Reservoir-induced Earthquakes Research Articles

Landslide hazard cascades can trigger earthquakes.

While earthquakes are well-known to trigger surface hazards and initiate hazard cascades, whether surface hazards can instead trigger earthquakes remains underexplored. In 2018, two landslides on the Tibetan plateau created landslide-dammed lakes which subsequently breached and caused catastrophic outburst floods. Here we build an earthquake catalog using machine-learning and cross-correlation-based methods which shows there was a statistically significant increase in earthquake activity (local magnitude ≤ 2.6) as the landslide-dammed lake approached peak water level which returned to the background level after dam breach. We further find that ~90% of the seismicity occurred where Coulomb stress increased due to the combined effect of direct loading and pore pressure diffusion. The close spatial and temporal correlation between the calculated Coulomb stress increase and earthquake activity suggests that the earthquakes were triggered by these landslide hazard cascades. Finally, our Coulomb stress modeling considering the properties of landslide-dammed lakes and reservoir-induced earthquakes globally suggests that earthquake triggering by landslide-dammed lakes and similar structures may be a ubiquitous phenomenon. Therefore, we propose that earthquake-surface hazard interaction can include bidirectional triggering which should be properly accounted for during geological hazard assessment and management in mountainous regions.

A Comprehensive Numerical Model for Reservoir-Induced Earthquake Risk Assessment.

The assessment of seismic risk and the prevention of earthquake occurrences during reservoir operation present significant challenges in terms of accurate determination. This study aims to address this issue by developing a numerical model. The primary objective is to estimate the vulnerability of different fault types to reservoir impoundment. This model integrates essential parameters such as fault dip and the relative orientation between the reservoir and potential earthquakes, and it is structured within a risk framework using polar coordinates. Through comprehensive computations, we evaluate the alterations in elastic stress and fluid pore pressure resulting from water impoundment. This is achieved by employing a fully coupled two-dimensional poroelastic approach. Furthermore, our model incorporates relevant seismic data to enhance its accuracy. The findings of our study underscore that the critical factor lies in the fault's precise positioning with respect to the reservoir. The risk associated with a fault is contingent upon both its location and its orientation, emphasizing the importance of these factors in determining hazardous zones.

Dynamic damage evolution of bank slopes with serrated structural planes considering the deteriorated rock mass and frequent reservoir-induced earthquakes

To investigate the dynamic damage evolution characteristics of bank slopes with serrated structural planes, the shaking table model test and the numerical simulation were utilized. The main findings indicate that under continuous seismic loads, the deformation of the bank slope increased, particularly around the hydro-fluctuation belt, accompanying by the pore water pressure rising. The soil pressure increased and then decreased showed dynamic variation characteristics. As the undulation angle of the serrated structural planes increased (30°, 45°, and 60°), the failure modes were climbing, climbing-gnawing, and gnawing respectively. The first-order natural frequency was used to calculate the damage degree (Dd) of the bank slope. During microseisms and small earthquakes, it was discovered that the evolution of Dd followed the “S” shape, which was fitted by a logic function. Additionally, the quadratic function was used to fit the Dd during moderately strong earthquakes. Through the numerical simulation, the variation characteristics of safety factors (Sf) for slopes with serrated structural planes and slopes with straight structural planes were compared. Under continuous seismic loads, the Sf of slopes with straight structural planes reduce stalely, whereas the Sf for slopes with serrated structural planes was greater than the former and the reduction rate was increasing.

Prediction and verification of earthquakes induced by the Xiluodu hydropower station reservoir

Research has been conducted on reservoir-induced earthquakes in China since the Xinfengjiang reservoir-induced earthquakes in the 1960s. Regulations now require the risk of reservoir-induced earthquakes to be evaluated in the pre-research stage of all hydropower projects. Although nearly 40 cases of reservoir-induced earthquakes have been reported in China, analyses comparing the changes in seismic activity following reservoir impoundment with predictions are rare. In this study, we compared seismic activities observed in the reservoir area before and after the impoundment of the Xiluodu hydropower station in terms of the spatial distribution, frequency, and focal depths of the earthquakes, and clarified the correlation between their frequency/timing and reservoir level after impoundment. We then concluded that the seismic events in the head region were karst-type earthquakes, while those in the second segment of the reservoir were tectonic earthquakes. The spatial distribution of the earthquake epicenters and the seismic intensities validated some of the results for the reservoir-induced seismic risk assessment for the Xiluodu hydropower station, indicating that the proposed earthquake triggers and predictive models are reasonable. This study can provide a valuable reference for investigating the mechanism (s) of reservoir-induced earthquakes, revising reservoir-induced earthquake hazard assessment codes, and predicting the hazard zones of reservoir-induced seismicity under similar conditions.

