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EXPERIMENTAL STUDIES OF PLASTIC-FLOW WAVES IN THE LITHOSPHERE (Ⅰ)
- Wang Shengzu, Li Jianguo, Zhang Zongchun
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2001, 23(3):
407-418.
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In order to study the propagation processes of plastic-flow waves in the lithosphere, plasticized rosin, i.e. the mixture of rosin (P) and plasticizer (P) with proper ratio of P/R, is used as analogue material for modeling ductile layer in the lithosphere. The mixture is poured into a rectangular shallow trough (280～300mm long and 190～200mm wide), forming ductile one layer model (1) with a thickness of 8～9mm, as shown in Fig. 1. The bottom (2) and walls (3 and 4) of the trough are made up of glass plates. Some models are covered by a 0.1～0.2mm-thick brittle layer composed of dried consolidated talc-powder slurry, forming brittle/ductile two-layer models. The driving boundary of the model, a movable wall (4) of the trough, is pushed by the springs (6), which are compressed by screw-pushers (7) (micrometer screws) or adding spacers. A number of measuring points (8) are placed on the models surface and their displacements are measured by using a coordinate-system micrometer with the horizontal accuracy of 0.002mm and vertical accuracy of 0.01mm. The tests are carried out at constant temperature with the errors within the range of ±0.5℃ for most of the models. According to the similarity criterion and considering the Stokes number, St(the ratio of viscous force to gravitational force) for viscous flow, the time ratio of prototype to model, Kt, is calculated from Kt=Kη/(Kρ,Kg,KL), and hence the velocity ratio KV=KL/Kt, where KηKρKg and KL are the ratios for viscosity, density, gravitational acceleration and length, respectively. The results of the experiments indicate that plastic-flow waves, being similar to gravity waves in viscous media, include "fast" and "slow" waves and both of them are the superposition of major and subsidiary waves. The major wave is similar to solitary wave or surge. The "fast" waves, including the major and subsidiary ones, are originated from the boundary of the model. The periods of them depend mainly on the pulsative driving period at the boundary, while the wave velocities and strain rates of them are not associated with the pushing velocity of the boundary. In terms of the theory of similarity, the velocities of the major "fast" waves inferred from the models are about 0.12～2.5km/a, corresponding or close in the orders of magnitude to those of some plastic-flow waves in the lower lithosphere, which control the migration of seismic activities (Wang et al., 1994). The major and subsidiary "fast" waves decay in general tendency with their propagation, showing the descend of strain rates, whereas they ascend again locally in a certain range of distances. As is inferred from the models, for instance, the strain rates ascend again in the range of 1 500～2 250km away from the driving boundary for the situation of the waves propagating in the lower lithosphere. Its upper limit corresponds roughly to the distance from the Himalayan collision boundary to the North China Plain. The strong seismic activities in the North China region may be associated, as one of the important factors, with the local ascent of strain rates in the propagation process of plastic-flow waves. Although what has been done in the experimental studies so far stays with qualitative or semi-quantitative simulation, the results of the physical modeling stated above have provided the powerful experimental evidences for understanding the generation and propagation of the plastic-flow waves in continental plate, which control the migration and fluctuation of seismic activities.
