JP3243499B2 - Earthquake prediction method by monitoring fault displacement and volume displacement - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for predicting the risk of earthquake occurrence by measuring and monitoring minute displacement of a fault and volume displacement of a bedrock, and more particularly to a deformation behavior associated with a slip phenomenon of a fault and an earthquake occurrence. The technical field of predicting earthquake occurrence by measuring and evaluating the earthquake.

[0002]

2. Description of the Related Art Conventionally, in the field of earthquake engineering or geophysics, to predict or predict the occurrence of an earthquake,
Measurements of seismic waveforms of prequakes propagating in the rock and volumetric strain related to the strain energy in the rock have been performed. In general, seismometers and accelerometers are used as vibration measurements related to such seismic motion in rock. Of these conventional measurements, seismic waveform measurement is based on the assumption that a microearthquake related to a small slip failure occurs before a slip phenomenon of a fault related to an earthquake occurs. The purpose of this study was to predict or predict the subsequent earthquake by measuring the waveform of a microearthquake propagating through the ground as the amplitude or vibration pressure of a vibrating resonator.

On the other hand, as the strain measurement related to the seismic motion in the rock, a method of evaluating the strain amount of the rock from the deformation of a borehole cavity drilled deep into the rock is generally employed. This conventional strain measurement is intended to predict or predict the occurrence of an earthquake by grasping the increasing tendency of the amount of rock strain related to the strain energy of the crust or plate, which causes the occurrence of an earthquake.

[0004] By the way, the earthquake is a phenomenon that, as revealed by plate technics, the strain energy accumulated by the movement of the crustal plate closely related to the flow of the mantle is rapidly released in a deep underground fault or the like. Many of these have occurred on existing faults, faults that have a history of earthquakes and rock failures at the plate-impacting transform (plate boundary).

[0005] In other words, in seismic engineering, it is a yield-slip phenomenon in a material-deficient portion. The fault yields under the stress condition in which the strain energy is induced near the fault, and the crustal potential energy due to the slip and motion due to vibration. It is said to exhibit stick-slip behavior that is sequentially released as energy. In order to grasp such yielding mechanically, it is necessary to directly detect the fault microscopic behavior and stress change in a transform in which the stress distribution and deformation behavior are qualitatively almost constant.
The scale etc. are inferred with high accuracy.

[0006]

However, the seismic waveform measurement by the conventional seismometers and accelerometers does not measure phenomena directly related to the mechanical behavior of the fault yielding. In this method, a slip deformation due to the yielding of a small-scale fault, which may occur in some cases, was assumed to be a precursor, and a prediction evaluation was attempted from the vibration. Conventional volumetric strain measurement in rocks is to grasp the rise of strain energy in the rocks, and the strain that causes yield slip on the fault is detected, but the data to know the limit of the yield is not did not become.

In other words, these conventional earthquake prediction methods based on seismic waveform measurement and volume strain measurement attempt to predict the seismic phenomenon, which is the yield slip of a fault, but directly measure the behavior and mechanism leading to the yield of the fault. Therefore, it was difficult to predict and evaluate the timing, range, scale, etc. of the earthquake with high accuracy from these conventional measurements.

[0008] As described above, there is no example of predicting the occurrence of an earthquake based on the measurement of the micro-behavior of the fault causing the occurrence of the earthquake and the volume displacement around the fault. There was no example of measuring and evaluating the relationship with the minute behavior. This is because, as mentioned above, conventional seismometers and accelerometers measure the occurrence of seismic ground motion, so unless a small earthquake regularly occurs before a large-scale earthquake occurs, it is a means of earthquake prediction. In addition to not being effective, conventional volumetric strain measurements in rock only measure the factors of earthquakes that are not related to fault yielding phenomena, so these conventional measurements predict the magnitude and timing of earthquakes. It could not be evaluated.

[0009] Further, if it is considered mechanically that the earthquake is the yield slip phenomenon of a fault which is a rock mass defect, it is indispensable to grasp the behavior process and mechanism leading to the yield for earthquake prediction. For the first time, direct measurement and understanding of microscopic behavior near a fault or in a transform (plate boundary surface) including a fault should provide clues for earthquake prediction.

