Based on the seismic data from temporary stations and regional stations in the northwestern area of Yunnan, the paper performs high-resolution detection and high-precision location on continuous waveforms recorded from February 25, 2018 to July 31, 2019 using waveform correlation methods and analyzes the seismicity characteristics of the Weixi-Qiaohou Fault in the northern section of the Red River fault zone. Studies have shown that the Weixi-Qiaohou Fault exhibits weak seismic activity currently, except for some special fault locations(such as terraces, intersections, etc.), but there may be a hidden steep-dip right-lateral strike-slip fault along the west side of the fault. Small earthquakes are frequent along the fault. The distribution of seismic activity and focal mechanism solutions indicate that this fault is a right-lateral strike-slip fault with a steep dip. Statistical parameters, such as seismic frequency, energy release rate and b-value, indicate that the seismic activity in the Weixi-Qiaohou Fault and its surrounding areas is relatively stable, and the regional stress enhancement is not obvious. The b-value is relatively high in most areas, and low b-value areas are mainly distributed in some special fault locations(such as terraces, intersections, etc.), but the scale is generally small. The statistical results of the ETAS model show that more than 40% of seismic activity may be affected by external factors such as deep fluid disturbance and remote strong earthquake triggering. This shows that the role of external trigger mechanisms in seismic activity cannot be ignored. The external triggering seismic activity factors are related to the disturbance of deep fluid activity and the dynamic triggering of long-distance strong earthquakes. Therefore, we believe that the Weixi-Qiaohou Fault is currently not active, but on the hidden branch fault to its west, small earthquake activity is clustering and has a tendency to increase. So, when assessing the seismic risk of the fault, comprehensive analysis shall be made on the activity of the main fault and the branch fault to its west.

Based on the seismic data collected from regional permanent stations and 6 temporal stations, we analyzed the seismic activity from October 2008 to July 2011 in Rongchang area. On the basis of HypoDD relocated results, we used Match&Locate method to detect and located the micro-earthquakes. We obtained the focal mechanism solutions of some earthquakes with M_L ≥ 3.5 by using CAP method. Then we analyzed the temporal-spatial distribution of earthquakes and discussed the characteristics of micro-seismicity before the M_L5.1 earthquake occurring on September 10, 2010. We totally detected 3 354 micro-earthquake events, which are nearly 5 times of the earthquakes in the seismic catalog issued by China Earthquake Networks Center. The magnitude of the detected events is mostly from M_L-1 to 1, and the focal depth is from 2 to 4km. The magnitude-frequency analysis shows that the catalog completeness is obviously improved after adding the detected earthquakes, with the lowest magnitude decreasing from M_L1.0 to 0.3. The earthquakes hypocenters are mainly clustered along faults or buried faults and in a dominant depth range consistent with the depth of injection wells, and also show a tendency of lateral extension from injection wells. The focal mechanism solutions of 9 earthquakes of M_L ≥ 3.5 presented reverse faulting, as the same as the preexisting faults, indicating that earthquakes were surely related to reactivation of the faults. The strike, dip and rate of the causative faults separated in wide ranges, which indicates not only obvious changes in structure and strike of preexisting faults but also the effect of increasing pore pressure on the local stress field. Before the M_L5.1 earthquake on September 10 of 2010, seismicity firstly showed clustering in time and covered the most part of the seismogenic fault in space. Then an obvious seismic quiescence occurred and lasted about 3 months. The phenomenon is consistent with the mechanism of creep sliding and resistance-uniformization along the fault zone, suggested on the basis of laboratory experiments, and it may be one of patterns of sub-instability along fault zone. However, such explanation needs to be further confirmed.

