Tohoku Earthquake and Tsunami

The 9.0M Tohoku Earthquake occurred at 13:46 (Malaysian time or 14:46 Japan Standard Time, JST) on 11 March 2011. The epicenter was reported to be some 129 kilometers off the east coast of the Oshika Peninsula, Tohoku, northern Honshu, with the hypocenter at a depth of 32 km (at 38.322°N, 142.369°E – USGS (2011) (Fig. 1). The earthquake triggered tsunami waves of up to 10 meters that struck the Japanese coast minutes after the quake.  Sendai and the neighbouring prefectures received the brunt of the tsunami which travelled up to about 10 km inland. Smaller tsunami waves occurred after several hours in many other countries. 

Figure 1 Seismic Hazard Map (After USGS 2011)

The earthquake caused massive and widespread damage in Japan, including damage to infrastructure (roads and railways) as well as fires in many areas, and contributed to a dam collapse. The tsunami caused loss of some 6,000 lives as well as damage to the stand-by diesel generators to pump coolants to bathe the fuel rods in the 6 reactors (3 reactors were shut-down when the earthquake hit) of the Fukushima Nuclear Power Plant, causing partial meltdown to at least three of its nuclear reactors.

As is common with all earthquakes, this earthquake was preceded by a 7.2M earthquake two days before, that is, on 9th March 2001, at approximately the same location. This earlier earthquake however did not cause a tsunami to occur and is now classified as a foreshock, among 3 other earthquakes of magnitude 6, with the main shock occurring on 11th March 2011. This main shock was followed by a series of aftershocks which caused collapse of buildings weakened by the main shock.

Considering the Tohoku Earthquake's magnitude of 9.0 (originally estimated at 8.9M, the USGS and Japanese seismologists had independently updated the magnitude to 9.0 from the previous estimate of 8.9), this would make it the most powerful earthquake to hit Japan in recent times and the fourth largest in the world since record-keeping began in 1900 (Fig. 2).

Figure 2 Great Earthquakes Since 1900. (Source: www.virtualuppermantle.info/2011-Sendai-Japan.htm)

Tohoku Earthquake

The Tohoku Earthquake, and the accompanying tsunami, was the result of a major thrust faulting along or near the convergent plate boundary where the Pacific Plate dips to the west beneath Japan (Fig. 3), the subduction zone being very active seismically. This map shows the rate and direction of motion of the Pacific Plate with respect to the Eurasian Plate near the Japan Trench. The rate of convergence at this plate boundary is high, at about 83 mm/yr (8 cm/year).

Figure 3 Subduction of the Pacific Plate beneath the Eurasian Plate. (Source: www.virtualuppermantle.info/2011-Sendai-Japan.htm)

Tsunami

The 3/11 Tsunami was believed to have occurred resulting from the 9.0M earthquake occurring along the subduction zone. The earthquake was believed to have occurred when the leading edge of the overriding plate broke free and sprung seaward, raising the sea floor and the sea-water above it. It was this uplift that initiated the tsunami. In addition to this, the bulge behind the leading edge would have collapsed, thereby thinning the plate and lowering coastal areas.

The energy released in the uplift would result in the formation of tsunami waves which would have raced toward nearby land, growing taller as it came in to the shore. Another part would have headed across the Pacific Ocean toward distant shores.

Conclusion

The magnitude of the Tohoku Earthquake and the resulting 10-m tsunami wave was unprecedented in the 130 years since Japan began recording its seismic events. The massive loss of lives and disrupted services would inevitably have an impact on Japan’s economy and its psyche in the short- to medium-term. However, with the resilience and the stoicism shown by the Japanese since the Earthquake (as well as the nuclear incidents), the nation will rise again to be an economically advanced nation within the next decade.

P. Loganathan M.I.G.M
27th March 2011