Seismic activities before and after the impoundment of the Xiangjiaba and Xiluodu reservoirs in the lower Jinsha River

The lower Jinsha River basin is located at the junction of Sichuan and Yunnan provinces in Southwest China, a region with intense tectonic movements and frequent moderate to strong seismic activities. Cascade hydropower stations have been constructed along the lower Jinsha River since 2012. However, research on the effect of the impoundment of large-scale cascade reservoirs in a river basin on local seismic activities is currently lacking. Accurately identifying earthquake locations is essential for studying reservoir-induced earthquakes. Analyzing the spatiotemporal migration process of seismic activities based on complete and precise earthquake relocation is fundamental for determining the fluid diffusion coefficient, constructing fault models for reservoir areas, identifying earthquake types, exploring earthquake mechanisms, and evaluating seismic hazards. The seismicity pattern in the Xiangjiaba and Xiluodu reservoir areas, where seismic activities had been weak for a long time, has changed with the successive impoundment of the two reservoirs, showing microseismic events and seismic clusters. We investigated the spatiotemporal characteristics of seismic activities in the Xiangjiaba and Xiluodu reservoir areas using the waveform cross-correlation-based double-difference relocation technique and the b-value analysis method. We discovered that seismic events after the impoundment of these two reservoirs exhibited different characteristics in different regions. The seismic activities at the Xiluodu dam quickly responded to the rising water level, with the seismic intensity decaying rapidly afterward. These events were concentrated in the limestone strata along both sides of the Jinsha River, with a shallow focal depth, generally within 5 km, and a high b-value of approximately 1.2. Such features are close to those of karst-type earthquakes. Microseismic activities frequent occur on the eastern bank of the Yongshan reservoir section downstream of the Xiluodu dam, with two parallel NW-trending earthquake strips visible after precise earthquake relocation. The MS5.2 earthquake near Wuji town on August 17, 2014, had prominent foreshocks and aftershocks distributed in a clear NW-trending 20-km-long strip, perpendicular to the riverbank. These seismic events had a low b-value of approximately 0.7. The orientation of the node plane revealed by the strike-slip focal mechanism of the mainshock is consistent with that of the strip formed by the foreshock-mainshock-aftershock sequence, indicating the existence of a NW-striking concealed fault. Seismic activities near the Yanjin-Mabian fault upstream of the Xiangjiaba reservoir area since 2013 were concentrated in a NW-trending strip, with several near EW-trending seismic clusters on its western side, and with the largest event having a magnitude of ML3.7. So far, the impoundment of the Xiangjiaba and Xiluodu reservoirs has not triggered seismic activities on the large Jinyang-Ebian and Yanjin-Mabian faults nearby.

Modelling seismicity pattern of reservoir-induced earthquakes including poroelastic stressing and nucleation effects

SUMMARY Abnormal seismic activities near reservoirs usually show a strong spatiotemporal correlation with the water filling history. Reservoir-induced seismicity is thought to be related to crustal pore pressure and stress changes caused by water impounded behind the dams. Though the Coulomb-type stress analysis helps illuminate areas that would be under risk following reservoir impoundment, it lacks the ability to explain the temporal characteristics of reservoir-induced seismicity. We present a numerical investigation of the seismicity rate evolution of reservoir-induced earthquakes. Our modelling employs a fully coupled 2-D poroelastic model to calculate the pore pressure and stress changes caused by water impoundment and incorporates the rate- and state-dependent friction law to investigate the seismicity rate. We demonstrate that shallow earthquakes are mainly caused by pore pressure increase, while poroelastic stress transfer takes the dominant role at depth. Whether a fault would be brought close to failure depends on its geometrical properties and its relative location to the reservoir. The temporal evolution of reservoir-induced earthquakes is primarily controlled by tectonic environment instead of the diffusion of pore pressure.

