The Depth Variations in the bValue of Frequency–Magnitude Distribution of the Earthquakes in the Garm Region of Tajikistanстатья
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Дата последнего поиска статьи во внешних источниках: 20 августа 2018 г.
Аннотация:The depth changes in the bvalue and density of the number of earthquakes in different magnitude bins (M ≥ 1.8, M ≥ 3.0, M ≥ 3.5) are analyzed using highly accurate seismological observations carried out in 1955–1991 at the Garm prognostic area in Tadjikistan. It is found that the observed bvalues are controlled by the variations in the proportion between weak and strong earthquakes. Two horizons with different patterns of the bvalue are identified in the Earth’s crust above and below a depth of 15–16 km. The bvalue in the upper and lower horizons is close to 0.8 and 1.2, respectively. The lower horizon is marked by almost complete absence of relatively strong earthquakes with M ≥ 3.0. The observed changes in the bvalue with increasing depth could probably be due to the increase in the strength of crustal material caused by the growth in tem perature and confining pressure in the depth interval from 0 to 15 km. The transitional interval between the upper and lower crustal horizons (~13–18 km), which is characterized by a sharp drop in seismic activity, can probably be associated with the zone of the phase transition of crustal material from an elastic brittle state to a plastic state, as suggested by some authors. Typically, the top of this zone hosts the hypocenters of the stron gest earthquakes in a given territory. The correlation is established between the crustal areas with low bvalues and the locations of the strongest earthquakes in the region. It is suggested that the threedimensional map ping of the bvalue can be helpful for estimating the location, depth, and maximal magnitude of the probable strong earthquakes in seismically active regions and can be used to assess seismic risks.