Triggered earthquake experiment method for simulating dive belt friction heterogeneous fault

Abstract

The invention discloses a triggered earthquake experiment method for simulating a dive zone friction heterogeneous fault, and aims to further improve the traditional laboratory earthquake fault model aiming at the confining speed-enhanced earthquake-free area of a speed-weakened earthquake-generating area and the fault structure distribution which generally exists in the dive zone and has nonuniform friction. A friction heterogeneous fault model is established, a modal vibration exciter with an ejector rod is adopted to apply low-amplitude disturbance to a fault, an earthquake is triggered under the condition that the fault is in a critical stress state, far-field earthquake triggering research on the friction heterogeneous fault is achieved, and the research on far-field earthquake triggering in allusion to the friction heterogeneous fault is expected to be provided for revealing complex sliding and fracture behaviors in a dive zone. And key laboratory evidence is provided.

Description

Technical Field

[0001] This invention relates to the field of geophysics, and more specifically to a method for simulating a triggered earthquake experiment on a subduction zone friction heterogeneous fault. Background Technology

[0002] Whether it's the prevalent slow-motion phenomenon found in the Cascadia subduction zone or the complex rupture behavior in the Chilean subduction zone, it's now widely recognized that creep or locked zones, where anechoic slip occurs, are common in subduction zone faults. These zones significantly influence the gestation and propagation of seismic ruptures, thus controlling the overall seismicity of the subduction zone. This complex phenomenon is typically explained by the rate-state friction law, which describes the friction constitutive model of faults. Here, parameters a and b are the frictional evolution parameters of the fault. Only when ab < 0 can the fault potentially experience unstable slip, i.e., an earthquake, in which case the fault exhibits velocity weakening. Conversely, when ab > 0, the fault exhibits velocity strengthening, resulting in stable slip. In other words, the frictional properties of the fault significantly affect seismic rupture in subduction zones. The non-uniformity of the frictional properties on the fault affects the propagation range of the rupture. Seismic nucleation only occurs in velocity weakening zones; velocity strengthening zones do not support seismic nucleation and inhibit rupture propagation. Laboratory seismological studies of frictional heterogeneous faults can supplement the complex slip and rupture behavior of subduction zones, and further explore the laws governing earthquake rupture of complex faults.

[0003] However, current understanding of the gestation mechanisms of earthquakes in frictionally heterogeneous faults containing velocity-enhancing zones is still limited. Existing studies mostly use fault gouge materials to simulate the distribution of frictionally heterogeneous faults. This leads to a serious problem: the non-uniform compaction of fault gouge results in uneven stress distribution. The controlling effect of stress on faults is widely recognized and extensively studied, so it will not be elaborated upon here. This means that the traditional method of using rock fault gouge to simulate frictional heterogeneity cannot decouple the two controlling factors of stress and frictional heterogeneity, and is therefore unreasonable. Summary of the Invention

[0004] In view of this, the present invention provides a laboratory simulation method for triggered earthquakes of heterogeneous frictional faults. It targets the seismogenic region with weakened velocity and the seismogenic region with enhanced velocity, which is a common feature of frictional heterogeneity in subduction zones. This method further improves the traditional laboratory earthquake fault model and studies the gestation and rupture nucleation characteristics of heterogeneous faults containing velocity-enhanced regions. The aim is to provide key laboratory evidence for revealing the complex slip and rupture behavior in subduction zones.

[0005] To achieve the above objectives, the present invention adopts the following technical solution: A method for simulating triggered seismic experiments on frictional heterogeneous faults in subduction zones includes the following steps: Step 1: Construction of a straight and smooth fault model Obtain a square board and bevel it along the hypotenuse of the square plane to form two identical triangular beveled models. The opposite surfaces of the two beveled models are the fracture surfaces. Polish the two fracture surfaces to form a smooth surface. Then, drill a round hole on one side of the fracture surface of one of the beveled models, close to the fracture location. Step 2: Construction of the Frictional Heterogeneous Fault Model Velocity enhancement regions are symmetrically set on the fracture surfaces of two obliquely cut models. A Teflon layer is sprayed on the velocity enhancement region of one fracture surface, and a Teflon layer is sprayed on the velocity enhancement region of the other fracture surface. After drying, a polyvinyl chloride film is covered. The outer parts of the velocity enhancement regions of the two fracture surfaces are roughened to form velocity weakening regions, thus completing the construction of the friction heterogeneous fracture model. Step 3: Drive the fault model to the critical stress state The model is loaded, and after being driven to a certain stress state by force in the X and Y directions at the loading end, the force in the X direction is kept constant, and the force in the Y direction is driven by displacement to reach the critical state where rupture and instability are about to occur. Step 4: Far-field earthquake triggering study of frictional heterogeneous faults Low-amplitude dynamic disturbances were released by hammering a circular hole with a vibrator, triggering the fault to become unstable and rupture. The data were recorded and studied.

[0006] Preferably, the square plate in step one is made of polymethyl methacrylate, a material that is readily available.

