Shock, aftershock

The Tohoku earthquake was unique in many ways and took the Japanese unawares.

THE massive earthquake of March 11 that wreaked widespread devastation over a 500-kilometre-long stretch of the eastern, or Pacific, coastline of Japan is the largest-ever recorded in Japanese history. The magnitude 9.0 earthquake occurred at 05-46 UTC/GMT (2-46 p.m. local time), with its epicentre (38.3N 142.4E) at 130 km off the east coast of Oshika peninsula in the Tohoku region of Honshu island, about 370 km north-east of Tokyo, and its hypocentre at a depth of 32 km (picture 1). (Initially reported as M 7.9 by the Japan Meteorological Agency (JMA) and the United States Geological Survey (USGS), the magnitude was quickly upgraded to 8.8, then to 8.9 and finally to 9.0 on March 14.)

This makes the the 2011 off-the-Pacific coast of Tohoku Earthquake, as it has been termed officially by the JMA, the fourth largest in the world since 1900. The quake triggered a violent tsunami with huge waves submerging a very vast area way inland and razing to the ground homes and other structures.

Aftershocks, many of which in other parts of the world would be major disasters in themselves, continue to pile on the nation and heap misery on its people. According to the JMA, as of 03-00 GMT on March 16, aftershocks of magnitude greater than 7.0 occurred three times, greater than 6.0 occurred 48 times, and greater than 4.5 about 500 times. The largest aftershock, with a magnitude of 7.5, occurred at 06-25 GMT on March 11. Such aftershock activity, according to the JMA, is very high compared with past major earthquakes in the island nation.

The aftershocks occurred off the coast in the large area that constitutes the contiguous prefectures (States) of Iwate, Miyagi, Fukushima, and Ibaraki (picture 2). In the 65 years after the end of the Second World War, this is the toughest and the most difficult crisis for Japan, Prime Minister Naoto Kan stated.

Japan's islands lie across four major tectonic plates: the Pacific plate, the North America plate, the Eurasia plate and the Philippine Sea plate. The earthquake occurred as a result of thrust faulting (picture 3) near the 2,200-km-long subduction zone interface plate boundary between the Pacific and North America plates. Here, since the crust around Japan is less dense and lighter than the crust in the Pacific, the Pacific crust goes down.

According to the USGS, at the latitude of the earthquake, the Pacific plate moves approximately westwards with respect to the North America plate at a velocity of 83 mm/year. This is a fairly high convergence rate compared with the 3 cm/yr of the Indian plate thrusting under the Himalayas and hence makes for a highly seismically active subduction zone. The Pacific plate thrusts underneath Japan (northern Honshu) at the Japan Trench and dips to the West beneath Eurasia (pictures 4 a & b). This motion pulls the upper plate down until it breaks and springs back with great force and releases an enormous amount of energy. The break in this instance was estimated to be several hundreds of kilometres long and it caused the sea floor to spring back by several metres, leading to the massive earthquake.

According to seismologists, a quake of this magnitude usually has a rupture length of about at least 450 km and requires a long and relatively straight fault line. Interestingly, however, the plate boundary and the subduction zone in this region are not very straight, and an earthquake of a magnitude more than 8.5 was considered extremely unlikely. This is perhaps indicative of why the Japanese were found wanting in foreseeing its possible impacts. It should be remembered that the Richter magnitude scale is a logarithmic one and the energy released goes up 30 times for every unit increase.

500-km-long rupture

According to the JMA, the fault zone may have ruptured over a length of 500 km across Iwate prefecture in the north to Ibaraki prefecture in the south with a width of 200 km and a depth from the surface of over 50 km. The JMA analysis seems to suggest that the main event itself comprised a set of three events and it may have had a mechanism similar to that of another large earthquake in 1869 with a magnitude of 8.6, which too resulted in a tsunami.

Pictures 5(a), (b) and (c) show the historic seismicity since 1900, with the gold star indicating the epicentre of the Tohoku quake. The set of orange circles in 5(a) (which shows earthquakes in the region in 2011) to the east of the epicentre are locations of an M 7.2 quake that occurred on March 9 and its aftershocks. Given the fact that the M 7.2 quake was very close to the big one of March 11 (about 40 km away), the proximity of its aftershocks, and the fact that there were three earthquakes of M 6+ earlier on the same day, it is now apparent that these were foreshocks to the great earthquake. Unfortunately, scientists do not know yet how to identify that a foreshock is actually a precursor to a large earthquake. The designation as a foreshock is always made only in hindsight.

