Tectonic Loading And The Interaction Among The Large Earthquakes On Xianshuihe Fault

Earthquake preparation, occurrence and postseismic adjustment are generallyconsidered to be a process of strain accumulating and releasing in seismogenic fault.Evolution of Coulomb failure stress changes can significantly improve ourunderstanding on the preparation process and the dynamical mechanisms ofearthquakes. Earthquakes are affected by the Coulomb failure stress changes bothcaused by tectonic loading and prior events, especially caused by either greatearthquakes on the same fault or other nearby shocks. Previous investigations haveproved that a positive correlation exists between the areas with Coulomb failure stressincreasing and the occurrence of subsequent earthquakes. Seismologists have alsofocused on the qualitative analysis of earthquake hazard based on the evolution ofCoulomb failure stress. With the development of laboratory techniques, the frictionlaw found in the laboratory observations can also be used in the seismic hazardanalysis. Consequently, researches quantitatively calculate the probability ofseismicity disturbed by Coulomb failure stress change, by applying the constitutivelaw and combing with the background seismicity.Xianshuihe fault zone is the eastern boundary of the Sichuan-Yunnanfaulted-block in southwestern China, which behaves as a huge left-lateral strike-slipactive fault system, where the most frequent seismicity in continental China occurs. Ithad experienced two active seismicity periods (1700~1816,1893~present). TakingXianshuihe fault zone as the study area, firstly, the static Coulomb stress triggeringamong the earthquakes with M≥6.7along the fault zone was investigated, then usingthe multi-layered viscoelastic media, the evolution of Coulomb failure stress on themajor fault segments was calculated due to coseismic, postseismic, and interseismicdeformation. In addition, by applying the rate-and state-dependent constitutiveapproach, combing with the background seismicity obtained by the earthquakecatalogue and large earthquake recurrence respectively, the probability of seismicitydisturbed by Coulomb stress change was calculated, and the earthquake hazard wasdiscussed furthermore. The main contents and acknowledge in the thesis are as follows:1. Theoretical calculation and analysis of the Coulomb failure stress changes.Take an earthquake with magnitude of7as example, I analyzed the spatialdistribution of Coulomb stress changes for a specific receiver fault which caused bydifferent types of seismogenic faults, investigated the sensitivity of Coulomb stresschange on strike-slip fault to the value of the apparent coefficient of friction, andcompared the postseismic deformation characteristics caused by the viscoelasticrelaxation simulated by the Maxwell body and the Burgers body.2. The interaction among the large earthquakes among the Xianshuihe fault zone.Initially, using a multi-layered viscoelastic media, I calculated the Coulomb stresschanges caused by the coseismic dislocation and viscoelastic relaxation of previouslarge earthquakes, verified the Coulomb stress triggering of earthquakes with theseven M≥6.7along the Xianshuihe fault since1893. In addition, to take into accountthe impact of the large earthquakes on the Xianshuihe fault zone and its adjacentfaults, I studied the24large earthquakes how to affect the large earthquakes on theXianshuihe fault. The result showed that, the occurrence of all of seven large eventson Xianshuihe fault since1893were triggered by prior events.3．The dynamic evolution of Coulomb failure stress changes on the Xianshuihefault zone. I comprehensively considered the coseismic dislocation and theviscoelastic relaxation caused by the seven large earthquakes on Xianshuihe fault, andthe interseismic stress accumulation modeled by introducing "virtual negativedisplacements" and caused by the tectonic loading of10major fault segments. Ifurther simulated the evolution of Coulomb stress changes with time on the majorfault segments over150yr due to coseismic, postseismic, and interseismicdeformation, and discussed its relationships with the large earthquakes, and analyzedthe seismic hazard qualitatively. The result showed that, the occurrence of all of sevenlarge events on Xianshuihe fault since1893was driven by a series of previous largeearthquakes and tectonic loading, and coseismic dislocation, viscoelastic relaxationand interseismic tectonic loading significantly affect the stress state on the fault.I studied the effect of coseismic, postseismic viscoelastic relaxation caused by the24large earthquakes on Xianshuihe fault zone and its adjacent faults, and thetectonic loading of10major fault segments since1816, influenced on the largeearthquakes on the Xianshuihe fault. Compared with the results which onlyconsidered large earthquakes and tectonic loading on the Xianshuihe fault, thephenomenon which large earthquakes generally advanced by the increase of Coulombstress was more pronounced.4. Quantitative calculation of the probability of seismicity based on the stressperturbation and the seismicity.(1) Taken Coulomb stress changes per year as stressperturbation which caused by the coseismic dislocation and postseismic viscoelasticrelaxation of7M≥6.7large earthquakes on Xianshuihe fault zone and17M≥7largeearthquakes occurred in its adjacent regions.(2) Achieved the background seismicitybased on the catalogue, and transformed it to the rate of large earthquake with amagnitude of6.7. In additional, used the material of large earthquake recurrence andthe method of “time-dependent” earthquake potential and prediction to obtain thebackground seismicity.(3) Based on the Coulomb stress change per year and thebackground seismicity, by applying the rate-and state-dependent constitutiveapproach, I simulated the probability of large or great earthquakes on the10segmentsof Xianshuihe fault, then further discussed the earthquake hazard for each segments.The result shows a higher probability of earthquake and seismic hazard in the middlesouth section of Xianshuihe fault.