日本MW9.0地震后沂沭断裂带两侧块体相对运动及对地震活动的影响

基于连续GPS数据，利用滑动块体模型研究了日本M_W9.0地震前后沂沭断裂带两侧块体连续的相对运动状态，并研究其对区域地震活动的影响，结果表明日本地震以来：1）两侧块体呈右旋走滑兼挤压状态，平均走滑、挤压速率分别为0.9±0.1 mm·a^-1和-0.7±0.1 mm·a^-1，相比日本地震之前，运动过程更具起伏特征，可能与日本地震前后俯冲带两侧板块间相互运动状态的改变有关；2）沂沭断裂带两侧地区地震活动频次N、总释放能量折算震级M、地震活动度S值、地震b值与两侧块体相对运动的相关系数分别为0.66、0.69、0.74、-0.6，T检验显示相关性显著.在研究区地震能量集中释放阶段两侧块体相对运动方向和研究区主压应力方向一致，相对运动速率和地震活动强度变化具有同步特征，两侧块体相对运动对区域地震活动具有控制作用；3）莱州序列和乳山震群的发生可能与两侧块体相对运动促进的局部区域应力调整有关.

Relative motion between the two blocks on either side of the Yishu fault zone after the 2011 Japan MW9.0 earthquake and its effect on seismic activity

Associated with the subduction of the Pacific plate, the Japan M_W9.0 earthquake occurred on 11 March 2011, and caused millimeter to centimeter coseismic horizontal displacement in Northeast China and North China. The Yishu fault zone (the Shandong segment of the Tancheng-Lujiang fault zone), a deep feature in eastern China, tectonically related with the Japan Trench system, also affected by the subduction of the Pacific plate, receives attention after this great shock. Previous work shows that the differential motion and interaction between active blocks play a direct effect on their seismic activity. As the boundary of the North China block and Ludong-Huanghai block, it has been a concerned issue that the transmitting of tectonic stress between two blocks on either side of the Yishu fault zone after the Japan M_W9.0 earthquake.
This work is based on the continuous GPS observations on the two sides of the Yishu fault zone and a model of sliding blocks. The purpose is to study the relative motion between the two blocks aforementioned and its evolution process and its relation with the regional seismic activity.
1) Since the Japan M_W9.0 earthquake, the two blocks exhibits a relative right-lateral strike-slip and compression state which accords with Yishu fault zone's dextral strike-slip and thrust under the NEE-directed regional principal compressional stress field, with mean rates was 0.9±0.1 mm·a^-1 and -0.7±0.1 mm·a^-1, respectively. Compared to the state before the earthquake, the process after the earthquake showed more characteristic in up and down motion ranging 0.5~2.5 mm·a^-1, which was likely related to the change of the relative motion between the two plates on either side of the subduction zone before and after the Japan M_W9.0 earthquake.
2) There was a better correlation between seismic activity and relative motion of the two blocks when earthquakes were selected in a retrieval circle, of which the center was the midpoint of the Yishu fault zone with a radius of 250~500 km. The best correlation coefficients between seismic activity parameter frequency N, total releasing energy equivalent magnitude M, seismicity S value, seismicity b value and the two blocks' relative motion are 0.66, 0.69, 0.74, and -0.6, respectively, which were significant by T-test. The two blocks' relative motion direction when the earthquake energy released centrally was consistent with the principal compressive stress direction of the study area and the rate of the relative motion had a synchronism character with the seismic activity intensity. It indicates that the two blocks' relative motion had some controlling effect on the main seismic activity situation of the Yishu fault zone and its two sides after the Japan M_W9.0 earthquake.
3) As typical cases, the analysis of the mechanism of the Laizhou earthquake sequence and Rushan earthquake swarm shows that they may be both caused by the tectonic stress adjustment of their local areas which were probably promoted by the two blocks' relative motion.
The research results above are of significance to understand the crustal motion state of the Yishu fault zone and the blocks on its two sides, and the dynamic mechanism of the seismic activity situation of the study area.