The Impact of Rising Reservoir Water Level on the Gravity Field and Seismic Activity in the Reservoir Area: Evidence from the Impoundment of the Three Gorges Reservoir (China)

Using the test–retest data of the relative gravity field and earthquake monitoring catalog of the Three Gorges Reservoir (TGR) from October 2001 to October 2009, this paper systematically analyzes the changes in the gravity field in the head area of the reservoir and the temporal and spatial distribution characteristics of seismic activity during the impoundment process. It also employs the surrogate reshuffling tests to calculate the cross-correlation between the reservoir water level and the seismic activity sequence and discusses the influence of the rising reservoir water level on the gravity field and seismic activity in the reservoir region. Then, by constructing a three-dimensional finite-difference model based on the theory of fluid–solid coupling, the mechanism of reservoir-induced earthquakes is discussed from the aspects of direct reservoir water load and reservoir water infiltration. The results show that: (1) The rising reservoir water level has had a critical impact on the gravity field and seismic activity in the reservoir’s head area. The cumulative changes in the gravity field from October 2001 to November 2008 show that water impounding has led to a huge banded positive anomaly of gravity along the river near Xiangxi, which reached 450 × 10−8 ms−2. The seismicity activity dominated by micro-earthquakes after a 135 m water level rose rapidly, and the monthly average earthquake frequency increased from 2.00 before the impoundment to 92.60 after the 175 m stage. (2) From the beginning of the impoundment to the experimental impoundment stage of 175 m, the time series correlation test result between the monthly frequency of earthquakes and the water level of the reservoir also changed from uncorrelated before the water storage to correlated when the time lag was 0 months at a 95% confidence threshold. This indicates that the seismic activity obviously has a direct relationship with the load pressure produced by the rapid rise of the reservoir water level, which causes the instability of the mines, karst caves, shallow rock strata, and faults within 10 km along the river and near the reservoir bank, and consequently induces earthquakes. (3) As the TGR enters the 175 m high-level operation stage, the cross-correlation test confirmed that the seismic activity and the reservoir water level show negative correlation characteristics under the time lag of 4 to 5 months, indicating that the seismic activity has a lagging response to the reservoir water level change. The continued infiltration of the reservoir water, followed by the softening of the faults and other actions, triggered the Xiangxi M4.1 earthquake at the center of the four quadrants of gravity anomalies near Xiangxi on 22 November 2008. The Xiangxi segment of the reservoir and its periphery, a triangular geological region where the Xiannvshan faults, the Jiuwanxi fault, and the Yangtze River meet, might be at risk of having reservoir-induced tectonic earthquakes.

Lithospheric Equilibrium, Environmental Changes, and Potential Induced-Earthquake Risk around the Newly Impounded Baihetan Reservoir, China

The Baihetan hydropower station is the second largest hydropower station worldwide. It began to store water in April 2021. We conducted a dense hybrid gravity and GNSS survey at 223 stations, obtained the free-air and Bouguer gravity anomalies, inversed the lithospheric density structure, and calculated the isostatic additional force (IAF) borne by lithosphere in the reservoir area. Moreover, we studied the gravity change and Coulomb stress change around the Baihetan reservoir due to impoundment. The main findings are the following. (1) Hybrid gravity and GNSS observations significantly improved the spatial resolution of the gravity field, and the maximum improvement reached up to 150 mGal. (2) A new method for risk assessment of reservoir-induced earthquakes is proposed from the perspective of lithospheric equilibrium. It was found that there is an IAF of −30 MPa at approximately 20 km upstream of the Baihetan dam, and the risk of a reservoir-induced earthquake in this area warrants attention. (3) It was found that the Coulomb stress variation on the Xiaojiang fault near Qiaojia at a depth of 10 km exceeded the threshold for inducing an earthquake (0.1 bar).

Advances in the identification of reservoir-induced earthquakes

Due to the construction of large hydropower stations and the impoundment of reservoirs, the earthquakes occurred in or around the reservoir area are called reservoir-induced earthquakes, which have the characteristics of low magnitude, shallow source and high intensity. The construction of large reservoirs plays an important role in promoting the local economic development, improving the water conditions of the people and affecting people’s life and property. Therefore, the identification, prediction and judgment of reservoir induced earthquake is an important technical support to ensure the construction and operation safety of hydropower stations. Over the years, many scientific and technological personnel have carried out active and large amount of research. This paper reviews and sorts out the macroscopic identification of reservoir induced earthquakes, including the mechanism of reservoir induced earthquakes, the cases of reservoir induced earthquakes, and the identification of reservoir induced earthquakes. Some problems and future research directions of reservoir induced earthquakes are discussed.

