Induced Earthquake Monitoring of the Three Gorges Reservoir

The Three Gorges Project(TGP) on the Yangtze River(YR) is the largest water power station in the world;it is now attracting the worldwide attention. Possessing comprehensive utilization benefits mainly for flood control,power generation and navigation improvement,TGP will be a vital and backbone project in harnessing and developing of the YR. TGP has been commenced in 1993,the main channel of Yangtze was diverted on Nov.8th,1997 and whole project was completed in 2009. The Three Gorges(TG) reservoir has been formed since June 2003 and impounded to 175 m on Oct.26th,2010. The reservoir has a total length of 660 km and an average width of 1.0-1.5 km,with a total waters area of 1048 km2. In pace with the raised pool level,the enlargedstorage,the probability of reservoir earthquake would be enlarged. According to the magnitude records of historical maximum earthquake and properly weighted,defined the maximum reliable earthquake of TG reservoir would be ML6 magnitude,existed the probability of ML 5.0-5.8 induced earthquake around the Xiannvshan and Jiuwanxi fault zone(18 km upstream of the dam) . However,the highest intensity around the dam site would not exceed Degree VI,whereas the main structures of TGP were designed with an anti-seismic intensity of Degree VII,hence the reservoir induced earthquake would not form direct threat to the main structures of TGP. After the impoundment and operation of TG reservoir,the frequency and intensity of earthquake has been increased in some extent,yet the earthquake activities at TG region still kept in an original weak earthquake activities status. We should strengthen the TG reservoir induced earthquake monitoring and forecast,prevent the earthquake and geological calamity,ensure the safety of dam construction and operation,form the harmonious society as well as guarantee the long term security. This paper explained the mechanism,kinds,causes,development and calamities of the earthquakes in the world,collected a lot of TGP,China and World's earthquake and geology materials. Such as 105 year(Jan.1906-June2011) earthquake records in the world(Tab.1) ,13 main reservoir induced earthquake records in China(Tab. 2) ,19 main reservoir induced earthquake records in the world(Tab.3) ,14 year(1996-2009) reservoir earthquake monitoring records in TGP(Tab.4) . Analyzed the cause,calamities of the main earthquakes as well as its alarm and forecast. May it provide reference for readers and beg your guide.

Research status of earthquake forecasting in hydraulicfracturing induced earthquakes

In the new types of industrial activities including unconventional energy extraction associated with shale gas and hot dry rock,gas reservoir operations,CO2 geological storage,undergoing research on induced earthquake forecasting has become one of the forward positions of current seismology.As for the intense actual demand,the immature research on induced earthquake forecasting has already been applied in pre-assessment of site safety and seismic hazard and risk management.This work will review systematically recent advances in earthquake forecasting induced by hydraulic fracturing during industrial production from four aspects:earthquake occurrence probability,maximum expected magnitude forecasting,seismic risk analysis for engineering and social applications and key scientific problems.In terms of earthquake occurrence probability,we introduce statistical forecasting models such as an improved ETAS and non-stationary ETAS and physical forecasting models such as Seismogenic Index(SI)and hydro-mechanism nucleation.Research on maximum expected magnitude forecasting has experienced four stages of linear relationship with net injection volume of fluid,power exponential relationship and physical forecasting regarding fault parameters.For seismic risk analysis,we focus on probabilistic seismic hazard assessment and quantitative geological susceptibility model.Furthermore,this review is extended to key scientific problems that contain obtaining accurate fault scale and environmental stress state of reservoir,critical physical process of runaway rupture,complex mechanism of fault activation as well as physical mechanism and modeling of trailing effect.This work in understanding induced earthquake forecasting may contribute to unconventional energy development and production,seismic hazard mitigation,emergency management and scientific research as a reference.