However, the conventional measurement does not directly measure the fault which is the source of the earthquake, but supposes that these measurements are performed on the ground surface or underground rock using a conventional displacement gauge or pressure gauge. In addition, there is a problem that it is necessary to measure phenomena affected by loose areas on the ground surface and underground cavities, weathering of rocks and cracks, microcracks, groundwater, and the like, and it is not possible to evaluate microscopic phenomena.

SUMMARY OF THE INVENTION The present invention has been made in view of the above-described conventional problems, and directly measures a microscopic behavior near a fault where an earthquake occurs or in a transform including the fault. The process and mechanism that causes the yield-slip phenomenon can be understood, and earthquake prediction can be made even if a small earthquake before a large-scale earthquake does not occur regularly. It is an object of the present invention to provide an earthquake prediction method by monitoring fault displacement and volume displacement which can be predicted and evaluated with high accuracy.

[0012]

In order to solve the above-mentioned problems, the present invention provides a method for predicting an earthquake by monitoring fault displacement and volume displacement, wherein a fault plane in a boundary transform such as a crustal plate or the like where an earthquake occurs is located. Involvement of the deformation behavior of the process leading to the yield-slip of an earthquake at or near an active fault plane near or at the surface and the stress change derived from the volumetric displacement of a borehole in a rock mass that causes an earthquake It is measured in the transform, and is characterized by predicting the risk of earthquake occurrence from the behavior related to rock microfracture and plastic deformation on the fault plane immediately before the occurrence of the slip phenomenon.

[0013] A borehole is drilled across a fault plane in which strain energy causing an earthquake is accumulated or a fault plane included in the boundary plane, and the relative change between the two rocks is three-dimensionally sandwiched by the fault plane. A three-dimensional micro-displacement measurement data and three-dimensional micro-displacement measurement data are installed, and a three-dimensional micro-displacement measurement device is installed. From the measurement data, the micro deformation of the fault and the stress change of the rock near the fault are measured and evaluated.

From the microdeformation of the fault and the stress change of the rock near the fault, the correlation between the displacement vector in the direction in which the slip phenomenon of the fault occurs and the stress change in that direction is derived, and the small displacement in the direction perpendicular to the fault plane is obtained. The correlation of the stress change is also derived. Further, the results of the above-described measurement and evaluation are arranged as changes with time, and the deformation rate in the correlation between the two is also monitored.

The mechanical mechanism of earthquake occurrence is that a fault that has undergone slip rupture has yielded again and exhibits a slip phenomenon, and local stress concentration on the fault plane before reaching the yield limit. Rock microfracture and plastic deformation phenomena appear. Instead of the conventional indirect method of measuring stress waves and electromagnetic waves generated by such micro-destruction and plastic deformation, it directly monitors the micro-deformation of the fault and the stress change based on the volume displacement near the fault. By this means, these precursor phenomena are identified as changes in the deformation rate and the like, and the occurrence time and occurrence risk of the earthquake are evaluated.

The borehole is drilled across a fault plane existing in the rock, and a fault displacement measuring device installed in the borehole is:
The three-dimensional measurement of the relative small displacement between the measurement unit and the sensor unit, which are independently fixed and installed in the rock mass sandwiching the fault, and the volume displacement measurement device installed in the borehole has no fault A three-dimensional volume displacement is obtained by measuring the relative small displacement between the measurement unit and the sensor unit, which are fixedly installed independently in a continuous rock mass, and the minute displacement of the borehole wall around the sensor unit.

[0017]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, embodiments of the present invention will be described in detail with reference to the accompanying drawings. FIG. 1 shows that in a borehole 3 formed by drilling across a fault 2 existing in a transform 1 which is a plate boundary layer in a rock, a fault micro-displacement meter 4 is installed with the fault 2 interposed therebetween. An example is shown in which a volume displacement meter 5 is installed on a continuous rock mass portion that does not sandwich the fault 2 of FIG.