Geological impact of the Three Gorges Reservoir on the Yangtze River in China

The Three Gorges Reservoir (TGR), located on the Yangtze River in China, currently holds the title as the biggest man-made water reservoir in the world. However, with its unprecedented size, construction has brought with it natural gas blowout incidents in nearby mining wells, a geological phenomenon that had not been considered before. These incidents had resulted in the loss of hundreds of lives as well as millions of dollars in properties. Through investigation, various hypotheses which suggest that the blowouts were indeed related to the reservoir are proposed. In addition, TGR’s size has made it a suitable target for quantifying and understanding various well-established side-effects that result from the creation of new reservoirs. Such ‘common’ effects that are considered include reactivated landslides and reservoir-induced earthquakes. From this, it is proposed that initial impoundment of water increases landslide susceptibility for a period, whilst being independent of rainfall. From this paper, it could be better understood how large-scale reservoirs are able to impact surrounding areas geologically.

Effective stress law for rock masses and its application in impoundment analysis based on deformation reinforcement theory

Reservoir impoundment is related to several hydraulic engineering concerns, including irreversible valley contractions, landslides and reservoir-induced earthquakes. However, these phenomena, such as valley contractions, are hardly to be explained by the conventional method. The scientific understanding of water effects during impoundment and their hazards to hydraulic structure are needed. The effective stress law for fissured rock masses is introduced in the elasto-plastic model employing the Drucker-Prager criterion and implemented in the three dimension (3D) nonlinear finite element method (FEM) program Three-dimensional FINite Element (TFINE). The slope deforms towards river-way during impoundment since the increasing pore pressure in fissures changes stress state and leads to additional plastic deformation in the rock materials. The value of Biot coefficient and the influence of water on rock materials are discussed in detail. Thus, the mechanism of slope deformation during the impoundment of Jinping-I arch dam is revealed, and the deformation is accurately measured. The application of the effective stress law provides a method to consider stress assessment, deformation evaluation and stability estimate of hydraulic structures during the impoundment process. This is a beneficial exploration and an improvement of hydraulic engineering design.

Estimation of Coulomb stress changes and induced earthquake occurrences prediction in the region of Three Gorges Reservoir, China

In the region of Three Gorges Reservoir (TGR) in China, there has been occurrence of several frequent earthquakes of moderate intensity since reservoir impounding occurred in 2003. These earthquakes are generally believed to be induced by reservoir impoundment and water-level variations. Usually, the geo-stress will change, when natural earthquakes occur. Following this principle, this paper adopted the rate and state theory to simulate and estimate Coulomb stress changes in the TGR region and obtained the pattern of Coulomb stress changes with time and the event sequence as well as the distribution of Coulomb stress changes in space. First, the TGR regional catalogue was analyzed and processed, leading to quantification of the magnitude of completeness and all of the parameters that are used in the stress–seismicity inversion process, including the reference seismicity rates, characteristic relaxation time, fault constitutive parameters, and stress rates. Second, the temporal evolution of the stress changes in different time windows was computed and analyzed, and it was found that there is an association between the Coulomb stress changes and rates of increase in the cumulative number of earthquakes. In addition, the earthquake occurred in November 2008 (M S = 4.1) was analyzed and attempted to simulate the distribution of stress changes in space through the stress–seismicity inversion model. The results proved that the modeled area coincides with the historical area of earthquakes that occurred after 2008. Finally, a prediction was made about the earthquake productivity rates after 2015, which showed a declining earthquake rate over time that ultimately returned to the background seismicity. This result is essentially in agreement with Omori’s law. To conclude, it is rational to use the stress-inversion method to analyze the relationship between induced earthquake seismicity and local stress changes as well as to simulate the area of earthquake occurrence and productivity rates of reservoir-induced earthquakes.

