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Japan: why the killer quake was never expected

R. PRASAD

Subduction zones, where one plate dives under another, are the regions where major quakes occur. But they are hardly monitored

— Photo: AP

Lack of instruments: Many lives could have been saved if the strain build-up was monitored by sea-based instruments located near the trench.

The 9 magnitude Tohoku-Oki megathrust earthquake that rocked Japan on March 11 was caused by a sudden slip of the plate interface of a relatively small area of 400 km length and 200km width.

That no seismologists had expected such a powerful quake to strike Japan (off the coast of Sendai) now is well known. Though precise prediction of quakes has not been possible till date, how and why did the scientists get it so wrong?

In the Japan Trench, the Pacific plate dives (subducts) under the Okhotsk plate. If the subduction (diving) is indeed smooth then no stress would build up, and therefore no earthquakes would happen.

Plates locked

Unfortunately the plates do not always subduct smoothly. At times they get locked or stuck to each other in some portions of the fault resulting in stress build-up.

This had indeed happened in the case of the Tohoku-Oki megathrust.

Being situated in the Pacific ring of fire, Japan is prone to quakes and the country is heavily monitored for quakes by land-based instruments and GPS. Unfortunately, as in the case of other countries, only a couple of faults are present on land.

Hardly monitored

More than 90 per cent of known tectonic plate boundaries in the world are present under water and at great depths. Believe it or not, these deadliest regions where major quakes occur are hardly monitored. This has been the case even in the case of Japan.

Land based instruments have limitations in assessing the actual stress build-up at the locked regions of the fault. In fact, published data did not reveal any locking in the fault region where the March 11 quake occurred.

The loss of human lives could have been minimised and the deadly nuclear accident thwarted if there were monitors based near the trench to reveal the build up of stress. Indeed, Japan has a few sonar transponders located on the seafloor to provide ground-deformation data.

According to a paper published in Nature today (June 23), data from these transponders was not collected frequently, as should have been the case. So what prevented Japan from collecting the data?

The prohibitive cost, the paper notes. True, as these instruments are located on the seafloor, the ships first identify their locations using global positioning system (GPS) and then use the triangulation method to locate the exact location of each transponder. The exercise when carried out regularly and at regular time intervals can help in understanding the change in the instruments' location.

While manufacturing and deploying these transponders cost several hundreds of thousands of dollars, the use of ships to collect the data costs an additional hundred thousand dollars.

According to Andrew V. Newman, the author of the paper, the total cost would work out to as much as half a million dollars!

That was the reason why Japan did not collect the data from these transponders regularly.

Is there a way out to make it cheaper and hence know the stress accumulation in the faults at the subduction zones?

Possible solutions

Dr. Newman thinks there is. Locating the sonar transponders without using ships is one way out. According to him, real-time transmission of data from the seafloor instruments can be made possible by hooking them to existing underwater cables. Probably even laying new cables for data transmission should not be ruled out.

The other possible solution he suggests is the use of buoys for collecting data from the transponders and transmitting them to satellites. Such a system is actually being developed at the Scripps Institution of Oceanography in San Diego, California.

The time has come to develop a cheaper system to collect and convey the data either in real-time or at regular intervals.

Such a system will enable scientists to densely populate the seafloor, especially the trenches where subductions occur, with sensitive instruments. The Japan Trench alone would need 100-400 such sensors.

According to the author, the total cost to equip the Japan Trench with sensors would be around 5 million to 20 million dollars. This pales in comparison with the over $300 billion projected cost of the Japan tragedy.

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