CN115857003A

CN115857003A - Inclined fault FSI super-shear earthquake risk estimation method

Info

Publication number: CN115857003A
Application number: CN202211507120.1A
Authority: CN
Inventors: 唐荣江; 甘露; 李福生
Original assignee: Yangtze River Delta Research Institute of UESTC Huzhou
Current assignee: Yangtze River Delta Research Institute of UESTC Huzhou
Priority date: 2022-11-29
Filing date: 2022-11-29
Publication date: 2023-03-28

Abstract

The invention belongs to the technical field of geological disaster risk assessment, and discloses an FSI (fault tolerant indicator) super-shear earthquake risk estimation method for an inclined fault, which simulates fault fracture propagation under different sliding angles and determines a sliding angle threshold value for generating FSI super-shear fracture; evaluating the orientation generated by the FSI super-shear fracture according to the asymmetry of the FSI super-shear fracture on the inclined fault; calculating low-frequency peak velocity PGV under different scenes, judging the relation between PGV distribution and fault direction, and evaluating the seismic oscillation distribution range of the upper disc of the inclined fault; and judging the influence of the relative relation between the sliding direction of the upper disc and the fracture propagation direction on the earthquake motion strength so as to guide an earthquake hazard evaluation strategy.

Description

Inclined fault FSI super-shear earthquake risk estimation method

Technical Field

The invention belongs to the technical field of geological disaster risk assessment, and particularly relates to an FSI (fault location indicator) super-shear earthquake risk estimation method for an inclined fault.

Background

The velocity of seismic fractures is typically sub-rayleigh wave velocity, however, in certain stress conditions, super-shear fractures may be induced. Super-shear fractures can produce mach waves whose radiated energy forms a pair of obliquely propagating beams away from the fault, which can result in a large range of energy distribution or strong ground vibration away from the fault, leading to a wider seismic hazard. Therefore, identifying whether one fault has the possibility of generating super-shear fracture or not under the actual condition is particularly important for reducing the risk of earthquake disasters to human society and improving earthquake-proof and disaster-reduction technologies.

Prior research work has shown that propagation of super-shear fractures has been observed in several large scale walk-glide earthquakes, where fractures occur predominantly in mode II (propagation direction parallel to the glide direction) and most of the fractures reach the earth's surface, with relatively straight fault trajectories. The generation of super-shear fractures can be explained by the Burridge-Andrews (BA) mechanism, i.e., when the fracture propagation direction coincides with the slip direction, the stress induced by the S-wave preceding a sub-Rayleigh fracture can induce a sub-fracture that propagates forward at a super-shear velocity, gradually pulling away from the original fracture. Furthermore, some studies suggest that as long as the fault is long enough, a flat free surface always tends to induce transformation of the super shear fractures. This free-surface-induced (FSI) hypershear fracture is triggered primarily by the SV-P transition seismograph, and is completely different from the fracture behavior induced by the BA mechanism. If a dip fault, i.e., a reverse fault or a normal fault, is considered, the mode II direction coincides with the fault dip direction, in which case the super-shear fracture is less observable even if it occurs. If observed along the fault strike, dip fault fractures propagate predominantly in mode iii (i.e., the propagation direction is perpendicular to the fault slip direction), however mode iii has been shown to prevent FSI super shear fractures from occurring.

From the above, in order to determine whether a fault has the capability of generating a super shear earthquake, it can be determined according to whether the fault is a walk-slip earthquake, the fault length, and whether the fault trajectory is straight. For dip faults (such as pure faults or reverse faults), the super-shear earthquake is difficult to appear and is difficult to observe even if the super-shear earthquake appears, however, for dip faults (assuming that the fracture reaches the ground surface), the mode III and the mode II are included in any direction and belong to a mixed mode. The oblique slip fault contains two modes II and III, and whether FSI super shear fracture is generated or not has no better judgment standard at present. In view of this, a technical method for effectively evaluating whether the oblique fault generates FSI super shear fracture and distribution thereof, and the generated influence is lacked at present.