[0007] Preferably, the vertical distance between the center of the circular hole in step one and the fault plane is 1-2 times the thickness of the plate. Since it is a low-frequency disturbance, in order to better trigger fault instability, the circular hole, i.e., the trigger point, needs to be set as close as possible to the fault plane, but not too close, as this will interfere with other locations and make it difficult to accurately control the nucleation location. Therefore, a vertical distance of 1-2 times the thickness of the plate is used to ensure accurate control.

[0008] Preferably, the depth of the circular hole in step one is 1 / 2 of the thickness of the plate.

[0009] Preferably, in step two, the Teflon layer thickness is ≤100μm; the grinding roughness is controlled to keep the elevation fluctuation of the fractured grinding surface within 40-60μm. A thinner Teflon layer can prevent excessive local thickness from affecting the overall stress distribution, and control the paint coating thickness within 100 micrometers, thereby greatly reducing the impact on stress distribution while meeting the desired effect.

[0010] Preferably, in step three, a biaxial loading system is used to drive the loading end in the X and Y directions to the same stress level at the same loading rate.

[0011] Optionally, the rated excitation force of the low-frequency exciter in step four is 200N.

[0012] Furthermore, the vibrator is equipped with a vertically moving push rod, which is connected to the horizontal rod by a Scott-Russell linkage mechanism. The push rod is perpendicular to the horizontal rod, and the end of the horizontal rod abuts against the circular hole.

[0013] Preferably, the data mentioned in step four includes the stress drop, friction coefficient, and seismic period of the frictional heterogeneous fault.

[0014] As can be seen from the above technical solution, compared with the prior art, the present invention discloses a method for simulating a triggered seismic experiment on a subduction zone friction heterogeneous fault, which has the following beneficial effects: This invention utilizes the unique speed-enhancing frictional properties exhibited by Teflon and PVC films, taking advantage of their extremely thin thickness, insensitivity to loading, and uniform stress distribution across the entire fault plane, to construct a frictional non-uniform fault model for rock-like materials.

[0015] Furthermore, compared to the existing biaxial loading experimental-driven spontaneous earthquake research, which suffers from the defect of unstable earthquake nucleation location and significant randomness, the technical solution of this invention is to control the earthquake rupture to occur at the same location in the velocity weakening section, and to study the specific evolution process of the earthquake rupture propagating to the velocity strengthening section. Then, a low-amplitude modal exciter with a top rod is used to trigger the fault model in the critical stress state, thus realizing far-field earthquake triggering research for frictional heterogeneous faults. Attached Figure Description

[0016] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the provided drawings without creative effort.

[0017] Figure 1 This is a structural diagram of the oblique-cut model; Figure 2 The diagram shows the relative position of the obliquely cut model and the structural diagram of the fault plane; Figure 3 This is a diagram showing the connection between the frictional heterogeneous fault model and the far-field triggering device. Figure 4 This is a comparison diagram of the frictional evolution of a non-homogeneous frictional fault and a smooth homogeneous fault. Detailed Implementation

[0018] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention. Example

[0019] As attached Figure 1-3 As shown, the experimental method for simulating spontaneous earthquakes in a subduction zone with frictional heterogeneous faults includes the following steps: Step 1: Process a straight and smooth fault model; Using polymethyl methacrylate (PMMA), a rock-like material commonly used in laboratory earthquakes, and taking advantage of its uniformity, flexibility, and ease of processing, a square plate with a length of a cm, a width of a cm, and a thickness of b mm was formed. The entire plate was then beveled along the hypotenuse of the square plane to form two identical triangular plates. The plane containing the hypotenuse is the fault plane. The fault plane was then polished to form a smooth surface. Subsequently, a circular hole with a fixed diameter and a depth of b2 mm was drilled on one side of the fault plane, close to the fault plane and perpendicular to it at a distance of b-2b mm. This hole serves as the triggering position for subsequent far-field triggering using an exciter. Step 2: Set up a fault friction heterogeneous distribution that confines the velocity enhancement zone to the velocity weakening zone; Based on the existing flat and smooth fault surface, a frictional non-uniform fault model containing velocity-enhancing and velocity-weakening regions is further constructed. A velocity-enhancing region with a total length of 2 cm is symmetrically set with the fault plane center as the origin. The specific method is as follows: other areas are covered, and a thin layer of Teflon paint is evenly sprayed onto a specified area of ​​2 cm length on two triangular fault planes. The coating thickness is less than 100 micrometers. After 24 hours of drying, a 2 cm × b mm polyvinyl chloride (PVC) film is cut and covered onto one side of the Teflon area to prevent adhesion during subsequent loading. The remaining exposed PMMA areas without Teflon coverage are uniformly sanded with high-grit sandpaper to a roughness controlled to maintain the surface elevation fluctuation of the fault within 40-60 μm, forming a velocity-weakening region. The fault model is divided into three sequentially arranged regions: velocity-weakening, velocity-enhancing, and velocity-weakening. This completes the construction of a frictional non-uniform fault model containing velocity-enhancing and velocity-weakening regions, using a natural subduction zone fault as a template. Step 3: Use a biaxial loading system to drive the fault model to the critical stress state. The model is loaded using a biaxial loading device. The stress distribution of biaxial loading is uniform, which can effectively eliminate the influence of stress non-uniformity on the problem of interest. Force is applied in the X and Y directions at the loading end to drive the loading end to the same stress level in the X and Y directions at the same loading rate. The force in the X direction is kept constant, and the force in the Y direction is driven by displacement to reach the critical state where rupture and instability are about to occur. Step 4: Using a low-frequency dynamic excitation device to conduct far-field seismic triggering research on frictional heterogeneous faults. A 200N vibrator with a top rod is used as a low-amplitude excitation source. The vibration amplitude is controlled by a vibration controller to be at a suitable value that can induce seismic rupture. A Scott-Russell linkage mechanism is used to connect the vertically moving top rod of the vibrator with a horizontal rod that enables far-field triggering. The vertical vibration of the top rod of the vibrator is converted into the horizontal impact displacement of the horizontal rod, realizing the connection between the low-amplitude vibrator and the trigger position. When the vibrator is started, the horizontal rod hammers the pre-drilled circular hole near the fault position, releasing the low-amplitude dynamic disturbance and triggering the fault to become unstable and ruptured. After an earthquake is triggered, data collected by a dual-axis servo press is used to analyze changes in friction coefficient, earthquake period, and other parameters.

