Scientist J R Kayal explains the why the monster quake that hit Japan caused such havoc and why it gave rise to a tsunami that compounded the misery. At 2.46 pm Japan time, an earthquake M8.9, Lat:37.68N, Long:143.03E, depth: 20 km below the ocean’s surface, off the coast Tohoku Earthquake, destroyed most parts of the Miyagi Prefecture, about 370 km north from Tokyo. The devastation was compounded by the earthquake shaking as well as by the tsunami, and more so with the nuclear power plant disaster which is very unfortunate!Click on NEXT to read further…

The 2001 mega thrust earthquake at a shallower depth in the subducting Pacific plate, due to the accumulated stress over past few hundred years, thrust up the overriding plate and generated the tsunami. A huge earthquake of M9.0 occurred with an average slip 10-15 m at the earthquake source, that bounced back a huge sea wave of the order of 100-200 km of wave length (length between the crest and trough of the sea wave) with 1-2 m height. This sea wave moves at a speed of (gh) 1/2 where g is the acceleration due to earth’s gravity and h is the seawater depth. So, with an average 3 km deep seawater the wave moves with a speed of 300-400 km per hour. When the waves reach to the coast, the huge (kinetic) energy converts into the Tsunami, i.e the sea wave jumps to a height of 10-15 metres and inundate the land up to 1 km or more distance. This is what had happened due to the recent four mega thrust earthquakes at the Chile trench in 1960, Aleutian trench, Alaska in 1964, Andaman-Sumatra trench in 2004, and now at the Japan trench in 2011. In the subduction zones, the deeper 100-600 km earthquakes within the subducted plate would not create such devastation. The normal faulting as well as the strike faulting earthquakes, though could be at a shallower depth (20-50 km), normally does not generate M9.0 earthquakes.Seismic and tsunami hazardsThe energy release of a great earthquake M8.0 is of the order of one million atom bomb of the Hiroshima type. And the energy release of an event of M 9.0 is about 30 times of M8.0; in other words the 2001 Japan event is equivalent to occurrence of 30 earthquakes M8.0 at a time. So the seismic hazard is unimaginable, and along with such seismic hazards the tsunami just wash out everything within 1-2 km in the coastal land. That is what we are seeing in Japan now.