Disclosure of Invention

In order to judge whether the inclined fault has the possibility of generating the advanced shear fracture, the invention is based on a finite element method, considers the numerical simulation of the three-dimensional earthquake dynamic fracture process under the inclined plane fault, judges the FSI advanced shear fracture according to the fault type and the basic geometrical form by analyzing how the fault type change (particularly the sliding angle change) affects the generation and the propagation of the FSI advanced shear fracture, and further judges the strong vibration distribution.

The invention discloses an FSI super-shear earthquake risk estimation method, which comprises the following steps:

step 1, constructing a super-shear seismic geometric model, and simulating fault fracture propagation under different sliding angles;

step 2, determining a sliding angle threshold value of global super shear fracture conversion according to the simulation condition, and predicting whether super shear fracture occurs;

step 3, evaluating the orientation generated by the FSI super-shear fracture according to the asymmetry of the FSI super-shear fracture on the inclined fault;

step 4, calculating the low-frequency peak velocity PGV under different scenes, judging the relation between the PGV distribution and the fault direction, and evaluating the seismic oscillation distribution range of the upper disc of the inclined fault;

and 5, judging the influence of the relative relation between the sliding direction of the upper disc and the fracture propagation direction on the earthquake motion intensity, and further guiding an earthquake hazard evaluation strategy.

Further, the sliding angle variation range is-90 degrees to 90 degrees, and five types of faults are obtained in sequence: pure faults, oblique slip normal faults, pure slip faults, oblique slip reverse faults and pure reverse faults; and constructing initial stress fields under all scenes, and projecting the regional stress fields under different scenes onto the fault plane to obtain the shear stress with the same size and different directions and the normal stress with the same size, so as to eliminate the influence of the fault stress intensity difference under different scenes on the dynamic fracture process.

Further, two opposite fracture propagation directions along the fault direction are definedDirection, if the tilting component of the upper disk or the lower disk is upward, the sliding direction of the block is defined as positive direction, and the other direction is defined as negative direction; velocity of rupture v _r By

Is calculated to be out of>

Is the spatial gradient operator, t _r (xi) is defined as the moment when the sliding rate exceeds the threshold value 0.01m/s at a certain point xi on the fault plane.

Further, FSI super shear fracture on a diagonal fault has asymmetry; in the positive direction, unsustainable super-shear fracture is generated, and the smaller the sliding angle is, the shorter the duration time of the super-shear fracture is, and the super-shear fracture gradually disappears when the sliding angle is lower than 40 degrees; in the negative direction, global super shear fractures are generated, which disappear immediately at sliding angles greater than 43 ° or less than-33 °.

Further, in step 4, the forward direction of the inclined fault is more widely distributed than the negative direction, and the secondary sub-Rayleigh fracture may generate a similar mach cone and cause the seismic energy of the disc on the inclined fault to be widely distributed.

Further, in the step 5, the earthquake motion is obviously enhanced in the direction that the sliding direction of the upper disc is consistent with the cracking direction; in the case of a slip fault, the directional effect of the fracture is most concentrated in the direction of fracture propagation, while the directional effect of the slip fault on the surface of the earth is most concentrated on the mass moving upward above the seismic source, i.e. the upper or lower wall.

The beneficial effects of the invention are as follows: the invention provides a risk evaluation method of an FSI (offset shear induced vibration) super-shear earthquake of an inclined fault for the first time, which can carry out qualitative and quantitative evaluation on the generation or non-generation of the super-shear earthquake and the distribution range of strong vibration. For a natural active fault, the rough fault geometry and fault type can be estimated according to the surface sliding rate and the structural stress field, and the possibility of generating super-shear fracture and which region can encounter stronger seismic motion can be qualitatively evaluated according to the method provided by the invention; by further numerical simulation, the result can be further quantified, e.g. how much shock acceleration is most likely to be generated. The method can forecast the medium-term and long-term dangerousness of a certain natural fault, furthest reduce the risks of earthquake disasters to the human society, and further guide and make an improved earthquake hazard evaluation strategy.

