Tensile Shear Cracks Research Articles

Study on Crack Propagation and Coalescence in Fractured Limestone Based on 3D-DIC Technology

To deeply understand the influence of crack inclination angle on crack propagation and coalescence in fractured limestone, uniaxial compression tests were carried out on limestone specimens with prefabricated cracks. The strain field evolution diagram of the failure process of the specimens was obtained using 3D digital image correlation technology (3D-DIC technology). This, in combination with the crack propagation diagram, was used to analyze the entire failure process of the limestone specimens. The test results show that the evolution process of the principal strain field agrees well with the process of crack initiation, propagation, and coalescence. The crack development process is the process of the high strain zones consistently propagating and also the process of micro-cracks appearing, developing, and nucleating to form macro-cracks. With the increase in the parallel crack inclination angle, the stress concentration zone of the intermediate crack transfers from both ends of the crack to the middle. Meanwhile, the coalescing crack type between the parallel crack and the intermediate crack changes from a coexisting tensile crack and tensile-shear crack to a single tensile crack. With the increase in the parallel crack inclination angle, the failure of the fractured limestone specimens changes from simple splitting or tensile failure to the coexistence of tensile-shear fracture and splitting. 3D-DIC technology provides an effective method to study crack propagation and coalescence during rock failure.

Experimental and DEM study on failure behavior and stress distribution of flawed sandstone specimens under uniaxial compression

Rock materials contain various scales of pre-existing flaws that greatly influence the stability of rock engineering. The study of strength and failure behavior of pre-flawed rock enhances the understanding the fracture mechanism of rock. In the present study, uniaxial compression tests were carried out on sandstone specimens containing three pre-existing flaws at various inclination angles. The experiment results indicated that the stress–strain curves, mechanical paprameters, acoustic emission and failure pattern of pre-flawed specimen were affected by the pre-existing flaws. To verify the test results, a PFC modelling was conducted on pre-flawed specimen using a set of calibrated parameters. The stress–strain curve, mechanical paprameters and failure mode of specimen containing three flaws were reproduced in the numerical simulation. Three types of crack, i.e., tensile crack, shear crack and mixed tensile-shear crack, were identified based on the laboratory micro observations and numerical displacement field. As the flaw angle increased from 0° to 90°, the tensile stress concentration gradually deteriorated and transferred from the middle to the tip of pre-existing flaw. The evolution of internal stress surrounding the pre-existing flaw during the crack initiation, propagation and coalescence process was further discussed. This experimental and numerical study aims to investigate the crack characteristic and internal stress evolution in fractured rock.

Experimental Study on Dynamic Mechanical Properties and Fracturing Behavior of Granite CSTFBD Specimens in SHPB Tests

Abstract The purpose of this paper is to study the dynamic fracture mechanics characteristics and fracturing behavior of cracked-straight-through flattened Brazilian disk specimens using the Split Hopkinson Pressure bar system. The researchers also used Particle Flow Code 2D for simulating the crack process and achieved consistent outcomes. In this experiment, rock specimens were made from Beijing Fangshan granite. The results of the experiment determined the effects of impact pressures and loading angles on the crack mode and mechanical properties of the specimens. These specimens have attained the stress equilibrium state prior to failure. With greater impact pressure, there will be faster changes in the corresponding equivalent tensile stress, while the peak value remains relatively high under the same loading angle. With different loading angles, the change rate of the equivalent tensile stress remains the same; however, the time required to reach the maximum stress becomes different. For a 30° loading angle, the energy dissipation ratio in accordance with the same impact pressure is lower than compared to that of a 0° loading angle. In this experiment, several cracks have been produced under a different range of impact pressures and loading angles, which can be classified into six types. They include two types of tensile cracks, shear cracks, and tensile-shear cracks. At a 0° loading angle, mode I fracture occurs, and at 15° and 30° loading angles, a mixed-mode I-II fracture occurs on the specimen. Furthermore, triangular fracture zones occur on the two end faces of the specimens under the loading force due to the concentration of local stress during the force process.

