Statistic relationship between earthquake magnitude and gravity changes and their mechanism analysis of different categories of earthquakes
Based on the 107 earthquakes of Ms≥4.0 collected,the statistical characteristics of the gravity changes anomaly before earthquakes in Chinese continent were studied.The statistical relationship between earthquake magnitude Ms and the gravity changes and the nature of the earthquake was explored.Reference indicators for the scale parameter S,magnitude parameter G,and time parameter T of gravity changes for Ms5.0,Ms6.0,Ms7.0,and Ms8.0 earthquakes have been given.The gravity changes before thrust,strike-slip,and normal earthquakes were compared and analyzed.Furthermore,by integrating the hypothesis of fluid-matter migration and the results of petrophysical experiments,the physical mechanisms of gravity changes before earthquakes and their connection with the meta-instability stage and the energy for earthquake gestation were discussed.The main conclusions are:(1)The relationship between gravity change parameters before earthquakes and earthquake magnitude appears to be nonlinear across all studied cases.Specifically,for earthquakes of magnitudes Ms5.0,Ms6.0,Ms7.0,and Ms8.0,the reference indicators for scale parameter S are 67,111,185,and 305 km;for magnitude parameter G,they are 49,67,92,and 125 μGal;and for time parameter T,they are 1.6 year,2.5 year,3.7 year,and 5.2 year,respectively.(2)The type of earthquake has some influence on the gravity changes before the earthquake,and the difference is especially obvious for earthquakes with Ms≥7.0.In terms of the scale parameter S of gravity change,the largest is for strike-slip earthquakes with Ms≤6.0,followed by normal earthquakes and then thrust earthquakes;thrust earthquakes are the largest with Ms＞7.0,followed by strike-slip earthquakes.In terms of the magnitude parameter G of gravity change,for magnitude Ms＜7.0,slip-type earthquakes are the largest,with thrust and normal earthquakes slightly lower;for magnitude Ms≥7.0,thrust earthquakes are the largest,followed by strike-slip earthquakes.In terms of the time parameter T of gravity change,normal earthquakes may correspond to longer times,followed by thrust earthquakes,and strike-slip earthquakes have the shortest times.(3)The possible physical mechanism of the gravity change before earthquakes are as follows:during the late stage of earthquake development or the meta-instability stage,the accumulated stress and strain in the surrounding area of the epicenter reach their peak,leading to the increase,expansion,and connection of rock fractures,which causes deep mantle material with rheological properties to migrate towards the crust,while the pre-existing fluid in the crust undergoes large-scale migration due to unequal stress distribution,ultimately causing a response in the surface gravity field.(4)The increase in pressure on the fault plane of a thrust earthquake will increase the frictional strength of the fault and inhibit rupture,leading to a higher parameter of gravity change.The increase in tension on the fault plane of a normal earthquake reduces the frictional strength of the fault and promotes rupture,resulting in a smaller parameter of gravity change.The longer time of gravity change may be related to the weaker driving force of tensile tectonics in mainland China.(5)The current mobile gravity measurement is constrained by the spatial and temporal density(point spacing of ≥30 km,time scale of ≥0.5 year),so that the gravity change.before earthquakes of Ms＜5.0 are generally not observed.Earthquake prediction based on gravity change anomalies mainly target earthquakes of Ms≤5.0,and the larger the magnitude Ms,the better the prediction efficacy.
万方数据
维普
