Serpentine minerals are among the minerals commonly found in the Earth’s subduction zones, and their unique physicochemical properties have a significant impact on subducting geodynamics. Friction experimental studies of serpentine minerals are essential to gain a deep understanding of the frictional sliding stability of serpentine-containing faults in subduction zones as well as explaining the complicated misalignment behavior of faults in subduction zone. Previous laboratory research has produced an abundance of results, and this work addresses two main aspects: the stable states of occurrence and interconversion relationships of serpentine minerals, and the parameters affecting the frictional strength and sliding stability of serpentine minerals. First of all, studies on the stable endowment state of serpentine minerals and the interconversion relationship show that different types of serpentines diaplay different stable phases under different conditions. Chrysotile and lizardite are stable at low temperatures, and the stability fields of both chrysotile and lizardite roughly overlap, but chrysotile is in a substable state. Antigorite is stable at high temperature conditions, such as subduction zone mantle wedges containing high pore fluid pressure conditions, and undergoes a transition from lizardite to antigorite with increasing temperature. Secondly, studies on the factors controlling the frictional strength and sliding stability of serpentine minerals have shown that temperature, pore fluid, and the effective normal stress are all critical factors, for example, an increase in temperature can significantly increase the frictional strength of lizardite and chrysotile. In addition, the friction strength of serpentine minerals shows an obvious pressure dependence, and it was found through previous experimental studies that the friction strength of chrysotile exhibits a high-pressure sensitivity, and that the friction strength of antigorite gradually increases with increasing temperature under low fluid pressure conditions, showing an obvious temperature strengthening phenomenon. In contrast, the change in frictional strength of antigorite with temperature under high-pressure fluid pressure conditions is diametrically opposed to the results of low-pore fluid pressure conditions, which shows a clear temperature weakening phenomenon. Previous studies have also found that antigorite-undergoes a dehydration reaction with increasing temperature under lower fluid pressure conditions, and then exhibits unstable velocity weakening phenomenon, while antigorite exhibits velocity weakening phenomenon under low shear deformation rate under high-pressure fluid conditions. By analyzing the variation of friction-slip stability of antigorite with the shear slip rate can help us to better explain the phenomenon of subduction-zone slow-slip. Overall, experimental studies of the friction of serpentine minerals provide a key experimental basis for a deep understanding of subduction zone geologic processes. The results of these studies are scientifically important for predicting earthquakes and explaining the evolution of the Earth’s internal tectonics and subduction zones, providing strong support for research and practice in the field of geosciences.

In 2018, a short-period seismic network was set up in Eryuan area of Yunnan Province to carry out continuous field observation of the sub-instability process of the earthquake. The relevant data of the Yangbi M_S6.4 earthquake sequence are mainly from the waveforms recorded by this network, combined with some other stations from Yunnan regional seismic network. The Yangbi earthquake sequence shows that the events in this area began to occur intensively on May 18. A total of 2 000 earthquakes with M>0.1 were recorded from May 18 to 23, including 770 foreshocks.

Seismicity analysis shows that two clusters of foreshocks occurred successively in the adjacent area of the main earthquake in the northwest segment of the rupture strip within 3 days, then in the subsequent impending period(within 1 hour before the main shock)about 60 events spread symmetrically from the center of the fracture zone to the ends. The spatial distribution of foreshocks in different periods shows the spatial migration of local fractures and accelerated expansion prior to the main shock. The spreading speed is about 5km/d from foreshock clustering process to 96km/d in impending earthquake period. The epicenter of the main shock is located at the edge of the cluster foreshocks and the northwest end of the final rupture zone. Subsequent aftershocks extend southeastward to the whole fracture zone in about half an hour, and the final fracture zone is more than 20 kilometers long, showing unilateral propagation of the rupture. Since 2018, b-value in the Yangbi area has been stable(0.9~1.1)for the past three years. After March this year, the b-value abnormally decreased to 0.6 before the main shock, reflecting that there was a significant process of continuous increase of local stress before the Yangbi earthquake.

The identification of short-term precursors and somehow definite information is one of the focus problems in earthquake prediction research. On the basis of the experimental results, Ma Jin proposed the theory of seismic meta-instability stage based on the characteristics of the load stress after the peak value from rock experiments and the corresponding change of related physical field, and considered that the degree of fault activity synergy was a sign to determine the stress state of the fault. When the fault activity changes from the expansion and increase of the stress releasing points in the early stage of meta-instability to the connection between the released segments at the late stage of meta-instability, that is, the quasi dynamic instability stage, the stress release on the fault will accelerate, and the acceleration mechanism is the strong interactions between the fault segments. In the context that the macroscopic stress state cannot be known directly, the original intention of the “meta-instability” test area is to try to capture the characteristic signal of the meta-instability stage described by the experimental phenomenon through the deformation and seismicity of the actual faults during the earthquake preparation process. It is clear that in this stage, the fault will continue to expand in the pre-slip zone theoretically, and it will enter into the quasi dynamic fracture expansion before the impending earthquake. This theory is obviously embodied in the foreshocks of this earthquake, forming the phenomenon of rapid migration of small earthquakes as mentioned above. From the current understanding of the meta-instability, it can be seen that the seismogenic fault is in the state of overall stress release at this stage, rather than the continuous increase of stress. Therefore, the decrease of b value before the earthquake shows that local faults have been activated and entered the final stage of nucleation process. The quasi dynamic spreading phenomenon before this kind of moderate-strong mainshock displayed by small earthquake activity can be identified as the precursor of a kind of earthquakes.