Cracking feature and mechanical behavior of shield tunnel lining simulated by a phase-field modeling method based on spectral decomposition

Disturbed by the surrounding engineering construction, the shield tunnel lining may appear mixed-mode cracks such as segment cracking and joint crushing, which seriously damage its mechanical behaviors. To simulate the shield tunnel lining with mixed-mode cracks, a phase-field modeling method based on spectral decomposition is proposed, which unifies the critical energy release rate and crack-driving strain energy. The indoor full-scale tests are carried out to study the failure mechanism of the segment and joint. The phase-field modeling method is verified by the tests, and the influence of loading path on the cracking feature and mechanical behavior of the segment and joint are analyzed by the proposed numerical method. The test and numerical simulation indicate that the crack in the segment gradually develops from I-type crack (tensile crack) to Y-type crack (tensile-shear crack), and the joint is crushed due to local compression, and the crushed region expands gradually with the increase of the eccentricity. As the eccentricity decreases, the ultimate bearing capacity of the segment and joint is improved, while the ductility decreases. Then, the phase-field modeling method is extended to the stratum-structure model. As the surface load increases, the joints of the shield tunnel lining are crushed, and the top and bottom segments are cracked. Meanwhile, the bending moment is reduced due to the redistribution of internal forces caused by the segment cracking and joint crushing.

Biaxial Shear Crack Propagation Modes of Rock-Like Specimens with Prefabricated Fissures and Their Strength Characteristics

A biaxial shear test is performed on prefabricated, single-fissure type, cubic rock-like specimens by using the TZW-500 rock direct shear apparatus to study the shear strength characteristics, crack coalescence, and propagation modes of the specimens with different geometric parameters. Results show that the crack coalescence and propagation modes of the rock-like specimens with prefabricated fissures can be divided into four types, namely, single main shear crack coalescence mode, main shear crack coalescence and secondary tensile-shear crack propagation mode, main shear crack coalescence and secondary shear crack propagation mode, and main shear crack coalescence and secondary tensile crack propagation mode. All modes are affected by the dip angle α and length l of the prefabricated fissure. When the dip angle of the prefabricated fissure is α∈[0°, 20°) or (70°, 90°], the cracks center on shear failure, and most shear cracks propagate along one end of the prefabricated fissure. At α∈(30°, 50°), the cracks bear the tensile-shear combined action, and the shear cracks propagate along the two ends of the prefabricated fissure. The peak shear strength of the rock-like specimens with prefabricated fissures is also closely related to the dip angle α and length l of the fissure. With the increase in dip angle α of the prefabricated fissure, the peak shear strength of each rock-like specimen decreases initially then increases, and the peak shear strength curve presents a similar “U” shape. At α∈[30°, 60°], the peak shear strength is within the peak-valley interval. When the length l of the prefabricated fissure is increased, the peak shear strength experiences a gradual reduction. When l > 20 mm, the peak shear strength is greatly influenced by l, but the influence is minimal when l ≥ 20 mm. At the same dip angle α and fissure length of l ≥ 20 mm, the correlation between peak shear strength and fissure width b is low.

Study of the Failure Mode of a Jointed Rock Mass due to a Stress Wave

The mechanical response and failure process of a jointed rock mass subjected to dynamic loading is very important for the safety and stability of rock engineering projects. In this study, we use RFPA2D-Dynamic, a rock dynamic failure process analysis platform, to establish a two-dimensional impact model of a jointed rock mass to analyze the mechanism of crack propagation in a jointed rock mass with preexisting cracks under dynamic loading. We discuss the influence of the stress wavelength and precrack inclination on the dynamic failure process and mode of the rock mass and compare this failure process with the failure model under static loading. The results show that the dynamic failure process and crack initiation type of a jointed rock mass are closely related to the stress wavelength. For a given peak, as the stress wavelength increases, the failure mode changes from local cracking that occurs above the precracks to a global instability caused by wing cracks. Meanwhile, as the wavelength increases, the shear cracks and mixed tensile-shear cracks generated at the two ends of the precracks are replaced by tensile cracks. The precrack inclination on a jointed rock mass mainly affects the strength of the jointed rock mass and the final failure mode. Specifically, when the joint inclination is small, the rock mass is severely damaged in the region above the precracks because the stress wave forms a region of cracks with a concentrated distribution. As the joint inclination increases, the damaged region becomes larger while the rock mass is less prone to failure; the strength of the rock mass gradually increases, and the wing cracks produced at the two ends of precracks propagate toward the upper and lower ends of the rock mass. However, when the stress wavelength is small, the precracks of different inclinations form cracks in the region above the precracks with a length similar to the precracks. For this condition, the propagation of the cracks is mainly controlled by the stress wavelength, while the influence of the inclination of the precracks is not significant. There is a significant difference between the failure modes of a rock specimen under dynamic loading or static loading because the stress wave produces a reflected tension wave in the direction parallel to the wave attack of the joint plane, which leads to spalling, while the wing cracks are more likely to occur under static loading.

