Upward motion of earthquake hypocenters in the Wadati-Benioff seismic zones

Abstract

Vertical migration of hypocenters of earthquakes (EQ) in the Wadati-Benioff focal zones is proved by special data processing. Migration is related to the ascent of earthquake hypocenters from the depths of the upper mantle (600‐700 km) to the Earth’s crust level. The total time of fluid ascent in the Tonga‐Bougainville and Java‐Sumatra zones is equal to 5.5‐6 and 10‐11 yr, respectively. This fact is important for explaining EQ triggering in the subduction regions and rift zones. In the last 10‐25 years, a wide range of investigations was performed to analyze short-range effects accompanying EQs, including electromagnetic, hydrogeochemical, and seismoacoustic phenomena in the Earth, and radiophysical and wave phenomena in the atmosphere and ionosphere. Important scientific projects were carried out in Japan, Russia (Kamchatka region), France, Italy, Greece, Indonesia, and a number of other countries [1]. The experimental results obtained in these works cannot be explained completely from the standpoint of the known models of EQ preparation for the following reasons: the last stage of the source preparation takes a short period of time (from a few days to a few weeks); the recorded effects are manifested sporadically in space and time; some effects are generated in the atmosphere, which gives grounds to suppose the effective transport of perturbations through the Earth’s surface; and electromagnetic effects are obviously related to fluid filtration through cracks and fractures in the crust. These peculiarities of the preparation process of the seismic source suggest the existence of local agents of geodynamic perturbation in a large volume of the Earth’s crust. New seismological data also indicate that the geological‐geophysical conditions are inhomogeneous in the preparation source. This is also confirmed by the instability of the solutions of the focal mechanism of the background EQ at the foreshock stage [2], the phenomenon of vertical migration of their hypocenters, small stresses in the Earth’s crust, and the longterm unstable (critical) state of the medium. Therefore, regardless of the large amount of collected empirical data, it was not possible to obtain an adequate scientific interpretation of the data from the standpoint of the existing models of EQ preparation. At present, the known general model of seismogenesis has been supplemented with a model in which the appearance and fluctuations of heat flux in the interior are the initial factors. They lead to a spatially scattered vertical migration of fluid formations, which form the conditions for rapid preparation of the seismic source owing to a strong reduction of stress in rock massifs under conditions of decreased strength of the medium and the existence of differently oriented active fractures and fissure zones in the Earth’s interior. In a short period of time, the variations in the volume and directions of near-surface migration in a nonuniform geothermal field lead to the main seismic shock and beginning of the aftershock sequence. The model suggested here includes the critical state of the medium as a necessary condition for migration without strong accumulation of stress at the last stage of the source preparation.