The temperature rise caused by frictional heating during seismic slip is able to indicate dynamic frictional properties of the seismic fault,which provides an approach to understand the dynamic process and energy budget of an earthquake.The residual indicators of frictional heating within the fault zone also can be taken as an evidence for seismic events.The vitrinite reflectance is a commonly-used geothermometer in the coal,oil and gas industries.It also has some potential applications in the studies of fault rock and fault mechanics.We studied vitrinite reflectance (VR) of fault rocks collected from surface outcrops of the Wenchuan earthquake fault zone in this paper.The measured data reveal that the VR of fault rocks are affected by fault motion,and there is a trend that the VR increases towards the fault core,which indicates the effects of frictional heating.The VR of fault rocks from the Bajiaomiao outcrop is much higher than those from the Shenxigou outcrop,which probably suggests the difference in fault activity at the two outcrops.Our study also suggests that systematic measurement of VR across the fault zone is helpful in identifying slip zones and determining their widths.From the VR measurement on an oriented specimen containing the slip surface of the Wenchuan earthquake from the Shenxigou outcrop,we observed anomalous high VR values in two black-colored slip zones of about 2mm in width near the slip surface.The numerical calculation shows that the maximum temperature rise on the fault plane near Shenxigou was probably less than 162℃ during the Wenchuan earthquake,which means the dynamic fault strength was quite low.These estimations are roughly in accord with the results from the high-velocity frictional experiments.

This paper briefly reviews the recent progress in high-velocity frictional experiments on rocks at State Key Laboratory of Earthquake Dynamics. In order to promote research on faulting and earthquake mechanics, a rotary-shear low- to high-velocity friction apparatus is installed at the laboratory, which is capable of producing plate to seismic slip velocities(～mm/yr to～m/s)and the capability of high-velocity friction bridges a technical gap of the laboratory. Focusing on the mechanical properties of the Longmenshan Fault zone, a series of high-velocity frictional experiments have neen operated by using the apparatus. The results indicate that the high-velocity frictional properties of the Longmenshan Fault zone are quite uniform and the significant high-velocity weakening must have promoted dynamic rupture propagation during the Wenchuan earthquake. The predominant mechanisms for fault weakening are processes related with frictional heating, including flash heating and thermal pressurization. Fault gouges exhibit very rapid healing by more than 0.4 in friction coefficient μ within 5～10s, and the rapid healing following seismic slip can be a cause for reduced aftershocks along major coseismic faults. Based on the present capability of the apparatus and the experimental results obtained, some topics are proposed for study on high-velocity friction of rocks in the near future including the modification of the rotary-shear low-to high-velocity friction apparatus and the related experiments.

In order to understand the role of underdeveloped immature small faults and conditions of the occurrence of medium-size earthquakes induced by fluid injection into deep reservoirs, we carry out an integrated study involving geological survey of small faults, seismological investigation of typical cases of injection-induced seismicity, and rock fracture test in laboratory. At first, we briefly summarize the general features of several important cases of injection-induced seismicity in gas/oil reservoirs in the Sichuan Basin, China. It is suggested that major induced earthquakes, especially those of a moderate size, result from the reactivation of pre-existing faults. We also present some pieces of field and petrologic evidence showing such small faults exist widely in reservoirs and are important channels for fluid migration. Then, we present experimental results with a focus on the formation and the after-slip of fault in typical sedimentary rocks under tri-axial compression. Finally, methodology for risk assessment and injection management is discussed based on insights gained from the integrated approaches.
Our results are helpful for understanding the question—why injection-induced seismicity is so significant in the Sichuan Basin. Major Pre-Triassic sedimentary rocks, including dolomite, shale, and dolomitic limestone are strong and demonstrating brittle fracturing behaviors. Such properties are necessary conditions for maintaining high level of reservoir stress and leading to seismic fracturing. Insights gained from this study may shed some lights to the general earthquake seismology and provide a better understanding of why damaging injection-induced earthquakes occur so that they can either be avoided or be mitigated. In general, existing of critically or sub-critically stressed faults of a dimension of a few kms is a necessary condition for M～5 level earthquakes. In addition, AE, or in other words, micro-seismicity monitoring is useful in risk assessment and injection management and should be fully utilized in injection applications.