Earthquakes induced by deep penetrating bombing

The data of M≥5 earthquakes occurred in one year before and after 4 deep penetrating bombs in the region within 500 km and 1 000 km from the shooting site are presented. The 4 bombs are those happened in 1999 Kosovo of Yugoslavia, the 1991 Baghdad of Iraq, the 2001 Tora Bora of Afghanistan, and the 2003 Kirkuk of Iraq, respec-tively. The data indicate that the deep penetrating bombs may have remotely triggered some earthquakes. The deep penetrating bombs in seismically active regions should be forbidden.

Spatio-temporal variations of shallow seismic velocity changes in Salton Sea Geothermal Field,California in response to large regional earthquakes and long-term geothermal activities

We measure spatio-temporal variations of seismic velocity changes in Salton Sea Geothermal Field,California based on cross correlations of daily seismic traces recorded by a borehole seismic network from December 2007 to January 2014.We find clear co-seismic velocity reductions during the 2010 M 7.2 El Mayor–Cucapah,Mexico earthquake at~100 km further south,followed by long-term recoveries.The co-seismic reductions are larger with longer post-seismic recoveries in higher frequency bands,indicating that material damage and healing process mostly occurred in the shallow depth.In addition,the co-seismic velocity reductions are larger for ray paths outside the active fluid injection/extraction regions.The ray paths inside injection/extraction regions are associated with smaller co-seismic reductions,but subtle long-term velocity increases.We also build 3D transient water flow models based on monthly injection/extraction rates,and find correlations between several water flow parameters and co-seismic velocity reductions.We interpret the relative lack of co-seismic velocity changes within the geothermal region as unclogging of fracture network due to persistent fluid flows of geothermal production.The long-term velocity increase is likely associated with the ground water depletion and subsidence due to net production.

Development of a new high resolution waveform migration location method and its applications to induced seismicity

Locating seismic events is a central task for earthquake monitoring.Compared to arrival-based location methods,waveformbased location methods do not require picking phase arrivals and are more suitable for locating seismic events with noisy waveforms.Among waveform-based location methods,one approach is to stack different attributes of P and S waveforms around arrival times corresponding to potential event locations and origin times,and the maximum stacking values are assumed to indicate the correct event location and origin time.In this study,to obtain a high-resolution location image,we improve the waveform-based location method by applying a hybrid multiplicative imaging condition to characteristic functions of seismic waveforms.In our new stacking method,stations are divided into groups;characteristic functions of seismic waveforms recorded at stations in the same group are summed,and then multiplied among groups.We find that this approach can largely eliminate the cumulative effects of noise in the summation process and thus improve the resolution of location images.We test the new method and compare it to three other stacking methods,using both synthetic and real datasets that are related to induced seismicity occurring in petroleum/gas production.The test results confirm that the new stacking method can provide higher-resolution location images than those derived from currently used methods.

The April 24, 2013 Changning M_s4.8 earthquake: a felt earthquake that occurred in Paleozoic sediment

The dense broadband seismic network provides more high-quality waveform that is helpful to improve constraint focal depth of shallow earthquake. Many shal- low earthquakes occurring in sediment were regarded as induced events. In Sichuan basin, gas industry and salt mining are dependent on fluid injection technique that triggers microseismicity. We method with regional records adopted waveform inversion to obtain focal mechanism of an Ms4.8 earthquake at Changning. The result suggested that the Changning earthquake occurred at a ESE thrust fault, and its focal depth was about 3 km. The depth phases including teleseismic pP phase and regional sPL phase shows that the focal depth is about 2 km. The strong, short- period surface wave suggests that this event is a very shallow earthquake. The amplitude ratio between Rayleigh wave and direct S wave was also used to estimate the source depth of the mainshock. The focal depth （2-4 km） is far less than the depth of the sedimentary layer thickness （6-8 km） in epicentral region. It is close to the depth of fluid injection of salt mining, which may imply that this event was triggered by the industrial activity.

