A crucial question in earthquake science is how earthquakes start. Field and experimental observations show a short period exists between the fault reaching peak stress and the coseismic event. Therefore, it is of fundamental significance to capture the spatio-temporal evolution of a fault’s deformation during this premonitory stage. It can help us understand how the rupture of an earthquake initiates and also provide precursory information. Stick-slip events or lab quakes can be produced in controlled conditions to mimic earthquakes in nature. In previous studies, we proposed the fault meta-instability model focusing on depicting this stage(hereinafter referred to as the meta-instability stage)and interpreting the transition from energy/stress accumulation to energy/stress release. We further divided the meta-instability stage into two substages, i.e., the quasi-static phase and a quasi-dynamic phase, corresponding to slow energy release and irreversible energy release elevated rate.
However, how the meta-instability stage can facilitate the final failure remains puzzled. In contrast, the meta-instability stage exhibits slow and mild deformation, while the coseismic stage is fast and violent. In order to bridge these two processes, it is essential to record the complete dynamic process of stick-slip events, including the premonitory and coseismic stage. Thus, the data acquisition system required must feature a high signal-noise ratio, high frequency, continuous recording, and dense instrumentation. In 2016, we developed an ultra-high-speed, multi-channel and continuous recording data acquisition system for deformation measurement(UltraHiDAM). UltraHiDAM has 64 channels, 16-bit resolution, and 4MHz sampling frequency, and can perform parallel continuous data acquisition. It is able to record strain signals and acoustic emissions continuously and synchronously at a high sampling frequency up to 4MHz for as long as a few hours. To our best knowledge, it is the first system that is capable of doing so.
Based on this system, we conducted a series of stick-slip experiments. We recorded the entire deformation process of the laboratory earthquake cycles, including the relatively slow deformation in the quasi-static phase(several seconds before the stress drop), the relative fast deformation in the quasi-dynamic phase(a few microns before the stress drop), and the complete process of the transient coseismic slip. High frequency continuous synchronous sampling allows us to reveal as many details as possible of unstable sliding transient processes, and analyze mechanical problems related to the seismic source.
We report results of stick-slip experiments using saw-cut bare-surface granodiorite samples. The main findings of this paper are summarized as follows: First, the substages can be further recognized based on the local deformation characteristics(Table 2). Second, strain and stress start to localize before the quasi-static phase; such localization’s acceleration indicates the whole fault has entered the quasi-static phase. Third, the strain field during the quasi-dynamic phase is characterized by a wave-like acceleration and reciprocating propagation(Fig. 9). Fourth, there is a short preparation period for each sub-stage of the quasi-dynamic process(Fig. 6). The existence of such preparation periods may help the imminent earthquake prediction. Finally, even for the stick-slip events captured on a simplified plane laboratory fault, the coseismic process can be multiple rupture events, each event has its own AE waveform that is distinguishable in time(Fig. 8).
The implications are that there is indeed precursory information during the different substages before the coseismic event, most of which are associated with the localization and propagation of strain and stress. An earthquake source’s actual mechanical process can be complex in terms of multiple stress drops and ruptures.

When P waves from distant earthquakes meet a velocity discontinuity in the earth's crust and upper mantle, they give rise to a series of converted PS waves besides PP refracted waves. It is possible to monitor the variation of the physical properties of the medium in the limited formation space above the transition zone of the seismogenic zone by measuring the time difference between the teleseismic PS converted wave and the first arrival P wave, that is, time-variation Δt_PS=t_PS-t_P. The advantage of this method is that the transition point of the teleseismic source with similar source is relatively stable at the transition interface, and the accuracy of the measured relative time is high, and the change of the medium in a small range of the seismogenic zone above the conversion interface can be monitored.
This paper studies the variation of the travel time difference Δt_PS in focal region before and after Wenchuan M_S8.0 earthquake. We select 2001 to 2012 as the research period, use teleseismic waveforms which occurred in the southern region of Sumatra and Hindu Kush area recorded by Sichuan station YZP and JJS. These teleseisms satisfy 5.0 ≤ M ≤ 6.5, and their waveform signal-to-noise ratio is high with clear initial P-wave motion. The epicentral distance of teleseisms is less than 3 degrees. Then we obtain the variation of the travel time difference Δt_PS between teleseismic PS converted wave and PP transmitted wave recorded during the study period in the two stations. The results show that there is a slow increasing trend of Δt_PS before 2006, and an obvious low value process of Δt_PS appeared in the period about 2 years before the Wenchuan earthquake. The maximum decline was about 0.2~0.3s, more than 4~5 times the measurement error. The low value has a certain degree of return about 2~3 months before the earthquake.
The change of arrival time difference indicates that the medium is in different states in different periods of seismogenic process. The sharp decrease in Δt_PS from 2006 to January 2008 may be due to the strong disturbance caused by the stress accumulation of the medium. At this stage, the velocity of P wave and S wave increases with the increase of stress, and the increase of S wave velocity will result in the decrease of Δt_PS. The change of Δt_PS is greatly affected by S wave velocity, so Δt_PS appears to decrease rapidly. Regarding the low value that has a certain degree of return about 2~3 months before the Wenchuan earthquake, the possible reason is that the release of stress is much higher than the accumulation of stress in meta-instability stage. At this stage, the velocity of S wave decreases and the decrease of S wave causes Δt_PS to increase. Then, the Wenchuan earthquake of magnitude 8.0 occurred. It is shown that the teleseismic converted wave method in this paper can monitor the variation of medium's wave velocity before large earthquakes, and it has a good prospect in seismic monitoring and worth further experimental study.