Stress changes due to the co-seismic slip on the source fault of the 2008 M_W7.9 Whenchuan earthquake and delayed response of inelastic deformation in the lower crust and upper mantle have an important role in the seismicity in Longmenshan area. After the Wenchuan earthquake,seismicity shows progressively increasing in a wide region. However,the south segment of the Longmenshan Fault did not show any significant change in seismicity,where positive Coulomb failure stress change(ΔCFS)was estimated under the elastic half-space model. Under such a background,the 2013 M_W6.6 Lushan earthquake occurred. This paper presents some preliminary results based on seismicity analysis and stress analysis using lithology models in which the lower crust and the upper mantle are suggested to be viscoelastic. The Wenchuan earthquake resulted in a miner negative coseismic ΔCFS in the hypocenter region of the Lushan earthquake. As a result of inelastic response the estimated ΔCFS reached the order of 0.2～0.4bar,a value sufficient to trigger earthquakes in critically loaded faults. We thus conclude that the Lushan earthquake provides a case of inelastic triggering of the Wenchuan earthquake. The 1970 M6.2 Dayi earthquake caused an obvious Coulomb stress shadow in its source area,which partly overlaps to the seismic gap between the ruptures of the Lushan and Wenchuan earthquakes. The stress shadow still exists although the area has been loaded by both the Wenchuan and Lushan earthquakes. We thus suggest that it is less likely that a great earthquake,which ruptures the entire gap,may occur in the near future if there are no other unknown factors.

Based on data collected from a temporary seismic network,in addition to some nearby permanent stations,we investigate velocity structure and seismicity in Rongchang gas field,where significant injection-induced seismicity has been identified. First,we use receiver functions from distant earthquakes to invert for detailed 1-D velocity structures beneath typical stations. Then,we use the double-difference hypocenter location method to relocate earthquakes of the 2010 M_L5.1 earthquake sequence occurring at the region. The relocated hypocenters show that the 2010 M_L 5.1 earthquake sequence was distributed in a small area surrounding major injection wells and clustered mostly along pre-existing faults. Major earthquakes show a focal depth less than 5km with a dominant depth of～2km,a depth of major reservoirs and injection wells. We thus conclude that the 2010 M_L 5.1 earthquake sequence might be induced by deep well injection of unwanted water at a depth～3km in Rongchang gas field.

In order to better understand the effect of phyllosilicate on fault strength, velocity-dependence of friction and seismicity, the velocity-dependence of friction for dry and wet muscovite-bearing halite gouge was studied by using biaxial friction configuration. Acoustic emission produced during the frictional sliding was recorded, and the microstructure of gouge zone was observed. The experiments show that dry gouge behaves stick-slip and velocity weakening at velocities of 0.1～100μm/s; Increasing σ₂ can enhance the transition to velocity strengthening and velocity-dependence transition occurs at velocity of 0.1μm/s, where fault behaves either stable sliding or stick-slip with much longer time than that in the velocity weakening region. Wet gouge behaves velocity strengthening at velocities of 0.05～0.01μm/s, velocity weakening at velocities of 0.1～10μm/s, and velocity strengthening again at velocities of 50～100μm/s. Each stick-slip event corresponds to one or a cluster of AE events for dry gouge, while there is no AE event corresponding to stable sliding for wet gouge. The microscope observation indicates that brittle fracturing and localized slip are predominant in the velocity weakening region and the velocity strengthening is controlled by distributed fracturing of halite under dry condition. While under wet condition, the two velocity strengthening regions correspond to different mechanisms. At higher velocities, the deformation of fault may be controlled by frictional sliding on the network developed by muscovite and uniform fragmentation of halite, and at lower velocities, the deformation of fault is also controlled by pressure solution of halite. Comparing to the results of halite gouge, it can be seen that the existence of muscovite has no effect on sliding mode and velocity-dependence for dry halite gouge, while it enhances the transition to stable sliding for wet halite gouge. The results we got provide basis for analyzing strength and stability of phyllosilicate-bearing faults. The stick-slip with longer time at transitional region confirms what observed in frictional experiments of halite gouge, which is significant for understanding mechanism of slow earthquakes.