Topographic seismic effects and avalanche hazard:A case study of Mount Siella(L’Aquila,Central Italy)

In mountainous areas,snow avalanches could be triggered by the shaking produced by earthquakes.The forces induced by the earthquake can cause an irregular increase of shear strength load down the slope,for the presence of complex surface and buried morphologies.Topographic irregularities generate maximum effects of waves amplification linked to wavelengths comparable to the horizontal dimension of the topographic feature.For this reason,the selected time-histories represent an appropriate input for the two-dimensional numerical response analyses when a dynamic phenomenon produce the resonant motion of a whole mountain.This represents an important earthquake-induced hazard in snow-covered mountain areas with high probability of seismic events.Some valleys are located in regions with scare ground motion data and investments on infrastructures are not always accompanied by adequate protection against earthquake-induced avalanches.The paper points out a simple deterministic approach for selecting a set of real accelerograms applied to a real case of Siella Mountain(Central Italy)where a large avalanche destroying a tourist facility of Rigopiano resort on 18 January 2017.The selected time histories were used as input for the two-dimensional numerical model of the subsoil to evaluate the topographic seismic amplification in ridge and compare it with the results of other authors.These methods suggest that morphology-related inertial effects should be considered as an overload action on snow layers when controlling multi-hazard studies and spatial planning.

Hydraulic Fracturing, Cumulative Development and Earthquakes in the Peace River Region of British Columbia, Canada

Unconventional petroleum development involving large volume fluid injection into horizontal well bores, referred to as hydraulic fracturing (HF, or fracking), began in the Montney Trend of northeast British Columbia, Canada, in 2005, quickly initiating earthquakes. Earthquake frequency increased substantially in the Montney by 2008, in relation to the number of wells fracked and the volume of injected frack water. A spatiotemporal filter was used to associate earthquakes with HF wells. A total of 439 earthquakes (M 1.0 - 4.6 (NRCAN catalogue) during 2013-2019 have close association with HF activity, of which 77% are associated with three operators. Fifteen percent of HF wells in the Montney are associated with these earthquakes, while 1.7% of HF wells are associated with M ≥ 3.0 earthquakes. There are strong linear relationships between the maximum earthquake magnitude each year and the annual volume of injected frack fluid. M ≥ 3.0 earthquakes are associated with large cumulative frack water volumes for antecedent time periods of 1 - 3 years, often with fluid injection by multiple operators. Eighty-seven percent of the Montney M ≥ 3.0 earthquakes have associated HF triggering events, but a few are sufficiently distant to be ambiguous. Distances from the induced earthquake epicentres indicate a variety of causal mechanisms are involved. It is concluded that ~60% - 70% of M ≥ 3.0 earthquakes are induced by hydraulic fracturing. HF-induced earthquakes can be considered in part related to the cumulative development density from multiple proximal operators and cumulative antecedent fluid injection over periods ranging from a few months to a few years. It is probable that induced earthquakes of M > 5 will occur in the future. There are significant public safety and infrastructure risks associated with future HF-induced earthquakes in the Peace River area. To carry out HF operations effectively and safely, potentially destructive earthquakes must be avoided or mitigated. The Traffic Light Protocol mitigation system used in British Columbia appears unlikely to prevent large magnitude earthquakes. Risk avoidance therefore becomes important and could include the establishment of frack-free zones proximal to populations and critical infrastructure.

Earthquake-induced liquefaction hazard mapping at national-scale in Australia using deep learning techniques