Many of the large earthquakes in the continental crust nucleate at the bottom of the seismogenic zone in depths between 10 and 20km which is related to the broad so-called ‘brittle-to-plastic or brittle-to-ductile’ transition region. From the field studies and seismic data, we could know that the dominant deformation mechanism at the base of seismogenic zone is likely to be semi-brittle flow of fault rocks. The physical and chemical processes acting in the ‘brittle-to-plastic’ transition are of great interest for a better understanding of fault rheology, tectonic deformation of the continental lithosphere and the generation of strong earthquakes. So it’s of great significance to know more about this transition. Despite the importance of semi-brittle flow, only few experimental studies are relevant to semi-brittle flow in natural rocks. In order to study the semi-brittle deformation and rheological characteristics of granite, we performed a series of transient creep experiments on fine-grained granite collected from the representative rock of Pengguan Complex in Wenchuan earthquake fault area using a solid-medium triaxial deformation apparatus(a modified Griggs rig). The conditions of the experiments are under the temperatures of 190~490℃and the confining pressures of 250~750MPa with a strain rate of 5×10^-4s^-1. The temperature and pressure simulate the in-situ conditions of the Wenchuan earthquake fault zone at the corresponding depths of 10~30km. We observe the microstructures of the experimentally deformed samples under the scanning electron microscope(SEM). The mechanical data, microstructures and deformation mechanism analysis demonstrate that deformation of the samples with experimental conditions could be covered by three regimes: 1)Brittle fracture to semi-brittle flow regime. We could see the strain and stress curves of the samples characterizing with strain hardening behavior and without definite yield point under low temperatures and pressures, which correspond to the depths of 10~15km; 2)Brittle-ductile transition regime. The strain and stress curves of the samples tend to be in a steady state with definite yield point under temperature and pressure at the depths of 15~20km. The main deformation mechanism is cataclasis, and dynamic recrystallization and dislocation creep are activated; and 3)Ductile flow regime which is at depths of 20~30km. The strength of granite increases with depth and reaches to the ultimate at the depth of 15~20km, and then decreases with depth at 20~30km. Based on the analysis of strength of granite, microstructures and deformation mechanism, we conclude that the granitic samples deformed with the characteristics of transient creep, and the strength of Longmenshan fault zone reaches maximum at the depths of 15~20km where it is in the brittle-to-plastic regime. Based on the Mohr circle analysis, the rupture limit at depths of 15~20km is close to the limit of friction, and at the same time, this depth range is also consistent with the focal depth of Wenchuan earthquake. Therefore, it implicates that the deformation and strength of Pengguan complex granitic rocks should control the nucleation and generation of the Wenchuan earthquake.

Since the 21st century, the occurrence of tremor and slow-slip events in the subduction zones has increasingly attracted researchers' attention. It seems that minerals in the subduction zone which may be related to tremor and slow-slip events, have become a topic of concern. Hydrous minerals are generally lower in strength than anhydrous minerals, so the hydrous minerals in the subduction zone(e.g., hornblende, serpentine, talc, etc.)may control the frictional sliding behaviors in the subduction zones.
Although many geophysical data indicate that serpentinization may exist in the depth range around the mantle wedge, ocean drilling results indicate that hornblende is a common hydrous mineral in the mantle. In order to understand the frictional behaviors of hornblende as a common hydrous mineral in the subduction zones under hydrothermal conditions, we used pure hornblende as the material for simulating fault gouge samples. In a series of experiments with a certain confining pressure, the axial loading rate is between 0.04μm/s and 1.0μm/s. In these experiments, the velocity stepping tests were carried out under the confining pressure of 136MPa, and the pore pressure was 30MPa. The frictional sliding velocity is switched between 1.22μm/s, 0.244μm/s, and 0.048 8μm/s to obtain data on the response to the velocity change. The experimental results are as follows:
(1)In these experiments(C_P=136MPa, P_P=30MPa), except for the quasi-static oscillations at 505℃ and 607℃, at most experimental temperatures, hornblende fault gouge samples all show stable sliding behaviors. The steady-state friction coefficient ranges from about 0.70(607℃)to about 0.72(403℃), with an average value of about 0.71, and does not show systematic changes with temperature.
(2)In these experiments of hornblende fault gouge(C_P=136MPa, P_P=30MPa), velocity strengthening behaviors were observed at temperatures of 101 and 203℃. It changes to velocity weakening at 303℃. It is in transition state at 403℃, and changes to velocity weakening at 505℃, and the velocity weakening continues until the highest temperature in our experiment, 607℃. The absolute value of velocity weakening(b-a)is between 0.000 51 and 0.001 4, which is a weak velocity weakening.
We numerically fitted the experimental data and obtained the values of the friction constitutive parameters a, b, and D_c at each temperature.
Our results of data fitting show that the slowness law adequately reproduces both the non-oscillatory rate steps and the periodical slow slips. As a result, a and b-values for the series exhibit a similar trend up to 203℃. The maximum of a averaged over steps (~0.009) occurred at 101℃, associated with a step-averaged b of 0.001 3. As temperature increased to 203℃, the step-averaged a decreased rapidly to a level of 0.006 8, with the corresponding b-value of 0.005 3~0.005 5. The temperature at 303℃ is a turning point for the a-value from the decreasing trend to a monotonic increasing trend up till 607℃. In contrast to the a-value, the average b in the series shows a growing trend in the whole temperature range.
The average D_c was found to range from 12~23.5μm for non-oscillatory cases, with no systematic changes as related to temperature. Much smaller D_c of 2μm was inverted for the oscillatory slow slips at 505℃ and 607℃ in the series, indicating that it was the cause of heightened critical stiffness that approached the vicinity of the loading stiffness.
Hornblende has a weak velocity weakening, with (b-a)-value below 0.001 5, which is less than the (b-a)-value used in the numerical simulation of the slow-slip in the Cascadia subduction zone, indicating that the velocity weakening is very weak, and this weak velocity weakening is conducive to the generation of slow slip. Therefore, the degree of velocity weakening of hornblende is in line with the appropriate conditions for the occurrence of slow slip in the subduction zone. The slow slip event may occur in a wider range of effective normal stress.