Drawings

FIG. 1 is a simulation region geometry presentation of a model constructed in accordance with the present invention;

FIG. 2 is a pre-fracture peak contour plot and a graph of fracture velocity after normalization;

FIG. 3 is a slip rate snapshot of a walk-slip fault and a slip-ramp reverse fault;

FIG. 4 is a velocity snapshot of the surface of a walk-slip fault and a dip-slip reverse fault;

FIG. 5 is a schematic representation of the surface peak velocity profile at different slip dips;

fig. 6 is a flow chart of a method according to the present invention.

Detailed Description

In order that the manner in which the present invention is attained and can be understood in detail, a more particular description of the invention briefly summarized above may be had by reference to the embodiments thereof which are illustrated in the appended drawings.

As shown in FIG. 1, the method for estimating the risk of the FSI super shear earthquake of the inclined fault comprises the following steps:

The method for estimating the FSI (offset shear interface) super-shear earthquake risk of the inclined fault constructs a super-shear earthquake geometric model, simulates fracture propagation of faults with different sliding angles (or initial stress directions), comprises a pure slip fault, a pure dip slip fault and an inclined slip fault, and each scene uses the same geometric shape, friction relation, material property and earthquake source position. In each case, faults with an inclination of 45 ° intersect the earth's surface and extend north, as shown in panel (a) of fig. 1, which is a geometric representation of a simulated area, with nucleation located in most cases at the two end pentagons (± 35,0, -10) to simulate a single-sided fracture, and part of the simulated case-forming nuclei located at the middle pentagons (0, -10) to simulate a double-sided fracture, panel (b) of fig. 1 is a cross-sectional view of the fault from the west, and panel (c) of fig. 1 is a cross-sectional view taken along the north. Initiation of the rupture is usually done manually, and will occur spontaneously once nucleation is complete. The nucleation sites are five-pointed stars in the middle or on either side of the fault, depending on whether it is desired to simulate a double-sided fracture. Here again, linear-time weakening friction law is used to achieve seismic nucleation.

To exclude the influence of the rock material properties, only homogeneous models (P-wave velocity 6000m/S, S-wave velocity 3464m/S, density 2667 kg/m) are considered here ³ ). Using simple linear sliding to weaken the law of friction, characteristic sliding distance D _c ＝0.4。μ _s =0.45 and μ _f =0.2 is the static friction coefficient and the sliding friction coefficient, respectively. The friction boundary conditions allow the fault to slide and spatially rotate in any shear direction.

When the sliding front angle is changed from-90 degrees to 90 degrees, five types of faults can be obtained in sequence: pure faults, oblique slip normal faults, pure slip faults, oblique slip reverse faults and pure reverse faults. Different fault types correspond to different initial stress fields, and in order to construct the initial stress fields in all scenes, the regional stress fields in different scenes need to be projected onto a fault plane to obtain shear stresses with the same size and different directions and normal stresses with the same size, so that the influence of the stress intensity difference of the faults in different scenes on the dynamic fracture process is eliminated. The stress parameter was set at 1.5, above the theoretical threshold for the shear transition in the 3D model without free surface given by [ Dunham E m.2007.Conditions relating to the overcure of super surfaces under slip-weather failure. Journal of geographic Research,112, b7] (S = 1.19). This indicates that the artificially generated super-shear fractures are completely induced by the free surface.

The embodiment of the invention carries out 16 groups of earthquake dynamic fracture numerical simulation, and the sliding angle is changed from-90 degrees to 90 degrees. In all scenarios, the rupture velocity v _r By

(Bizzarri and Spudich, 2008), in which>

Is a spatial gradient operator, t _r (ξ) is defined as the moment in time at which the ξ slip rate exceeds the threshold value of 0.01m/s at a certain point on the fault plane. In each set, the fracture nucleates at coordinates (-35, 0, -10) and (35, 0, -10), respectively.