For analyzing the role that reservoir impounding plays in triggering earthquake, the process of diffusion of pore pressure and its mechanism of action should be understood firstly. The temporal distribution of seismicity, which occurred before the M_S8.0 Wenchuan earthquake, following the impoundment of Zipingpu reservoir is studied in this paper. Then the mechanisms of the occurrence and development of reservoir triggered seismicity are discussed. A comparative analysis of the temporal distribution of seismicity and the submerged area by reservoir impounding is carried out firstly. Then the influence of various factors on modeling is analyzed in detail. After calculating, the pore pressure change by the Zipingpu reservoir impoundment is obtained. The following observations are made:(1)Conspicuous swarms of earthquakes, of which the sources are located on the same fault of the M_S8.0 Wenchuan earthquake, occurred orderly with the impoundment of Zipingpu reservoir.(2)Because of the influences of the terrain and the medium, the range of effect of pore pressure change by the impoundmemt is limited and anisotropic. Hydraulic diffusivities(D)of 0.7 and 0.35m²/s along the fault strike and the fault dip are reached respectively by a semi-quantitative assessment. Of course, the qualitative pressure constraints on the surface are also applied for the modeling.(3)The calculation results show that the temporal distribution of seismicity near the Zipingpu reservoir is related with the pore pressure change. After the pore pressure reached the threshold of triggering earthquake, whether the pressure head change is high or not, the change rate of pressure head change plays a key role in the decrease or increase of seismicity. It means that the triggered seismicity by pore pressure is a dynamic triggering process.

The Fen-Wei rift is composed of a series of Cenozoic graben basins, which extends in an S-shape and strikes mainly NNE. Two distinct types of basins are defined in the Fen-Wei rift. The NEE-striking basins(or basin system) are bounded by active faults of mainly normal slip while the NNE-striking basins are characterized by their dextral strike-slip boundary faults. The adjacent NEE-striking basins(or basin systems) are linked by the arrangement of NNE-striking basins and horsts that is called the linking zone in this study. The segmentation of the Fen-Wei rift shows that the geometry and the activity of different rift segments are varied. The southern and northern rift segments strike NEE and are characterized by tensile movement while the central rift segment strikes NNE with transtensional motion. Previous field surveys show that the ages of the Cenozoic basins in the Fen-Wei rift are old in the southern rift segment, medium in the northern rift segment, and young in the central rift segment. The sizes of linking zones are large in the central rift segment, medium in the northern rift segment, and small in the southern rift segment. In addition, the east tip of Xinding Basin propagates towards NEE along the northern rift segment and the west tip of the basin grows towards NNE, while the shape of Linfen Basin is almost antisymmetric with respect to the Xinding Basin. However, the previous laboratory or numerical simulations cannot explain these features because they didn't pay enough attention to the control of the rift segmentation on the evolution of NEE-striking basins and their linking zones. In this study, based on the previous field studies, we study the fracture process of a clay layer under the segmented dextral transtension of the basement. The spatiotemporal evolution of the deformation field of the clay layer is quantitatively analyzed via a digital image correlation method. The experiment reproduced the main architecture of the Fen-Wei rift. The results show that:(1) The chronological order of basin initiation and the different sizes of linking zones in deferent rift segments are caused by the different obliquity angles(the angle between the rift trend and the displacement direction between the opposite sides of the rift) among the southern, northern and central rift segments.(2) The interaction between adjacent NEE-striking basins leads to the formation of NNE-striking linking zones.(3) The interaction between adjacent rift segments may cause the special distribution of Xinding and Linfen Basins. Thus, we propose that the differences of the Fen-Wei rift segments are mainly controlled by the different obliquity angles. The lack of considering the influences of pre-exiting structures leads to the limited simulation of the details within the southern and northern segments of the Fen-Wei rift. Further studies may improve the model if this is taken into account.

Stick-slip of fault in laboratory accompanies change of temperature. Temperature change is not only concerned with sliding friction, but also with the stress state of the sample. In this article, we use infra-red thermal imaging system as wide-range observation means to study the temperature variation of different stages during the deformation of sample. The rock sample for the experiment is made of granodiorite from Fangshan County with a size of 300mm×300mm×50mm. It is cut obliquely at an angle of 45°, forming a planar fault. Two-direction servo-control system was used to apply load on the sample. The load in both directions was forced to 5MPa and maintained constant (5MPa) in the X direction, then the load in the Y direction was applied by a displacement rate of 0.5μm/s, 0.1μm/s and 0.05μm/s successively. The left and below lateral of the sample were fixed, and the right and top lateral of the sample were slidable when loaded. The experiment results show not only the temperature change from increase to decrease caused by conversion of stress accumulation to relaxation before and after the peak stress, but also opposite variation of temperature increase on fault and temperature decrease in rock during instability stage. Most important of all, we have found the temperature precursor identifying the position of instability through the temperature variation with time along the fault. It shows that rate of temperature increase of instability position keeps relative high value since the stage of strongly off-linear stage, and accelerates in stage of meta-instability. After separating the effect of friction and stress, we found that temperature increase occurs in the rock near the fault instead of on the fault, which means the mechanism of temperature increase is stress accumulation. Temperature of fault at the instability position does not increase, which means the position is locked. We speculate that the position of locked area on fault with high stress accumulation near the fault may be the future instability position. It is of significance of studying temperature variation during stick-slip to the monitoring of earthquake precursors. Heat caused by friction of earthquake needs long time to transfer to the surface and could not be detected as a precursor. While the stress of surface rock near the fault would change as the stress of interior rock changes, which could cause detectable temperature variations. The research purpose of this article is to find special change positions before instability. As the temperature variations are caused by stress and slip of fault, the results are also meaningful to analysis of stress and displacement data related to earthquake precursors.