Spatio-temporal characteristics and focal mechanisms of deep low-frequency earthquakes beneath the Zao volcano, northeastern Japan

Deep low-frequency earthquakes (DLFs) beneath volcanoes are possible evidence for deep-seated magmatic activity in the mid-to-lower crust and uppermost mantle. After the 2011 Tohoku-oki earthquake (Mw 9.0), the number of DLFs beneath the Zao volcano in northeastern Japan began to increase. The hypocenters of the DLFs formed two clusters at shallower (20–28 km) and deeper (28–38 km) depths located near the side and lower parts of the high Vp/Vs zone, respectively. To understand the distribution and behavior of deep volcanic fluids, we examined the spatio-temporal characteristics and focal mechanisms of the DLFs beneath the Zao volcano. Systematic event detection and hypocenter determination based on waveform correlations were performed, which indicated that the activity of the deeper cluster intensified prior to the activation of DLFs in the shallower cluster. In addition, the focal mechanisms based on the averaged S/P wave power spectral ratios of the DLFs grouped by correlation were estimated. We found that the focal mechanisms of all DLF groups had a dominant isotropic component and that the maximum principal axis direction differed slightly from the regional stress field. The source process suggests a chain of tensile-shear cracks with different orientations, which were affected by the local stress field. Our results indicate that deep magma at different depths interacted on the time scale of the magmatic processes other than pressure diffusion.

Failure characteristics of surrounding rocks along the radial direction of underground excavations: An experimental study

For the in situ surrounding rocks of underground excavations, the radial stress corresponding to the minimum principal stress σ3 varies with increasing distance away from the excavated boundary. To characterize the rock failure along the radial direction of an excavation, true triaxial compression tests with different σ3 values (0.5, 5, 10, 15, 20, 25, and 30 MPa) were performed. The characteristic stresses (including the crack initiation stress σi, crack damage stress σd, and peak stress σs) and failure mode of the tested specimen under different σ3 conditions were systematically investigated. The experimental results indicate that with increasing σ3, the characteristic stresses increase monotonically and continuously. The increasing rate of the characteristic stresses with σ3 ranging from 0 to 10 MPa is higher than that with σ3 ranging from 10 to 30 MPa, indicating that low σ3 impose a greater effect on the characteristic stresses than high σ3. In addition, the failure mode of the tested specimen changes from tensile-shear failure mode I (with vertical tensile cracks and oblique tensile-shear cracks developing along the σ2 direction) to tensile-shear failure mode II (with oblique tensile-shear cracks developing only along the σ2 direction) to tensile-shear failure mode III (with oblique tensile-shear cracks along both σ2 and σ3 directions) as σ3 increases. Then, the failure characteristic of the surrounding rocks of the underground excavation was characterized based on the obtained experimental results. When maximum radial stress σr encountered near the excavated boundary is much smaller than the axis stress σa, the surrounding rocks will exhibit a “Ternary failure characteristic”, i.e., with an increase in the distance from the excavated boundary along the radial direction, the failure mode of the surrounding rocks after excavation changes from tensile/splitting failure mode to tensile-shear failure mode I and then to tensile-shear failure mode II. When the maximum σr encountered near the excavated boundary approximates the original σa, the surrounding rock masses will exhibit a “Quaternary failure characteristic”. Specifically, the corresponding failure mode along the radial stress direction away from the excavated boundary changes from tensile failure mode to tensile-shear failure mode I then to tensile-shear failure mode II and finally to tensile-shear failure mode III.

Experimental and numerical investigations on crack development in 3D printed rock-like specimens with pre-existing flaws

