Wednesday, 29 February 2012

The Nature of the 2011 Japan Earthquake and Tsunami

By Nadine Wager

On March 11th 2011 at 2.46pm, Japan was hit by its largest earthquake in history with a magnitude 9 on the Richter scale (previously the largest was 8.4 on the Richter scale) the same as around 2 million atomic bombs!

The earthquake is believed to be caused by a sudden movement, releasing the hundred years of tension generated by the Pacific plate subducting under the Eurasian plate. The earthquake was so large it shifted Japan 3 metres out to sea, dropped the Japanese coast over a metre and even knocked the earth off its current axis by 25cm, shortening the day by 1.8 micro seconds. The epicentre was located 100km off the coast of Sendai and the focus was 6km deep. The earthquake hit Sendai first with its P waves travelling at 6km a second and the always slower S waves travelling at 3km.

The earthquake affected all of the Eastern coast of Japan, taking around 100 seconds to reach its capital Tokyo and created ground shaking which lasted around 5 minutes, an abnormal time for an earthquake. Massive cracks were visible in the ground and during the shaking the ground was even seen to be expanding and contracting due liquefaction in some areas. Liquefaction occurs when the shaking of saturated sediment squeezes water to the surface.

Due to an epicentre located in the sea, and such a large explosion of energy from the earthquake a tsunami, over 10 metres high was created. The deep ocean and the massive moment meant the tsunami travelled at speed of 60kmph, taking only an hour to reach the coast. The tsunami, hitting Ofunato first then Sendai then Miyako, was superior to tsunami defences such as sea walls which had been weakened by the dropping of the coastline.

The tsunami is believed to have travelled up to 10km inland depending on the topography of the land, with flatter land allowing the tsunami to travel faster. In more developed places the water was seen to reach even higher height due to being squeezed between and over buildings. The tsunami travelled in both directions, toward Japan and out into the Pacific ocean, where places such as Hawaii receiving waves of 1 metre high.

Over 500 aftershocks were recorded during the week after the earthquake with some shakes, such as the one that killed 3 people and injured many others on 8th April, reaching a magnitude of 7.1. The map above (from the USGS) shows the sheer number of after shocks that have occured. Scientists have warned that Japan may face years of aftershocks as the fault continues to adjust and settle to its new position and pressures.

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