Since the strain energy of the crustal plate opposes the transform 1 which is the boundary surface where the crustal plate collides, the fault 2 in the above-described transform 1 before the slip yielding and the bedrock in the vicinity thereof Shows a stress state that is equivalent by multiplying the deformation coefficient depending on the elastic modulus and depth of the rock mass and the correction coefficient of the earth covering pressure. For this reason, the fault micro-displacement meter 4 and the volume displacement meter 5 can measure the micro-deformation of the transform 1 and the fault 2 and the volume change of the bedrock with high accuracy until immediately before the occurrence of the earthquake.

FIG. 2 is an enlarged sectional view showing the tomographic displacement meter 4 in FIG. The fault micro-displacement meter 4 includes a measuring unit 9 fixedly installed by fixing means 8 in one of the rocks 7 sandwiching the fault 2 in the borehole 3 formed by drilling the rocks 6 and 7 across the fault 2, The other rock mass 6 has a sensor unit 10 fixedly installed by the same fixing means 8. The sensor unit 10 includes at least three directions of displacement sensors 1.
1 and is a member separated from the measuring unit 9 but is installed in a state of being in contact with each other.

The measuring section 9 and the sensor section 10 are integrally inserted into the borehole 3 by a detachable fixed guide member (not shown), and are fixed by expansion and contraction means (not shown). Is fixed independently. The cylindrical member accommodating the measuring unit 9 and the sensor unit 10 is provided with at least two hole-wall pressing parts 8a around the circumference thereof, and the fixing means 8 is expanded and contracted by a hydraulic cylinder or a spring. It has a structure in which it is projected outward in the direction and is pressed and fixed to the rock surface of the hole wall.

The measuring section 9 is provided at the rear end of the first cylindrical member 12 whose front end is reduced in diameter, and has at least three measuring surfaces 9a which are flat and intersect at right angles to each other. On the other hand, the sensor unit 10 includes a second cylindrical member 13 following the first cylindrical member 12.
At least three displacement sensors 11, which are provided in at least three axial directions and are vertically opposed to the measurement surface 9a, respectively, are attached to the tip of the sensor. Can be measured.

The relative displacement between the first and second cylindrical members 12 and 13 is taken as a result of measurement in at least three axial directions orthogonal to each other, and these numerical values are converted into a tomographic plane and a coordinate system orthogonal to the plane. Accordingly, it is possible to evaluate a small slip vector (a deformation direction and a deformation amount) in the tomographic plane and a micro-displacement of the closing or the opening of the tomographic plane.

FIG. 3 shows an example of measurement by the tomographic displacement meter 4 in FIG. 2, and FIG. 4 shows an example in which coordinates of the tomographic plane are converted from the measurement result. Both of them represent a time-dependent change curve, and the vertical axis 14 in FIG. 3 represents the displacement amount by the triaxial displacement sensor 11 in the sensor unit 10 of the tomographic displacement sensor 4, and the horizontal axis 15 represents the elapsed time of the measurement. Is represented. In this displacement measurement, the displacement is measured as a relative change from the time of installation in the well 3, so that the start of the measurement is based on the displacement reference 1.
It becomes 6.

By converting the measurement data obtained in FIG. 3 into a tomographic plane and a coordinate system orthogonal to the tomographic plane, as shown in FIG. 4, a small slip vector (deformation direction and deformation amount) in the tomographic plane is obtained. It is possible to evaluate the occlusion of the tomographic plane or the minute displacement of the opening. The vertical axis 17a in FIG.
b represents the azimuth of the displacement vector, and the abscissa 18 represents the elapsed time of the measurement. For example, the maximum displacement of the slip displacement vector in the fault plane is represented by a curve 19, and the azimuth is represented by a curve 20; Further, a closing or opening displacement in a direction orthogonal to the tomographic plane is represented by a curve 21.

In the time curve of the maximum slip displacement in the fault plane of the curve 19, the inflection point 22 where the slope of the curve changes indicates that the deformation of the fault exhibits a behavior related to microfracture or plastic deformation in the fault plane. This shows that it is possible to evaluate the change in the deformation rate of the fault represented by the magnitude of this inclination.

FIG. 5 is an enlarged sectional view showing the volume displacement meter 5 in FIG. The volume displacement meter 5 includes a measurement unit 109 and a sensor unit 110 which are fixed and installed by a plurality of fixing means 108 in a continuous rock 6 which does not sandwich the fault 2 in a well 3 formed by drilling across the fault 2. Have.