The Yutian M_S7.3 earthquake occurred on February 12, 2014 in Xinjiang Uygur Autonomous Region, China and the epicenter is located in the western part of Altyn Tagh Fault. This is the second M≥7 earthquake following the March 21, 2008 Yutian M_S7.3 earthquake in the south of the Tarim Basin. Aftershocks of this Yutian M_S7.3 earthquake are distributed mainly along the NE direction, and that in the southwest part of the aftershock area presents a near-NS distribution. Most of the aftershocks including foreshocks (accounting for 85% of the total aftershock sequence) are densely distributed in the southwest of the aftershock zone, the vast majority of strong aftershocks (all of the M_S≥5 and 81% magnitude 4 earthquakes)are distributed in this area. Aftershocks of the first day are mainly distributed in this area and in a near-NS distribution. The aftershocks extended from west to east. The authors noted that there occurred several M5~M6 earthquakes in 1982, 2011 and 2012 along the near-NS direction of this Yutian earthquake and this Yutian earthquake filled up the empty section. Based on the regional tectonic environment, earthquake focal mechanism solutions and aftershocks distribution, etc., we analyzed the process of this earthquake and found that the earthquake occurred at the branch fault of Altyn Tagh Fault zone on the south margin of Xiaoerkule Basin. Affected by the tectonic stress of Altyn Tagh Fault zone, Xiaoerkule Basin suffers the local near east-west extension, earthquake rupture occurred first along the near-NS direction, the unlocking of this tectonic position promotes the left-lateral movement of the Altyn Tagh Fault, producing NE-oriented rupture, and stress transferring to the east. This paper also discusses the seismogenic structures of historical earthquakes above M_S7 occurring on the Altyn Tagh Fault zone and their impact on the fault zone.

The objective of this paper is to explore the current tectonic activity with satellite remote sensing thermal information by taking a case of the Wenchuan earthquake. Three items are accomplished as follows: 1)the process of evaluation of thermal field before and after Wenchuan earthquake is analyzed. The results indicate that there exist plenty of thermal information associated with the distribution of tectonics in the in-situ land surface temperature field, which is extracted from land surface temperature in which the effects of non-tectonic factors, such as topography, atmosphere and solar radiation are gotten rid of. 2)Combining with measurement of the shallow atmosphere temperature, the quantitative relationship between increment of land surface temperature and crustal stress-strain is preliminarily investigated. Results indicate that the increment generated by the crustal stress may obviously affect the shallow atmosphere temperature. Especially, the temperature-lowering zone has more reliability. There exist some temperature lowering zones along the boundaries of large geological blocks before and after Wenchuan earthquake, which reflects the relatively extensional movement (or stress relaxation) among these corresponding blocks. 3)Based on the co-seismic deformation, the comparative analysis is done between co-seismic deformation and thermal information. Results indicate that the tectonically adjusted area obtained from temperature field is largely accordant with that of co-seismic deformation. This shows that the variation of temperature along the boundaries of large geological blocks within the Tibet Plateau reflects the process of the adjustment of crustal deformation of the Tibet Plateau before and after Wenchuan earthquake. In summary, it is a possible approach to obtain the change of state of crustal stress by using the thermal method.

Identification of short-term and impending precursors, including the signal indicating earthquakes are inevitable, is one of focused issues in research of earthquake prediction. To explore this problem, modeling study of instability on a planar strike-slip fault was performed in the laboratory. It is based on the condition that the stress variation curves at a loading machine can reveal the stress state of the specimen and allow us to recognize its meta-instability stage. In terms of the advantage that the information from the loading machine can be compared with observations to the physical quantity of the sample, this work captures and analyzes the differences of temporal-spatial evolution processes of strain parallel and perpendicular to the fault in the stress linearity-off stage and meta-instability stage. The study suggests that a fault consists of relatively fragile and tough portions; the former usually are weakened first as expressed by pre-slip of the fault, and slow or small earthquakes indicative of beginning of strain release; while the latter become the locality of strain accumulation and fast instability finally, i.e. the future seismic source. The synergism process of a fault is actually a process of interaction between different portions of the fault. It is also a conversion from independent activities of each fault segment to synergism activity. The degree of synergism is an indicator of the stress state. It is based on the assumption that an earthquake results from sudden fast slip on a fault which relies on two primary conditions: one is the fault has a high synergism degree which facilitates connection between fault segments resulting in rapid slip of longer fault segments, and the other is sufficient strain is accumulated at some portions of the fault to overcome resistance of local tough portions on the fault. Usually the synergism process of a fault includes three stages: generation of strain-release patches, expansion and increasing of these patches, and mutual connection of strain-release areas. The first stage occurs when the stress curve deviates from linearity, strain variations of different portions of the fault begin to diverge, resulting in isolated patches of strain release and strain accumulation along the fault. In the second stage, related with the quasi-static instability of early meta-instability, those isolated areas of strain release increase and extend steadily. The third stage is equivalent to the late meta-instability that is a quasi-dynamic instability process when the sections of strain release on the fault accelerate to expand and strain levels of strain-accumulation areas accelerate to rise. The accelerated synergism begins at the time when the quasi-static state transforms into the quasi-dynamic state, of which the expansion mechanism of strain release segments changes, i.e. the expansion of isolated fault segments is replaced by connection between fault segments under interaction. At this time the fault is in a critical state and bound to generate earthquakes sooner or later. As a case study, based on the experimental results above and coupled with temporal-spatial evolution of earthquakes on the Laohushan-Maomaoshan Fault west of the Haiyuan Fault zone, this work analyzes the synergism process of the fault before the M6.2 earthquake on 6 June 2000 in this region.