Influences of the Three Gorges Project on seismic activities in the reservoir area

Reservoir-induced earthquakes related with the construction of the Three Gorges Project have attracted great concerns of the public. Since the first water impoundment on May 25, 2003, a number of earthquakes have occurred during the water storage stages, in which the largest was the Badong M5.1 earthquake on December 16, 2013. In this paper, the relationships between seismic activities, b value, seismic parameters, and reservoir water level fluctuations are studied. In addition, based on the digital seismic waveform data obtained since 2000, the focal depth changes and focal mechanism characteristics before and after the water impoundment are studied as well. These provide us important information to understand the earthquake mechanisms. The results show that these earthquakes are typical reservoir-induced earthquakes, which are closely related to water infiltration, pore pressure, and water level fluctuations. The majority of the micro and small earthquakes are caused by karst collapse, mine collapse, bank reformation, superficial unloading, and so on. The larger earthquakes are related to the fault structures to some extent. Due to the persistent effects of water impoundment on the seismic and geological environments around the reservoir and water infiltration into the rocks, the influences on the crustal deformation field, gravity field, seepage field, and fault medium-softening action may vary gradually from a higher strength to a weaker one. Therefore, it is possible that small earthquakes and few medium earthquakes (M≤5.5) will occur in the reservoir area in the future.

Dynamic stability analysis of the bedding rock slope considering the vibration deterioration effect of the structural plane

In traditional dynamic stability analysis of the rock slope, more attention has been given to the influence of the dynamic load, but less attention to the vibration deterioration effect of the rock structural plane subjected to cyclic shear. Based on previous research results, a mathematical model was developed to calculate the vibration deterioration effect of the structural plane, which was further applied to the dynamic stability analysis of the rock slope in this paper. The process of vibration deterioration was realized by programming with FISH programming language in FLAC3D, and the numerical calculation method was adopted to analyze the slope stability during the earthquake and the long-term stability of the slope subjected to frequent microseisms. The results show that the vibration deterioration effect of the structural plane is more obvious with the enlargement of seismic load. Thus, it is more necessary to consider the vibration deterioration effect for dynamic stability analysis of the slope subjected to strong earthquakes. Permanent reduction of the shear strength of the slope sliding plane could even be caused by a microseism due to the vibration abrasion of the initial fluctuation angle. Therefore, influence caused by frequent microseisms on the long-term stability of the slope cannot be ignored. The probability statistics method was used to process the time-history curve of slope stability coefficient, which led overall evaluation index of the slope stability to be associated with the reliability theory. Research results in this paper can provide some reference to analyze the dynamic stability during the strong earthquakes and the long-term stability under frequent microseisms (e.g. reservoir-induced earthquakes and aftershocks) for the bedding rock slope.

Evaluation of Seismic Events Occurred after Filling and Drawdown of the Reservoir at Song Tranh 2 HPP in Vietnam

Reservoir-induced earthquakes are a challenging issue for hydropower, and have occurred at many sites around the world. However, each event is unique in itself and depends on the geo-tectonics and geo-hydrology of the area in which the event is situated. This article focuses on seismic events at the Song Tranh 2 hydropower project located in Quang Nam province, Vietnam. The construction of the 96 meter high Roller Compacted Concrete (RCC) dam of this project was completed in August 2011. Approximately one year after commissioning, the dam began experiencing a serious leakage problem through the dam body. In addition, a series of earthquakes occurred near the project area, and continued for several months. The high intensity and magnitude of the earthquakes caused damage to the project and promoted fear among the local people living in the downstream valley. The issues drew significant media attention and thousands of articles about this project were written within a short time. As a result, dam authorities have been subject to extreme public pressure.This paper describes the earthquake events and difficult situation that both the local population and authorities faced in its aftermath. In addition, we analyze seismic events qualitatively,using data and information on the water filling and drawdown processes. Our analysis provides an insight into these seismic events, as we reconstruct the earthquake scenarios and test a hypothesis of earthquake occurrence. Future earthquake activities are also predicted and compared.DOI: http://dx.doi.org/10.3126/hn.v15i0.11285HYDRO Nepal JournalJournal of Water, Energy and EnvironmentVolume: 15, 2014, JulyPage: 16-20

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 seismicity before impoundment and that the epicenters were concentrated in the east and west sides of the Zigui Basin, most of which were natural tectonic earthquakes. 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 M s2. 0. Therefore, the intensity of the earthquakes 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-induced 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.

Discrimination of earthquakes and explosions using multi-fractal singularity spectrums properties

A new method of discrimination of seismic records from earthquakes and explosions is proposed which is based on using properties of their multi-fractal singularity spectrums. The efficiency of the method is illustrated by analysing seismic records from the region of Aswan Dam in Egypt. Current pattern of seismicity in the Upper Egypt is composed by three types of records: tectonic earthquakes, reservoir-induced earthquakes and seismic events generated by quarry blasts. To discriminate quarry blasts from earthquakes of both natures, multi-fractal analysis of records were used. Singularity spectrum support width and multi-fractal generalised Hurst exponents were calculated for all seismic events in the selected data set from a given area. The linear Bayesian discriminator using these characteristics of seismic records provides correct classification for 93 % of earthquakes records and for 99 % of quarry blasts records.

