On Physical Nature of the 70-km Seismic Boundary Caused by Tidal and Fluid Effects.

Statistic analysis of the EQ catalogs (ISC, NEIC, 1960-2006; Harward Catalog, 1976-2005) showed that the seismic boundary at the 70-km depth marked out often as a real boundary, which divides all events into two separate classes: non-deep seismic events (about 85% of all events) which respond to external perturbation effects, and the deep-focus events non-responded to an outer influence. The results of two series of statistical analyses were presented. All events were subdivided into two groups: shallow events (H<=Htr) and deep events (H>Htr), where Htr is threshold value of the EQ source depth. The statistical verification of hypothesis about within-year variability existence for the events of various energy levels [Sasorova, Zhuravlev, 2006] was carried out in the first series. It was disproved the null hypothesis about uniform EQ distributions in the course of year for shallow events. But it was confirmed the null hypothesis for deep EQ. The statistical verification of hypothesis about existence non-random component in time distribution of the EQ's between the northern and southern part of the Pacific [Sasorova et al, 2006] was carried out in the second series. It was found (according the distribution-free test) that nonrandom component does not exist for deep EQs. But it is clearly manifested in time distribution of the shallow events. In both cases it was found that the Htr boundary between the shallow and the deep events was arranged at the depth between 60 and 80 km (we let to vary Htr value from 15 to 300 km). Thus the EQ with sources located above this boundary are affected by external factors, which may trigger the process of EQ generation, while the external factors don't influence on the EQ sources located below this boundary. The drastic change of EQ source parameter values via hypocenter depth was also observed near the 70-km depth boundary [Rodkin, 2004]. Revealed parameter change is connecting with difference in the deep fluid behavior. The equation of state for water was analyzed. It was shown that water stays on a free state only at the depth less than 70 km (this depth value is correspond to the pressure equal to 2 GPa). Tidal forces generate in the Earth crust the areas with damage accumulation (shear and tension cracks). An ascending motion of the water fluid upward the fractures under the influence of the deep rock pressure and Sun-Moon tidal forces lead to interaction water fluid with damaged rock and to adsorption decreasing of the rock strength (Rebinder effect). In this case the fracture force threshold decreases in tens times. A water-saturated rock is medium with non-linear response to periodic effects. The weak effects of tidal forces were accumulated in such water-containing medium. In rocks where free state water is absent, tidal effects cannot be accumulated and non-linear mechanism of deformation storage is not realized.