Small Dip Angle Research Articles

Modeling the Hydraulic Fracturing Processes in Shale Formations Using a Meshless Method

Complex bedding properties and in situ stress conditions of shale formation lead to complex hydraulic fracturing morphologies. However, due to the limitations of traditional numerical methods, the simulation of hydraulic fracturing in shale formation still needs further development. Based on this, the liquid–solid interaction modes and the SPH governing equations considering liquid–solid interaction force have been introduced. The smoothing kernel function in the traditional SPH method is improved by introducing the fracture mark ξ, which can realize the simulation of rock hydraulic fracturing processes. The stress boundary of the SPH method is applied by stress mapping of “stress particles”, and the feasibility and correctness of the method are verified by two numerical examples. Then, the simulation of hydraulic fracturing processes of bedding shale formations are carried out. With the increase of horizontal stress ratio, the total number of damaged particles decreases, but the initiation and extension pressure increase gradually. The initiation stress of small bedding dip angles (θ < 45°) is larger than that of big bedding dip angles (θ > 45°). The hydraulic fracture propagation range at low horizontal stress ratio is wider and the fracture is along the direction of maximum principal stress, while the hydraulic fracture propagation range at high horizontal stress ratio is limited to the perforation. The hydraulic fracture will propagate through the bedding with small dip angles. However, when the bedding dip angle is larger, the hydraulic fracture will propagate along the bedding direction.

The Statistical Characteristics Analysis for Overvoltage of Elevated Transmission Line under High-Altitude Electromagnetic Pulse Based on Rosenblatt Transformation and Polynomial Chaos Expansion

A High-Altitude Electromagnetic Pulse (HEMP) could induce very fast transient overvoltage (VFTO) with nanosecond level rise time and mega-volt amplitude, which severely threatens the electrical devices connected to the elevated transmission line. An elevated transmission line with different locations may suffer different levels of HEMP threat since the dip angle could influence the polarization of the HEMP wave. The combination of Rosenblatt Transformation and Polynomial Chaos Expansion (R-PCE) is introduced in this paper. With this method, the efficiency of calculating the overvoltage of an elevated transmission line under HEMP is improved, speeding up 24.75 times. The influence of different factors (dip angle, elevated height, and earth conductivity) on the overvoltage of elevated transmission lines applied in power systems is calculated and analyzed. The numerical result shows with 99.9% confidence that the overvoltage would be over 3.7 MV of amplitude and 6.7 × 1014 V/s of voltage derivative, which is much more rigorous than a lighting pulse. We also find that elevated transmission lines may have a larger HEMP threat in a small dip angle area. The corresponding data are shown at the end of the paper, which could be useful for relative researchers in pulse injection experiments and reliable evaluation.

Design of Miniaturized Wideband Beam Deflection Conformal Array Antenna

Antenna beam deflection, along with miniaturization and wideband of the antenna is in demand for practical applications. In this paper, a cylindrical conformal array antenna with a small-tilt forward beam was designed. The microstrip antenna unit was loaded with the artificial electromagnetic structure, which reduced the size of the antenna unit. As a result, the center spacing of the array elements can be shortened with the same array element spacing. The beam deflection angle can be increased in this way without increasing the coupling effect between the parts. Changing the number of line array elements and the number of line arrays can regulate the beam width of E-field and H-field, respectively. The bandwidth of the antenna can be significantly extended by slotting the ground plane. This work implemented a cylindrical conformal array of the antenna's forward beam with a small dip angle using a cylindrical carrier as an example. The measurement results showed that the angle between the main beam and the carrier axis of the conformal antenna was less than 30°, the bandwidth was more than 30%, and the antenna volume decreased by 40.4%.