Modelling of reservoir-induced earthquakes: a multivariate adaptive regression spline

This paper employs a multivariate adaptive regression spline (MARS) for the prediction of the maximum magnitude (M) of reservoir-induced earthquakes based on reservoir parameters. MARS is a non-parametric adaptive regression procedure. It has the capability to determine the important sequence of inputs for the output. The comprehensive parameter (E) and maximum reservoir depth (H) are used as inputs for the MARS. The developed MARS gives an equation for the prediction of M. A comparative study is carried out between the developed MARS and other models and the results show that the developed MARS is a robust model for the prediction of M.

Multi-scale decomposition of point process data

To automatically identify arbitrarily-shaped clusters in point data, a theory of point process decomposition based on kth Nearest Neighbour distance is proposed. We assume that a given set of point data is a mixture of homogeneous processes which can be separated according to their densities. Theoretically, the local density of a point is measured by its kth nearest distance. The theory is divided into three parts. First, an objective function of the kth nearest distance is constructed, where a point data set is modelled as a mixture of probability density functions (pdf) of different homogeneous processes. Second, we use two different methods to separate the mixture into different distinct pdfs, representing different homogeneous processes. One is the reversible jump Markov Chain Monte Carlo strategy, which simultaneously separates the data into distinct components. The other is the stepwise Expectation-Maximization algorithm, which divides the data progressively into distinct components. The clustering result is a binary tree in which each leaf represents a homogeneous process. Third, distinct clusters are generated from each homogeneous point process according to the density connectivity of the points. We use the Windowed Nearest Neighbour Expectation-Maximization (WNNEM) method to extend the theory and identify the spatiotemporal clusters. Our approach to point processes is similar to wavelet transformation in which any function can be seen as the summation of base wavelet functions. In our theory, any point process data set can be viewed as a mixture of a finite number of homogeneous point processes. The wavelet transform can decompose a function into components of different frequencies while our theory can separate point process data into homogeneous processes of different densities. Two experiments on synthetic data are provided to illustrate the theory. A case study on reservoir-induced earthquakes is also given to evaluate the theory. The results show the theory clearly reveals spatial point patterns of earthquakes in a reservoir area. The spatiotemporal relationship between the main earthquake and the clustered earthquake (namely, foreshocks and aftershocks) was also revealed.

Finite element investigation of the poroelastic effect on the Xinfengjiang Reservoir-triggered earthquake

Coulomb failure stress changes (ΔCFS) are used in the study of reservoir-induced seismicity (RIS) generation. The threshold value of ΔCFS that can trigger earthquakes is an important issue that deserves thorough research. The Ms6.1 earthquake in the Xinfengjiang Reservoir in 1962 is well acknowledged as the largest reservoir-induced earthquake in China. Therefore, it is a logical site for quantitative calculation of ΔCFS induced by the filling of the reservoir and for investigating the magnitude of ΔCFS that can trigger reservoir seismic activities. To better understand the RIS mechanism, a three-dimensional poroelastic finite element model of the Xinfengjiang Reservoir is proposed here, taking into consideration of the precise topography and dynamic water level. We calculate the instant changes of stress and pore pressure induced by water load, and the time variation of effective stresses due to pore water diffusion. The ΔCFS on the seismogenesis faults and the accumulation of strain energy in the reservoir region are also calculated. Primary results suggest that the reservoir impoundment increases both pore pressure and ΔCFS on the fault at the focal depth. The diffusion of pore pressure was likely the main factor that triggered the main earthquake, whereas the elastic stress owing to water load was relatively small. The magnitude of ΔCFS on seismogenesis fault can reach approximately 10 kPa, and the ΔCFS values at the hypocenter can be about 0.7–3.0 kPa, depending on the fault diffusion coefficient. The calculated maximum vertical subsidence caused by the water load in the Xinfengjiang Reservoir is 17.5 mm, which is in good agreement with the observed value of 15 mm. The accumulated strain energy owing to water load was only about 7.3×1011 J, even less than 1% of the seismic wave energy released by the earthquake. The reservoir impoundment was the only factor that triggered the earthquake.