Using the cataclastic analysis method, this paper tries to make an analysis on the focal mechanism data of 486 small earthquakes that occurred at the epicenter of the Wenchuan earthquake and its surrounding areas in more than three years before the 2008 Wenchuan earthquake. The result shows that obvious stress change occurred at the seismic source and its surrounding areas around June 2007 before the Wenchuan earthquake, manifested in two high numerical value areas of abnormal stress state. Meanwhile, the formation process of the above areas was accompanied by the drop of stress level of the Longmenshan central fault. The ultimate strong earthquake occurred on the stress gradient belt between the high stress area and the low stress area. The evolution process of stress level before the Wenchuan earthquake indicates that earthquake nucleation phenomenon turned up before the strong earthquake. One result can be inferred that there was an abnormal process of accelerated movement of the whole Bayankala block before the Wenchuan strong earthquake.

Rongchang area had exhibited low levels of natural seismicity,and there was no record of earthquake with M_L>5 in the history.However,following the injection of unwanted water from gas production,seismicity has increased dramatically and showed progressive increase of magnitude since July 1988,and an earthquake of M5.2 occurred in 1997.Rongchang area is thus an ideal site for studying seismicity induced by deep well injection.Unfortunately,there was only one seismic station in the area,and the research was limited by the poor detectability and hypocenter location accuracy.In order to make a thorough investigation on the injection-induced seismicity in the area,a temporal seismic network was installed in July,2008 under the cooperation of the State Key Laboratory of Earthquake Dynamics,Geological Survey of Japan and Chongqing Earthquake Administration.The seismic network consists of 6 stations,by which waveforms are continuously recorded.As a result,both the detectability and location accuracy are improved greatly.This paper presents a brief summary of the cooperative project and some preliminary results of recent seismicity in the area.

We have performed a series of frictional experiments using a servo-controlled biaxial loading machine with three granodiorite block direct shear configuration.In the experiments,a small-amplitude sine wave of displacement is modulated to shear loading to study the effects of shear stress perturbation on stick-slip instability.The main results are as follows.Under constant normal stress and constant loading point velocity in shear direction,the sample shows regular stick-slip behavior.After the displacement perturbation is modulated to shear loading,the correlation between the timing of stick-slip and the perturbation increases as the perturbation amplitude increases.Stress drop and interval time of stick-slip events also tend to be scattered,and the critical amplitude for producing significant impact is around 0.05MPa.The tendency that the stress drop and interval time of stick-slip events are scattered with increasing perturbation amplitude is more obvious at higher normal stress,and the correlation between the timing of stick-slip and the perturbation also increases with increasing normal stress at the same perturbation amplitude.The effect of perturbation period on friction is not evident.The results imply that the change in Coulomb shear stress caused by strong earthquake may obviously change not only the occurrence time of earthquakes but also the earthquake magnitude on nearby faults.

To investigate the mechanism of seismicity due to impoundment of a reservoir,we propose a method for integrated study on regional/local seismicity and stress by both surface loading and pore pressure diffusion.As an example,possible role of Zipingpu Reservoir on nearby seismicity was studied in detail.The epicenter of the M_S 8.0 Wenchuan earthquake on May 12 of 2008 is very close to the Zipingpu Reservoir.On one hand,several statistical properties including event rate(n),cumulative energy release(E),seismic b-value in the magnitude-frequency relation,and spatial correlation length(SCL)of earthquakes occurring in Zipingpu area from 2004.8 to 2008.5.11 were estimated in detail.On the other hand,we quantitatively examined change of Coulomb Stress(ΔCFS)due to the impoundment of the reservoir.Both weight loading and pore pressure diffusion resulted in significant ΔCFS on the underlying Yingshu-Beichuan and Guanxian-Mianzhu Faults,which are considered as the source faults of the Wenchuan earthquake.Some clear correlations were verified between the local seismicity and stress change,thus we concluded that the impoundment of Zipingpu Reservoir clearly affected the local seismicity and it is worthwhile to further study if the effect played a role in triggering the Wenchuan earthquake.

