Study On Steady State Deformation And Seismic Hazard Induced By The 2011 Tohoku-oki Mw9.0 Earthquake

The 2011 Tohoku-Oki Mw9.0 earthquake occurred near the coast of northeastern Japan,which not only caused significant coseismic and postseismic deformation in near-field areas such as the main island of Japan,but also caused different degrees of deformation in far-field areas such as eastern China,South Korea and North Korea.Based on the viscoelastic sphere earthquake dislocation theory,combined with co-seismic data on land and seafloor and 5-10 years post-seismic data from 330 land GPS stations on the main island of Japan,we inverted the fault slip distribution of the 2011 Tohoku-Oki Mw9.0 earthquake.Then,we calculated the steady state horizontal displacements in the Japanese archipelago and eastern China and Coulomb failure stress changes on the main faults of Northeast and North China induced by the 2011 Tohoku-Oki Mw9.0 earthquake,and we analyzed the impact of the earthquake on the seismic hazard of Northeast and North China.First,we selected the continuous observation data of 330 land GPS stations on the main island of Japan.The time span of these observation data is 2 years before and 10 years after the earthquake.Using these observation data,we obtained the annual average displacement of 5?10 years after the 2011 Tohoku-Oki Mw9.0earthquake.The displacement generated during this period can be almost completely regarded as the viscoelastic relaxation of the mantle.Then,we used the viscoelastic sphere earthquake dislocation theory to simulate the displacements of 5?10 years after the earthquake.Comparing the observed displacement with the simulated ones,we found the optimal viscosity of the mantle is 9.0×10¹⁸Pa·s;Then,we used the viscoelastic Green function and combined co-seismic and post-seismic data to invert the fault slip distribution of the 2011 Tohoku-Oki Mw9.0 earthquake.The results showed that the maximum co-seismic dislocation of the 2011 Tohoku-Oki Mw9.0earthquake reached 62.72 meters,and the seismic moment of co-seismic slip was4.48×10²²Nm,and the corresponding moment magnitude was Mw 9.034.The post-seismic displacement caused by the viscoelastic relaxation provides information of the co-seismic slip,the inversion based on the co-seismic and post-seismic data based on the viscoelastic sphere earthquake dislocation theory can effectively improve the reliability of the fault slip distribution of the 2011 Tohoku-Oki Mw9.0earthquake.Finally,the inversion method proposed in this paper provides theoretical support for the post-seismic scientific investigation of major earthquakes where the co-seismic data is not enough:we can invert the fault slip distribution by densifying the observational network during the post-seismic period and obtaining enough post-seismic data,even if the co-seismic data is not enough.Steady state seismic deformation refers to the long-term accumulation of viscoelastic relaxation effects caused by earthquakes.Through the study of steady state seismic deformation,the overall postseismic deformation field of the earth can be studied,which is beneficial to analyze the ultimate response of the surrounding area to the earthquake.In this paper,we use spherical earth earthquake dislocation theory to calculate the steady state horizontal displacement caused by the 2011Tohoku-Oki Mw9.0 earthquake in Japanese and eastern China at first.The results show that the steady state horizontal displacement caused by the 2011 Tohoku-Oki Mw9.0 earthquake on the main island of Japan will reach a maximum of 8 meters,which is roughly twice the coseismic horizontal displacement;the steady state horizontal displacement in Northeast China will reach 0.8 meters,and in North China the steady state horizontal displacement will reach about 0.5 meters,which is dozens of times the co-seismic horizontal displacement.It can be seen that the strong earthquake has an extremely significant impact on the deformation field of the surrounding area.Then,based on results of previous research and combined with geological data,we established geometric and sliding structural models of 15 main fault zones in Northeast and North China,and then calculated the coseismic and steady state Coulomb failure stress changes caused by the 2011 Tohoku-Oki Mw9.0earthquake on the above fault zones using spherical earth earthquake dislocation theory.The results show that the coseismic Coulomb failure stress changes caused by the strong earthquake on the main fault zones in Northeast and North China did not exceed 1 k Pa;the steady state Coulomb failure stress changes significantly exceed the coseismic results.Except for the part of northern section of Yilan-Yitong fault zone and southern section of Nenjiang fault zone,the steady state Coulomb failure stress changes on the other fault zones did not exceed the seismic trigger threshold of 10 k Pa.Therefore,the 2011 Tohoku-Oki Mw9.0 earthquake has a limited impact on the seismic hazard of Northeast and North China,and will not significantly change the stress state and seismic activity of the main fault zones in the region.