To understand the mechanism of lower-crust earthquake and slow slips, it is necessary to study the frictional properties of mafic rocks and their major rock-forming minerals. Previous studies have performed a series of experimental researches on gabbro, basalt and their major constituents.
According to the results of previous experiments, frictional sliding of plagioclase under hydrothermal conditions(100~600℃)shows a property of velocity weakening, and the experimental results show that both the direct rate effect parameter(a)and the healing effect parameter(b)increase with temperature, a typical feature for thermally-activated processes. Velocity weakening means property of a shear band that has a stronger friction healing effect than the direct rate effect in the rate and state friction constitutive framework, and the healing effect(b value)in constitutive relation mainly reflects the increase in contact area with time under hydrothermal conditions, with some minor effect of structural changes. Since the microphysical mechanism of feldspar minerals at the contacts is mainly brittle cataclasis for temperatures below 600℃, the significant frictional healing effect in this case can only be explained by the mechanism of pressure solution. In order to determine if the dissolution process of plagioclase actually occurs on the laboratory time scale, we conducted hydrostatic experiments on plagioclase powder samples under hydrothermal conditions whereby frequent contact switch between particles seen in frictional sliding experiments can be avoided, making the observation on the dissolution sites possible.
Experimental temperatures were 400℃ and 500℃, with confining pressure of 90~150MPa, pore pressure of 30MPa, with 2mm initial thickness of fault gouge. The mechanical data show that a creep process occurred in the plagioclase fault gouge in the experimental temperature and pressure range; and the microstructures of the experiment show that precipitation of new grains is prevalent as the product of pressure solution process between plagioclase particles. At the same time, it is observed that the contact points have an appearance similar to fused, fuzzy structure as signatures of dissolution. The results of our experiments provide a definite experimental evidence for the healing mechanism in friction of plagioclase and for the theoretical relation between unstable slip and the pressure solution process.
The results of the experiments are summarized as follows:
(1)Drainage rate of pore water in plagioclase gouge was high in the first few hours of experiment, but gradually decreases over time for both temperature and pressure series of experiments slowing down to a steady state. This feature indicates that there is a creep process that evolves inside the plagioclase gouge.
In the temperature-series experiments, the drainage rate of the pore water in the plagioclase gouge at 400℃ is relatively low than the cases for higher temperatures. Thus, the applied temperature is positively correlated with the creep of plagioclase gouge.
(2)Scanning electron microscopy(SEM)observations of the experimentally deformed samples were performed on thin sections cut along the sample axis. Firstly, from the images of microstructure, it was found that the degree of particle fracture became more significant at a higher effective pressure, with smaller pore volume between particles. In the temperature-series experiments it was found that the degree of compaction of plagioclase gouge increased with increasing temperature. Precipitation of plagioclase grains in layered structures was generally observed in high-magnification images, indicating the presence of pressure solution processes. Contact points were also found to be in a state of ambiguity that seems to be a fused morphology, but the details of the structure remain to be determined by further observations.
The above results indicate that the pressure solution process of plagioclase particles can occur on a typical laboratory time scale, and the results of this study provide robust experimental evidences for the theory that links between pressure solution and the mechanism of frictional healing and unstable slips for plagioclase.