Aftershocks usually occur, geographically, near the main shock. During the main shock, the stress on the main quake's fault changes and that change produces most of the aftershocks. Sometimes the change in stress can even trigger aftershocks on other nearby faults. The aftershocks follow a predictable pattern as a group, although individual earthquakes themselves are not predictable, and they decay with time and also with distance from the main shock.

Picture 5(b) shows earthquakes greater that M 7 since 1900. There have been five M 8+ earthquakes in the region over the past 111 years, including the March 11 one. Hence, great earthquakes are not uncommon, but M 9.0 was unusually large and unexpected. Picture 5(c) shows that quakes near the Trench have shallow origin and increase to depths of 300 km to the west as the Pacific plate thrusts beneath Japan. It is the shallowness of the major quakes along the Trench that leads to massive tsunamis as well.

Seismic moment

The surface energy released by the earthquake is estimated to be about 2 x 1017 (200 thousand million million) joules, which was dissipated as shaking and tsunami energy. This is nearly twice the energy released during the 2004 M 9.1 Sumatra earthquake. The total energy, also called the seismic moment, was 3.9 x 1022 joules, which is equivalent to 9.32 trillion tonnes of TNT (trinitrotoluene), or about 600 million times the Hiroshima bomb. It is more than 200,000 times the surface energy but slightly less than the total energy released during the Sumatra quake. Seismic moment is proportional to the product of the extent of the slip on the fault and the area of the fault that slips.

What is surprising is how a rupture of just 500 km, compared with the 1,300 km rupture of the Sumatra quake, released energy of a similar magnitude. Seismic analysis of data has shown that the shorter rupture size was more than compensated for by the extent of the slip in the thrust fault between the two grinding Pacific and North America plates.

The large slip is believed to have occurred at three places in the north and south of the hypocentre, and also between Fukushima and Ibaraki prefectures. They seem to have slipped by a huge 20-40 metres, making it one of the largest fault movements ever seen, thus causing an enormous build-up of stress and its release.

The largest amounts of rupture occurred over 100 seconds but smaller displacements continued for another 75 seconds or so after the start of the earthquake. Scientists also believe that this energy release could have been transferred to other nearby faults. Also, this enormous built-up stress could be released through future ruptures in months or even years.

Astonishing impact

In geophysical terms, the release of such a huge amount of energy has had astonishing permanent geophysical impacts. It has moved portions of north-east Japan eastwards, pushing it closer to America by as much as 2.4 m and making the landmass wider than before. The massive slip also seems to have led to the dropping of a stretch of 500 km of the coastline vertically by 0.6 m.

According to Italy's Institute of Geophysics and Volcanology, the earthquake shifted the earth's axis by 25 cm. The redistribution of the earth's mass caused by the plate movements would have resulted in the shifting of the axis. Conservation of angular momentum forces the earth to then rotate a bit faster, which, calculations show, would have led to a shortening of the 24-hour day by 1.8 microseconds.

An earthquake of such a magnitude is expected to lead to such changes. Indeed, the M 8.8 Chilean earthquake last year shortened the day by 1.26 microseconds and the Sumatra quake did so by 6.8 microseconds.

The shaking intensity scale, instead of the Richter magnitude scale, for example the I-XII Modified Mercalli Intensity Scale, gives a measure of the actual impact of the earthquake in terms of ground motion at a given place. While Intensity IV represents shaking felt by most people, XII indicates total destruction.

Japan, however, uses its own intensity scale called Shindo (literally meaning degree of shaking') evolved by the JMA. It is a 10-unit measure, ranging from 0, a very light tremor, to 7, a very severe earthquake. Intermediate levels 5-6 and above can cause heavy damage.

According to its report of March 16, on the basis of the occurrence of aftershocks so far, the possibility of aftershocks with a maximum JMA Seismic Intensity of 5+ or higher was 40 per cent for the three-day period from 03-00 GMT March 16, followed by 20 per cent for the three-day period from 03-00 GMT March 19 to 03-00 GMT March 22.

Peak Ground Acceleration (PGA) is a measure of the force of the ground motion and an important parameter for earthquake engineering. The force that one experiences in daily life is the acceleration due to gravity (g), which is 9.8 m/s2, and gives us the feeling of our weight. According to the Earthquake Research Institute (ERI) of the University of Tokyo, a strong PGA of 2 m/s2 was observed in a broad area of the 500-km-long coastline from the mid-Iwate to Ibaraki coast.

