Natural Model Study of the Trigger Mechanism of Seismic Activity

Spreading of mid-ocean ridges may be considered as a trigger of the intraplate seismicity. There are two types of spreding: the ridge-push (impulse impacts) and the slab pull. These processes can disturb the metastability of a crustal fault-block medium and consequently cause low-magnitude platform earthquakes. A boulder-clay dam can be considered as a natural model of a fault-block medium. We used a marine and a lake dam to model both types of processes. We treat the load caused by sea tides as gradual external impact. Pulsed impacts were simulated by series of 100 hammer strokes. The goal of the experiment is to study mechanisms that convert external loads into the seismic emission in a fault-block medium. Our experimental results confirm the following geodynamic assumptions: - Both impact types induced the spike of microseism energy evolved in micro-impulse sequences; - The direction of the applied force, its modulus and the rate of temporal change determine the energy and the amount of micro earthquakes; - The time-lag between series of pulsed impacts and spikes of the seismic emission is significantly greater than the excitation time; - The conversion of external loads into the seismic emission is a nonlinear process. This assumption is based on the main part of seismic energy being emitted in frequency range, which is significantly higher than the frequency of the impacts; - The seismic emission occurs in the weakest place where a fault-block medium has the maximum stress concentration. Besides the modeling a series of micro earthquakes was recorded on a large moraine on the Solovetsky Islands in July 2005. This spike of seismic activity preceded the earthquake in 22.10.2005 (t0=17:46:44.2, lat=64.55, lon=41.00, ML=2.9, close to Arkhangelsk, Russia). Given that the Solovetsky Islands is located 300 km closer to the Mid-Atlantic Ridge than Arkhangelsk, it may be deformation from the spreading ridge that triggered these micro earthquakes.