In this paper, we report friction experiments performed on natural fault gouge samples embedded in granitic rock from drilled core by a project entitled "the Longmenshan Fault Shallow Drilling(LMFD)". Compared with other natural fault gouge, this yellow-greenish gouge(YGG)is dominantly chlorite-rich. The maximum content of chlorite reaches 47%in the YGG. To understand the frictional properties of the YGG sample, experiments were performed at constant confining pressure of 130MPa, with constant pore pressure of 50MPa and at different temperatures from 25℃ to 150℃. The experiments aim to address the frictional behavior of the YGG under shallow, upper crustal pressure, and temperature conditions. Compared with previous studies of natural gouge, our results show that the YGG is stronger and shows a steady state friction coefficient of 0.47~0.51. Comparison with previous studies of natural gouge with similar content of clay minerals indicates a sequence of strengths of different clay minerals:chlorite > illite > smectite. At temperatures up to 150℃ hence depths up to~8km in the Longmenshan region, the YGG shows stable velocity-strengthening behavior at shallow crustal conditions. Combined with the fact of strong direct velocity effect, i.e., (a-b)/a>0.5, faults cutting the present clastic lithology up to~8km depth in the Longmenshan fault zone(LFZ)are likely to offer stable sliding resistance, damping co-seismic rupture propagating from below at not-too-high slip rates. However, as the fault gouge generally has low permeability, co-seismic weakening through thermal pressurization may occur at high slip rates(>0.05m/s), leading to additional hazards.

We performed deformation experiments using Carrara marble in dry and wet conditions under temperature of 400~700℃ and confining pressure 300MPa with two different strain rates. Water contents of deformed samples were measured using FTIR spectroscopy. The microstructure and deformation mechanisms of samples were observed under optical microscopy, scanning electron microscopy and energy spectroscopy analysis. The mechanical data show that samples display strain hardening at 400℃, and transition to steady creep at temperature from 500~700℃. The strength of marble reduced gradually with elevated temperatures or decreased strain rate. However, water effect to the strength of the marble is significantly weak. Microstructures observed show that the deformation is cataclastic flow in dry samples, fracture and pressure solution in wet samples at 400℃. Samples underwent brittle-plastic transition at 500℃. Dislocation glide is major deformation mechanism for dry samples at 600℃. Dislocation climb and dynamic recrystallization are major deformation mechanism for wet samples at 600℃ and for all wet samples and dry samples at 700℃. Lower strain rate and higher water content could promote the process of pressure solution and diffusion as well as dynamic recrystallization.

The discovery of tremors on the lower crust portion of the San Andreas Fault has attracted more attention on the mechanical properties of the lower crust in recent years, and some experimental studies have been carried out to understand the mechanical behavior. Previous experiments under effective normal stresses of 200MPa have shown that pyroxene and plagioclase mineral separated from the gabbro and their mixtures all show velocity weakening in the lower-crust temperature range, which results in unstable slip when frictional sliding is the dominant deformation mechanism. This work is to examine whether the velocity-weakening behavior of plagioclase gouge also applies to relatively lower effective normal stress. Our experiments were performed under effective normal stress of about 100MPa, with a constant confining pressure control, with pore pressure of 30MPa and temperature of 100℃ to 600℃. We found that the frictional sliding of plagioclase are basically the same with the previous results obtained under effective normal stress of 200MPa, both of which show velocity weakening over the entire temperature range. The only difference is the out-of-trend drop of constitutive parameter a at 600℃ for the lower effective normal stress of 100MPa. It is thus concluded that reducing the effective normal stress has little effect on the sliding stability of plagioclase, and the previous conclusion made for mechanical behavior of the lower crust that unstable slips are possible therein also applies to the lower effective normal stress of 100MPa.

We investigated frictional sliding behavior of mixture gouges of quartz with various weight proportions of biotite and their structured equivalents with same weight proportions of biotite as layers embedded in quartz gouge. Our experiments were performed under effective confining pressure of 200MPa, pore pressure of 30MPa, temperature of 100℃ and the shear displacement rate of 1.22μm/s. The results show that for structured gouges with biotite layers embedded in quartz gouge as a weak structure, the strength has a power law decreasing trend with increasing weight proportions of biotite. The fault gouges can be weakened significantly by as little as 5wt% biotite, and 30wt% biotite corresponds to a beginning point of less sensitive strength change in response to increasing biotite proportion. On the other hand, the strength of mixed gouges shows a linear decreasing trend with increasing biotite proportion. Microstructures of deformed samples show that in mixed gouges, biotite and quartz are both sheared and grain size extremely reduced, and their contributions to overall strength have a close relation with their respective contents. However, in structured gouges, the shear deformation mainly occurred in the weak biotite layers with no shears crossing the quartz gouge. These results confirm the importance of the weak fabric in its effect on frictional strength. If the weak minerals form foliations and interconnected arrangements, it will lead to weakness of fault zones.

Through a series of analysis on the black gouge at Pingxi in Yingxiu-Beichuan Fault, we found about 9wt.%organic matter of kerogen type preserved in the black gouge. The gas chromatography mass spectrometry (GC-MS) was applied to the organic matter and five major alkane compounds were identified, namely, n-alkanes (C₁₄-C₂₁), acyclic isoprenoids(pristane and phytane), sterane, terpane and n-alkylcyclohexanes (C₁₀-C₂₁). Based on preliminary analysis on the organic compounds, we conclude that the organic matter in the black gouge should have deposited in a sea facies or in a saline lacustrine reducing environment, with features of long-time storage and high maturity degree. Through contrast experiments between original gouge samples and organic-removed gouge samples, we found that organic matter in the Pingxi black gouge can significantly weaken fault frictional strength and increase its sliding stability.