Australia is a relatively stable continental region but not tectonically inert,having geological conditions that are susceptible to liquefaction when subjected to earthquake ground motion.Liquefaction hazard assessment for Australia was conducted because no Australian liquefaction maps that are based on modern Al techniques are currently available.In this study,several conditioning factors including Shear wave velocity(Vs30),clay content,soil water content,soil bulk density,soil thickness,soil pH,distance from river,slope and elevation were considered to estimate the liquefaction potential index(LPI).By considering the Probabilistic Seismic Hazard Assessment(PSHA)technique,peak ground acceleration(PGA)was derived for 50 yrs period(500 and 2500 yrs return period)in Australia.Firstly,liquefaction hazard index(LHI)(effects based on the size and depth of the liquefiable areas)was estimated by considering the LPI along with the 2%and 10%exceedance probability of earthquake hazard.Secondly,ground acceleration data from the Geoscience Australia projecting 2%and 10%exceedance rate of PGA for 50 yrs were used in this study to produce earthquake induced soil liquefaction hazard maps.Thirdly,deep neural net-works(DNNs)were also exerted to estimate liquefaction hazard that can be reported as liquefaction hazard base maps for Australia with an accuracy of 94%and 93%,respectively.As per the results,very-high liquefaction hazard can be observed in Western and Southern Australia including some parts of Victoria.This research is the first ever country-scale study to be considered for soil liquefaction hazard in Australia using geospatial information in association with PSHA and deep learning techniques.This study used an earthquake design magnitude threshold of Mw 6 using the source model characterization.The resulting maps present the earthquake-triggered liquefaction hazard and are intending to establish a conceptual structure to guide more detailed investigations as may be required in the future.The limitations of deep learning models are complex and require huge data,knowledge on topology,parameters,and training method whereas PSHA follows few assumptions.The advantages deal with the reusability of model codes and its transferability to other similar study areas.This research aims to support stakeholders'on decision making for infrastructure investment,emergency planning and prioritisation of post-earthquake reconstruction projects.

全球海啸相关研究进展综述

With the advancement of the global economy,the coastal region has become heavily developed and densely populated and suffers significant damage potential considering various natural disasters,including tsunamis,as indicated by several catastrophic tsunami disasters in the 21st century.This study reviews the up-to-date tsunami research from two different viewpoints:tsunamis caused by different generation mechanisms and tsunami research applying different research approaches.For the first issue,earthquake-induced,landslide-induced,volcano eruption-induced,and meteorological tsunamis are individually reviewed,and the characteristics of each tsunami research are specified.Regarding the second issue,tsunami research using post-tsunami field surveys,numerical simulations,and laboratory experiments are discussed individually.Research outcomes from each approach are then summarized.With the extending and deepening of the understanding of tsunamis and their inherent physical insights,highly effective and precise tsunami early warning systems and countermeasures are expected for the relevant disaster protection and mitigation efforts in the coastal region.

Seismic hazard assessment of the Three Gorges Project

Seismic monitoring data for the past 50 years in the Three Gorges Reservoir area show that the reservoir head area is a typical weak seismic region with low seismieity before impoundment and that the epicenters were concentrated in the east and west sides of the Zigui Basin, most of which were natural tectonic earth- quakes. After impoundment, the seismic activity shifted to the segment between Badong and Zigui along the Yangtze River, mainly within 5 km of the reservoir bank. The seismogenesis was categorized into four types： Karst collapse earthquakes, earthquakes caused by Karst gas explosion, mining tunnel collapse earthquakes, and rock （terrane） slip earthquakes, all of which are related to the lithology, structure, and tectonics of near- surface geological bodies of the area. Compared with the seismicity before impoundment, the seismic frequency increase was remarkable, with most of the magnitudes below Ms2.0. Therefore, the intensity of the earth- quakes remained at a low level. On November 22, 2008, a magnitude 4.1 earthquake, the largest earthquake recorded since impoundment, occurred in Quyuan Town, Zigui County. The intensity and PGA of reservoir-in- duced earthquakes are higher than those of tectonic earthquakes with equal magnitude, but the peak intensity of reservoir-induced earthquakes is not likely to go beyond that of the estimated range from earlier studies.