Recently,the strong earthquakes in China mainland occurred mainly around the Bayanhar block. It is important to monitor the information of ongoing crustal activity at the key tectonic positions. We have developed a set of wireless equipment for measuring the ground temperature in field,and have established a network of measurement of the ground temperature along Xianshuihe Fault. Some changes of temperature were observed before and after the Lushan earthquake on April 20,2013.First of all,an apparent and persistent change of the ground temperature in Kangding appeared,starting from January 31,2013.This temperature variation corresponded with the occurrence of the small earthquakes around the observation station. According to the relationship between the temperature and stress,the abrupt change of ground temperature is essentially the geological stress adjustment. From the viewpoint of geological structures,both Longmengshan Fault and Xianshuihe Fault are the boundary faults of the Bayanhar block,but located at different boundaries,so,Kangding in Xianshuihe Fault is tectonically related to Lushan in Longmengshan Fault. Thus,the temperature change described above would possibly be the precursor of the Lushuan earthquake.

Within almost five years,the 2008 Wenchuan M_S 8.0 and 2013 Lushan M_S 7.0 earthquakes ruptured the Longmenshan Fault zone successively. The characteristics of earthquakes and their development tendency on this fault zone have been a focus of subject of research. This article attempts to explore some features of seismic preparation process of the 2008 Wenchuan event from temporal-spatial evolution of earthquakes along the Longmenshan Fault zone during more than 40 years.(1)The spatial range of the earthquake preparation,or seismic nucleation,is much smaller than that of co-seismic rupturing. It indicates that the seismic source,probably consisting of some small asperities or barriers,prepared on a finite fault segment can be connected and expand into a large-scale rupture section along the fault when the fast instability occurs at the source.(2)Prior to the 2008 Wenchuan giant shock,its preparation area had experienced a dense distribution of small earthquakes for eight years or more,while no conspicuous slip and deformation were observed on the surface. This implies that the seismogenic fault segment of the Wenchuan event on the Longmenshan Fault was undergoing probably compressive deformation,accompanied with cataclastic process. When the cataclastic deformation of the great-shock source reached a critical state,fault instability occurred along the fault with rapid rupturing. (3)To further study the variations of the main-shock area prior to the event,this article analyzes the temporal-spatial processes of small earthquakes around the main shock since 2004 recorded by a special seismic network in the Zipingpu reservoir. The results indicate that the scope of the seismicity expanding along the fault took place along the fault in October 2005 and October 2006,respectively,in accordance with the time when the reservoir reached its high water level. Among them,the second expanding from October 2006 covered a relatively large area and with relatively big magnitudes,implying great importance for the study of the final instability process of the 2008 Wenchuan huge earthquake. Besides,this paper discusses the correlation of the rupturing process of the 2008 Wenchuan giant event with the geometry of the fault and the reason why the 2013 Lushan earthquake occurred many years after the Wenchuan event rather than immediately following this giant shock like usual big aftershocks. The research results are helpful for understanding of seismogenic processes of major earthquakes of the thrust type.

Shanxi tectonic belt is a historically earthquake-abundant area and the focal distribution of the majority of the strong earthquakes is controlled by the local north-south oriented structures on the tectonic belt. Using the cataclastic analysis method(CAM),we performed an inversion analysis on the stress state of focal mechanism solutions of earthquakes which happened on Shanxi tectonic belt from 1967 to 2010.Results show that spatial distributions of the maximum principal compressive stress axis of Shanxi tectonic belt have changed over time,with two different predominant directions,NW and NE,in different periods of time. When the maximum principal compressive stress axis is oriented in NE direction,the stress state is registered as horizontally shearing and horizontally extension on the north-south and southeast oriented local segments in turn. When the maximum principal compressive stress axis is oriented in NW direction,the stress state of north-south and northeast tectonic segments is primarily registered as horizontally shearing.

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.

The spatial-temporal evolution process of strain field and acoustic emission(AE)events was investigated during the deformation of 5° bend faults,with 96-channels strain acquisition system and 16-channels distributed AE acquisition system in the laboratory.The loading was applied by controlling the Y-displacement and holding the X-load in a biaxial servo-control loading system, and the Y-loading rate was altered by 0.5μm/s,1μm/s,0.5μm/s and 0.1μm/s in sequence.The observation results show that: (1)quasi-periodic stick-slip always occurred under different loading rates, and the smaller the loading rate,the greater the period and stress drop; (2)low energy AE events increased before faults slid,but high energy AE events appeared as faults slid.AE events distributed near the bends and the upper and lower fault segments which were located by arrival time of AE wave.From the AE location results,AE sources mostly scattered in bend zones,and upper and lower fault segments,and the fault instability appeared first near bend point,then the alternative activities happened between upper and lower fault segments.Large instability took place in the lower fault segment,finally; (3)High strain concentration zone located near bend point and fault segment.And it is significantly different that mean strain and maximum shear strain increment changed alternately at the inside and outside of bend during strain accumulation and release stage; (4)Strain observation results illustrate that mean strain release first occurred near the bend,then released in the whole fault.It would be a critical instability condition for a bend fault.The observation to bend faults is important and helpful to investigate fault activity state.

The stick-slip process of pre-cut bending faults with a 5°angle at bending point between the two fault segments is investigated by use of fault displacement measurement,strain tensor analysis and acoustic emission(AE)technique in the laboratory.The dynamic process and corresponding properties of physical evolution are discussed.The experimental results from bending faults show that: 1)A negative relationship was revealed between the logarithms of the stick-slip cycle and the logarithms of loading rate; 2)Under different loading rate,most of instabilities of bending faults are earthquake doublets,and the interval time between the two sub-events are primarily from 100ms to 200ms; 3)For different observational approaches,even if with the same sampling rate,the differences of the coseismic response were observed,such as the significant strain weakening stage indicated by strain measurements,but there was no significant change in fault displacement before fault instability; and 4)AE sources obviously migrated along faults during fault sliding.More dynamic information about fault instability process is needed to know the mechanism of strong earthquakes and the features of aftershocks.