Computed Tomography Observation and Image-Based Simulation of Fracture Propagation in Compressed Coal

In this study, the fracture propagation characteristics and associated mechanisms of coal are investigated by using computed tomography (CT) observation and image-based simulation. The spatial distribution and the structural morphology of original fractures provide significant influences on the failure behavior of fractured coal. The fractures with small dip angles and large openings result in more-obvious fracture closure and stable propagation stages, while failure pattern is more sensitive to those with large dip angles. The coal tends to experience brittle failure, which transits from a splitting to mixed-splitting faulting mode because of the difference in original fracture distribution. The final failure fracture network originates mainly from the propagation of original fractures, driven by localized tensile stress. Fracture interaction and mineral influence tend to increase the complexity in the failure fracture network. Moreover, image-based numerical models are established on the basis of CT reconstruction, where the spatial distribution and the structural morphology of original fractures are properly considered. Numerical modeling reproduces similar stress–strain responses and failure fracture networks to that observed in the experiment. The predicted distribution of tensile stress shows a similar evolution trend to the failure fracture network, implying that the fracture propagation of coal is dominated by tensile failure. Shear cracks emerge mainly after the large fracture running through the coal sample has been formed.

Research on transition corridor boundary of distributed propulsion VTOL fixed wing

The transition state is the most critical and dangerous state of the VTOL fixed wing aircraft in the whole flight process. In this paper, a transition corridor for a distributed propulsion VTOL fixed-wing aircraft is studied based on the lift characteristics of the wing and the power constraints of the power unit. Firstly, according to the sliding flow theory, the dynamic characteristics models of the lift fan system in the front part of the fuselage and the distributed duct system in the rear part of the fuselage were established by introducing the influence factor of the duct, and verified with the test data. Secondly, according to the lift characteristics of the wing, the transition curves of the aircraft at different angles of attack are calculated, in which the transition curves corresponding to the zero-lift attack angle and stall attack angle constitute the lift characteristics transition corridor of the distributed propulsion VTOL fixed-wing aircraft. Finally based on the dynamic performance of the power unit model, calculate the lift characteristic transition each state point in corridor corresponding power demand, according to the power limit of the lift fan system, distributed duct system and power unit total power limitation, get distributed to promote vertical take-off and landing a fixed wing aircraft power limitation in lifting features and power unit under the condition of complete transition corridor. The final results show that the minimum forward velocity is inversely proportional to the attack angle. The power required by the tail distributed duct system will exceed the limit when the aircraft is in low speed and small dip angle transition. For the power limit boundary required, the power limit condition of individual component is stricter than the total power limit condition. The research results of this paper can provide some reference for the transition corridor research of such VTOL fixed-wing aircraft, and on this basis, the subsequent work such as parameter sensitivity analysis and control system design of transition corridor can be carried out.

Seismicity and Stress State in the Ryukyu Islands Subduction Zone

Based on the newly compiled and mostly complete unified earthquake catalogue for China’s seas and adjacent areas, further information was obtained about the structural shape and dip angle of the Benioff zone in the Ryukyu Islands subduction zone during the different subduction stages. In addition, using the damped regional stress tensor inversion method, we were able to investigate the complex stress field characteristics and the dynamic significance of the shallow and intermediate earthquakes in the Ryukyu Islands subduction zone. The results show that the tectonic stress field of the Ryukyu Islands subduction zone was extensional along the subduction direction in the northern area of the Tokara Strait and was compressional along the subduction direction in the southern area of the Tokara Strait. The R value of the shallow stress field of the Okinawa Trough was low, and the σ3 was stable in the NNW direction with a small dip angle (>30°). The type of stress field in the shallow part of the Okinawa Trough transitioned from strike-slip type to normal fault type from north to south, reflecting the difference in the degree of development of the trough, and the southern segment of the trough began to transform into the expansion stage. The northeastern portion of the study area and southeast Taiwan constituted the high R value (0.68–0.87) region where the σ2 had tensile components. The stress state was biaxial tension–uniaxial compression, and the principal compressive stress was determined to be in the SEE direction with a large dip angle (>30°). The σ1 in northeast Taiwan exhibited a nearly vertical (>60°) plunge, while the σ2 and σ3 were nearly horizontal. The σ2 was thrust in the ENE–WSW direction, and the σ3 was extended in the NNW direction. Through this research, a greater understanding has been gained of the seismicity characteristics and shape of the Ryukyu Islands subduction zone. Supplementary research has also been completed on the focal mechanism solution and stress field of the Ryukyu Islands subduction zone. Finally, this research is important for earthquake hazard analysis and earthquake engineering safety evaluation in this area.

