Decoding the variation laws of the deformation field before strong earthquakes has long been recognized as an essential issue in earthquake prediction research. In this paper, the temporal and spatial distribution cha...Decoding the variation laws of the deformation field before strong earthquakes has long been recognized as an essential issue in earthquake prediction research. In this paper, the temporal and spatial distribution characteristics of deformation anomalies in the northeastern margin of the Qinghai-Tibetan Plateau before and after the Menyuan M_(S)6.9 earthquake were studied by using the Fisher statistical test method. By analyzing the characteristics of these anomalies, we found that: 1) The deformation anomalies are mainly distributed in the marginal front area of the Qinghai-Tibetan Plateau, where short-term deformation anomalies are prone to occur due to a high gradient of gravity;2) The deformation anomalies along the northeastern margin of the Qinghai-Tibetan Plateau are characterized by spatial propagation, and the migration rate is about 2.4 km/d. The propagation pattern is counterclockwise, consistent with the migration direction of M_(S)≥ 6.0 earthquakes;3) The time and location of the Menyuan earthquake are related to the group migration of earthquakes with M_(S)≥ 6.0. Finally,based on the results of gravity field variation and the theory of crust stress wave, the law of deformation anomaly distribution was discussed. We suggest that both the deformation propagation along the northeastern margin of the Qinghai-Tibetan Plateau and the earthquake migration are possibly associated with the variation of the stress field caused by subsurface mass flow.展开更多
Twenty-seven FHDZ-M15 combined geomagnetic observation systems(each of which is equipped with a fluxgate magnetometer and a proton magnetometer)had been installed in the China geomagnetic network before the 2008 Wench...Twenty-seven FHDZ-M15 combined geomagnetic observation systems(each of which is equipped with a fluxgate magnetometer and a proton magnetometer)had been installed in the China geomagnetic network before the 2008 Wenchuan earthquake,during which coseismic disturbances were recorded by 26 fluxgate magnetometer observatories.The geomagnetic disturbances have similar spatial and temporal patterns to seismic waves,except for various delays.Six proton magnetometer observatories recorded coseismic disturbances with very small amplitudes.In addition,fluxgate magnetometers registered largeamplitude disturbances that are likely to have included responses to seismic waves.However,two problems remain unresolved.First,why do these geomagnetic disturbances always arrive later than P waves?Second,why do the geomagnetic disturbances have spatial and temporal directivity similar to the main rupture direction of the earthquake?Solving these two problems may be crucial to find the mechanism responsible for generating these geomagnetic anomalies.展开更多
This article analyzes the relationship between the water level and the water-tube tilting in Shuangyang lake,based on the differential deformation features reflected by the NS and EW components of the water-tube tiltm...This article analyzes the relationship between the water level and the water-tube tilting in Shuangyang lake,based on the differential deformation features reflected by the NS and EW components of the water-tube tiltmeter.The results show a good spatiotemporal consistency between the variation of water level and the NS tilt component,which is considered to be affected by the magnitude and duration of the water level variation in Shuangyang Lake.The article uses Landsat remote sensing image data to extract the water boundary of Shuan-gyang Lake,and takes advantage of the finite element numerical simulation method to build three-dimensional models for different geological structural conditions of the Shuangyang seismostation.The simulation results show that when the underground medium is granite,the effect of water level variation on the vertical displacement of the surface is non-directional.With a 50-m soil layer in Model 2,the simulated NS tilt variation is equivalent to the actual observed water-tube tiltmeter NS component when the water level variation is 0.44 m and 0.8m.When the variation of water level reaches 2.0m,the simulation result of the NS component is 79.6 ms,which is slightly larger than the observed result of 60.32 ms.However,the simulation results show that the variation of the EW component is significantly smaller than that of the NS one.Due to the fact that the Shuan-gyang lake is long in the NS direction and short in the EW direction,the existence of the soil layer tends to generate ground deformation along the NS direction in the vicinity of the lake after the increase of water level,thereby resulting in the difference of the ground deformation in the two directions.展开更多
In this study,a series of undrained cyclic torsional shear tests were conducted to investigate the effect of cyclic loading frequency f on the pre-liquefaction(shearing contractive(SC)period and initial shearing dilat...In this study,a series of undrained cyclic torsional shear tests were conducted to investigate the effect of cyclic loading frequency f on the pre-liquefaction(shearing contractive(SC)period and initial shearing dilative(ISD)period)and post-liquefaction(late shearing dilative(LSD)period)deformation properties of saturated Fujian sand.The secant shear modulus G and damping ratioλin the entire cyclic loading process,and the unloading tangent shear modulus G_(L1)and flow deformation tangent shear modulus GL2 in the ISD and LSD periods were adopted to quantitatively characterize the evolution of hysteresis loop with an increase in shear strain amplitude ca.The test results show that the effect of f on G of saturated Fujian sand in the SC period is not apparent.However,all the G-γ_(a),G_(L1)-γ_(a),and GL2-ca curves in the ISD and LSD periods showed a downward trend with an increase in f.This study also proposes a modified method for calculatingλto compensate for the analytical error caused by the non-closure of hysteresis loop.Compared with the classical curves that mainly applied in geotechnical engineering,theλfirst increases and then decreases with the increase ofγa.Furthermore,theλevaluated by the modified method is approximately 10%–15%more than theλevaluated by the traditional method when theλreaches its peak value.展开更多
基金sponsored by the National Natural Science Foundation of China (41330314)。
文摘Decoding the variation laws of the deformation field before strong earthquakes has long been recognized as an essential issue in earthquake prediction research. In this paper, the temporal and spatial distribution characteristics of deformation anomalies in the northeastern margin of the Qinghai-Tibetan Plateau before and after the Menyuan M_(S)6.9 earthquake were studied by using the Fisher statistical test method. By analyzing the characteristics of these anomalies, we found that: 1) The deformation anomalies are mainly distributed in the marginal front area of the Qinghai-Tibetan Plateau, where short-term deformation anomalies are prone to occur due to a high gradient of gravity;2) The deformation anomalies along the northeastern margin of the Qinghai-Tibetan Plateau are characterized by spatial propagation, and the migration rate is about 2.4 km/d. The propagation pattern is counterclockwise, consistent with the migration direction of M_(S)≥ 6.0 earthquakes;3) The time and location of the Menyuan earthquake are related to the group migration of earthquakes with M_(S)≥ 6.0. Finally,based on the results of gravity field variation and the theory of crust stress wave, the law of deformation anomaly distribution was discussed. We suggest that both the deformation propagation along the northeastern margin of the Qinghai-Tibetan Plateau and the earthquake migration are possibly associated with the variation of the stress field caused by subsurface mass flow.