So far,although many reservoirs have been built,few of them have triggered seismicity.In order to study the mechanism of triggered seismicity so as to provide useful reference data for site selection of dams,we analyze the effects of hydro-geological structures and gravity loads in reservoir areas,and draw some conclusions:1)Rapid-response type seismicity may occur when there are some abandoned mines or karst caves;2)Based on the results of numerical calculation,Coulomb stress on different faults changes differently when the gravity load of the reservoir increases or decreases.The calculation results change with the stress field,the fault parameters and the relative position of the fault and reservoir.When the reservoir is on the down-thrown block and the fault dip is large,the fault may be more unstable because of the gravity load.In this case,Coulomb stress may increase locally on normal dip-slip faults,but on whole reverse dip-slip fault.Under the same fault occurrences,the Coulomb stress change of a normal dip-slip fault is larger than that of the reverse dip-slip fault.When the coefficient of friction is 0.6,the quantity of Coulomb stress change induced by gravity load is about 1/4 as much as that induced by pore water pressure in the fracture of the saturated karst cave.So,more attention should be paid to pore water pressure in the fracture.

The relationship between the thermal field and strain field during deformation of fault is the physical basis to clarify whether satellite infrared information or the ground temperature field can be used to study fault activity.This study attempts to discuss these problems by laboratory experiments.A two-direction servo-control system was used to apply load on the samples with compressive and tensile en echelon faults.An infrared thermal image system and a contact-type thermometer recorded synchronously variations of the brightness temperature field of infrared radiation and temperature field during deformation of the rock specimens.A digital CCD camera and the digital speckle correlation method(DSCM)was used to collect and analyze images to obtain the evolution processes of displacement and strain fields.The experimental result shows as follows:1)the temperature is the highest of the jog area of the compressive en echelon faults,whereas that is the lowest of the extensional en echelon faults prior to failure of the jog area.The record by DSCM displays that the mean strain of the jog area is the largest for the compressive en echelon faults,while that is the smallest for the extensional en echelon faults.These mean that the temperature field has clear responses to the opposite stress states at the jog areas with two kinds of en echelon faults,and thus provides an indicator for determining whether the fault has slid;2)the en echelon faults experience two deformation stages from the stress building up and fault propagating in the jog area to the unstable sliding along the fault.Correspondingly,the mechanism of heating-up turns from strain heating into frictional heating.Three kinds of phenomena have been observed at the jog area and its vicinity during the stage of transformation.They are temperature drop,fast fluctuation of temperature,and pulses of temperature rising.Mechanism of these phenomena is discussed;3)these variations of thermal field at the jog area are followed by swift rise of temperature along the fault.The onset of temperature rise along fault occurs 2～3 seconds prior to its unstable slip.However,the temperature drop of the jog area happens about 20 seconds before the unstable slip of the fault and the appearance of temperature rising pulses is 10～20 seconds earlier than that of the fault slip.They are precursors to unstable slip of the fault.These experimental analyses demonstrate that observations and studies on the thermal variations at the sensitive portion of a fault comparing with other relevant data are of great importance for detecting precursors ahead of unstable slip of active faults.It is planned to design an observing project along a suitable fault.

The knowledge of electrical prosperities of mantle minerals can help us to learn the distribution of the electrical conductivity in the interior of the earth and the polarization mechanism of the earth's material.The experimental results of electrical conductivity of mantle minerals were summarized and analyzed.We reviewed several important influential factors on the electrical conductivity of mantle minerals.The conduction mechanism of dry mantle minerals is different to that of wet mantle minerals.There is a small polaron conduction mechanism for dry mantle minerals,however,the conduction mechanism of wet mantel minerals is attributable to free proton.The results of typical electrical conductivity of mantle minerals were plotted and compared,and several electrical conductivity depth models were also introduced.We concluded that the water should play an important role in the electrical conductivity and the conductivity depth models.At last,we also point out the prospects of research in the future.

Migration of strong earthquakes(M≥7) along the North-South Seismic Belt since 1500 AD shows three patterns: roughly similar time and distance interval migration from N to S, multi-pattern migration from S to N and clusters of strong earthquakes occurring in some periods within the whole seismic belt. Based on analysis of strong earthquakes in the past hundred years, the activity of the North-South Seismic Belt is related to the strong earthquake activity of the South Asia Seismic Belt elongating from Burma to Sumatra, Indonesia. Strong earthquakes along the former belt often occur several months or years after the quakes on the later belt. The above-mentioned migration characteristic of strong earthquakes is likely caused by the northward collision and subduction of the India Plate as well as coaction between the Qinghai-Tibet Plateau and the stable and hard Ordos and Alashan Massif at the northeastern margin of the Plateau. The South-to-North migration of strong earthquakes with different time intervals and different migrating rates may directly reflect the uneven, irregular pushing of the northeastern corner of the India plate, and the gradually northward transmitting and expanding of the related stress as it accumulates and strengthens. While the North-to-South migration of strong earthquakes with long time intervals and uniform rate may relate to the movement of the further deeper materials, or to the interaction between the strongly-deformed Qinghai-Tibet Plateau and the northern hard massif. Perhaps it results from the successive, north-to-south, lateral-slipping and rotational-twisting movement of the strip massifs constituting the Qinghai-Tibet Plateau. As for the phenomenon that strong earthquake clusters occur constantly in a certain time on the North-South Seismic Belt, it may imply that the deformation induced by India Plate indenting strongly into Eurasia continent on the eastern margin of the Qinghai-Tibet Plateau has been strengthened. And the clustering of strong earthquakes on North-South Seismic Belt and South Asia Seismic Belt also confirms that the northern collision zone and the eastern subduction zone of India Plate, as a whole, have impact on the activity of the earthquakes on the Chinese North-South Seismic Belt and Burma, Andaman till Sumatra.