In this paper,the rupture propagation along reverse fault in bedrock towards the overlying sediment layer has been studied using modeling tests and the evolution of the vertical displacement field on the surface of sediment layer has been measured using fringe projection technique,in which the effects of fault displacement,thickness of sediment layer and fault dip have been analyzed.The research results indicate that fault in bedrocks propagates gradually towards the overlying sediment layer and a gradient zone of vertical displacement(i.e.a deformation zone)surrounding the projection line of the upper end of fault in bedrock is formed on the surface of sediment layer as the fault slips,given a constant thickness of sediment layer and a constant fault dip.The larger the fault displacement is,the larger the width of the deformation zone and the deformation intensity are.However,the width of deformation zone keeps constant and only the deformation becomes more concentrated as fault displacement increases after the fault propagates to the surface of sediment layer.Under the same fault displacement and the same fault dip,the thicker the sediment layer is,the wider the deformation zone controlled by the fault in bedrock is,i.e.the larger the influencing extent of the fault is.When the thickness of sediment layer exceeds a critical thickness,an extensional rupture may occur on the surface of the sediment layer caused by uplifting of the sediment layer.Under the same thickness of sediment layer and the same fault displacement,the width of the deformation zone on the surface of sediment layer decreases but the deformation intensity increases as fault dip increases.The experiment results imply that the critical fault displacement,the critical thickness of sediment layer and the fault dip play important roles in determining the surface deformation and rupture of earthquake generated by buried reverse fault.The results obtained in this paper are valuable for further understanding the characteristics of surface deformation caused by Wenchuan earthquake.

This paper briefly reports our newly findings on remains of earthquake surface ruptures along and near the transition segment(between Lizhou and Xichang)of the Anninghe and Zemuhe Fault zones in western Sichuan and analyzes their relations to historically documented large earthquakes.During geological surveys in recent years we find unknown-aged surface ruptures at three sites on and near the transition segment of the Anninghe and Zemuhe Fault zones.Through analyzing both characters of the surface ruptures and their locations in heavily damaged areas of relevant historical earthquakes in the studied region,we distinguish that ruptures at two of the three sites,north of Yangfushan and west of Dapingzi,should be remains of the surface rupture zone of the large 1536 earthquake.The findings of the remains at these two sites suggest not only that the southern portion of the 1536 rupture zone was yielded along the transition segment of the Anninghe and Zemuhe Fault zones,but also that the southernmost end of the 1536 rupture zone would have arrived at or been very close to Xichang.While,ruptures at and near the other site,east of Lijinbao,north of and not far from Xichang,should be remnants of the surface rupture zone of another historical large earthquake occurring in 1850.These remnants further prove that the most northwestern end of the 1850 rupture zone would have arrived somewhere at least several kilometers north of Xichang.It thus can be inferred from the evidence given in this paper that the major fault at and near Xichang was once ruptured by both the 1536 and 1850 earthquakes.Such an inference is compatible with the described phenomena in literal historical records that very heavy destruction and disaster happened in the city of Xichang during both the 1536 and 1850 earthquakes.

It is important for active fault research and seismic hazard assessment in urban area to study the existence of buried active fault and its possible effect on the earth's surface.Many geophysical and geo～chemical techniques have been developed for exploration of buried active fault,and advanced remote techniques have been used in measurement and analysis of deformation on the earth's surface caused by earthquake on buried fault.However,the relationship between buried seismogenic fault and surface deformation and rupture caused by earthquake needs to be further studied.It is helpful for understanding rupture propagation along buried fault in loose sediment to study the evidence of fault sliding in the overlying sediments with simulation experiments.In this paper,rupture propagation along strike-slip fault in bed rock towards the overlying sediment layer has been studied with simulation experiment and deformation field on the surface of sediment layer has been observed using digital speckle correlation method,in which the effects of fault displacement and thickness of sediment layer were mainly analyzed.The research results indicate that a deformation and rupture zone composed of tension and shear fractures along the projection line of fault in bed rock is gradually formed in the sediments as the fault slips,given a constant thickness of the sediment layer.The larger the fault displacement is,the larger the width of deformation zone and the deformation intensity are.However,the width of deformation zone will keep constant when fault displacement exceeds a critical value,but the deformation will only become more concentrated as fault displacement increases.For a same fault displacement,the thicker the sediment layer is,the wider the deformation and rupture zone controlled by the bed rock fault will be,i.e.the larger the influencing extent of the fault is.However,when the sediment layer exceeds a critical thickness,the deformation pattern of sediment layer is changed.There exists no deformation zone consistent with the strike of fault in bed rock but a large-scale tensile shear rupture zone oblique to the fault strike.The experimental results imply that the critical fault displacement and the critical sediment thickness play important roles in determining the surface deformation and rupture caused by buried fault generated earthquake.