Strong ground motion

Pictures 6(a) to (d) show a visualisation of the seismic wave propagation of the Tohoku earthquake, using data from the high-sensitivity seismograph network (Hi-net) located all over Japan. The intensity of ground motion is indicated by the colour intensity and its height. The strong ground motion due to the earthquake reached Oshika peninsula in 35 seconds from the time of origin of the earthquake and propagated to the whole north-eastern region of Japan in 70 seconds. After about 300 seconds, the whole Japanese archipelago was shaken for more than a few minutes.

As mentioned earlier, the shallow nature of earthquakes at the Japan Trench frequently causes tsunamis to occur, and the M 9.0 Tohoku earthquake also resulted in a massive tsunami, causing heavy destruction along the Japan's north-eastern Pacific coastline. The tsunami propagated across the Pacific. A tsunami is caused by the sudden vertical motion of the grinding plates with the Pacific plate going down and the North America plate bouncing up a bit. This up and down motion of plates is particularly strong in subduction zones, and even M 6+ earthquakes can cause a tsunami around Japan.

The sudden motion displaces a large volume of water, which causes a tsunami (picture 7). Tsunami waves travel very fast; the speed depends on the depth of water. In the open ocean, where depths are about 5 km, the speed can be as high as 220 m/s. When the depth becomes one-tenth of that closer to the shore, it falls to about 70 m/s.

The reason why a tsunami is so destructive is that the rapidly moving waves slow down as they approach the shoreline. But in the deep ocean the water is still moving in at very high speed. This results in a piling up of water near the shore, and the sea level can rise by several metres (even up to 10-20 m). Tsunami waves also have a very long wavelength (hundreds of kilometres) because of which the wall of water stays that way for a long period of time. This process is accentuated by the fact that coastal regions, as mentioned earlier, also sink.

The Hawaii-based Pacific Tsunami Warning Centre (PTWC) issued tsunami warnings to all the Pacific countries, the entire north and south coast of North and South America from Alaska to Chile. While tsunami waves did reach most of these places, they did not have any significant impact. For instance, in Chile, whose coast is the farthest, at 17,000 km from Japan, a 2 m tsunami wave did strike its coast. But with the epicentre so close to the east coast of Japan, the impact of the tsunami on Japan was altogether of a different magnitude. The extent of damage caused by the double whammy of earthquake and tsunami, but mostly by the tsunami, has been massive.

Besides the PTWC, the JMA, too, issued tsunami warnings based on its network of DART (Deep-ocean Assessment and Reporting of Tsunamis) buoys in the Pacific. Immediately after the quake, the first warning already carried the serious warning for a major tsunami wave with a maximum of 6 m for Miyagi prefecture and 3 m for Fukushima. But within half an hour, the warning for these areas respectively was changed to 10 m and 6 m. And within 45 minutes, the warnings were for 10 m and more tsunami waves for Iwate, Miyagi, Fukushima and Ibaraki prefectures on the eastern coastline.

Vast stretches of Japan along the Pacific, including the Hokkaido island, were issued warnings of a serious tsunami threat of 6-8 m waves striking their regions. Initial estimates put the arrival times (pictures 8 a to d) as 10-30 minutes, and as warned, the waves had devastated the vulnerable coastal prefectures under half an hour of the earthquake, between 06-12 and 06-21 GMT, with wave heights ranging from 3 m to 7.3 m.

As it published the tsunami data observed from its observation sites, the JMA added: At some parts of the coasts, tsunamis may be higher than those observed at observation sites. And, indeed, some reports do suggest that tsunami heights in parts could have been 10 m or more.

On the basis of a tsunami source model developed by the USGS, and assuming the fault size to be 600 km x 250 km and the fault slip to be 17 m, Takashi Furumura of the ERI has carried out a simulation of the tsunami of the March 11 Tohoku earthquake. The snapshot sequence in picture 8 captures the simulated propagation of the tsunami to the east coast of Japan.

Although Japan has invested hugely in building anti-tsunami walls up to 12 m high along as much as 40 per cent of its coastline, the Tohoku tsunami seems to have completely overwhelmed these defences, destroying many of them in the process. A major effect of this was the very serious impact on the nuclear power plants (NPPs) in the Fukushima prefecture, with the imminent danger of radiation fallout from the damaged reactor.