New Delhi: The devastating earthquake measuring 8.9 on the Richter scale that struck Japan on Friday had its epicentre near the east coast of Honshu. The epicentre was at a depth of 24.4 kilometres.
Only a couple of days earlier, another earthquake of magnitude 7.2 succoured about 130 kilometres east of Sendai, a city with population over one million that experienced strong shocks. The epicentres of both the earthquakes lie on close proximity and experts at the Incorporated Research Institutions of Seismology believe that the March 6 quake was a fore-shock of the great earthquake of March 11.
Earthquakes of such scale are not very uncommon in Japan, in fact there are have been five earthquakes of a similar scale since 1900.
The March 11 earthquake was caused due to of thrust faulting. In thrust faulting rocks placed lower in the earth's crust get pushed over the overlying layers. This happened along or near the boundary where the Pacific Plate, an oceanic tectonic plate beneath the Pacific Ocean, moves under Japan. The convergence rate at the Pacific Plate's boundary near Japan is much higher than other zones.
Great earthquakes at shallow depth beneath the ocean pose a major tsunami threat, as experienced in Japan soon after the quake.