The paper first summarizes the advances in detection of volcano activity using satellite thermal infrared remote sensing and discusses the feasibility and means to monitor the thermal anomaly caused by volcano activities through satellite thermal infrared remote sensing technology. Then, some applied methods are put forward to remove the influences of geological environments and meteorological variation on earth surface thermal infrared radiation for extracting volcanic thermal anomaly. These methods include getting rid of the influences of ground environment factors, such as terrain, vegetation and rock types by establishing brightness temperature variation models of volcanic region and eliminating the influences of weather by difference calculation of brightness temperature between volcanic region and adjacent contrast region. Finally, the paper gives a study case of Changbai Mountains volcano. By using NOAA AVHRR data of the three years of 1999, 2003 and 2004, the authors analyzed and interpreted the thermal infrared brightness temperature images of Changbai mountain volcano region, and calculated the annual variations of brightness temperature of Changbai Mountains Tianchi Lake and adjacent contrast region. The result shows that the spatial distribution of brightness temperature in Changbai mountains volcano area is mainly controlled by hypsography and physiognomy, and presents a “tundish” shape. From Tianchi Lake to its peripheral places, the brightness temperature increases gradually, but the Tianchi Lake is a clear hot spot in cold background. The temporal variation of brightness temperature in this region is mainly influenced by seasons and presents seasonally changeable characteristics. The comparison analyses of annual variations of brightness temperature between Tianchi Lake and contrast region reveal that the brightness temperature of Tianchi Lake in 2003 and 2004 increased markedly with a magnitude about 2K compared with that in 1999. We suppose this elevated temperature is likely to be a reflection of increasing activity of Tianchi volcano in recent years. Furthermore, this result is consistent with that obtained by Changbai Mountains Tianchi Volcano Observatory in the past 4 years. These indicate that the satellite thermal infrared remote sensing is a feasible and effective measure for detecting volcano activity, and its application in detection of volcano activity deserves more detailed research.

The twentieth century's deadliest M7.8 Tangshan,China earthquake killed more than 240,000 people because it took place beneath a city of dense population. Therefore a great amount of various phenomena relating to this seismic event have been observed and documented. It was accepted as the seismo-generic fault of the earthquake when an about 8km-long intermittent surface fissure zone passing through the city was located,although it has been puzzling researchers that the inferred fault does not seem to be great enough for such a strong earthquake. The right-lateral displacements about the fissure zone were acknowledged to represent the strike-slip of the fault. Consequently,studies with focus on this fissure zone have obstructed researchers from paying much attention to other discoveries beyond the city. For example,field surveys reported some large subsidence areas produced by the earthquake with remarkable boundary fissures but no special articles have been published on them. We began our investigations in order to locate the subsidence areas in aerial photographs that were taken immediately after the main shock and came up with the shocking discovery of a previously unknown great seismic fault of the earthquake. The newly revealed great fault is at least 90km and has a coseismic vertical dislocation of 3m at the surface. What is more,it possesses the characteristics of a normal listric fault. The great fault can be divided into two parts. The southwestern part is proved by field investigation,leveling data,aerial photography and geophysical exploration,while the existence of the northeastern part is exhibited with evidences in water-flow distribution and seismic reflection profile. Distribution of aftershocks and leveling deformation agree perfectly with the sense of this great fault.

Compared with the strong noise background,intensity of efficient signal is very small when using thermal radiation to obtain the current tectonic activity. Increasing range of thermal radiation resulting from fault activity accords with dimension of inversion precision of thermal radiation of land surface,and information of fault activity is submerged by inversion error of thermal radiation of land surface. Besides,it is difficult to discriminate the information of fault activity when the data precision is high. This paper develops a new method,i.e. the Mean Grad Method (MGM),to solve the above-mentioned problems. First,we average the long-time data to gain a higher data precision at an expense of losing time information,then,distinguish the space distribution of the current tectonic activity from thermal radiation field of land surface by space grad of thermal radiation according to the difference between influence of atmosphere and tectonic activity on land surface. This method can offer some features on variation of current tectonic activity with time and space and is helpful for plotting out earthquake danger area.

The thinking of earthquake research in China should be shifted from the viewpoint of fault to active block (MA Jin, 1999). ZHANG Pei-zhen et al. divide the active blocks in Chinese mainland and its adjacent area into two degrees: the first degree refers to active-block regions and the second degree refers to active blocks. The former contains 6 block regions, e.g. the Qingzang (Qinghai-Tibet) region, etc. and the latter contains more than 20 active blocks, e.g. the Lhasa block and so on. We attempt to analyze the characteristics of geological structure and focal mechanism of group strong earthquakes that occurred recently in Chinese mainland from the block viewpoint on the basis of the two-degree active blocks. The strong earthquakes (M≥7 in the west and M≥6 in the east) occurring in China of 1977—2003 can be roughly divided into 9 groups. In summary, the strong earthquakes occurring in the recent 10 years still have the grouping feature and most of them are located in the boundary zones between active-block regions or active blocks. Moreover, their focalmechanism solutions are quite similar to each other, except for the earthquakes in the 4th group (the earthquakes that occurred in the Beibu Gulf and the Taiwan Straits can be considered as an individual case) and in the 5th group (the earthquakes that occurred in Mandalay-Diannan block near the plate boundary are not regarded as intraplate earthquakes). Based on the study of horizontal strain field in Chinese mainland and its surroundings with GPS data, we point out in the paper that group strong earthquakes have their own genesis for the similar motion pattern and dynamic origin. From the above analysis, we conclude: (1) The rule of strong earthquake occurrences in groups is still effective after more than 10 years practice, and it is an applicable method for locating earthquakes in the medium and short-term earthquake predictions. (2) The regional characteristics of group strong earthquakes enable us to predict the location of earthquakes in a smaller range on the boundary zone between the second-degree blocks in a first-degree block region or between two first-degree block regions. (3) Except plate-margin area, the group strong earthquakes have consistent focalmechanism solutions. This indicates that they have similar kinetic mechanisms and dynamic processes, or perhaps, we can say, that they develop monolithically and occur successively.