High-precision relocation of the aftershock sequence of the January 8, 2022, MS6.9 Menyuan earthquake

The 2022 Menyuan MS6.9 earthquake, which occurred on January 8, is the most destructive earthquake to occur near the Lenglongling (LLL) fault since the 2016 Menyuan MS6.4 earthquake. We relocated the mainshock and aftershocks with phase arrival time observations for three days after the mainshock from the Qinghai Seismic Network using the double-difference method. The total length and width of the aftershock sequence are approximately 32 km and 5 km, respectively, and the aftershocks are mainly concentrated at a depth of 7–12 km. The relocated sequence can be divided into 18 km west and 13 km east segments with a boundary approximately 5 km east of the mainshock, where aftershocks are sparse. The east and west fault structures revealed by aftershock locations differ significantly. The west fault strikes EW and inclines to the south at a 71º–90º angle, whereas the east fault strikes 133º and has a smaller dip angle. Elastic strain accumulates at conjunctions of faults with different slip rates where it is prone to large earthquakes. Based on surface traces of faults, the distribution of relocated earthquake sequence and surface ruptures, the mainshock was determined to have occurred at the conjunction of the Tuolaishan (TLS) fault and LLL fault, and the west and east segments of the aftershock sequence were on the TLS fault and LLL fault, respectively. Aftershocks migrate in the early and late stages of the earthquake sequence. In the first 1.5 h after the mainshock, aftershocks expand westward from the mainshock. In the late stage, seismicity on the northeast side of the east fault is higher than that in other regions. The migration rate of the west segment of the aftershock sequence is approximately 4.5 km/decade and the afterslip may exist in the source region.

Influence of natural fractures on propagation of hydraulic fractures in tight reservoirs during hydraulic fracturing

Natural fractures in tight reservoirs play an important role in affecting propagation of hydraulic fractures during hydraulic fracturing. In this study, simulated hydraulic fracturing is implemented on nine artificial samples. Within these artificial samples, the pre-existing fractures either showing different dip angles or exhibiting different distances from the simulated wellbore are created to investigate the influence of different types of natural fractures on fracture propagation in tight reservoirs during hydraulic fracturing. Based on the experimental results, propagation of hydraulic fractures varies with the presence of pre-existing fractures having different dip angles during the hydraulic fracturing. Specifically, hydraulic fractures easily go through the pre-existing fractures with high dip angles, while they are hindered by pre-existing fractures with small dip angles. In addition, propagation of hydraulic fractures changes with the distance of pre-existing fractures from simulated wellbore. With a small distance from simulated wellbore, hydraulic fractures are redirected by showing bifurcation from the pre-existing fractures. As the distance increases, hydraulic fractures are less affected by pre-existing fractures. On the contrary, effective stress is the main factor influencing propagation of hydraulic fractures. To validate the laboratory result, microseismic monitoring technology was used detect the influence of natural fractures on propagation of hydraulic fractures in tight reservoirs during a hydraulic fracturing process on Longmaxi Formation in the Fuling area of Sichuan Basin of China. This study is expected to inspire new understanding of the influence of natural fractures on propagation of hydraulic fractures during hydraulic fracturing in tight reservoirs.

An Improved Form of 2D SPH Method for Modeling the Excavation Damage of Tunnels Containing Random Fissures

The excavation damage of deep tunnels is one of the most important factors contributing to the failure of tunnel structures. In order to investigate the influence of tunnel shapes and fissure geometries, the kernel function in the traditional SPH method has been improved, which can realize the brittle fracture characteristics of particles and can be called the Improved Kernel of Smoothed Particle Hydrodynamics (IKSPH-2D). Meanwhile, the random fissure generation method in IKSPH has been put forward. Different tunnel shapes, fissure geometries, and locations are considered during the simulation of tunnel excavation, and results show that (1) the typical “V”-shaped shear damage zones appear after the tunnel excavation, which is consistent with engineering practice. Meanwhile, tunnel excavation also has an “activating” effect on the preexisting fissures. (2) The stability of circular-shaped tunnel is the best, while horseshoe shaped tunnel is worse, and the “U”-shaped tunnel is the worst. (3) Fissures with small and large dip angles have the greatest influence on the stability of tunnel excavation. With the increase of fissure numbers and lengths, the tunnel tends to be instable. (4) The IKSPH method gets free from traditional grids in FEM, which can dynamically reflect the fracture processes of tunnel excavation. Meanwhile, developing 3D IKSPH parallel program will be the future directions.