Rock masses contain various discontinuities such as flaws, cracks, join sets and more, each significantly affect the crack initiation, propagation and coalescence patterns of new cracks which dominates the ultimate failure of geostructures. The interaction of the pre-existing flaws with each other and with the newly formed cracks is complicated which demands a comprehensive look into the phenomenon. This paper focuses on the applicability of the 3D printing technology coupled with the digital image correlation (DIC) and the bonded particle model (BPM) in replicating real behaviour of natural rocks containing pre-existing flaws. Systematic flaws configurations are considered including single, coplanar, partially overlapped and fully overlapped arrangements to enable a comprehensive coalescence analysis. The results show that the BPM is capable of precisely modelling the crack initiation location, type (tensile, shear or mixed-mode) and coalescence type detected by the DIC. Moreover, it is shown that the 3D printed specimens can reproduce the previously identified cracks and coalescence types. By the analysis of the displacement fields, a new coalescence type is detected and introduced and named as type X, which is a mix of wing crack and quasi coplanar secondary shear crack. The analysis of the displacement vectors revealed five types of cracks: shear opposite slip crack, tensile opposite crack, tensile compliant crack, shear compliant crack, and mixed tensile-shear crack. Furthermore, the comparisons made between the peak loads of the experiments and the simulations show a god agreement in terms of peak load magnitude and flaws inclination angle dependency.

Cracking process and acoustic emission characteristics of sandstone with two parallel filled-flaws under biaxial compression

Discontinuities widely exist in natural rocks. To investigate the progressive micro-cracking process and failure mechanism of fissured rocks, a series of biaxial compression tests were conducted on sandstone specimens containing two parallel filled flaws using acoustic emission (AE) analysis synchronized with digital image correlation (DIC) monitoring. Experimental results show that the peak strength and elastic modulus of sandstone decrease first and then increase with the change in the ligament angle from 0° to 150°, achieving a minimum at 60°. The flaws remarkably facilitate crack coalescence under low lateral stress, such as at 2.5 MPa and 5 MPa. However, with an increase in lateral stress to 10 MPa, the crack coalescence is less influenced by the presence of pre-existing flaws. The AE events produced by flawed sandstone during the loading process conform to the Hurst statistical law. Fractal analysis shows that the lateral confinement reduces the irregularity of ultimate fracture geometry. Based on the AE dominant frequency features, the micro-tensile cracks, micro-shear cracks and micro-tensile-shear cracks are distinguished. The results show that with an increase in lateral stress, the percentage of micro-tensile cracks are constrained, but the number of micro-shear and mixed tensile-shear cracks increases. In addition, the micro-shear cracks preferentially appear in flawed sandstone specimens under high lateral stress as compared with specimens subjected to low stress.

Modeling the entire progressive failure process of rock slopes using a strength-based criterion

To simulate the entire process of the progressive failure of rock slopes, a series of techniques are incorporated into the original NMM (numerical manifold method). To reflect the stress concentration near the crack tip, the Williams' displacement series is adopted to enrich the global displacement function of the NMM. Furthermore, the most recently proposed strength-based LT criterion, which can account for tensile cracks, tensile-shear cracks and compressive-shear cracks, is adopted to determine the crack propagation direction and length. Three typical numerical examples, including a Mode-I crack problem, a Mode-II crack problem and a Brazilian disc problem are adopted to verify the numerical model. The numerical results indicate that the numerical model is capable of accurately simulating the Mode-I crack propagation problem, the mode-II crack propagation problem and the failure process of Brazilian disc. Furthermore, the numerical model is adopted to investigate the entire progressive failure process of two rock slopes. The corresponding results indicate that the numerical model can not only simulate the propagation and coalescence of multiple cracks in rock masses but also the opening/sliding of rock blocks along discontinuities. The proposed numerical model warrants further investigation.

Seismic amplification of the anti-dip rock slope and deformation characteristics: A large-scale shaking table test

This paper studied the dynamic response and failure characteristics of the anti-dip rock slope through a large-scale shaking table test. An anti-dip rock slope model with a length of 2.2m, width of 0.8m and height of 1.6m has been constructed in a rigid model box. The similarity law and dimensional analysis have been applied to determine similarity relations and loading conditions. The results indicated that the acceleration was amplified with the increase of the model elevation and shaking intensity. The upper slope showed an exceedingly strong response to horizontal acceleration, meanwhile the lower part showed a stronger response to vertical acceleration. Both the horizontal and vertical accelerations sharply increased at shallow depths of the slope surface. Rock mass dislocations occurred at the slope crest during the initial deformation stage, and subsequently downward toppling took place at the slope crest. Tensile-shear cracks presented in upper slope and went through the model blocks, and intense acceleration responses were recorded where the rock mass was severely deformed. Finally a curved-linear sliding surface was formed. It is notable that shallow landslide was not accidental phenomenon in the lab experiment.

