Comparative Characteristics of Seismic and Deformation Effects for Three Great Subduction Megaearthquakes

The comparative characteristics of seismic and deformational effects of three great subduction megaearthquakes in Sumatra in 2004 (Mw = 9.2); in Maule, Chile, in 2010 (Mw = 8.8); and in Tohoku, Japan, in 2011 (Mw = 9.0) are considered. In all cases the main rupture at the time of the earthquake was located in the subduction zone on the surface of the oceanic lithospheric plate plunging gently beneath a continent or an island arc. The process of destruction in each focus is characterized by a megathrust, with a gentle inclination angle of ~8–18o, according to the definitions of its focal mechanism, in almost full compliance with a gentle dipping of the oceanic lithospheric plate. The displacement occurred in a locked area of the subducting plate extended for several kilometers below the ocean bottom to a depth of 30–40 km. In all three cases, the maximum coseismic slip, obtained according to the data of geodetic measurements, occurred in the upper 25 km of the locked area, while its lower part radiated coherent short-period seisms. The trace of the rupture on the surface, marked by the aftershock area, ranged from 400–600 km in the case of Tohoku and Maule earthquakes up to ~1500 km during the Sumatra earthquake. The rupture in the Maule and Tohoku earthquakes was bilateral, with its approximately symmetric propagation from the epicenter, while in the Sumatra earthquake the rupture spread unilaterally relative to the epicenter from southeast to northwest. The propagation time of the rupture also varied. If in the first two cases it was 140–160 s, for the Sumatra earthquake it lasted 500–600 s. The largest tsunami wave, up to 40–60 m in height, was recorded during the Tohoku earthquake, extending for more than 200 km along the coast of Sanriku province. The rupture during the megaearthquakes under discussion is not confined in depth to the locked seismogenic area, marked by the area of the aftershock nearest in time and the zone of maximum coseismic slips determined by the geodetic GPS measurements. The rupture continues aseismically (postseismic slip) in the transition zone from brittle to brittle-plastic slip to depths of ~60–80 km. In addition, there is evidence that the displacements on the megathrust can continue slightly deeper to the region of the brittle–plastic slip. Such episodic events of slow slip (“silent” or “slow” earthquakes) and “seismic tremor” were recorded in southwestern Japan and southern Chile. These differences in seismic and deformation effects can serve as evidence of the change in frictional properties with depth along the surface of the megathrust. It should also be recognized that the strength barriers and asperities on the megathrust surface expressed in certain geological structures or yet unclear nature of segments of high frequency radiation can manifest themselves in the character of distribution of the accompanying seismicity during the great subduction earthquakes.