The seismogenic fault of Wenchuan earthquake is a high-angle reverse-slip fault. It is necessary for the sliding of such a high-angle reverse fault and the triggering of the Wenchuan earthquake on it to have special mechanical conditions at the deep part of fault. In this study, we investigated the deformation mechanism of cataclastic-mylonite rocks in ductile shear zones found in the Yingxiu-Beichuan Fault. The deformation temperature and the flow stress of brittle-plastic transition of fault were estimated by the deformation fabrics of quartz. The water contents and the distribution of major minerals in mylonite were measured using Fourier transform infrared spectroscopy(FTIR). The fluid inclusions were measured using Raman and microprobe. The rehological structures of brittle-plastic transition of the Longmenshan Fault zone under different fluid pressure and strain rate conditions were constructed to discuss the role of the high fluid pressure in the seismogenic and occurrence mechanics of Wenchuan earthquake.
The studies showed that inhomogeneous ductile deformation occurred in the brittle-plastic transition of the Yingxiu-Beichuan Fault. The complex deformation characters of quartz display different deformation temperatures in the ductile shear zone. The quartz in fine-grained mylonite was deformed by the grain boundary migration and recrystallization, implying the deformation temperature is from 500 to 700℃. The quartz in porphyroclastic mylonite was deformed by the subgrain rotation and recrystallization, implying the deformation temperature is from 400 to 500℃. The earlier quartz veins and healed cracks were deformed by the bulges and recrystallization, implying the deformation temperature is from 280 to 400℃. The later quartz veins which cut the earlier quartz veins were deformed by the cataclastics, indicating the deformation temperature is from 150 to 250℃. The deformation of quartz shows that the ductile shear zone experienced multi-phase brittle-ductile transitions. Based on the grain size of recrystallized quartz, the ductile flow stress of the fault is estimated to be 15～80MPa. The trace amount water in quartz and feldspar exists in the forms of hydroxyl in crystals, grain boundaries water and fluid inclusions water, and the water contents are higher with increasing strain of rocks, with a changing range from 0.01wt% to 0.15wt%. A lot of secondary fluid inclusions were found in the quartz in the brittle-plastic transition of fault, which were captured during crack healing. Based on measurement of the fluid inclusions, the capture temperature of the fluid inclusions is from 330 to 350℃, and fluid pressure is about 70～405MPa, the corresponding fluid pressure coefficient is estimated to be from 0.16 to 0.9, which stands for the characters of fluid inclusions captured during cracks healing process related with co-seismic and postseismic slip of fault.
Rheological structure was constructed based on the analysis data and flow law of wet quartz, as well as variation of fluid pressure and strain rate during periods of inter-seismic to earthquake nucleation, and after-slip to post seismic. Rheological structure shows that the strength of fault and depth of brittle-plastic transition change with strain rate and fluid pressure during inter seismic, earthquake nucleation, and after-slip period, and the depth of brittle-plastic transition is fit to the deformation mechanism of quartz, and the depth of transition of velocity weakening to strengthening of fault slip, as well as the focal depth of Wenchuan earthquake, which display that the Yingxiu-Beichuan Fault has the probability of weakening of sliding velocity and qualification of earthquake nucleation. However, the existing high fluid pressure in fault could be the most important factor for the high-angle reverse fault slip and triggering the Wenchuan earthquake.

The co-seismic surface ruptures of the May 12,2008 Wenchuan earthquake in Bajiaomiao and Shengxigou were developed mainly at the outcrops of carbon mudstones of Xujiahe formation of Triassic system. The black color and textures of co-seismic gouge are similar to old gouge and bed rock. We excavated trenches along the surface ruptures and collected samples of wall rock, fault breccia, old fault gouge and co-seismic gouge. All samples were analyzed quantitatively by X-ray diffraction. The main rock-forming minerals and clay minerals of co-seismic gouge are similar to old gouge, but their content is different, which shows the co-seismic gouge was formed based on old gouge. The wall rock and fault breccias adjacent to co-seismic gouge are carbon mudstones. The mineral composition and texture of the fault zone are obviously simpler than that of the northern part of the surface ruptures of Yinxiu-Beichuan Fault. The major minerals of co-seismic gouge are quartz and clay minerals, containing a few amount of feldspar, without calcite; a small amount of dolomite was found in co-seismic gouge at Shenxigou, and the content of dolomite is much lower than that in bed rock and old gouge. The marked character of new gouge is abundant in clay minerals, and the content of clay minerals decreases in turn from co-seismic gouge to old fault gouge, fault breccia and wall rock. The main clay minerals are illite and illite/smectite (I/S) mixed layer, containing a few amount of chlorite; a few kaolinite was found in co-seismic gouge of Shenxigou, the bed rock and gouge of Bjiaomiao did not contain kaolinite. Mineral characters of co-seismic gouge are different from old gouge. The old gouge contains calcite and dolomite, and the co-seismic gouge contains a few amount of dolomite and without calcite; the old gouge does not contain illite. However, for co-seismic gouge, mineral characters are different between black and white gouges, the content of illite in black gouge is higher than that in white gouge. In this study, the main clay minerals are I/S mixed layer, illite and chlorite, which is similar with San Andreas Fault and Chelunpu Fault. However, kaolinite content is extremely low in this fault, only trace kaolinite was found in the co-seismic gouge of Shenxigou. The high content of I/S in co-seismic gouge shows that the rich K+fluid participated in the seismic fault slip. All of these characters show that minerals of co-seismic fault gouge in this study are somewhat different with that of San Andreas Fault and Chelunpu Fault.

