Dr Bruce Malamud
Japan lies a couple hundred km from where tectonic plates are colliding at a convergent boundary, with one plate being dragged (subducted) beneath the other. These tectonic plates are very thick, ranging from 10km to 50km and are in movement with respect to each other. At Japan, they can move at rates of eight to nine centimetres (3.1 to 3.5 inches) each year. This may not seem a lot, but imagine moving two gargantuan pieces of the earth so they collide with each other, year after year, a few inches more every year. The result is a collision, with one plate forced to go beneath the other. It is not friction free (say like a piece of plastic being pushed under another piece of plastic) but sticks. If enough stresses build up from this sticking, then eventually there will be a sudden slip; this produces an earthquake. Japan is well known for being particularly susceptible to subduction zone earthquakes, with immense pressures brought to bear by the sticking tectonic plate zones nearby it, and the slipping results in many earthquakes, some of them very big.
Why do earthquakes happen? Guardian interactive
On Friday 11 March 2011 at 2.46pm (local time), at a point 32km below sea level, and about 129km (80 miles) east of Sendai Japan, there was a slip in part of the subduction zone. This is where the Philippine Plate was being subducted below the North American Plate, at an angle of about 15 degrees. This resulted in an earthquake of moment magnitude nine and a rupture of a fault roughly parallel to the east coast of Japan over several hundred kilometres.
British Geological Society - Earthquake details, including videos, seismographs and maps
Equivalent to the energy released in half a megaton nuclear bomb.
Equivalent to 1000 times the energy released in a magnitude seven earthquake, or one thousand half megaton nuclear bombs. If we converted this to energy, this would be roughly enough to power every home in the USA for 50 days.
One of the questions that has been asked by many is whether there have been more frequent large earthquakes in the last few years. Let’s take as a large earthquake one with moment magnitude seven. The number of earthquakes per year with moment magnitude greater than or equal to seven varies certainly, year to year, but the average from 1900 to present is about 17 magnitude seven or greater earthquakes per year (compared to about one magnitude eight or greater earthquake). If we just look at 1990 to 2010, then the average was about 15 magnitude seven or greater earthquakes per year. And if we look at the last three years, then the average is also 15 of this size earthquake per year. So, no, the actual number of very large earthquakes is not increasing over time. It fluctuates year to year, with some years less and some years more.
After an earthquake occurs along a fault, stress is released in parts. But then, part of this stress is redistributed to other parts of the fault. This means that they are now more likely to become unstable, with many subsequent earthquakes. Aftershocks can continue for weeks and months after the main shock (the biggest earthquake in the sequence), sometimes even years.
When an earthquake occurs, it releases stress that has built up over time, along a fault. However, in addition to releasing stress, it redistributes the stress along that fault, and sometimes these will be redistributed to other nearby faults. In the case of the Japan earthquake, approximately 400km of fault was affected. With the redistribution of stress, aftershocks occur, for weeks, to months (and sometimes years) after the main shock. The magnitude nine earthquake in Japan will result in aftershocks occurring all along the fault on which the original earthquake occurred. Some scientists say that one can expect aftershocks as much as one unit less than the original shock. So in this case, we might expect aftershocks of all sizes, but as big as a magnitude 8 (which would be in itself a concern of potentially triggering a tsunami).
The main shocks that occurred in Christchurch on 22 February 2011 and off the coast of Japan on the 11 March 2011 are not related. They were on very different fault systems, almost ten thousand km apart.
