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2011年MW9.0的日本东北地区近海大地震在日本海沟附近产生了最大超过50m的同震滑动,这可能导致该区域进入了应力近乎完全卸载的状态。为检验上述假设,我们利用综合大洋钻探计划在震后1年左右时间得到的钻孔资料,以及对震前应力状态的推断结果,确定了陆楔前缘的原地应力状态。根据孔壁崩落法对水平应力方向与应力大小的估算结果,以及在破裂向海沟轴方向传播过程中同震位移增大的现象,可推断原地水平应力在地震过程中变小了。这种应力变化显示出板块界面的前缘发生了相对自由的滑动,该结论与地震引起了断层弱化和发生了近乎全部的应力降等观测结果是吻合的。
The 2011 northeastern Japan earthquake off the coast of Japan in MW9.0 produced a coseismic slip of up to more than 50 m near the Japanese trench, which could have caused the region to enter into a state of nearly complete unloading of stresses. To test the above assumptions, we use the borehole data obtained from the integrated ocean drilling program about a year after the earthquake and the inferred stress state before the earthquake to determine the in-situ stress state of the front of the wedge. According to the estimation of the direction and stress of horizontal stress and the increase of coseismic displacements during the propagation of the crack to the axis of the trench, it can be inferred that the horizontal stress in situ becomes smaller during the earthquake. This change in stress shows that the leading edge of the slab interface is relatively free to slip. This conclusion agrees well with the observation that the earthquake caused the weakening of the fault and almost all of the stress drop occurred.