As frictional sliding of rocks is closely related to faulting and earthquake activities,it is essential to properly describe its constitutive relations.Dieterich proposed a preliminary rate-dependent constitutive law based on his experimental studies.After the refinement by Ruina,the constitutive relation has come to the form that we are currently familiar with,known as the rate-and state-dependent constitutive laws.The parameter a-b in the friction laws has been shown to be important in controlling the stability of frictional sliding.Analysis of small perturbation around a steady state for a spring-slider system(linear analysis)shows that the system is stable no mater how much the stiffness is in the condition of velocity strengthening(a-b>0);and sliding instability occurs only in the condition of velocity weakening(a-b<0).Under the framework of rate-and state-dependent friction laws,recent experimental work has been performed on gabbro gouge under both dry and hydrothermal conditions.This work is to study the different contributions of major mineral constituents of gabbro to the overall sliding behaviour of gabbro gouge under hydrothermal conditions.The experiments were conducted on pyroxene and plagioclase gouges separated from a gabbro rock sample with a triaxial system using gas as the pressure medium.A mineral powder layer of 1 mm thick was placed along an inclined saw-cut in a 20-mm-dia～meter cylinder sample to simulate a fault with gouge.The experiments were mainly conducted under pore pressure of 30MPa and effective normal stress of 200MPa and a series of experiments of plagioclase under effective confining pressure of 100MPa were also conducted for testing the reproducibility.At temperatures up to 607℃,standard slip rate steps switching between 1.22μm/s and 0.122μm/s were applied to obtain the rate dependence of friction.Slower rate steps switching between 0.224μm/s and 0.0488μm/s were also applied to explore possible change of sliding behaviour in slow slip rates.The steady state rate dependence a-b of plagioclase shows only negative values,and the steady state rate dependence of pyroxene gouge is negative in most of the temperature range except that at～200℃.Both of these results are quite different from the results of gabbro gouge documented in a previous study.Accordingly,the velocity-strengthening behavior of gabbro gouge above 510℃ may be caused by the presence of minor minerals such as hornblende,mica,and others in gabbro.Values of friction coefficient were picked for comparison.For plagioclase gouge,the friction coefficient shows an increasing trend with increasing temperature in the low temperature range and a decreasing trend with increasing temperature in the higher temperature range.The maximum is attained at～300℃.For pyroxene gouge,the coefficient has no systematic variation due to temperature elevation and varies around an average of 0.74.The results under hydrothermal conditions in this study are radically different from the oven-dried case in a previous study,and this indicates that water has a strong influence on the stability of frictional sliding.

Mechanical behaviors of frictional sliding can be described well by rate-and state-dependent friction laws.This paper reviews factors that control sliding stability of crustal rocks,and summarizes as follows:(1)analysis of small perturbation around steady state sliding(linear analysis)shows that an important condition for self-sustained instability to occur is a negative rate dependence of friction(a-b<0).Under such a condition,earthquake slips can nucleate on active faults.(2)Measurement of water content in crustal rocks implies that both "dry" and "wet" conditions in the lower crust are possible,and previous creep experiments show that deformation mechanisms under dry conditions are notionally brittle fracture and faulting.(3)Attention has been paid to gabbro under high pressure and temperature as a number of strong earthquakes were discovered to have occurred in the lower crust.Experiments on dry gabbro indicate that velocity weakening may be the typical sliding behavior with temperature ranging from 420℃ to 615℃.Synthetically considering the relevant conditions such as "dry" condition that may exist in relatively cool interior of continental lower crust,and mafic rock that is not likely to deform by plastic flow under such conditions,we may extrapolate this result to actual tectonic settings where the environments are more complex.This may explain why earthquakes occur in the lower crust in some areas.

