Nonstationary dynamic control of seismic activity of platform regions by mid-ocean ridges

The plot of temporal variation in the seismic activity level in the central and eastern North American platform (NAP) is shown to be similar to that for the Mid-Atlantic Ridge (MAR). This fact was previously noted for Fennoscandia [Skordas et al., 1991]. The characteristic features of the MAR plot recur approximately every three years for Fennoscandia and every four to eight years for the NAP. These data indicate that the mid-ocean ridge largely controls the seismic activity of the adjacent platforms. The control is provided by the ridge push force. As a result of variations in this force due to the nonstationary process of dike intrusion in the axial zone of the ridge, disturbances of the stationary stress-strain state of the lithosphere migrate from the ridge. Using the Elsasser model, the observed time shift can be used for estimating the viscosity of the asthenosphere, amounting to 1017 Pa s with an accuracy of ±30% in the case considered. The disturbance amplitudes decaying away from the ridge are high enough to change the seismic activity of the adjacent platforms.