Ring-Shaped Seismicity Structures Forming before Large Earthquakes and the Great Earthquakes in the Western and Eastern Pacific

Abstract

We consider the characteristics of seismicity before large earthquakes and the great earthquakes in the periphery of the Pacific Ocean. It is found that earthquakes with M_w = 7.0–9.2 that occurred in 1964–2016 were preceded by the formation of ring-shaped seismicity structures, usually in two depth ranges: 0–33 and 34–70 km. We obtain correlations between sizes of shallow (L) and deep (l) seismicity rings, threshold magnitudes (M_th1 and M_th2, respectively), and the time of their formation (T₁ and T₂, respectively) on magnitudes M_w of the mainshocks. It is shown that the sizes of ring-shaped structures at any given M_w for earthquakes in the western margin of the Pacific are significantly smaller than for those in the eastern margin. However, the values of M_th1 and M_th2 are close for these two regions. Parameters T₁ and T₂ vary considerably depending on the event, but on average they are ~27–30 years. It is supposed that the formation of ring-shaped structures is related to the migration of deep-seated fluids, while the difference between characteristics of these structures in the western and eastern margins of the Pacific are caused by different contents of fluids in the crust and upper mantle of the respective regions. This conclusion agrees with the available data on the peculiarities of aftershock processes of large earthquakes in the considered regions.