip stability is characterized by velocity strengthening at temperatures of 100 similar to 300 degrees C and velocity weakening at temperatures of 400 similar to 500 degrees C under normal loading rate (1 similar to 0. 2 mu m . s(-1)). The results find that loading rate can also affect the stability of pseudotachylyte gouges. At the temperature range of 100 similar to 300 degrees C, pseduotachylyte exhibits velocity strengthening behavior at experimental standard velocity of 1 similar to 0. 2 mu m . s(-1) and converts to velocity weakening behavior at slow loading rates (0. 2 similar to 0. 04 mu m . s(-1)). Based on the experimental results, it is inferred that the depth of earthquake nucleation is 10 similar to 30 km at the Aninghe fault. Comparing with the frictional experiments of granite gouge, we found that the frictional strength of pseudotachylyte is higher than that of granite, implying that it is impossible for the large earthquake to re-occur at the same location if the last co-seismic rupture was healed by pseudotachylyte. However, it could increase the possibility of unstable slip and nucleation of earthquake, also promote propagation of co-seismic dynamic rupture if there exists pseudotachylyte in fault zone.

The "Anninghe seismic gap" reflects fault-locking and associated strain-accumulation along with the central part of the Anninghe Fault over the last 30 years, and is considered a high possibility location for future earthquakes. In this study, we performed frictional experiments of natural granite gouges from the Anninghe Fault core under hydrothermal conditions. Velocity-stepping experiments were performed at effective normal stress of 200 MPa, pore fluid pressure of 30 MPa, temperature from 25 to 600, and shear velocities between 0. 04 mu m.s(-1) and 1 mu m.s(-1). Results show that the friction coefficient (about 0. 663 similar to 0. 704) increases at 100 similar to 400 degrees C followed by a decrease at 400 similar to 600 degrees C. The slip stability response is characterized by velocity strengthening at temperatures of 25 similar to 100 degrees C and velocity weakening at temperature of 200 similar to 600 degrees C. The upper temperature limit for unstable sliding of granite gouges is 600 degrees C, which is much higher than the 350 similar to 400 degrees C upper limit of unstable sliding temperature given by previous data. The results also show that the stability of granite fault is related to the loading rate at the same temperature, which the velocity-weakening region is more likely to occur at the lower rate or slow velocity (0. 04 mu m.s(-1)). Based on the experimental results and combined with the geothermal gradient in Sichuan-Yunnan Block, it is inferred that the depth of seismic nucleation is 10 similar to 30 km in the middle and northern section of the Anninghe Fault, which implies that the earthquake gap should be a kind of interseismic locking along the Anninghe Fault.

脆塑性转化带对于研究岩石圈变形、断层强度和变形机制以及强震的孕育和发生具有重要意义.文中采用汶川地震震源区彭灌杂岩中具有代表性的细粒花岗岩样品,在固体压力介质三轴实验系统上开展了高温高压非稳态流变实验研究.实验设计模拟了汶川地震区地壳10～30km深度的实际温度和压力,温度为190～490℃,压力为250～750MPa,应变速率为5×10-4s-1,利用扫描电镜对实验样品进行微观结构观察.实验力学数据、微观结构及变形机制分析表明,在相当于地壳浅部10～15km深处的低温低压条件下,表现为应变强化,样品具有脆性破裂-半脆性流动的变形特征;在相当于地壳15～20km的深度条件下,随着应变量增加,应力趋于稳态,样品具有脆塑性转化特征;在相当于地壳20～30km的深度条件下,样品具有塑性流动特征.当样品处于半脆性域时发生非稳态流变,主要变形机制为碎裂作用,同时激活了动态重结晶作用、位错蠕变等塑性变形机制.样品强度随着深度不断增大,在深度为15～20km时达到极大值,深度为20～30km时强度逐渐减小.因此,花岗岩的强度随深度的变化规律与微观结构及变形机制均表明,在实验温度和压力条件下,花岗岩具有非稳态流变特征,在15～20km深处,龙门山断裂带处于脆塑性转化带,花岗岩强度达到最大值,该深度与汶川地震的成核深度一致,显示出彭灌杂岩的强度和变形对汶川地震的孕育和发生具有控制作用.