Pressure calibration for solid medium vessel under high pressure and high temperature is an important step before apparatus is employed,because precise pressure calibration directly determines the preciseness of measurement of experimental pressure.Sound and effective methods are premises of pressure calibration.Pressure calibration includes confining pressure calibration and axial load calibration.The best way of axial load calibration is to estimate axial friction by multi-cycle of piston-in and piston-out.There are two key points during the test:(1)ensuring the hit-point of piston and sample:The hit-point is determined by an intersection of two beelines,one is the linear fit to the part of load-displacement curve of piston contacting with soft metal,the other is the linear fit to the part of load-displacement curve of sample's elastic deformation;(2)confirming dynamic friction:The dynamic friction which is dependent with displacement is established by the linear fit to the part of load-displacement curve of piston contacting with soft metal.Then,the final axial calibration includes cutting the load-displacement curve before hit-point,and correcting the load-displacement with dynamic friction.The best method for calibrating confining pressure is mineral phase transition,such as quartz-coesite,albite-jadeite + quartz,fayalite + quartz-ferrosilite and farringtonite-Mg₃(PO₄)₂-Ⅱ,because those phase transitions are more stable and the transition equations are widely used in previous calibrating confining pressure.It is proposed that quartz-coesite and albite-jadeite + quartz be used as pressure standards for the piston-cylinder apparatus in the pressure-temperature range of 2.5～3.2GPa,500～1200℃ and 1.6～3.2GPa,600～1200℃.Tests were performed for calibrating the axial friction using 2GPa confining pressure vessel.The sample is gabbro.Two experiments have been performed:(1)a number of cycle-experiments with different piston-rate under 500MPa and 820℃,1000MPa and 900℃,1000MPa and 25℃;(2)under 500MPa and 1000℃,firstly cycle-experiments were conducted,and then piston rate reduced from 2×10^-4mm/s to 5×10^-5mm/s after rock sample's plastic deformation,and the rate dependence of creep observed.The result of the experiment shows that the factors which affect the dynamic friction are confining pressure,temperature and piston rate.Confining pressure is the main factor,dynamic friction increases with the increase of confining pressure.Temperature and piston rate only influence on intercept.Hence,axial calibration should be conducted under specific experimental condition.Finally,axial calibration of an experimental load-displacement curve was conducted.

Previous studies showed that the deformation of dry mafic lower crust is semi-brittle,which may locate in transition of semi-brittle slip to semi-brittle flow. Therefore,we perform tests on brittle-plastic transition of four kinds of dry and wet mafic rocks in order to comprehend the mechanical behavior of continental lower crust. The experimental confining pressure ranges between 450～500MPa,and strain rate is 1×10^-4s^-1. The results of experiments show that the samples of dry Jinan gabbro (sample C),dry Yanqin diabase (sample D) and wet Yanqin diabase (sample D) have experienced deformation regimes of faulting,cataclastic flow,semi-brittle flow and plastic flow under 300～900℃; and dry Panzhihua fine grained gabbro (sample A) and fine-to medium grained gabbro (sample B) have experienced deformation regimes of semi-brittle flow and plastic flow under 700～900℃. The temperature of brittle-ductile transition of dry gabbro is 100℃,higher than that of dry diabase. The temperature of brittle-plastic transition of all dry mafic samples is 700℃,but the microstructures of semi-brittle flow are variable. For example,grain-size reduction and preferred orientation of plagioclase and clinopyroxene occurred in deformed diabase,displaying the typical structures of protomylonite. However,this kind of microstructure did not appear in the deformed samples of gabbro. In contrast to the semi-brittle flow regime,the strength and microstructure of all dry mafic samples are basically the same in the plastic flow regime under higher temperature,when dislocation glide becomes the predominant deformation mechanism. The main effect of water on brittle-plastic transition of mafic rocks is embodied by both the change of strength and temperature of brittle-ductile transition and brittle-plastic transition. The strength of wet diabase is much lower than that of dry gabbro and diabase at the experimental condition. The temperature of both brittle-ductile transition and brittle-plastic transition of wet diabase is 100℃ lower than that of dry diabase,and 200℃ lower than that of dry gabbro. The temperature of transition from semi-brittle flow to plastic flow of wet samples is much lower than that of dry samples.

Experiment on frictional sliding of simulated fault gouge is of great significance to the understanding of earthquake nucleation and natural shear zone structures. The present paper deals with deformational structure of simulated gabbro fault gouge and its origin,for the purpose of understanding fault activity in deep crust. The experiment also shows that various shear structures are well developed in the simulated fault gouge. Among them,the boundary shear,R₁ shear,Y shear and T gash are relatively well developed,but R₂,P and X shears are scarcely observed. R₁ shear has an angle of about 11°to the boundary surface,indicating the formation of a set of Coulomb fracture during the process of shear deformation. Observation of the thin sections shows that the density of R₁ shear increases with increasing normal stress,while under the same normal stress condition the density of R₁ shear increases with increasing temperature. Microstructural observations show that under temperature condition of 200℃the fault gouge is dominated by brittle deformation,involving grain crushing,cataclastic flow by shear comminution,and the banded alignment of dark colored minerals such as biotite and pyroxene along the shear plane. Between the temperature of about 200～400℃,preferred orientation of felsic minerals is gradually formed,defining mylonite like foliation. Above 500℃,the densely distributed R₁ shears occurr in the strongly deformed region,and they tend to coalesce with Y shears rather than to cut through the gouge layer,leading to the well development of Y shears. In these high temperature conditions,plagioclase begins to deform plastically,and the foliation defined by felsic minerals separates from that defined by dark-colored minerals,resulting in a typical S-C fabric.

