粘弹性数值模拟龙门山断裂带应力积累及大震复发周期

2008年5月12日在低地形变速率的龙门山断裂带上突发汶川强震,引发人们对该地震孕震机制的思考。本文根据GPS观测资料确定边界条件,通过三维粘弹性数值模拟探讨了汶川地震的孕震机理,计算了该区域岩石圈的应力增加速率和积累过程,以及汶川地震同震应力变化与震后应力松弛,在此基础上估算了汶川8.0级大地震的复发周期。数值模拟结果表明:印度板块对欧亚板块的推挤造成青藏高原的物质东流,高原中、下地壳物质在龙门山断裂带处遭到相对坚硬的四川盆地的阻挡之后,部分中、下地壳物质在龙门山断裂带下堆积产生应力集中。两个重要因素为应力集中提供了重要控制作用:其一是青藏高原中、下地壳较低的粘滞系数与四川盆地中、下地壳较高的粘滞系数的差异,其二是从青藏高原到四川盆地的Moho面深度在龙门山断裂带的突变。低应变速率的龙门断裂带岩石圈在数千年时间尺度的应力积累过程中,脆性上地壳的应力随时间近乎线性增长,并且上地壳深部的应力增长率超过浅部,6000年内应力积累最大量达到-21.6MPa,应力增长速率为-0.0036MPa/a;而柔性的中、下地壳以及岩石圈上地幔的应力在增长一段时间之后趋于稳定。在空间上,龙门山断裂带受到的压应力从断层西南向北东方向逐渐减小,而剪应力从西南到北东方向逐渐增大,应力状态有利于地震发生时断层的破裂方式从西南的逆冲运动向北东的逆冲兼走滑运动的方式发展。通过应力积累与地震应力降的计算得到汶川8.0级大地震的复发周期约为5400年。

Stress Accumulation on Longmenshan Fault and Recurrence Interval of Wenchuan Earthquake Based on Viscoelasticity

Of interest is the stress accumulation prior to the devastating May 12, 2008 Wenchuan earthquake (Ms 8.0) which occurred on the Longmenshan fault area with slow strain accumulation. This study discusses this problem through 3-D numerical modeling based on viscoelasticity with the GPS data used to quantify the boundary conditions. We calculate the stress accumulation in the lithosphere of the Longmenshan falut area before the earthquake, and the co-seismic stress change and the pos-seismic stress relaxation to estimate the Wenchuan earthquake recurrence interval. It is shown that the eastern movement of Tibet Plateau resulting from the India-Asia collision is obstructed at Longmenshan fault area by the hard Sichuan Basin. The soft materials of middle and lower crust of Tibetan Plateau accumulate there resulting in stress concentration. Two factors are controlling this stress concentration. The first is the sharp reduction of Moho thickness at the Longmenshan fault from Tibetan Plateau to Sichuan Basin. The other factor is the large difference in the viscosity of the middle and the lower crust between Tibetan Plateau and Sichuan Basin. During inter-seismic period stress increases almost linearly in brittle upper crust of the Longmenshan fault, and stress increasing rate increases with depth in upper crust. The maximum stress accumulation is about 21.6MPa with increasing rate of of 0.0036MPa/a at the bottom of the upper crust after 6000 years. But stress increases exponentially in the ductile middle and the lower crust and the upper lithosphere mantle. After stress accumulates for 6000 years, we get this stress state on the Longmenshan fault. The normal stress decreases, but the shear stress increases from Southwest to Northeast in upper crust on the Longmenshan fault. This stress state explains the motion of the fault in thrust in the SW and in strike-slip motion in the NE along the Longmenshan fault. The recurrence interval of Wenchuan earthquake is estimated about 5400 years corresponding to the time needed to accumulate the magnitude of the stress drop of the earthquake.