A proposed classification of the Earth’s crustal areas by the level of geodynamic threat

It is accepted as a well-known fact that in different places on the Earth’s crust,a similar anthropogenic impact causes a dissimilar response.Seismic zoning maps are not designed to predict such geodynamic hazards as rock bursts,induced earthquakes,reactivation of tectonic faults,etc.,and therefore require careful adjustments in places of intense impact on the subsurface strata.In this regard,we consider the classification of the Earth’s crustal areas according to the degree of geodynamic hazard,i.e.its potential geodynamic response to anthropogenic intervention.This classification is based on the concept that there exists a critically stressed layer within the Earth’s crust.It is believed that such a critically stressed layer within the Earth’s crust extends from the Earth’s surface to a certain depth,and each point depends on the nature of the interaction between crustal blocks of different hierarchical levels.From this perspective,anthropogenic impact,such as mining operations,represents a direct impact upon the critically stressed zone.We recognize the hypothesis that the thicker the critical stressed rock layer,the stronger the response might be to anthropogenic intervention,as it has more accumulated energy.Four categories of geodynamic threat have been found and mapped.To verify this classification,the manifestations of the geodynamic hazards were studied.The intensity of geodynamic hazard increased from the first area to the fourth area.The phenomenon of large induced seismic events with hypocenters at great depths is explained on the basis of this theory,and could be associated with anthropogenic impacts from the surface directly on the regional zone of the critically stressed rock massif.The approach can be used to assess the geodynamic consequences of human exposure to the Earth’s crust.

Source characteristics of the mainshock and aftershocks of the 2019 Changning earthquake sequence:Implications for fluid effects

The 2019 M_(S)6.0 Changning earthquake occurred in the tectonically stable Sichuan Basin,where the epicenter and its adjacent areas are important shale gas and salt mine production fields,resulting in hot debates on whether the seismogenic mechanism of the 2019 Changning earthquake is related to human activities.As source characteristics and fluid pressure can provide important constraints on whether an earthquake is induced,weinvestigate the seismogenic mechanisms of the mainshock and 9 MW≥4.0 aftershocks.In overall,the mainshock and the majority of the aftershocks are characterized by relatively shallow focal depths(1‒4 km)and significant non-double-couple(non-DC)components.However,the mainshock and the aftershocks differ in two aspects:(1)the compensated-linear-vector-dipole components dominate the non-DC components of the mainshock,whereas the isotropic components dominate the most aftershocks;(2)the fluid overpressure of the mainshock is over 30 MPa,whereas the fluid overpressure of the most aftershocks is less than 10 MPa.Thus,we propose that the mainshock is triggered by weakened fault strength with long-term fluid injection,and that its large non-DC components are associated withcomplex rupture processes.Comparatively,the aftershocks may be triggered by postseismic stress transfer by combining the Coulomb failure stress changes in the poroelastic medium.Our results highlight the possible role of fluid in the occurrence of the Changning earthquake sequence.

Calculation of landslide occurrence probability in Taiwan region under different ground motion conditions

In this study,Bayesian probability method and machine learning model are used to study the real occurrence probability of earthquake-induced landslide risk in Taiwan region.The analyses were based on the 1999 Taiwan Chi-Chi Earthquake,the largest earthquake in the history in this Region in a hundred years,thus can provide better control on the prediction accuracy of the model.This seismic event has detailed and complete seismic landslide inventories identified by polygons,including 9272 seismic landslide records.Taking into account the real earthquake landslide occurrence area,the difference in landslide area and the non-sliding/sliding sample ratios and other factors,a total of 13,656,000 model training samples were selected.We also considered other seismic landslide influencing factors,including elevation,slope,aspect,topographic wetness index,lithology,distance to fault,peak ground acceleration and rainfall.Bayesian probability method and machine learning model were combined to establish the multi-factor influence of earthquake landslide occurrence model.The model is then applied to the whole Taiwan region using different ground motion peak accelerations(from 0.1 g to 1.0 g with 0.1 g intervals)as a triggering factor to complete the real probability of earthquake landslide map in Taiwan under different peak ground accelerations,and the functional relationship between different Peak Ground Acceleration and their predicted area is obtained.