Experimental investigation on the strain energy evolution mechanism of granite specimens with a single flaw

Damage and fracture of rocks is a comprehensive result of strain energy dissipation and release. In order to study the energy evolution mechanism of hard brittle rock masses during failure process, uniaxial compression tests on the granite specimens with a single pre-existing flaw are carried out with digital image correlation method (DIC). The results indicate that: (1) The stress fluctuation before structural failure corresponds to the sudden increase of the dissipated energy, the essence is that rock masses occur obviously damage and cracking. (2) For medium flaw dip angles, the stress fluctuation occurs repeatedly in stages, whose strain energy is dissipated gradually; for large or small dip angles, the stress fluctuation is almost not appeared, and the energy dissipation is insignificant. (3) With the increase of flaw dip angles, the dissipated energy ratio at the peak stress first increases and then decreases showing an inverted U-shaped, resulting in a similar law of rock damage but a contrary law of strength; the corresponding elastic energy ratio is U-shaped of decreasing first and then increasing, causing the structural failure changes from dynamic to static and then to dynamic again. The results can provide theoretical references for the formation mechanism and prevention method of dynamic disasters of rock underground engineering.

Buckling failure analysis for buried subsea pipeline under reverse fault displacement

Buckling failure analysis of buried subsea pipeline under reverse fault displacement was investigated by the Vector Form Intrinsic Finite Element method in this paper, considering nonlinear soil–pipeline interaction in slit with sand and the multifold nonlinearities of nonlinear material, large deformation, large displacement and self-collision/contact. Two typical conditions discussed were the operation condition and the empty waiting state during installation, maintenance, and operation. Effects of soil properties, pressure loadings, diameter–thickness ratios and dip angles on local buckling formation, tensile strain and cross-sectional distortion were investigated. The results show that subsea pipelines obtain a large deformation capacity and the flexure curve shape is usually S-shape. The external pressure is more unfavorable than the internal pressure since it aggravates the compression strains but alleviates the tension strains. A small fault dip angle results in serious axial compression and can lead to local collapse in the empty waiting states. For big diameter–thickness ratios or great pressure level, external pressure will cause local collapse and even buckle propagation along the pipeline. Seven different modes of collapse and propagation are discovered in the empty waiting state. They can be classified by the first collapse location, buckle propagation direction and further propagation sustainability. The cross-sectional shapes after collapse and during propagation are mainly determined by the compression or tension state around the circum. These results can be used for the development of performance-based design methodologies for buried subsea pipelines.

Numerical Investigation on the Hydraulic Fracture Evolution of Jointed Shale

Joints are a common structure of heterogeneous shale rock masses, and in situ stress is the environment in which heterogeneous rock masses can be found. The existence of joint plane and confining pressure difference influences the physical properties of shale and propagation of fractures. In this study, jointed shale specimens were simulated under different confining pressures to explore the failure patterns and fracture propagation behavior of hydraulic fracturing. Different from the common research of hydraulic fracturing on signal parallel joint rock mass, the simulations in this study considered three points (parallel joint, multi‐dip angle joint, and no‐joint points). The effects of the single‐dip angle joint, multi‐dip angle joint, and confining pressure difference on the hydraulic fracture evolution and stress evolution of the jointed shale were studied comprehensively. The confining pressure difference coefficient proposed in this study was used to accurately describe the confining pressure difference. Results indicate that the larger is the confining pressure difference, the stronger is the control of the maximum principal stress on fracture evolution; by contrast, the smaller is the confining pressure difference, the stronger is the control of the joint plane on fracture evolution. Under the same confining pressure difference, the hydraulic fracture propagates more easily along the small dip angle joint plane. As the value of the confining pressure difference coefficient moves closer to zero, the hydraulic fracture propagates randomly, the tensile stress region around the fracture tip widens, and the joint planes fractured by tensile increase. This study can offer valuable guidance to the design of unconventional reservoir reconstruction.