For convenience, two opposite fracture propagation directions along the fault run are defined herein. If the tilting and sliding component of the upper disk or the lower disk is upward, defining the walking and sliding direction of the block as a positive direction; and the other direction is defined as the negative direction. Fig. 2 is a graph of the pre-rupture peak contour plot (black line at 3s intervals) and the rupture velocity after normalization. The left and right patterns of the same row have the same sliding angle, nucleating at (35, 0, -10) and (-35, 0, -10), respectively. The arrow on the right indicates the sliding direction of the upper disc relative to the lower disc. The model in the solid box represents the positive direction and the model in the dashed box represents the negative direction. By comparing the fracture rates in the different scenarios shown in fig. 2, it can be seen that the transition from overcutting fracture occurs only when the current slip angle exceeds a certain threshold, and that there is a large difference in the overcutting fracture distribution in the two opposite directions of the trend.

For the positive direction of smaller slip angles (in solid lines in FIG. 2) -33-43, unless the slip angle is close to 0 (pure slip layer), most of the super-shear fractures are concentrated only near the surface. This observation is consistent with the conclusion of [ Duan B.2010.Role of initial stress terms in fracture dynamics and ground motion: enzyme study with simulations for the Wenchuan earth source. Journal of geological Research,115, B05301, doi. As the absolute value of the sliding angle increases, the super shear fracture gradually disappears. In some cases (e.g. rake = ± 19.47 °, ± 29.96 °), the super-shear fracture regains sub-Rayleigh wave velocities after propagating a short distance near the shallow surface. This phenomenon is defined as unsustainable super shear failure, indicating that the propagation distance of super shear failure produced by a diagonal fault may not be long enough to be difficult to observe.

However, in the negative direction (within the dashed line in fig. 2), the free surface induced sub-disruptions may extend to the entire depth of pregnancy, similar to the case of pure slip fault (RA 0). This phenomenon is defined as global super shear failure. The global super shear fracture disappears immediately once the absolute value of the sliding angle exceeds a certain threshold (e.g., the sliding angle is greater than 43 ° or less than-33 °). These phenomena indicate that the fault slip component effectively suppresses the transition of the super shear fracture. Since the sliding angle thresholds of the slant-sliding reverse fault and the slant-sliding normal fault are 43 degrees and 33 degrees respectively, the slant-sliding reverse fault is easier to generate global super shear fracture than the slant-sliding normal fault.

To reveal the difference in fracture propagation in two opposite directions along the fault trend, a dynamic fracture snapshot (rake =0 °,20 °, -20 ° (fig. 2)) on the fault plane of the partial scene is shown. Here a scenario with a sliding angle of 19.47 ° is chosen, which has two types of super-shear transitions at the same time: global ultra shear fracture and unsustainable ultra shear fracture.

