VERY SLOW-MOVING CRUSTAL STRAIN DISTURBANCES

A model of discrete crustal blocks, separated by a thin weak transition zone whose stress-strain relations contain derivatives of real order implying a memory mechanism in the medium, seems to give a good explanation for the low velocity values (0.03-15 cm/s) of slow moving crustal strain disturbances. Tilt and strain anomalies observed before moderate and large earthquakes are thought to be generated in the focal zone of the forthcoming earthquake, due to the spatial and temporal redistribution of the physical processes that occur during the preparation time of the earthquake. On the basis of experimental results it has been assumed that these anomalies propagate from the hypocentral area. In addition post-seismic deformations show the effects of propagation. The anomalies observed have a duration and a velocity ranging from minutes to months and from O.O5 cm/s to 10 cm/s, respectively. The velocity of migration of post-seismic deformation also lies in this range. The velocity of propagating strain disturbances is obtained as a function of the frequency and of the linear dimension of the oscillating blocks. The velocity values calculated using this model range from 0.1 cm/s to 10 cm/s, for viscosities of 109-1011) g cm-1 s-2+z (0.1≤ z ≤ 0.5) and blocksizes (1-4) · 106 cm, at frequencies of 10-5-10-8 s-1. © 1990.