Quantifying and modelling the effects of pre-existing basement faults on folding of overlying strata in the Surat Basin, Australia: Implications for fault seal potential

Ancient basement fault geometry has a systematic effect on the folding of overlying sedimentary successions. Using aeromagnetic, Bouguer gravity, as well as 2D and 3D seismic datasets, the characteristics of the basement structure underlying the Jurassic-Cretaceous Surat Basin were examined and used to contextualise the nature of its folded strata. Using Fault-Forward modelling software, the deformation of the basement structural layers was modelled and analysed for its influence on the reactivation of faulting. The results show that the major fault trends in the eastern Surat and Bowen basins consist of five key features with N- and NE-trending orientations: the Goondiwindi, Moonie, Undulla Transition Zone, Burunga-Leichhardt, and Cockatoo fault systems. Most of the major fault deformation ceased in the Late Triassic. However, some faults (i.e., the Moonie and Goondiwindi fault systems) were variably reactivated along their lengths during the Jurassic. Three models of pre-existing basement reactivation are proposed to explain the observed variability in folding style with application to different parts of the basin. Ancient fault systems with dip angles greater than 45° were readily reactivated and produced tight monocline structures with high strain (ε > 0.35) and fault throw (>60 m) (e.g. Moonie Fault System), while faults with small dip (<30°) or large apical angles generated folds with low strain and uplift of the hanging wall. This variability in fault reactivation according to differences in fault throw potentially influences across-fault juxtaposition of strata and sealing potential for CO2 storage. Modelling such as this should be undertaken elsewhere in situations where trishear structural domains are expected, to better understand and predict the deformation of overlying stratal successions.

Using gravity gradients to estimate fault parameters in the Wichita Uplift region

SUMMARY The geological setting of southwestern Oklahoma and northeastern Texas is an ideal example of an aulacogen, the result of the tectonic evolution of a failed rift of the North American continent during the Palaeozoic era (540–360 Ma). The Wichita Province forms the uplifted basement portion of this Southern Oklahoma Aulacogen (SOA). The major fault zones to its north and south are clearly evident in gravity gradient maps produced by the recently constructed Earth Gravitational Model 2008 (EGM2008). Fault parameters, such as the dip angle, location and density contrasts have been estimated from profiles of seismic data and local gravimetry in the 1990s. On the other hand, gravitational gradients that are derived from EGM2008 and then combined to form the differential field curvature are particularly indicative of linear structures such as dip-slip faults. They are used here exclusively, that is, without additional geophysical constraints, in an optimal, least-squares estimation based on the Monte Carlo technique of simulated annealing to determine dip angle and location parameters of the major faults that border the Wichita Uplift region. Results show that these faults have small dip angles, in basic agreement with the low-angle faults inferred from seismic studies. The EGM2008 gradients also appear in some cases to provide an improved map of the major faults in the region, thus offering a strong constraint on their location.

Analysis of Seismic Stability of an Obsequent Rock Slope Using Time–Frequency Method

Obsequent rock slopes are often thought to be more stable than consequent slopes and passive sliding is unlikely to occur under earthquake loading. However, failures in obsequent rock slopes were indeed observed in recent large earthquakes. This paper presents a time–frequency solution to the seismic stability of obsequent rock slopes fully considering the time–frequency characteristics of earthquake waves. Large-scale shaking table tests were conducted to illustrate the application of the time–frequency method to an obsequent rock slope containing multiple weak layers with a small dip angle. The seismic stability of the obsequent rock slope is analyzed combining the time–frequency method, outcomes from the shaking table test, and conventional pseudo-static and dynamic numerical analyses. The results show that passive sliding can develop in the obsequent rock slope when taking the time–frequency components of the earthquake waves and the vertical seismic force into account. The middle–upper part of the obsequent rock slope is more vulnerable to seismic damage. The slope bulges under the earthquake loading; the maximum permanent surface displacement occurs at the middle–upper part of the slope, rather than the slope crest. Additionally, the response of seismic safety factor lags behind the responses of acceleration and surface displacement.