The measuring section 109 is provided at the rear end of the first cylindrical member 112 whose front end is reduced in diameter.
a. On the other hand, the sensor section 110 is the first cylindrical member 11
2 is provided on the second tubular member 113 following the at least three displacement sensors 1 facing the wall surface of the well 3.
At least six displacement sensors 11a and at least three displacement sensors 111b provided in at least three axial directions opposed to the measurement surface 109a are attached, and the minute displacement of the well wall surface and the minute displacement of the relative distance between the two fixed members are measured. It can measure in three dimensions.

The stress state in the rock is shown from the measurement results of the radial displacement around the second cylindrical member 113 in at least three directions and the relative displacement between the two fixed members in at least three axes in the member axial direction. It can be evaluated as six volume displacement tensors or six volume strain tensors related to the six stress tensors.

FIG. 6 shows an example of measurement by the volume displacement meter 5 in FIG. 5, and FIG. 7 shows an example of a main stress change curve derived from the measurement results, and an example in which the shear stress in the slip plane direction in the fault plane is evaluated. 8 are respectively shown in FIG. 8, and both show a time-dependent change curve. The vertical axis 23 in FIG. 6 is a displacement sensor 111 in at least six directions in the sensor unit 110 of the volume displacement meter 5.
The horizontal axis 24 represents the elapsed time of the measurement. In this displacement measurement, the displacement is measured as a relative change from the time of installation in the well 3, and the volume strain can be easily converted from the ratio with the measurement interval in the well 3.

Six stress tensors are derived from the measurement data in at least six directions obtained in FIG. 6, and from the evaluation that the three stresses represented by the shear stress tensor become 0, the main stress change amount is determined. It is derived as shown in FIG. The vertical axis 25 in FIG. 7 represents the magnitude of the principal stress, the horizontal axis 26 represents the elapsed time, and the principal stress represents the amount of change since the start of the measurement. Here, the curve 27 represents the change in the maximum principal stress, the curve 28 represents the change in the minimum principal stress, and the curve 29 represents the change in the intermediate principal stress.

Deriving the shear stress tensor according to the direction of the azimuth of the slip displacement vector in the fault plane, the change in the shear stress in the direction of the small slip vector in the fault plane is evaluated as shown in FIG. Can be. The vertical axis 30 in FIG. 8 represents the magnitude of the stress, and the horizontal axis 31 represents the elapsed time of the measurement. The inflection point 33 where the slope of the curve changes in the time-dependent change curve 32 of the shear stress in the fault plane indicates that the deformation of the fault exhibits a behavior related to microfracture and plastic deformation in the fault plane. Similarly to the evaluation of the change in the deformation rate of the time curve of the slip displacement shown in FIG. 4, it is possible to evaluate the tendency of the change in the shear stress change rate of the fault represented by the magnitude of the slope.

In FIG. 9, the vertical axis 34 represents the maximum slip displacement in the fault plane shown in FIG. 4, and the horizontal axis 35 represents the shear stress change in the fault plane shown in FIG. The correlation curve 36 is shown. In the correlation curve 36, the slope of the curve represents the shear stiffness 37, and the curve 38 in which the stiffness has changed indicates that the deformation of the fault exhibits a behavior related to microfracture or plastic deformation in the fault plane. By evaluating the inflection point 39 of the deformation rate in the shear rigidity of the fault, it becomes possible to recognize the risk of earthquake occurrence in the fault.

Further, based on the change of the deformation rate in this curve, the risk of earthquake occurrence can be predicted by deriving the maximum shear stress 40 in the yielded state from the predicted curve 41 that converges. By constantly monitoring the small displacement of the fault and comparing it with the expected curve,
It is possible to evaluate the risk of earthquake occurrence with high accuracy.

The strain energy distribution derived from the numerical analysis of the stress in the stratum using the displacement data of the ground surface measured and evaluated by satellite observation and the like, and the three-dimensional minute displacement on the fault plane in the transform. It is also possible to compare the potential energy derived from the slip displacement of the rock sandwiching the fault and to predict the risk of slip deformation on the fault plane from the process of strain energy release.

