This paper discusses results obtained by observing rock rigidity changes over time and space in seismoactive crustal and mantle regions in the subduction zone of the Pacific plate beneath Eurasia. Data on P-wave first arrivals from earthquakes listed in bulletins of the seismic station network worldwide are used. By discriminating between brittle vs. ductile failures in earthquake foci, we assess the rigidity parameter of rock mass in which earthquake focus occurs. Our method affords scanning rigidity both through time and to depth. Discussed are scanograms of rigidity vs. time for a layer of fixed thickness and width along the spreading zone and scanograms in time-depth coordinates for two subduction zone-transverse crust-mantle sections along arc segments. It is concluded that i the rock rigidity control method using the character of P-wave first arrivals from local earthquakes has proved its workability in the complex tectonic setup of the subduction zone of the Pacific plate, ii crustal and mantle rock rigidity changes over time with a period of ca. 6 years, which coincides with the period of the Chandler wobble of the earth's axis, iii the apparent rock rigidity is lower in the crust than in the mantle, due likely to a greater healing rate of source ruptures in the mantle, iv crustal rigidity of the Pacific plate is lower than that of the Eurasian plate, and v the contact zone between the plates exhibits a transverse nearly horizontal layering of the rigidity distribution. We believe that the pioneering results from applying our method will prove useful to experts in tectonics, geodynamics, and seismicity in the study region.
rock rigidity distribution, subduction zone, Pacific plate.
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