Effect of fine-grained dipping interlayers on mechanical behavior of tailings using discrete element method

Due to the presence of fine-grained interlayers (frequently called lenticles), the stability of tailings dams can deteriorate. In this study, a two-dimensional numerical model with flexible boundary was constructed, and biaxial tests were carried out for samples with interlayers at different dip angles. The influence of the interlayer inclination on the mechanical properties of tailings was analyzed. The simulation results revealed that the shear strength of tailings with interlayers decreases with the increase in the interlayer dip angle. Obvious anisotropy of shear strengths was observed due to the presence of interlayers. All the deformations of the samples with interlayers were well captured in the DEM simulations. Bulging deformations were found in the samples with interlayers at dip angles of 0°, 15° and 30°. Shear slip along the fine interlayer plane was generated in the samples with interlayers with dip angles of 45° and 60°. The principal orientation of the contact unit normal and the normal contact force aligns with the loading direction for the samples with interlayer at small dip angle (i.e., 0°). However, for the sample with interlayer at large dip angle (i.e., 60°), the principal orientation trends to the direction normal to the interlayer.

Strength and deformation of tailings with fine-grained interlayers

Due to the presence of fine-grained interlayer structures (usually called lenticles), the stabilities of tailings dams deteriorate. In this study, the strength and deformation of tailing samples containing fine-grained interlayers were studied by undrained tests. Based on the Coulomb theory, the slip mechanism of tailing samples was investigated. The results indicate that the stress-strain relationship of the samples that contain interlayers of large dip angles (i.e., 45° or 60°) is typical strain softening. The stress-strain relationship of the samples, with the interlayers dipping with small angles (i.e., 0° or 15°), is strain hardening. The mechanical parameters, e.g., shear strength, internal friction angle and cohesion, decrease with the increase in the dip angle of the interlayer. In tests, three different deformation patterns, i.e., bulging, slip and composite deformation, were observed from the samples with interlayers of small, large and medium dip angles, respectively (the medium dip angle is 30°). The slip occurred along the interlayer plane of the samples. When the angle of the interlayer is near 60°, the strength and deformation of the samples are controlled by the fine-grained interlayer. The findings of this study can provide reference for the stability evaluation of anisotropic tailing dams and other similar structures.

Effects of formation dip angle and salinity on the safety of CO2 geological storage - A case study of Shiqianfeng strata with low porosity and low permeability in the Ordos Basin, China

The safety of CO2 geological storage is a key problem that hinders its implementation. In particular, the formation dip angle and salinity can directly affect the spatial migration distribution, storage form and storage amount of CO2. A three-dimensional simulation model was established to evaluate the effects of the formation dip angle and salinity on the safety of CO2 geological storage. The simulation results showed that a larger dip angle resulted in a greater CO2 migration distance. The greatest CO2 migration distances with formation dip angles of 0°, 5° and 10° were 60%, 73.3%, and 86.7%, respectively, compared with a formation dip angle of 15° in the 200th year of CO2 migration. A larger formation dip angle was not conducive to CO2 geological storage. When the salinity was greater, smaller values were obtained for the CO2 liquid phase mass fraction, CO2 gas phase, liquid phase and total storage amount. With salinity values of S,3/4S,1/2S,1/4S, and 0.03, the proportions of liquid-phase storage amount in the salt-free strata were determined as 26.2%, 38.0%, 54.3%, 74.5%, and 88.1%, respectively, after CO2 migration for 200 years. Higher salinity resulted in less dissolved CO2 and decreased CO2 geological storage safety. After considering the combined influence of the dip angle and salinity, we found that the effects of the dip angle were obvious on the unit pressure and gas phase saturation, and the effect of salinity on the CO2 liquid phase mass fraction was significant. A higher dip angle and salinity greatly decreased the CO2 geological storage safety. Thus, a reservoir with a smaller dip angle and lower salinity should be selected as a site for CO2 geological storage in future studies.

Geometry and tectonic deformation of the seismogenic structure for the 8 August 2017 &lt;i&gt;M&lt;/i&gt;&lt;sub&gt;S&lt;/sub&gt; 7.0 Jiuzhaigou earthquake sequence, northern Sichuan, China