As most earthquake focal depths are concentrated in the upper crust (e.g. Chen et al.,1983),previous studies on rate dependence of frictional strength have focused on granite that is typical in the upper crust (Lockner et al.,1986; Blanpied et al.,1995). However,recent reassessment of focal depth of many earthquakes revealed that some of them have been generated in the continental lower crust (Maggi et al.,2000). Moreover,teleseismic imaging of the structure across the San Andreas Fault also suggested the existence of deep fault that cuts through the crust (Zhu,2000). With this background,we need to pay more attention to intermediate and mafic rocks in the lower crust. Frictional resistance and the related rate-dependence of such rocks are critical to understand the mechanical behavior of deep faults that cut the lower crust. Specifically,this is crucial for answering two important questions,namely: 1) Is it likely for earthquake to nucleate in the continental lower crust? 2) How much stress can the lower crust support? We conducted experiments on frictional sliding of oven dried gabbro gouge at elevated temperatures with a triaxial testing system using gas as the pressure medium. A gabbro gouge layer of 1 mm thick (particle size of the gouge<76μm) was placed along an inclined saw-cut (35°to the loading axis) in a 20-mm-diameter cylinder sample to simulate a fault with gouge. Two series of experiments have been conducted with normal stresses of 200MPa and 300MPa,respectively,with temperatures up to 616℃. For both normal stresses,the rate-dependence of friction is either positive or approaching to neutral rate-dependence. With normal stress of 300MPa,the values of steady state rate dependence are positive in most of the temperature range but show a minimum around 315℃ (approaching to zero). Variation of rate dependence under normal stress of 200MPa is less pronounced. Based on theoretical analysis on slip nucleation (e.g. Dieterich,1992) and these experimental results,it is unlikely that unstable slip nucleates in larger scales under similar conditions as set in the experiments.

In summarizing the metamorphic temperature and pressure at hydrostatic condition of ultra high pressure rocks obtained from both field geology and experiments, we find that this problem is needed to further discuss, because the collision tectonic zone is not under hydrostatic conditions but under the action of differential stress. In this paper, we reassess and analyze the data of high temperature-high pressure experiment on quartz-coesite transition at differential stress condition made by Hirth and Tullis (1994). The results show that coesite is observed in both the semibrittle faulting and semibrittle flow regimes under temperature condition of 500～700℃ and pressure condition of 1.20～1.25GPa. Coesite is present mainly at the top and bottom of the tested samples adjacent to the pistons, as well as along fracture zones and along grain boundaries oriented perpendicular to σ₁ within the sample. The confining pressure (1.20～1.25GPa) required for quartz coesite transition in the presence of large differential stress is much lower than that (2.5～3GPa) at hydrostatic pressure condition. Obviously, the effect of differential stress is of great significance in the experiments. It is found that garnet in eclogite might be plastically deformed, indicating that differential stress do exist in collision tectonic zone, while the upper limit of the tectonic differential stress is constrained by the strength of rocks. Accordingly, differential stress is of great significance to ultra high pressure metamorphism. It is suggested, therefore, that systematic high temperature high pressure experiments are the essential and effective way to further investigate this problem.

Rheological parameters and deformation mechanisms of rocks are the basis for estimating crustal strength by using frictional constitutive equations and power law creep equations. In the past 30 years, substantial progress has been made in high-temperature and high-pressure experimental studies, which provide a large numbers of data on rheological parameters of crustal minerals and rocks, as well as new insights into the deformation mechanisms. In this paper, we summarize systematically the existing experimental data, and study crustal rheology of North China by using these data combined with focal depth distributions in this region. The results show that the upper crust as represented by granite and low-grade metamorphic rocks is deformed in brittle faulting and frictional sliding regime, the strength of which is controlled by friction on faults; the middle crust comprising felsic-gneiss, as well as the upper layer of lower crust composed of intermediate granulite behave in plastic flow regime; the lower layer of lower crust consists of dry mafic granulite, which behaves in brittle-plastic flow transition regime. The composition and rheological stratification of the crust in North China may cause decoupling of different crustal layers and provide mechanical conditions for strong earthquake generation. In addition, they may also serve as the bottom boundaries for blocks of different scales.

Seismic tomographic data show that crustal low-velocity layers and mantle uplift do exist in a vast area along Beijing-Tianjin-Tangshan-Zhangjiakou, where a lot of large earthquakes had occurred. However, such low-velocity layer and mantle uplift can not be found in Ordos region, where no strong earthquake had occurred. Geological study shows that the compositions of the crust are similar in these two areas. The main difference of the two areas is that rifting had occurred in Beijing-Tianjin-Tangshan-Zhangjiakou area at Cenozoic. We suggest, therefore, that the crustal low velocity layers in North China were controlled by the Cenozoic rifting and mantle uplifting, which caused the rise of temperature and hence giving rise to plastic deformation of lower part of the middle crust and lower crust. The lattice preferred orientation of minerals formed by plastic deformation can cause wave anisotropy, while subgrain substructure produced by dislocation creep may reduce the elastic wave velocity of rocks. The low velocity layers in lower part of the middle crust and lower crust, therefore, should be the result of plastic deformation of rocks. The low velocity layers in the upper part of the middle crust, however, might represent the low angle detachment or ductile shear zones formed during rifting, while low velocity might be resulted from anisotropy of rocks and possibly the effect of fluid. The crustal structure of Ordos region is similar to "ice-water" structure, and that of Beijing-Tianjin-Tangshan-Zhangjiakou area is like "sandwich". A large number of studies show that the crustal weak layers (lower velocity and plastic deformation layers) enhance the decoupling of stronger layers, and this may play an important role in the generation of large earthquake.