Scientists at the University of Cambridge have released the conclusions of a new study, explaining why the tsunami that struck the coasts of Japan last year was so massive. Their research indicates that the effects of elastic energy produced during the tremor were doubled by a gravitational energy release.
On March 11, 2011, a megathrust earthquake of magnitude 9.0 occurred at a depth of just 32 kilometers (20 miles), at a distance of about 70 kilometers (43 miles) east of the Oshika Peninsula, in Tōhoku.
The seismic event was quickly cataloged as one of the five worst tremors in recorded history, and the worst-ever to hit Japan. It produced a massive tsunami wave that reached a top height of 40.5 meters (133 feet), thankfully only at several locations.
Following the massive fault line slip, the entire island of Honshu, of the four main islands in the Japanese archipelago, was moved about 2.4 meters (8 feet) to the east. Even the axis of the planet shifted by around 10 to 25 centimeters (4 to 10 inches).
Since then, experts have been trying to figure out why the tsunami was so intense. Researchers found that a joint release of gravitational and elastic energy was the main culprit behind the massive wave.
The Tōhoku tsunami baffled the international scientific community because it defied predictions that resulted from current models of how deep-sea tremors affect the water above. Full details of the new study appear in the August 24 issue of the journal Earth and Planetary Science Letters.
The excessive water movement that was recorded on March 11, 2011 was most likely caused by the collapse of a huge portion of soft material on the ocean floor, immediately after the earthquake occurred. This led to significantly-worse effects than the tremor alone would have caused.
“As the plates move against each other, the rocks on their boundaries slowly bend under the pressure, until they eventually crack and slide on faults,” Cambridge Department of Earth Sciences professor James Jackson explains. He carried out the work with colleague Dan McKenzie.
“When they do, there is an upwards and outwards movement that takes just a few seconds: a movement of 10 meters is a large earthquake and out at sea this causes a tsunami,” Jackson goes on to explain.
However, this phenomenon cannot account for the 60-meter movement seen in the case of the Japanese tsunami. “This suggests that something else was taking place to increase the movement several fold,” the team co-leader adds.
The decisive factor appears to have been the accumulation of debris in an unstable wedge. The seafloor structure was produced by the gradual grinding of the two tectonic plates that collide near the earthquake's epicenter. The entire wedge collapsed during the seismic event, boosting the tsunami's power.