[0020] The biaxial press can record the horizontal force Fx and vertical force FY in real time during the earthquake preparation stage, the coseismic stage after triggering, and the post-earthquake healing stage. After acquiring and further analyzing the full-cycle laboratory earthquake data, the fault macroscopic friction coefficient μ=

[0021] like Figure 4 As shown, the evolution of the friction coefficient over time reveals that the frictional heterogeneous fault formed by the velocity-enhanced zone exhibits unique phenomena such as high frequency, low friction, and frequent small earthquakes.

[0022] Furthermore, by changing parameters such as the exciter parameters and the position of the trigger hole, we can study the nucleation and fracture evolution of friction heterogeneous faults under different conditions.

[0023] The embodiments in this specification are described in a progressive manner. For the apparatus disclosed in the embodiments, since they correspond to the methods disclosed in the embodiments, the description is relatively simple; relevant details can be found in the method section. The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for simulating triggered seismic experiments on frictional heterogeneous faults in subduction zones, characterized in that, Includes the following steps: Step 1: Construction of a straight and smooth fault model Obtain a square board and bevel it along the hypotenuse of the square plane to form two identical triangular beveled models. The opposite surfaces of the two beveled models are the fracture surfaces. Polish the two fracture surfaces to form a smooth surface. Then, drill a round hole on one side of the fracture surface of one of the beveled models, close to the fracture location. Step 2: Construction of the Frictional Heterogeneous Fault Model Velocity enhancement regions are symmetrically set on the fracture surfaces of two obliquely cut models. A Teflon layer is sprayed on the velocity enhancement region of one fracture surface, and a Teflon layer is sprayed on the velocity enhancement region of the other fracture surface. After drying, a polyvinyl chloride film is covered. The outer parts of the velocity enhancement regions of the two fracture surfaces are roughened to form velocity weakening regions, thus completing the construction of the friction heterogeneous fracture model. Step 3: Drive the fault model to the critical stress state The model is loaded, and after being driven to a certain stress state by force in the X and Y directions at the loading end, the force in the X direction is kept constant, and the force in the Y direction is driven by displacement to reach the critical state where rupture and instability are about to occur. Step 4: Far-field earthquake triggering study of frictional heterogeneous faults Low-amplitude dynamic disturbances were released by hammering a circular hole with a low-frequency vibrator, which triggered the instability and rupture of the fault. The data were recorded and studied.

2. The method for triggering earthquake experiments to simulate frictional heterogeneous faults in subduction zones according to claim 1, characterized in that, The vertical distance between the center of the circular hole and the cross-section in step one is 1-2 times the thickness of the plate.

3. The method for triggering earthquake experiments to simulate frictional heterogeneous faults in subduction zones according to claim 1, characterized in that, The depth of the circular hole mentioned in step one is 1 / 2 of the thickness of the plate.

4. The method for triggering earthquake experiments to simulate frictional heterogeneous faults in subduction zones according to claim 1, characterized in that, In step two, the Teflon layer thickness should be ≤100μm; the grinding roughness should be controlled to keep the elevation fluctuation of the topo-grinding surface within 40-60μm.

5. The method for triggering earthquake experiments to simulate frictional heterogeneous faults in subduction zones according to claim 1, characterized in that, In step three, a biaxial loading system is used to drive the loading end to the same stress level in the X and Y directions at the same loading rate.

6. The method for triggering earthquake experiments to simulate frictional heterogeneous faults in subduction zones according to claim 1, characterized in that, The data mentioned in step four include stress drop, friction coefficient, and seismic period of the frictional heterogeneous fault.

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