To reveal the geometry of the seismogenic structure of the Aug. 8, 2017 MS 7.0 Jiuzhaigou earthquake in northern Sichuan, data from the regional seismic network from the time of the main event to Oct. 31, 2017 were used to relocate the earthquake sequence by the tomoDD program, and the focal mechanism solutions and centroid depths of the ML ≥ 3.5 events in the sequence were determined using the CAP waveform inversion method. Further, the segmental tectonic deformation characteristics of the seismogenic faults were analyzed preliminarily by using strain rosettes and areal strains (As). The results indicate: (1) The relocated MS 7.0 Jiuzhaigou earthquake sequence displays a narrow ~ 38 km long NNW-SSE-trending zone between the NW-striking Tazang Fault and the nearly NS-striking Minjiang Fault, two branches of the East Kunlun Fault Zone. The spatial distribution of the sequence is narrow and deep for the southern segment, and relatively wide and shallow for the northern segment. The initial rupture depth of the mainshock is 12.5 km, the dominant depth range of the aftershock sequence is between 0 and 10 km with an average depth of 6.7 km. The mainshock epicenter is located in the middle of the aftershock region, showing a bilateral rupture behavior. The centroid depths of 32 ML ≥ 3.5 events range from 3 to 12 km with a mean of about 7.3 km, consistent with the predominant focal depth of the whole sequence. (2) The geometric structure of the seismogenic fault on the southern section of the aftershock area (south of the mainshock) is relatively simple, with overall strike of ~150° and dip angle ~75°, but the dip angle and dip-orientation exhibit some variation along the segment. The seismogenic structure on the northern segment is more complicated; several faults, including the Minjiang Fault, may be responsible for the aftershock activities. The overall strike of this section is ~159° and dip angle is ~59°, illustrating a certain clockwise rotation and a smaller dip angle than the southern segment. The differences between the two segments demonstrate variation of the geometric structure along the seismogenic faults. (3) The focal mechanism solutions of 32 ML ≥ 3.5 events in the earthquake sequence have obvious segmental characteristics. Strike-slip earthquakes are dominant on the southern segment, while 50% of events on the northern segment are thrusting and oblique thrusting earthquakes, revealing significant differences in the kinematic features of the seismogenic faults between the two segments. (4) The strain rosettes for the mainshock and the entire sequence of 31 ML ≥ 3.5 aftershocks correspond to strike-slip type with NWW-SEE compressional white lobes and NNE-SSW extensional black lobes of nearly similar size. The strain rosette and As value of the entire sequence of 22 ML ≥ 3.5 events on the southern segment are the same as those of the MS 7.0 mainshock, indicating that the tectonic deformation here is strike-slip. However, the strain rosette of the entire sequence of 10 ML ≥ 3.5 events on the northern segment show prominent white compressional lobes and small black extensional lobes, and the related As value is up to 0.52, indicating that the tectonic deformation of this segment is oblique thrusting with a certain strike-slip component. Differences between the two segments all reveal distinctly obvious segmental characteristics of the tectonic deformation of the seismogenic faults for the Jiuzhaigou earthquake sequence.

Thermal State, Slab Metamorphism, and Interface Seismicity in the Cascadia Subduction Zone Based On 3‐D Modeling

AbstractGiant earthquakes have repeatedly ruptured the Cascadia subduction zone, and similar earthquakes will likely also occur there in the near future. We employ a 3‐D time‐dependent thermomechanical model that incorporates an up‐to‐date description of the slab geometry to study the Cascadia subduction thrust. Results show a distinct band of 3‐D slab dehydration that extends from Vancouver Island to the Seattle Basin and farther southward to the Klamath Mountains in northern California, where episodic tremors cluster. This distribution appears to include a region of increased dehydration in northern Cascadia. The phenomenon of heterogeneous megathrust seismicity associated with oblique subduction suggests that the presence of fluid‐rich interfaces generated by slab dehydration favors megathrust seismogenesis in the northern part of this zone. The thin, relatively weakly metamorphosed Explorer, Juan de Fuca, and Gorda Plates are associated with an anomalous lack of thrust earthquakes, and metamorphism that occurs at temperatures of 500–700°C near the Moho discontinuity may represent a key factor in explaining the presence of the associated episodic tremor and slip (ETS), which requires a young oceanic plate to subduct at a small dip angle, as is the case in Cascadia and southwestern Japan. The 3‐D intraslab dehydration distribution suggests that the metamorphosed plate environment is more complex than had previously been believed, despite the existence of channeling vein networks. Slab amphibolization and eclogitization near the continental Moho depth is thus inferred to account for the resultant overpressurization at the interface, facilitating the generation of ETS and the occurrence of small to medium thrust earthquakes beneath Cascadia.