Changes Of Crustal Stress/Strain And Geodynamics

The stress and strain fields inside the Earth are changing all the time,and they are intimately related to geodynamic processes.This thesis is directed at understanding the relationship between changes of crustal stress/strain and various dynamic processes on the Earth(e.g.,tide,hydrologic process and earthquake).We approach the problem observationally and theoretically.In the observational front,we decipher strain signals recorded by borehole strainmeters in western North America and explore physics of various dynamic processes such as tide,hydrologic process and earthquake through quantitative analyses of those recorded anomalous strain signals.In the theoretical front,we establish a physical framework and calculate stress changes inside the Earth induced by surface loading,gas injection/extraction in Hutubi(China)gas field,and North Korea's nuclear test.We further analyze triggering effects of these stress changes on the nearby seismicity.In the observational front,we decipher crustal deformations and explore their physical mechanisms using data recorded by the Plate Boundary Observatory borehole strainmeters.(1)We observe an abnormally strong(1-2 orders of magnitude larger than theoretical prediction)daily-cycle S1 strain tide in the strainmeters,and find that it is caused by elastic loading of barometric pressure and thermoelastic deformation of surface temperature.(2)We detect a strong(2 microstrain,1-2 orders of magnitude larger than tidal deformation)hydro-related localized long-period strain signal in the strainmeters,and through poroelastic simulations,we discover that the observed long-period strain and its relationships with rainfall and pore pressure could be explained by a rainfall-hydrological diffusion-poroelastic deformation mechanism.(3)We observe abnormal postseismic strain variations that exhibit larger amplitude than the coseismic strain and that last several months.By comparing the postseismic strain with the pore pressure,we find that the postseismic strain is jointly caused by postseismic pore pressure changes and aseismic fault creep triggered by the earthquakes.In the theoretical front,we calculate Earth's stress induced by various forces,includ.ing Earth's surface loading,gas injection/extraction in Hutubi gas field and North Korea's nuclear test,and estimate their effects on nearby seismicity.(4)We are the first to build a preliminary database of loading-induced stress changes on the Earth in a global scale and at depths,by considering six loading/unloading processes:hydrologic loading,atmospheric pressure,ocean water,solid Earth tide,pole tide and postglacial rebound.This database contains stress changes in various temporal and spatial scales,and could motivate more comprehensive studies to seek clearer physical insights on triggering mechanisms of various tectonic events,such as earthquake,slow-slip event and volcano.(5)We calculate stress perturbations caused by gas injection/extraction in Hutubi gas storage field and North Korea's 2017 nuclear test,and explore their triggering effects on the small earthquakes on nearby faults.