Fig. 3 is a snapshot of the slip rate at the slip fault (i.e., panel a) and the dip reverse fault (i.e., panel b, panel c slip angle =19.47 °) at 7, 10, 13, 16, 19 and 22s, with the middle five-pointed star representing the nucleation site; in the negative direction on the slip fault and the skew fault, a global super shear transition is generated. In the positive direction on the diagonal fault, an unsustainable local super-shear fracture and a linear sub-Rayleigh secondary fracture are generated. As can be seen from fig. 3, the secondary fracture front induced by the oblique fault may propagate near the free surface at an extra shear velocity and return to a sub-Rayleigh velocity at the deep part, which can also be seen in the fracture velocity of fig. 2 at a sliding angle of 19.47 °. It is noted that near the surface of the earth, the fracture front propagating at an ultra-shear velocity (fig. 3 panel c) is curved, similar to the ultra-shear fracture front spreading southerly (fig. 3 panel b), whereas the secondary fracture front forms a straight line of inclination (19 s, 22 s) as the fracture propagates. This phenomenon occurs in almost all diagonal faults, especially in the positive direction, and appears as a series of parallel diagonal lines (within the red dashed line in fig. 2) in the break time contour. The formation of a secondary linear sub-shear fracture is independent of the super-shear transformation, since the same fracture can be generated in a model with a sliding angle of 68 °, but no super-shear transformation occurs, indicating that for a diagonal fault, the free surface may induce a secondary fracture propagating at a sub-shear velocity. Some models (fig. 2) with slip angles of 9.55 °, ± 19.47 °, ± 29.96 °, can produce three types of fractures simultaneously: including sub-Rayleigh fracture, secondary FSI hypershear fracture and linear secondary sub-shear fracture, the three fracture fronts form two "kinks" at the intersection. By comparing the time-to-failure contours within the solid box of FIG. 2, it can be seen that the linear sub-shear failure front becomes steeper and plays a more dominant role as the sliding angle increases.

The fracture front appears differently in the two opposite propagation directions, and this asymmetry may lead to a corresponding difference in strong ground vibration. FIG. 4 is a snapshot of the velocities of the earth's surface at 7, 10, 13, 16, 19 and 22s for a walk-slip fault (panel a of FIG. 4) and a dip-slip reverse fault (panels b, c of FIG. 4), with the solid white lines representing the surface trajectory of the fault, the dashed lines representing the projection of the fault edge, and the five-pointed stars representing the epicenter position; as can be seen from fig. 4, the propagation of the super-shear fractures generates a significant mach cone in the negative (south) direction of the pure slip and the oblique slip fractures (panels a, b of fig. 4), which can induce global super-shear fractures. It is surprising that the linear sub-Rayleigh fracture dominates when the fracture propagates in the positive (north) direction and generates a strong surface mach wave (c). Because the super-shear fractures are distributed only within a narrow range of the surface in this direction, but generate a stronger mach wave than the global super-shear fractures.

In order to reveal the seismic motion distribution characteristics of the oblique slip fault, the low frequency peak velocity results (PGV) of a part of the scene are calculated, as shown in fig. 5, which is a distribution diagram of the peak velocities of the earth surface at different slip inclinations, the white solid line represents the trajectory of the fault on the earth surface, the dotted line represents the projection of the buried position of the fault, and the five-pointed star represents the epicenter position. Due to computational memory constraints, the minimum grid spacing is set to L _g =200m, which results in loss of high frequency components of the seismic waves. Considering that the length of 9 grid points is just one wavelength, the maximum frequency can be calculated by:

where λ = L _g *8,V _s ＝3464m/s.

The source is assumed to be located at the center of the fault (0, -10) in order to analyze the asymmetric behavior of the double-sided fracture. As can be seen from fig. 5, the wider high PGV distribution is in the positive direction than the PGV distribution in the negative direction of the skew layer. In most scenarios in this direction, linear sub-Rayleigh fractures dominate, appearing as tilted parallel lines. Previous studies have shown that overcut fractures cause much more extensive and stronger seismic motion than sub-shear fractures, however this contradicts the rake =42 °,68 ° scenario in fig. 2, since they have almost no overcut distribution in the positive direction (fig. 2). In addition, in the positive direction, the PGV distribution generated by the slip-slip reverse fault is significantly larger than that of the slip-slip forward fault and the slip fault. It was found that due to the non-perpendicular dip of the fault, the seismic motion is significantly enhanced in the direction where the upper disc sliding direction coincides with the fracture direction, since the reflected waves from the free surface reinforce the seismic motion in the vicinity of the reverse fault. In the case of a slip fault, the directional effect of the fracture is most concentrated away from the seismic source, while the directional effect of the dip fault on the surface is most concentrated away from the mass (upper or lower wall) moving upward from the seismic source. For example, the rake =0 ° scenario produces a bilateral global super shear rupture, while the north PGV is significantly larger than the south, when the upper disc sliding direction points exactly to the north, while other skew faults also exhibit similar characteristics. In summary, the spatial distribution of seismic high risk areas on a dip layer strongly depends on the magnitude of dip and slip angles.