Acoustic emission (AE) technique is one of the important methods in rock mechanics and experimental seismology. Improving AE data acquiring and processing technology is a basic work in research of the features of AE during the process of rock deformation. Following the progress of the AE systems in State Key Laboratory of Earthquake Dynamics,more and more AE data were obtained with higher recording speed and higher resolution for every experiment. It is a challenge to store such growing data reasonably and deal with it efficiently up to now. Researchers who work in the stand-alone mode have to copy a lot of raw AE data files with different format from remote computer to local PC. Storage space and time are thus squandered away. In addition,every user has to rewrite some processing programs using his own language for the row data,as a result,human resource is wasted. The commercial data service systems are not designed for laboratory study. According to the special demands,AE data service system based on web is designed and realized in the laboratory. Experiment AE data and processing method can be accumulated with the system. On the AE data access interface which is independent of equipments,researchers can acquire and process data easily. The system enhances the frequency of data use and the efficiency of data processing because it allows more and more people to participate in processing and analyzing the AE data. The system not only satisfies the needs of researcher and programmer,but also overcomes the previous shortcoming in the operating mode of AE data management. The most important thing is that the resolvent of AE data service system can be extended and applied to other experiment data to improve work efficiency.

The crustal movement shows obviously non-linear characteristics in time and space,and the tectonic deformation and related phenomena are undoubtedly affected by such non-linear mechanical process. Experimental study on frictional behavior under non-linear loading is helpful for understanding the effect of stress change on earthquake mechanism. In this paper,the frictional experiments on medium-scale granodiorite samples containing 45? precut surface have been conducted by using a servo-controlled biaxial rig,and the effect of lateral stress (σ₂) on friction has been studied. The experimental results indicate that the interval and stress drop of stick-slip events increase with increasing normal stress on sliding surface under constant loading velocity in the direction of maximum compression stress σ₁ and constant lateral stress σ₂.However,the relation is greatly changed when stress perturbation of sine wave with high frequency and small amplitude is added to σ₂. Stress drop and interval of stick-slip events are obviously scattered. In particular,the magnitude of stress drop may tremendously increase. Strain measurement indicates that the perturbation of lateral stress increases the heterogeneity of strain distribution and release along fault,and stick-slip events with large stress drop and that with small stress drop are corresponding to strain release along whole fault and that along parts of fault,respectively. Compared to the previous experiment result of stress perturbation in σ₁,the effect of perturbation in σ₂ on stick-slip is more obvious,meaning that change in normal stress may affect fault instability more greatly than change in shear stress. The experimental results imply that not only the effect of change in coseismic Coulomb stress on seismicity should be analyzed,but more attention should be paid to the possible effect of change in normal stress on fault when the fault interaction is considered.

This paper discusses the mechanism for seismic quiescence before large earthquakes based on experimental results of acoustic emission during rock deformation. Under the condition of biaxial compression and constant loading point velocity,samples simulating discontinuous faults,including compressional en-echelon faults,fault with macro-asperity,and model-Ⅲ shear fault,show relative quiescence of acoustic emission before slip instability,which is characterized by obvious decrease of occurrence rate and release of strain energy of acoustic emission events. The analysis indicates that the predominant deformation mode during this stage is creep sliding along fault zone,especially along the newborn fault segment in the discontinuous area,causing decrease of differential stress in the sample. The reason for occurrence of this stage is that a relatively uniform distribution of resistance along a fault zone is necessary for its slip instability,but uneven surface of newborn fault segment does not meet this condition,so the creep sliding is needed for resistance uniformization along the fault zone. Because there exist only very small asperities along the fault zone and the stress relaxation occurs in the whole sample during this stage,acoustic emission activity becomes weak. Based on the experimental results,it is suggested that the process of creep sliding and resistance-uniformization along fault zone is one possible mechanism for seismic quiescence before large earthquake.

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.