The next big quake

The quake that rocked Japan on March 11,triggering tsunamis from its north-eastern coast near Sendai,brought with it over a hundred after-shocks,among them the fear of an impending recurrence in Tokyo or elsewhere. The magnitude-9 quake,the largest one to ever hit Japan,occurred at the interface between the Pacific and the North America plates,two of the four major tectonic plates that Japan and its neighbouring islands rest on. “Think of the earth as an egg,with the plates forming the crust,the yolk as the solid core and liquid in between. When a part of the crust breaks away and collides with or slides under another part,the impact of the event will cause ruptures and disturbances along the fault lines,” says Kusala Rajendran,a professor at the Centre for Earth Sciences,Indian Institute of Science,Bangalore,who studies tectonics and crustal processes.

An earthquake in a subduction zone—a region where an oceanic plate slides beneath another oceanic or continental plate—is no surprise. And the latest quake,too,follows a pattern,says Professor Rajendran. The Japan Trench has witnessed nine events of magnitude 7 or more since 1973,many of them just a few kilometres from the location of the recent quake. In fact,three of the most recent earthquakes in Japan with a magnitude of eight or more were a result of the Pacific plate (a volatile plate,which moves 30-90 mm per year) moving beneath the North America plate. “It’s like a delicately balanced pile of logs. When you remove one log,the entire structure is disturbed. It may seem to settle down at an angle,but sooner or later,it’s sure to come crashing. Very soon,you might hear of volcanoes erupting in the region and more earthquakes. A major earthquake is never a solitary event—it causes a redistribution of stresses,” says the seismologist.

The Japan earthquake slightly re-distributed the earth’s mass,shifting its figure axis (the axis on which the planet’s mass is balanced) by about 6.5 inches. What is not known,however,is whether the Japan quake can trigger seismic disturbances elsewhere in the world. In the past year,the Pacific plate has caused three major earthquakes along its corners—Chile,New Zealand and Japan. In the March 13 issue of Newsweek magazine,Simon Winchester writes that the next big quake is likely to occur at the fourth corner of the plate—the San Andreas fault along the west coast of North America. Many seismologists are,however,wary of drawing such conclusions. “Distant triggering has not been understood properly,” says Professor Rajendran.

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Other speculations about impending earthquakes are more calculated. At the Cascadia subduction zone,which extends all the way from the Vancouver Island to north California,seismologists expect a tsunamigenic earthquake to occur soon. Having deduced from ancient Japanese records and the age of dead trees in the area that the fault has historically experienced a tsunami every 300 years,the last being the ‘orphan tsunami’ of 1700,they predict an impending earthquake of magnitude 9 or higher.

Professor Rajendran has conducted similar studies in Port Blair,one of the areas that suffered the brunt of the Indian Ocean tsunami-earthquake of 2004. “From dating the dead trees from a subsided mangrove,we found evidence of a 2004-like tsunami that occurred a thousand years ago,” she says.