The Kunlunshan M_S 8.1 earthquake of Nov.14,2001 is the biggest seismic event in China's continent in the past five decades. For this event,some researchers have developed an analysis method of brightness temperature difference inside and outside the Eastern Kunlun Fault,and have drawn an important conclusion. They found that before the earthquake (beginning from Oct.2001),the brightness temperature along the seismogenic fault became higher than that outside the fault,and affirmed that this phenomenon was the impending precursor of the 2001 Kunlunshan M_S 8.1 earthquake. However,our comparison study on the IR images of 2001 with those of 1999 has revealed that the same phenomenon has occurred also in 1999,in which no earthquake has been recorded. The Eastern Kunlun Fault is interpreted and analyzed by using NOAA thermal infrared (IR) remote sensing images combining with numerical processing of IR brightness temperature. The comparison of IR images in seismically quiet period of 1999 with those before and after the Kunlunshan M_S 8.1 earthquake in 2001 has indicated that seasonal meteorological factor greatly affects the change of IR images along seismogenic fault. In early winter,the IR brightness temperature along the fault is equal or even higher than that outside the fault. Moreover,the comparison between brightness temperature along the Eastern Kunlun Fault and along the Altyn Tagh Fault has also revealed that during the transitional period between autumn and winter,the disturbance of meteorological factors on ground surface are significantly greater than the IR of ground object itself. We suggest,therefore,that the anomalous increase of ground temperature before the occurrence of the M_S 8.1 Kunlunshan earthquake on Nov.14,2001 incorporated natural phenomenon of seasonal change,while the identification of anomaly related to earthquake needs a further study.

Some researchers believe that satellite thermal infrared anomaly before earthquake is related to gas releasing of the earth. Accordingly,whether the gas releasing phenomena can be observed by satellite thermal infrared remote sensing becomes a key problem. In this paper,two sites where gas releasing phenomena of the Earth occurred assuredly were selected for the study of this key problem. One is the Kaixian County where high pressure and high concentration gas blow-out event of the No.16 oil well took place,and the other is around the epicenter of the west of Kunlunshan Pass M_S8.1 earthquake where gas releasing has been observed to continuously occur along the Kunlunshan Fault even one year after the earthquake. The characteristic features of satellite thermal infrared images before and after gas releasing process have been analyzed,and the thermal infrared anomaly related to gas releasing or gas blow-out has been identified. Furthermore,the feasibility to observe gas releasing phenomena by satellite thermal infrared was discussed. The result shows that the gases blew out from the oil well and the fire from ignited gases are distinctly reflected on satellite thermal infrared images,but the gas releasing phenomena along the Kunlunshan Fault after the M_S8.1 earthquake are poorly displayed on thermal infrared image,so that the infrared thermal anomaly related to gas releasing along this fault is difficult to be recognized by visual interpretation. The comparison of brightness temperature,however,has revealed that the brightness temperature in the gas releasing sites along the fault is higher than that of the surrounding areas either before or after the earthquake.

For extracting the information about fault activity and seismicity from thermal infrared radiation,two crucial problems should be taken into consideration: 1) Aerosphere exists between solid earth and the satellite based sensor,and thus influences the infrared energy of land radiation when it passes through the aerosphere to arrive at the sensors. Moreover,the atmosphere itself is involved in thermal radiation,too. Therefore,to extract the geophysical information from thermal radiation,we have to begin with the estimation of atmospheric influence; 2) for identifying the anomalies induced by fault activity in thermal radiation,we should be able to distinguish the normal radiation from anomalous radiation,as anomaly can only be identified in reference with normality. This paper discusses these problems from the following aspects: 1) define the concepts of land surface brightness temperature (LSBT) and its annual variation field. LSBT would be an important physical concept in extracting the geophysical information from thermal radiation. LSBT should be assigned to the category of radiation energy of land surface,rather than the traditional "temperature" concept. Once the emissivity is known,the temperature can easily be calculated according to Planck law; 2) introduce the split window method for calculating the LSBT and the influence of atmosphere. The calculation shows that the influence of atmosphere on brightness temperature received by satellite based sensor is about ?10K; 3) the annual variation field of LSBT in China is extracted from the LSBT data by using the wavelet and split-window methods; the data used in this study come from the observations of NOAA/AVHRR from 1981 to 2001. The results of this study may provide a basis for further analyzing the anomaly field of thermal radiation.