基金The National Key R&D Program of China(2017YFC1500502)provides the funding
文摘Twenty-seven FHDZ-M15 combined geomagnetic observation systems(each of which is equipped with a fluxgate magnetometer and a proton magnetometer)had been installed in the China geomagnetic network before the 2008 Wenchuan earthquake,during which coseismic disturbances were recorded by 26 fluxgate magnetometer observatories.The geomagnetic disturbances have similar spatial and temporal patterns to seismic waves,except for various delays.Six proton magnetometer observatories recorded coseismic disturbances with very small amplitudes.In addition,fluxgate magnetometers registered largeamplitude disturbances that are likely to have included responses to seismic waves.However,two problems remain unresolved.First,why do these geomagnetic disturbances always arrive later than P waves?Second,why do the geomagnetic disturbances have spatial and temporal directivity similar to the main rupture direction of the earthquake?Solving these two problems may be crucial to find the mechanism responsible for generating these geomagnetic anomalies.
基金sponsored by National Key Research and Development Program of China(2018YFC0807000)The Spark Program of Earthquake Technology of CEA(XH20070Y)The Earthquake Tracking Task of CEA(2021010221).
文摘This article analyzes the relationship between the water level and the water-tube tilting in Shuangyang lake,based on the differential deformation features reflected by the NS and EW components of the water-tube tiltmeter.The results show a good spatiotemporal consistency between the variation of water level and the NS tilt component,which is considered to be affected by the magnitude and duration of the water level variation in Shuangyang Lake.The article uses Landsat remote sensing image data to extract the water boundary of Shuan-gyang Lake,and takes advantage of the finite element numerical simulation method to build three-dimensional models for different geological structural conditions of the Shuangyang seismostation.The simulation results show that when the underground medium is granite,the effect of water level variation on the vertical displacement of the surface is non-directional.With a 50-m soil layer in Model 2,the simulated NS tilt variation is equivalent to the actual observed water-tube tiltmeter NS component when the water level variation is 0.44 m and 0.8m.When the variation of water level reaches 2.0m,the simulation result of the NS component is 79.6 ms,which is slightly larger than the observed result of 60.32 ms.However,the simulation results show that the variation of the EW component is significantly smaller than that of the NS one.Due to the fact that the Shuan-gyang lake is long in the NS direction and short in the EW direction,the existence of the soil layer tends to generate ground deformation along the NS direction in the vicinity of the lake after the increase of water level,thereby resulting in the difference of the ground deformation in the two directions.
基金supported by the National Outstanding Youth Science Fund Project of the National Natural Science Foundation of China(Grant No.51722801)National Natural Science Foundation of China(Grant No.52078020).
文摘In this study,a series of undrained cyclic torsional shear tests were conducted to investigate the effect of cyclic loading frequency f on the pre-liquefaction(shearing contractive(SC)period and initial shearing dilative(ISD)period)and post-liquefaction(late shearing dilative(LSD)period)deformation properties of saturated Fujian sand.The secant shear modulus G and damping ratioλin the entire cyclic loading process,and the unloading tangent shear modulus G_(L1)and flow deformation tangent shear modulus GL2 in the ISD and LSD periods were adopted to quantitatively characterize the evolution of hysteresis loop with an increase in shear strain amplitude ca.The test results show that the effect of f on G of saturated Fujian sand in the SC period is not apparent.However,all the G-γ_(a),G_(L1)-γ_(a),and GL2-ca curves in the ISD and LSD periods showed a downward trend with an increase in f.This study also proposes a modified method for calculatingλto compensate for the analytical error caused by the non-closure of hysteresis loop.Compared with the classical curves that mainly applied in geotechnical engineering,theλfirst increases and then decreases with the increase ofγa.Furthermore,theλevaluated by the modified method is approximately 10%–15%more than theλevaluated by the traditional method when theλreaches its peak value.
