2004年苏门答腊地震粘滞性松弛效应对华南地区地壳水平活动的影响

2004年苏门答腊地震后, 不同学者根据不同观测数据(地震波、 GPS), 得到了此次地震的断层滑动模型。 反演过程中使用半无限空间模型时, 无法利用远场观测数据进行约束, 势必影响远场形变的解释。 基于Hoechner等使用的断层几何模型和GPS同震位移数据, 本研究利用球体位错理论反演方法反演了2004年苏门答腊地震断层滑动模型, 得到的矩震级为9.24, 最大滑移量为30.4 m, 由于考虑了曲率的效应, 该模型在远场同震位移的计算结果与GPS数据吻合较好。 然后, 选取了2001—2004年和2004—2007年两期的GPS水平位移速度场, 研究2004年苏门答腊地震对华南地区地壳水平活动的影响, 从两期的GPS水平位移速度场差异可以看出地震后华南块体有向西南方向的运动趋势, 华南块体受到此次地震明显的震后影响。 最后, 基于反演得到的断层模型, 利用Tanaka等提出的粘弹性球体位错理论对华南块体两期GPS水平位移速度场差异进行模拟, 得到华南块体内部粘滞性系数为2×10¹⁹ Pa·s, 当考虑地幔粘滞性松弛效应后, 两期的速度场差异的均方根值由3.2 mm减少为1.9 mm。 可见在研究2004年前后中国大陆GPS水平位移速度场时, 若继续以华南块体为基准, 需考虑此次地震的地幔粘滞性松弛效应。

Effect of Viscoelastic Relaxation following the 2004 Sumatra Earthquake on Horizontal Crustal Movement in South China

After 2004 Sumatra earthquake, different authors obtain the fault slip models from different observation data, such as seismic waves and GPS data. However, the inversion of the co-seismic model from observed co-seismic displacements can not use far-field data as the constraint while adopting half-space medium, and then the inverted model is difficult in explaining far-field data. In this paper, based on the fault geometry model and GPS data provided by Hoechner et al., we inverted the slip model of 2004 Sumatra earthquake by using the inversion algorithm based on spherical dislocation theory. We found that the moment magnitude of the fault model is 9.24, and the maximum slip reach 30.4 m, calculated co-seismic displacements of our fault model agree well with far-field GPS observations after considering the effect of earth curvature. Then, in order to study the effect of 2004 Sumatra earthquake on crustal movement in Southern China, we select two periods of GPS velocity fields (2001—2004 and 2004—2007). From the differences of them, we can see Southern China block has a significant trend of southwest movement after this earthquake, which implied South China block has been affected by the viscoelastic relaxation of this earthquake significantly. Using the viscoelastic dislocation theory of Tanaka, we simulated the differences of two periods of GPS velocity fields in south China block based on our fault model. We found that the optimal viscosity of this area is about 2×10¹⁹ Pa·s when taking the effect of viscoelastic relaxation into account, the RMS (root mean square) of the differences reduced from 3.2 mm to 1.9 mm. In studying crust deformation before and after the 2004 Sumatra earthquake, the viscoelastic relaxation of this earthquake should be considered when based on south China block continuously.