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.

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.

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.

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.

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.

Based on the experimental results of two similar rocks having high/very low microcrack density under two extremely different loading conditions: fast constant stress rate (～6 MPa/m) loading and creep (at ～95% of the fracture strength) loading,the role of pre-existing microcracks and loading condition on the fracturing properties of granitic rocks was examined. The detailed spatio temporal distribution of acoustic emission (AE) events in these samples were monitored by the use of a high-speed multi-channel waveform recording system,which can record AE waveforms from 32 sensors mounted on the surface of tested sample without a major loss of events even for an event rate on the order of several thousand events per second. Event rate,b-value in magnitude-frequency relation,and hypocenter distribution of AE events characterize three typical long-term stages of microcracking activity during the fracturing process: primary,secondary and nucleation. Precursory anomalies on cracking activity such as event rate and b-value in the nucleation stage were observed. In general,a higher pre existing microcrack density or a lower loading stress-rate results in a longer nucleation stage,and therefore,increases the predictability of catastrophic failure event. Further,the physics of these stages is analyzed using the subcritical crack growth model for crack population of fractal size distribution based on the stress-aided corrosion theory.

An experimental study on the spatial distribution of the focuses of acoustic emissions of Inada granite (coarse grained granodiorite) under triaxial compression is reported in this paper. It is pointed out that AE locations is fractal with a dimension of 2.2 in average,lower than the result of Oshima granite (fine grained granodiorite),which is 2.7,under similar deformation condition. For Inada granite a band-limited fractal had been observed. Fractal dimension changed around grain size of the rock (R_C=5mm) Fractal dimension D_s and D_l (below and above the grain size,respectively) had different developing tendency. D_s and D_l can be considered as indicators of the inhomogeneity of stress field in corresponding scale.

By the spatial scanning of the frequency-magnitude distribution of earthquakes in Beijing-Tianjin-Tangshan-Zhangjiakou area,it is found that there are three types of the distribution of earthquakes in this area. Type Ⅰ is consistant with a linear b-value model and types Ⅱ and Ⅲ are consistant with a characteristic earthquake model. The area,in which large earthquakes have never occurred in last 22 years,often has a distribution of type Ⅰ,and the area,in which large earthquakes have occurred,often has a distribution of type Ⅱ and Ⅲ. Result of the rock mechanical experiments shows that the frequency-magnitude distribution of acoustic emission events in rocks has a high b-value in the process of fracturing and fracture joining,and has a lower b-value in the process of frictional slip accompanied with stick-slip. But the frequency-magnitude distribution in a whole process of rupturing and slipping of intact rocks is consistant with the characteristic earthquake model. Therefore,it is evident that the frequency-magnitude relationship may represent a stage of structural deformation for an area. The nonlinearity of frequency-magnitude distribution might give us more information about the geometrical characteristics of the rigional structure. The emergence of nonlinearity of F-M distribution in area with moderate magnitudes but without large magnitudes in the considered period may indicate that the area is in a state much closer to the critical point for the unstable sliding.