Connection between earthquakes and deep fluids revealed by magnetotelluric imaging in Songyuan,China

Songyuan is the most earthquake prone area in northeast China.Since 2006,earthquakes have occurred in the area in the form of swarms,with a maximum magnitude of M_L5.8.There is much controversy about the cause of the Songyuan earthquakes.We attempted to determine the cause using a three-dimensional electrical conductivity structure inverted from a regional network of magnetotelluric data in the Songyuan area.The L-BFGS inversion method was applied,with a fullimpedance tensor data set used as the inversion input.Combined with an evaluation of the earthquake locations,the resistivity model revealed a northeast-oriented hidden fault running through the Songyuan earthquake area(SEA),which was speculated to be the preexisting Fuyu-Zhaodong Fault(FZF).Our resistivity model also found an apparent lithospheric low-resistivity anomaly beneath the earthquake area,which breached the high-resistivity lithospheric mantle and stalled at the base of the crust.A petrophysical analysis showed that this lower crustal low-resistivity anomaly was most likely attributed to hydrated partial melting,which could release water into the lower crust during later magma emplacements.While weakening the strength of the FZF,these ascending fluids also increased the pore pressure in the fault,further reducing the shear strength of the fault.Shear stress action(a fault strike component of the east-west regional compress),together with possible near-surface disturbances,may drive the fault to slip and trigger the earthquakes in Songyuan.It is possible that the continuous replenishment of fluids from the deeper mantle forces the Songyuan earthquakes into the form of swarms.We infer that the Songyuan earthquakes could be attributed to a combination of preexisting faults,regional stress,and deep fluids associated with plate subduction,and near surface disturbances might induce the earthquakes in advance.The Songyuan earthquakes are inherently induced earthquakes,fed by deep fluids.

Evaluation of fault stability and seismic potential for Hutubi underground gas storage due to seasonal injection and extraction

The Hutubi gas field was put into production in 1998 and then converted into an underground gas storage(UGS)facility in 2013,and since then a cluster of earthquakes associated with seasonal injection and extraction activities have been recorded nearby.To evaluate the fault stability and seismic potential,we established a pseudo-3D geomechanical model to simulate the process of seasonal injection and extraction.Reservoir pore pressures from 1998 to 2019 were obtained through multiphase reservoir simulation and validated by history matching the field injection and production data.We then imported pore pressures into the geomechanical model to simulate the poroelastic perturbation on faults for over 20 years.The fidelity of this model was validated by comparing the simulated surface deformation with global positioning system(GPS)measured data.We used Coulomb failure stress(CFS)as the indicator for the likelihood of fault slippage.The simulation results show that the location of the induced earthquake cluster was within the positive Coulomb stress perturbation(DCFS)area,in which fault slippage was promoted.In addition,DCFS at the earthquake location kept increasing after the injection began.These findings could explain the induced earthquakes with the Coulomb failure stress theory.Furthermore,we conducted a parameter sensitivity study on the dominant factors such as the maximum operating pressure(MOP),frictional coefficient,and dip angle of the pre-existing fault.The results indicate that the magnitude of DCFS caused by seasonal injection and extraction decreases with distance;MOPs are constrained to 32.9,36.2,and 39.5 MPa according to different DCFS thresholds;the critical dip angle ranges are 0-20°and 80°-100°;and strengthening the fault friction can either increase or decrease the seismic potential.This study can help determine the MOP for Hutubi underground gas storage(HTB UGS)and provide a framework for simulating the potential causes of induced seismicity for other sites.

Numerical Analysis on the Influence of Thickness of Liquefiable Soil on Seismic Response of Underground Structure

Seismic response of underground structure in liquefiable soils was analyzed by means of fully coupled dynamic finite element method.The soils were simulated by a cyclic mobility constitutive model,which is developed at the base of modified Cam-Clay model with some concepts such as stress induced anisotropy,overconsolidation and structure.It is verified that the constitutive model can perfectly described the dynamic character of both liquefiable sand and non-liquefiable clay.Special emphasis was given for the influence of thickness of liquefiable soil on the seismic response.Results showed that soils at both sides of the structure flowed toward the bottom of the underground structure with the occurrence of liquefaction,which led to the uplift of structure.The uplift of underground structure increased with the increasing of thickness of liquefiable soils.