岩石圈塑性流动波的实验研究(Ⅰ)

EXPERIMENTAL STUDIES OF PLASTIC-FLOW WAVES IN THE LITHOSPHERE (Ⅰ)

实验采用了适当配比的塑化松香 ,模拟岩石圈延性层 ,研究边界驱动条件下塑性流动波的传播过程。实验表明 ,塑性流动波类似于粘性重力波 ,并有“快波”和“慢波”之分 ,二者分别由主波和辅波叠加而成。主波类似于孤立波或涌波。根据相似原理 ,“快波”主波的外推波速约为 0 .12～2 .5km/a ,在波速量级上大致相当或接近于岩石圈下层某些控制地震迁移的塑性流动波。

The results of physical modeling for plastic-flow waves in the lower lithosphere indicate that in addition to 'fast' waves, which roughly correspond to the plastic-flow waves with velocities of 10 0～10 2km/a in the prototype, the 'slow' waves with significantly lower velocities also exist in the model compressed at its driving boundary. The experimental procedure and similarity principle for the 'slow' waves are essentially the same with those for the 'fast' waves (Wang et al., 2001). The measuring markers placed on the model's surface are arranged along a longitudinal line with constant initial spacing of about 10mm. The 'slow' waves are also decomposed into major and subsidiary waves, and both of them are viscous gravity waves: the major wave is similar to solitary wave or surge, and the subsidiary wave is manifested as wave group. Note that the 'slow' waves described here are mainly induced by the 'fast' waves when they arrive somewhere with a distance X 0 away from the boundary, where X 0 is called wave-generating distance. The value of X 0, as shown in Fig. 6, tends to get larger with increase of the nominal relaxation number , a ratio of the duration of test to the relaxation time of media in the ductile layer. As a matter of fact, it is impossible for 'fast' waves to induce the 'slow' waves when the distance X 0 required is too large and the wave-propagation range is limited. On the contrary, the 'slow' waves can be considered as the waves originated at plate boundary as the distance X 0 is approximate to zero. As inferred from the models according to the similarity principle, the 'slow' waves are associated with some slow processes of tectonic deformation in the geological history of the prototype, which are characterized in orders of magnitude by wave velocities of 10 -1 ～10 0m/a, time intervals of 10 0Ma and spatial spans of 10 2～10 3km. They control the long-term fluctuation and migration of seismic activities and influence the evolution of tectonic movements. The horizontal strains (along longitudinal axis) Δ ε and vertical strains Δ ε z are estimated in terms of the measurements of horizontal and vertical displacements of the measuring markers, while the Poisson's ratio and volumetric strain increment of each marker pair can be calculated for each time step. As a result, the average Poisson's ratio υ =0.465 (approximate to 0.5) and the average volumetric strain increment Δ ε v=-0.002 7 (its absolute value is less than the strain measuring error of 0.002 9) are obtained, indicating the volume of the model without considerable variation. It means that the strain response along the horizontal direction is mainly transformed into the variations of the thickness of the layer and the elevation of the model's surface, and it is therefore confirmed that the 'slow' waves are similar to viscous gravity waves. The strain rates in the lower lithosphere inferred from the models are 10 -15 ～10 -14 /s in orders of magnitude, which are comparable to those in tectonically active regions. However, the pushing velocities of the driving boundary inferred from the models, 4.1～12.9m/a, are significantly greater than those in the prototype, for instance, about 0.05 m/a for the Himalayan driving boundary. It implies that the physical modeling stated in this paper is qualitative or semi-quantitative.