[0035]

As described above, according to the earthquake prediction method of the present invention, a borehole is bored across a fault plane in which strain energy causing an earthquake is accumulated or a fault plane included in the boundary plane. Then, a fault displacement measuring device that measures the relative change between the two rocks three-dimensionally across the fault plane is installed, and a volume displacement measuring device that evaluates the stress change from the measurement of the volume change in the rock near the fault is installed. Then, from these three-dimensional micro displacement measurement data and three-dimensional micro volume displacement measurement data, the micro deformation of the fault and the stress change of the rock near the fault are measured and evaluated.

From the microdeformation of the fault and the stress change of the rock near the fault, a correlation between the displacement vector in the direction in which the slip phenomenon of the fault occurs and the stress change in that direction is derived. The correlation of the stress change is also derived, and the results of the above measurement and evaluation are arranged as a change with time, and the deformation rate in the correlation between the two is also monitored.

As described above, in the fault or the transform including the fault which causes the earthquake, the behavior related to the microfracture of the rock and the plastic deformation on the fault surface immediately before the occurrence of the slip phenomenon is directly measured. In addition, it is possible to understand the behavioral processes and mechanisms that cause the yield slip phenomenon of the fault. In addition, by identifying these precursor phenomena as changes in the deformation rate or the like, it becomes possible to evaluate the time and risk of occurrence of an earthquake. Therefore, it is possible to predict earthquakes even if small earthquakes before large-scale earthquakes do not occur regularly,
Further, it is possible to predict and evaluate the occurrence time, range, and scale of the earthquake with high accuracy.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view showing the installation state of a tomographic displacement meter and a volume displacement meter.

FIG. 2 is an enlarged sectional view showing an installation state of a tomographic displacement micrometer.

FIG. 3 is a graph of a measurement example using a tomographic displacement micrometer.

FIG. 4 is a graph in which a measurement result obtained by a tomographic displacement meter is coordinate-transformed with respect to a tomographic plane.

FIG. 5 is an enlarged cross-sectional view showing an installation state of a volume displacement meter.

FIG. 6 is a graph of a measurement example using a volume displacement meter.

FIG. 7 is a graph of a main stress change curve derived from a measurement result by a volume displacement meter.

FIG. 8 is a graph showing an evaluation of a shear stress in a slip vector direction.

FIG. 9 is a graph of a correlation curve between a maximum slip displacement and a change in shear stress.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Transform 2 Fault 3 Hole 4 Fault micro displacement meter 5 Volume displacement meter 8 Fixing means 9 Measurement part 10 Sensor part 11 Displacement sensor

Continuation of the front page (72) Inventor Yuki Onodera 3-103 Onuma, Kasukabe-shi, Saitama Examiner, Tera, Terra Co., Ltd. Toru Hongo (56) References JP-A-9-145849 (JP, A) JP-A-57- 40608 (JP, A) Table 8-8-504941 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) G01V 1/00

Claims (1)

(57) [Claims] 1. A three-dimensional structure of a fault to be measured by installing a fault displacement measuring device in a borehole pierced across a fault plane in a boundary plane transform such as a crustal plate or an active fault plane near the surface of the ground. Risk of earthquake occurrence based on monitoring data of micro-displacement and three-dimensional micro-volume displacement in borehole, which is measured by installing a volume displacement measuring device on a continuous rock part that does not sandwich the fault in the borehole drilled near the fault plane A small slip vector and a small opening displacement or a small occlusion displacement of the fault plane derived from the three-dimensional micro displacement, and a micro ground in the rock near the fault plane derived from the three-dimensional micro volume displacement. From the pressure change, the correlation between the small deformation in the slip direction of the fault plane and the direction perpendicular to the fault plane and the small ground pressure change is evaluated, and the results of these measurement evaluations are arranged as changes over time, and the correlation is further evaluated. By monitoring the deformation rate at the same time, the rock micro-fracture and plastic deformation behavior on the fault plane immediately before the slip plane occurs as a seismic phenomenon as an earthquake can be evaluated by the above-mentioned correlation and the change of the deformability and deformation rate over time. Earthquake prediction method by monitoring fault displacement and volume displacement, characterized by recognizing as change of earthquake.