环太平洋强震引起的稳态形变场及其地球动力学含义

Steady-state deformations caused by strong earthquakes in circum-Pacific seismic belt and its geodynamic implications

地震引起的稳态形变由同震形变和震后地幔黏滞性松弛效应的累积两部分组成.本文首先利用球形地球地震位错理论计算了智利MW8.8地震引起的同震水平位移和震后地幔黏滞性松弛效应,发现近场以同震变形为主,远场变形主要来自震后地幔黏滞性松弛效应的累积.接着,本文计算了环太平洋地区373个MW7.0及以上中浅源强震引起的同震和稳态水平位移和稳态应变,发现环太平洋强震引起的稳态水平位移在地震带附近由同震破裂主导,随着震后时间的增加,地幔黏滞性松弛效应逐渐扩展到整个太平洋板块,在太平洋中心区域亦可达分米量级,稳态水平位移的方向大致与太平洋板块、欧亚板块和美洲板块的运动方向一致;环太平洋强震使得整个太平洋板块呈现出明显的西北-东南方向的拉张应变,在东太平洋海隆处引起的稳态拉张应变超过10-7,拉张方向与东太平洋海隆的走向近似垂直.上述拉张性稳态应变表明,环太平洋地震引起的地幔黏滞性松弛效应降低了洋中脊扩张的难度,是引起海底变形的重要机制之一.环太平洋强震在中国东北地区产生的稳态水平位移年变化率与GNSS水平速度场在方向上大体一致,但在量级上占观测结果的20%~70%,表明环太平洋强震引起的地幔黏弹性松弛效应是现今中国大陆GNSS速度场的一个重要组成部分.

The steady-state deformation caused by earthquake consists of co-seismic deformation and the cumulative effect of post-seismic mantle viscoelastic relaxation. Firstly, we use the dislocation theory in a spherical Earth model to calculate the co-seismic horizontal displacement and post-seismic mantle viscoelastic relaxation effect caused by the Chile MW8.8 earthquake, we find that near-field deformation is mainly dominated by co-seismic deformation, while far-field deformation primarily results from the cumulative effect of post-seismic mantle viscoelastic relaxation. Subsequently, we calculate the co-seismic and steady-state displacement fields and strain fields caused by 373 shallow-focus earthquakes with moment magnitude > MW7.0 occurred in the circum-Pacific region. It is found that the steady-state horizontal displacements caused by those strong earthquakes are dominated by co-seismic ruptures near the circum-Pacific seismic belt. As post-seismic time increases, the mantle viscoelastic relaxation effect gradually extends across the entire Pacific plate, reaching decimeter levels in the central Pacific region. The direction of steady-state horizontal displacements is generally consistent with the movement directions of the Pacific, Eurasian, and American plates. Strong earthquakes in the circum-Pacific region induce significant northwest-southeast extensional strain across the entire Pacific plate, with steady-state extensional strain exceeding 10-7 at the East Pacific Rise, and the direction of extension is approximately perpendicular to the orientation of the East Pacific Rise. The extensional steady-state strain indicates that the mantle viscoelastic relaxation effect caused by circum-Pacific earthquakes reduces the difficulty of mid-ocean ridge expansion. The steady-state horizontal displacement rate in Northeast China caused by circum-Pacific strong earthquakes is generally consistent with the GNSS horizontal velocity field in direction, but accounts for 20%~70% of the observed results in magnitude. This suggests that the mantle viscoelastic relaxation effect caused by circum-Pacific strong earthquakes is an important component of the current GNSS velocity field in mainland of China.