Crack Initiation and Coalescence Behavior of Two Non-parallel Flaws

Crack initiation and coalescence behavior of rock or rock-like specimens containing artificial flaws under uniaxial compression have been subjects of intensive investigation in the past. Most of these investigations however focused on crack initiation and coalescence between two or more parallel flaws. Although there have been few experimental studies on non-parallel flaws, these studies did not address the influence of geometrical factors such as ligament length and ligament angle on the crack initiation and coalescence behavior of non-parallel flaws. To investigate whether the individual geometrical factors have similar effects on the crack initiation and coalescence behavior of both parallel and non-parallel flaws, we conducted uniaxial compression tests to investigate crack cracking and coalescence processes in rock like material containing two non-parallel flaws. The paper presents the influence of individual geometrical factors on the crack initiation process and coalescence pattern of non-parallel flaws. Initiation of primary first cracks from all the tips of the two flaws did not occur simultaneously in all the flaw configurations. The flaw configuration of the non-parallel flaws influences the crack initiation, crack trajectories and coalescence behavior. The crack coalescence pattern changes with an increasing ligament angle from indirect to shear crack or mixed tensile-shear crack to tensile crack coalescence. The chance of direct coalescence is reduced with an increase in ligament length. In conclusion, the crack initiation and coalescence behavior of prismatic rock-like specimens with non-parallel flaws, as influenced by the geometrical factors, are analogous to the cracking and coalescence pattern observed in specimens with parallel flaws.

Experimental study on cracking behaviour of moulded gypsum containing two non-parallel overlapping flaws under uniaxial compression

Failure of rock mass that is subjected to compressive loads occurs from initiation, propagation, and linkage of new cracks from preexisting fissures. Our research investigates the cracking behaviour and coalescence process in a brittle material with two non-parallel overlapping flaws using a high-speed camera. The coalescence tensile crack and tensile wing cracks were the first cracks to occur from the preexisting flaws. The initiation stresses of the primary cracks at the two tips of each flaw were simultaneous and decreased with reduced flaw inclination angle. The following types of coalescence cracks were identified between the flaws: primary tensile coalescence crack, tensile crack linkage, shear crack linkage, mixed tensile-shear crack, and indirect crack coalescence. Coalescence through tensile linkage occurred mostly at pre-peak stress. In contrast, coalescence through shear or mixed tensile-shear cracks occurred at higher stress. Overall, this study indicates that the geometry of preexisting flaws affect crack initiation and coalescence behaviour.

Crack coalescence between two non-parallel flaws in rock-like material under uniaxial compression

Crack coalescence between parallel flaws has been extensively studied in brittle rock and rock-like materials. Due to the nature of rock masses that contain more than one joint set, the cracking process cannot be completely studied using specimens that contain parallel flaws. To address this area of research, crack coalescence between two non-parallel flaws is studied numerically using parallel bonded-particle models in which one flaw does not overlap or partially overlaps the other (varying α) and in which one flaw completely overlaps the other (varying β). Five types of linkage are observed between two flaws: tensile crack linkage, tensile crack linkage with shear coalescence at tip, shear crack linkage, mixed (tensile-shear crack) linkage and indirect crack linkage. The geometries of the two non-parallel flaws strongly influence the crack trajectories and coalescence patterns. At large angles of α (135°) and β (60°), coalescence occurs more easily by tensile crack(s) before the peak stress is reached. The stress distribution in bridge area of the non-parallel flaws is more complicated than that of the parallel flaws. This difference affects the stress for crack initiation as well as the pattern for coalescence.

AE Characterization of Cracking Mode in Sandstone Uniaxial and Brazilian Tests

Laboratory results from sandstone Brazilian splitting tests and uniaxial compression tests based on acoustic emission (AE) monitoring indicated that the acoustic emission parameters analysis method can be applied to analyse the characteristics of acoustic emission and to classify the crack modes in rock materials. It concluded that more than 99 per cent of the whole cracking signals in Brazilian tests were classified as tensile mode, and no shear cracks occurred. And more than 65 per cent of the AE signals in uniaxial tests were tensile-shear crack mode, along with about 30 percent of tensile mode and 5 percent of shear mode, and shear cracks only occurred in the unstable crack extension stages; tensile-shear cracks are the main crack modes in the crack stable extension stage.