After a series of complicated modifications the surface temperature of the Earth can be measured. Usually this surface temperature is referred to as land surface temperature. The heat effect that engenders this temperature variation may come from the climate and subsurface heat sources. Among them,the subsurface heat sources are of particular importance to earthquake and active fault studies. Unfortunately,the heat effect of climate is much larger than that of the subsurface heat sources so that it may conceal the information of subsurface thermal activity. Heat Penetrability Index (HPI) method is proposed in this paper for measuring subsurface thermal activity. The first assumption for HPI method is that the sun is heating the land surface synchronously. The second is the horizontal variation of heat exchange rates of rocks on top of subsurface heat sources. Provided that the rocks are heated,then the variation of surface temperature can be observed. According to the afore mentioned concepts and thermodynamics,the HPI can be deduced through correlation analysis as expressed by: where u and v represent surface temperature in two different areas,n is the sample number in the scanning window and D is the Heat Penetrability Index. When subsurface heating event takes place,D will go up,and vice versa. Some examples from experiment and satellite infrared image analysis are presented to test the effectiveness of HPI method.

The calculation of land surface temperature from thermal radiation requires the emissivity value of rock. However,there are lots of rocks in the Earth,and even the rocks of the same type may have obviously different emissivity. Recent methods for measuring the emissivity of rock are relatively complex,and most of them depend excessively on environmental condition. Therefore,when plenty of emissivity data are needed,it is necessary to develop a simple method of measurement. In fact,emissivity is a constant at room temperature,and the radiation of instrument itself and environment can enjoin the inversion as an unknown quantity. Then emissivity can be obtained by least square method through measuring the radiation of rock in a series of temperature and radiation conditions. On the basis of this method,the emissivities of 16 rock samples were measured. The square error of the results keeps in about 0.01,mostly less than 0.01,and the correlation coefficient of all linear fits is larger than 0.99.

The correlation between deformation and temperature is tested in detail on five rock types commonly observed on the Earth's surface. It is proven by both experimental and theoretical analysis that during the elastic deformation stage,the temperature of the rock is directly proportional to the loading force. According to the first and second laws of thermodynamics and Maxwells equation,the following relation can be deduced: σ-σ₀=-(cσρ)/α ln [T/T₀] or T/T₀= exp [-α/cσρ(σ-σ₀)] where σ is stress,T is temperature in Kelvin scale,α is modulus of heat strain,c σ is heat capacity and ρ is density. The results of the experiment are consistent with that of classic thermodynamics. Under Earth's surface condition,i.e. ordinary temperature and pressure,the maximum temperature increment of the sample elastically deformed in an open system is around 0.2K,and the normal rate of temperature rise is approximately 3mK/MPa. At the moment of fracturing,the temperature increment of the samples is much greater than that during elastic deformation,ranging from a couple degrees to more than ten degrees. The temperature increments caused by the elastic deformation of rocks are so minute that not only will they have any effect on climate,but cannot be identified by satellite infrared detectors. Therefore,new methods have to be explored for distilling faint information from infrared images,while the other thermo anomalies aside from elastic deformation should be carefully reviewed.

The infrared signals produced during rock deformation are very faint and tiny,so that the measurements of them require excellent capabilities of detection system and strict working environment. The reliability of the results of infrared experiment,therefore,depends greatly on the reasonable technical specifications of infrared photodetector,high sensitivity recorder for temperature field and effectively controlled thermal background. All of the key parameters are discussed in detail in this paper based on laboratory infrared observation. The features for an infrared camera include spectral band,digitizing resolution,view field etc. For capturing the optimal infrared images,it is suggested that the spectral range should be 7～13μm,the observation range should be from 0.5m to infinity,and high-density focal plane array should be employed as far as possible. In order to confirm the temperature from infrared image,a surface temperature recorder with multi thermometers should be employed as an eyewitness working alongside the camera. A realistic experiment system has been developed based on the afore mentioned discussion. Aside from the traditional functions of infrared camera,the system is fitted with high-speed data transmission component. The infrared focal plane array has 320×240 pixels,and 25 frames of temperature field data are saved into the hard disk per second. Some steel beam bending and steel cylinder compressing experiments were made for testing the system. The tests have proved that the system is reliable and the data quality is improved greatly.

The propagation of acoustic waves through periodically layered media (PLM) is experimentally investigated for the purpose of determining how the velocities and waveforms depend on periodical structures and detection frequency. In our experiments the PLM were made of stacks of plastic and steel plates with different periodical spacing. The acoustic wave frequency used in our experiments was 240MHz. Velocities and waveforms were measured by Digitization Wave Meter with 12-bit resolution and 40MHz sampling rate. The results show that the velocity transition from the time averaged relation to the long-wave approximation (LWA) is related to the critical ratio of λ_i/d_i(i=1,2) (λ_i-the wavelength of wave going through medium i; d_i-the thickness of medium i in PLM) rather than to λ/d (the ratio of wavelength to the thickness of rhythmic layer). The LWA occurs when λ_i/d_i>5(i=1,2),and the velocities (V) approach to the Reuss bound (V^R),whereas the time averaged velocities hold when λ_i/d_i<4(i=1,2). We attribute the behavior of V→V^R to "softer shape" distribution of different rigid components in PLM. The waveforms are also frequency dependent. With larger thickness of rhythmic layer,the attenuation of the first arrival is low and the multiple waves reflected from interfaces can be observed. In the case of LWA,however,the transmitted waves tend to have relatively simple waveforms. This behavior implies that simple waveform does not always reflect simple medium structure through which the wave propagates. As λ_i/d_i～6 (i=1,2),the transmitted wave is obviously attenuated,and consequently it is equivalent to a shield layer for the wave. Therefore,the influences of medium structures on velocities and waveforms should be taken into account when interpreting deep structures and compositions based on seismic data. Tectonic nature of intracrustal low velocity zones is one of the hotly debated topics. An important implication of our experimental results is that PLM are potentially low velocity zones. If extremely reflective deep crust is caused by alternating layers,the relatively lower velocities observed in deep crust should probably be attributed to the rock properties that we wish to image,as well as to its structures,scales and detection frequencies.