In conclusion, the invention provides a systematic oblique fault FSI super-shear earthquake risk assessment method for the first time based on numerical simulation results of a plurality of earthquake dynamic fracture processes, wherein the method finds the asymmetry of FSI super-shear fracture on an oblique fault and assesses the azimuth generated by super-shear; the super-shear fracture difference of the oblique slip normal fault and the oblique slip reverse fault is summarized, and the possibility of generating super-shear and the magnitude of seismic oscillation can be evaluated; extracting the size of a key threshold value for conversion of the global super shear fracture, and predicting whether the super shear fracture is generated or not; finding and utilizing the relation between PGV distribution and fault direction to evaluate the seismic motion distribution range; and (3) finding the influence of the relative relation between the sliding direction of the upper disc and the fracture propagation direction on the earthquake motion strength, further guiding an earthquake hazard evaluation strategy and predicting the possible earthquake hazard distribution on the inclined fault.

The above description is only a preferred embodiment of the present invention, and is not intended to limit the present invention, and all equivalent variations made by using the contents of the present specification and the drawings are within the protection scope of the present invention.

Claims

1. A method for estimating the risk of an FSI super shear earthquake in a diagonal fault is characterized by comprising the following steps:

step 4, calculating low-frequency peak velocity PGV under different scenes, judging the relation between PGV distribution and fault direction, and evaluating the seismic oscillation distribution range of the upper disc of the inclined fault;

2. The method for estimating the risk of the oblique fault FSI super-shear earthquake as claimed in claim 1, wherein the variation range of the sliding angle is-90 ° to 90 °, and five types of faults are obtained in sequence: pure faults, oblique slip normal faults, pure slip faults, oblique slip reverse faults and pure reverse faults; and constructing initial stress fields under all scenes, and projecting the regional stress fields under different scenes onto the fault plane to obtain the shear stress with the same size and different directions and the normal stress with the same size, so as to eliminate the influence of the fault stress intensity difference under different scenes on the dynamic fracture process.

3. The method of claim 2, wherein two opposite fracture propagation directions along the fault trend are defined, if the slip component of the upper plate or the lower plate is upward, the slip direction of the block is defined as a positive direction, and the other direction is defined as a negative direction; velocity of rupture v _r By

Is calculated in that>

Is a spatial gradient operator, t _r (xi) is defined as the moment when the sliding rate exceeds the threshold value 0.01m/s at a certain point xi on the fault plane.

4. The method for estimating the FSI hyper-shear seismic risk of the inclined fault according to claim 3, wherein the FSI hyper-shear fracture on the inclined fault has asymmetry; in the positive direction, unsustainable super-shear fracture is generated, and the smaller the sliding angle is, the shorter the super-shear fracture duration is, and the lower the super-shear fracture duration is, and the super-shear fracture gradually disappears; in the negative direction, global super shear fractures are generated, which disappear immediately at sliding angles greater than 43 ° or less than-33 °.

5. The method for estimating the risk of the FSI super-shear earthquake of the inclined fault as claimed in claim 1, wherein in the step 4, the positive direction of the inclined fault is more widely distributed with high PGV than the negative direction, and the secondary sub-Rayleigh fracture can generate a similar Mach cone and cause the seismic wave energy on the disc of the inclined fault to be widely distributed.

6. The method for estimating the risk of an FSI hyper-shear earthquake of a deviated fault according to claim 1, wherein in the step 5, the earthquake motion is significantly enhanced in a direction in which the sliding direction of the upper disc coincides with the fracture direction.