Cracking process of rock mass models under uniaxial compression

Anisotropic strength and deformability of the rock mass with non-persistent joints are governed by cracking process of the rock bridges. The dependence of cracking process of jointed rock masses on the two important geometrical parameters, joint orientation and joint persistence, was studied systematically by carrying out a series of uniaxial compression tests on gypsum specimens with regularly arranged multiple parallel pre-existing joints. According to crack position, mechanism and temporal sequence, seven types of crack initiations and sixteen types of crack coalescences, were identified. It was observed that both tensile cracks and shear cracks can emanate from the pre-existing joints as well as the matrix. Vertical joints were included and coplanar tensile cracks initiation and coalescence were observed accordingly. For specimen with joint inclination angle β=75°, it was found that collinear joints can be linked not only by coplanar shear cracks but also by mixed tensile-shear cracks, and that a pair of them can form a small rotation block. Seven failure modes, including axial cleavage, crushing, crushing and rotation of new blocks, stepped failure, stepped failure and rotation of new blocks, shear failure along a single plane and shear failure along multiple planes, were observed. These modes shift gradually in accordance with the combined variation of joint orientation and joint persistence. It is concluded that cracking process and failure modes are more strongly affected by joint orientation than by joint persistence, especially when joint inclination angle is larger than 45°. Finally, variations of macroscopic mechanical behaviors with the two geometrical parameters, such as patterns of the complete axial stress-axial strain curves, peak strength and elastic modulus, are summarized and their mechanisms are successfully explained according to their different cracking process.

Mechanism of High Frequency Shallow Earthquake Source in Mount Soputan, North Sulawesi

DOI: 10.17014/ijog.v6i3.122Moment tensor analysis had been conducted to understand the source mechanism of earthquakes in Soputan Volcano during October - November 2010 period. The record shows shallow earthquakes with frequency about 5 - 9 Hz. Polarity distribution of P-wave first onset indicates that the recorded earthquakes are predominated by earthquakes where almost at all stations have the same direction of P-wave first motions, and earthquakes with upward first motions.In this article, the source mechanism is described as the second derivative of moment tensor, approached with first motion amplitude inversion of P-wave at some seismic stations. The result of moment tensor decomposition are predominated by earthquakes with big percentage in ISO and CLVD component. Focal mechanism shows that the recorded earthquakes have the same strike in northeast-southwest direction with dip about 400 - 600. The sources of the high frequency shallow earthquakes are in the form of tensile-shear cracks or a combination between crack and tensile faulting.

Tensile-shear crack model for the mechanism of volcanic earthquakes : Shimizu, H; Ueki, S; Koyama, J TectonophysicsV144, N1–3, Dec 1987, P287–300

Tensile-shear crack model for the mechanism of volcanic earthquakes : Shimizu, H; Ueki, S; Koyama, J TectonophysicsV144, N1–3, Dec 1987, P287–300

A tensile — shear crack model for the mechanism of volcanic earthquakes

Temporary seismic observations were carried out for about 1 month at Miyakejima Island on the Izu-Bonin arc after an eruption on October 3, 1983. More than 3000 seismic events were observed and they are classified into 4 different categories: short-period earthquakes, long-period earthquakes, isolated tremors and continuous tremors. Two active source regions of the short-period earthquakes were located along the old caldera rim of the Miyakejima volcano. One region was very close to the eruptive fissures. Another was about 1–2 km northwest of the fissures. Almost all the earthquakes in the former region showed dilatant first motions of P-waves at all the stations in the island. On the other hand, almost all events in the latter region showed compressive first motions of P-waves at all the stations. The first motions of P-waves for these earthquakes cannot be explained by the quadrant-type pattern due to a double-couple force system. Nucleation of a tensile crack coupled with a shear crack (a tensile-shear crack model) is proposed as a source mechanism of the short-period earthquakes. Moment tensor elements of tensile and shear cracks have been determined by the least squares method using observed P- and S-wave data. Results suggest that tensile cracking (opening or closing) is dominant for generating those earthquakes in comparison with shear cracking. The strikes of the tensile cracks for the earthquakes are shown to be nearly parallel to the strike of the fissures. The earthquakes that occurred in the northwestern region of the fissures are considered to be generated by a sudden opening of tensile-shear cracks due to the excess pressure of intrusive magma. In contrast, the earthquakes occurring beneath the fissures are probably generated by a sudden closing of tensile-shear cracks, and this suggests that the magmatic pressure under the fissures rapidly decreased after the eruption. Moment tensor analysis of the tensile-shear crack model in this study demonstrates the non double-couple source mechanism for volcanic earthquakes related to magmatic activity.