Japan has many measures in place to mitigate the damage to earthquakes:
Warning people, so as to minimize loss of life. When an earthquake occurs, it initially sends out waves that travel through the earth (body waves). The fastest of these waves involve compression and are called P-Waves or primary waves. Slightly slower than these waves, are S-Waves or secondary waves, which are very destructive, because they shake things from side to side. Since there is a time difference between the P-Waves and the S-Waves arriving, once P-Waves are detected, a warning system sounds, escalators in Japan are stopped, trains come to a stop, gas main valves are shut off, and people run for cover. They might have as much as a minute, and this small amount of time is enough to bring the trains to a stop and people get to cover, before the more destructive secondary waves arrive
Emergency Drills. From a young age, children are taught what to do in the case of an earthquake, so they are prepared to go to certain places and take cover
Emergency rations. People keep emergency rations in their homes, enough for a couple of days in case they are without food or water for a period of time
Building codes. Houses and buildings are built to ‘sway’ with the back-and-forth of earthquake waves. The idea is to build things so that if the earthquake is very severe, rather than the building collapsing (if it were made of brick, or concrete) it will either sway without breaking, or if it sways too much, it will deform, and bend, but still give people time to get out
Japan's earthquake preparation has spared it from a far worse fate
Japan earthquake: 'It's as if these big concrete buildings are made of jelly'
When the earthquake occurred, it resulted in a rough rectangle several hundred km long by about one hundred km wide slipping by varying amounts, some places as much as 17 metres. This slippage deep in the earth resulted in a mass of earth at the sea floor’s bottom being uplifted, over tens of seconds. You can simulate this by taking a clear plastic bag (water tight!), filling it with water, and giving a sudden jolt to its bottom with the palm of your hands. You’ll see the top of the water become disturbed. This huge mass of earth being uplifted suddenly gave a jolt to the water at the bottom of the ocean, which was transferred km upwards to the top of the ocean. This energy, along a long line of several hundred km, resulted in waves, not that high (10s of centimetres), with one set of waves going towards Japan’s coast (very near by, so very little warning for people there) and the other set going farther out into deeper water in the Pacific, and eventually travelling around the world (three times). Tsunami waves themselves are not very high in the deep ocean (tens of cm) but when they approach land, they can become tens of metres, and be very destructive. They also travel very fast in the deep ocean, as fast as a jet-liner, but when they get to shallower waters they slow down.
How earthquakes trigger tsunamis – BBC bang Goes the Theory explanation
The Why files: Tsunami: The killer wave
Japan has an extensive system of coastal barriers so that when high waves come, they are prevented from coming to where people are living. In the case of Tohoku event, the waves were higher than the Japanese expected, so the water went over their barriers. Japan also has an early warning system (with regular drills) so that if a big earthquake occurs, they examine whether or not it might cause a tsunami, and if they think it will, they alert the population within minutes of the earthquake occurring. However, people have to then be able to go somewhere (higher ground if they are on land in low lying areas; out to deeper water if they are in a boat), and if the warning time is too short, they won’t have time to do this.
Japan earthquake: Can you tsunami-proof a country? - BBC
For a complete prediction, we need to tell people when a disaster will occur, where, and how big. As scientists, we have a good idea of where large events might occur based on written and instrumental records of past events. So for instance, we know that Japan is near subduction zones, and that there is an extensive history of earthquakes in the past, so we know that Japan is likely to experience earthquakes. Based on these past records, we can also forecast the chance that a given size or larger earthquake might occur, in a given year. This is called probabilistic hazard forecasting, and has been very useful in telling us about how big we might expect, on average, each year. But true prediction is much more difficult, where we tell people that ‘next week there will be an earthquake of magnitude of nine'. Although scientists have been trying for many years to predict earthquakes (the when and how big), they so far have not succeeded, but are still working at it.
Why earthquakes are hard to predict – New Scientist
The earthquake protection in Japan worked well, both in terms of houses, buildings, and people remaining calm. Tsunami protection however might change. There might be changes in Japan in how houses are constructed, so that they protect both against the forces of earthquakes and tsunamis. And, there may be changes in where people are allowed to live. The other change might be a rethinking of the defences that are available for allowing large waves of water to get onto the land. However, these defences are already very expensive, so it is not clear if these can be made even bigger.
Bruce was interviewed in March 2011.
Dr Bruce D Malamud is a Reader in Natural and Environmental Hazards in the King’s College London Department of Geography. His research includes wildfires, floods, earthquakes, landslides and heavy-metal contamination. He is President (2007 to 2011) of the Natural Hazards Division of the European Geosciences Union.
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