The magnitude nine earthquake that struck off the coast of Honshu has been officially named the Tohoku Earthquake. An earthquake of that size was not expected.
Thursday 17th March 2011
Last week’s Tohoku earthquake, which was located off the coast of the Japanese island of Honshu, was larger than expected. It was thought that the trend of magnitude 7 earthquakes along the Japan Trench would continue, but an event of magnitude 8.5 or greater was not expected. Initially thought to be magnitude 8.9, the earthquake has been revised to magnitude 9.0 making it the world’s fourth most powerful earthquake since 1900; ranking behind the Chile earthquake of 1960 (magnitude 9.5), Alaska 1964 (magnitude 9.2), Sumatra 2004 (magnitude 9.1) and equally ranked with the Kamchatka quake of 1952. Initial estimates suggest that a 400 km-long stretch of the Japan Trench may have moved during the quake.
The undersea earthquake struck 129 km east of Sendai (177 km east of Yamagata, 373 km north-east of Tokyo), Honshu at 6:46 p.m. on Friday 11th March 2011 New Zealand Daylight Time. The quake occurred at 2:46 p.m. local time and 35 minutes later the first large tsunami wave swept ashore at Miyako and Kamaishi where waves over 4 metres in height were recorded. A three metre wave swept inland at Ofunato just 29 minutes after the earthquake.
The Japan Meteorological Agency has reported a maximum wave height of 7.3 metres at Soma and the official wave heights at Kesennuma and Fukushima are still unknown.
The massive waves breached tsunami defences giving people little time to escape. In the days immediately following, the extensive tsunami damage hampered rescue efforts with roads blocked by massive piles of cars, houses, boats and other debris. Cold weather reduced the survival chances for people trapped in the debris, and snow has fallen at several locations during the past week.
Whilst the earthquake caused significant damage, the massive tsunami caused even more, wiping out entire towns and large parts of several cities. Fires contributed to the damage.
With infrastrucure seriously damaged by the earthquake and tsunami, fuel has been at a premium, and supplies have been short in north-west Honshu and quickly ran out in some places. With transport seriously compromised, food and bottled water supplies have been very short across a significant area.
While trying to commence rescue efforts in areas affected by a massive earthquake and tsunami and in cold conditions, catastrophe has dogged the heels of officials with nuclear emergencies declared at three separate nuclear power stations on the island of Honshu. The most significant incidents have occurred at Fukushima Dai-ichi (Fukushima number 1) nuclear power station located in the town of Okuma (the Fukushima II nuclear power station is located 10 km to the south).
Fukushima I is a complex of six boiling water nuclear reactors, the first of which came online in 1970. Reportedly only three of the reactors were critical and on-line at the time of the earthquake, with units 4, 5 and 6 down for maintenance. Units 1, 2 and 3 shut down automatically after the earthquake but the tsunami, which the Japan Meteorological Survey still lists as of unknown height at Fukushima, invaded the complex and damaged the emergency generators needed to maintain cooling water flow to the reactors and supply power for management of the power station.
Unit 1 quickly began overheating and radioactive steam was vented to avert an explosion. The local population within 10 km of the plant was ordered to evacuate. A large explosion occurred the following day and the evacuation zone was extended to 20 km.
On March 14th an explosion blew off the roof off the building housing reactor number 3. The following day reactor number 2 lost coolant for several hours and an explosion occurred within the reactor. At about the same time, fire broke out in the building housing reactor 4. It was later revealed that a pool in the unit 4 building was used for storing spent fuel. The crisis has deepened at unit 4 with the storage pool boiling dry this morning New Zealand time, allowing the fuel rods to heat and reactivate. Significant nuclear contamination is feared.
A 20 kilometre evacuation zone remains around Fukushima Dai-ichi and residents situated 20-30 km from the plant have been told to stay indoors and close ventilators.
Japan now faces an unprecedented crisis. Reeling from a massive earthquake and tsunami, with infrastucture compromised by tsunami damage and electricity shortages, bitten by the cold weather, and now a nuclear emergency that it is feared could result in contamination reaching Tokyo under certain weather conditions. Most activity is now focussed on unit 4 where the heating of the spent fuel rods could trigger a nuclear emergency similar to that which occurred at Chernobyl in 1986. The worst case scenario would be for a chain reaction to be triggered in the stored fissile material at the power plant affecting an area up to 300 km from the power station.
There is considerable uncertainty over whether large populations should be evacuated, or whether this is even possible with railways compromised, fuel supplies constrained and food in short supply in the worst-affected areas.
Japan is thought to be the best earthquake-prepared country in the world. Major earthquakes are expected at any time, so the population is prepared through regular exercises. Tsunami-prone areas have been identified and novel attempts have been taken to deflect or slow some of the anticipated waves or strengthen important plant.
The steadily worsening situation since Friday’s earthquake has dumbfounded most observers. Clearly something has gone seriously wrong with planning, response or both, and should ring alarm bells for emergency managers around the world.
Some key points have already been identified and are discussed in greater detail below.
Earthquakes over magnitude 8.5 were not expected to occur on the Japan Trench.
The tsunami height may have exceeded expectations.
The Fukushima Dai-ichi nuclear power station may have been subjected to ground acceleration beyond its design specification.
The Fukushima Dai-ichi reactor number 1 was scheduled for de-commissioning early this year but a ten-year extension was granted by regulators in February.
Earthquakes over magnitude 8.5 were not expected to occur on the Japan Trench
This has some scientists scratching their heads. There is no earthquake prediction science today, but a lot of work has been put into studying the likelihood of an earthquake occurring at a certain place. Several parts of the globe have more earthquakes than normal which provides a wealth of information about past events. This data is examined to look for patterns and, when they emerge, these can be projected into the future as a possible scenario. When combined with examination of the physical environment, the relatively short period of recorded history can be projected further back to look for more patterns of recurrent earthquake activity.
Fortunately, Japan has had a long period of civilisation and written records of historic earthqukes go back a long way. The modern earthquake catalogue includes nine events of magnitude 7 in the Japan Trench subduction zone since 1973 and what is thought to be a magnitude 8.4 quake in 1933. Like last week’s earthquake, the 1933 event and a smaller magnitude 7.6 event in 1896 were more notable for the tsunami damage rather than the earthquake damage, with maximum wave heights of 38 metres in 1896 and 29 metres in 1933. Another large offshore earthquake in 1611 also produced large waves.
In the year 869 the Sendai region was swept by a large tsunami that has been identified from both written records and an onshore sand layer.
Analysis has shown that the Sanriku coastline of north-east Japan is particularly vulnerable to tsunami because it has many deep bays that amplify tsunami waves to great heights. Tsunami-amplifying bays have also been identified on New Zealand coasts.
With history listing so many magnitude 7 events, a magnitude 8.5 was not anticipated, and there was no strain build-up that indicated the potential for one. In fact, it was the Tokai earthquake, expected to strike further south near Tokyo, that was the most-feared threat. The Tokai quake has a long, regular pedigree, and is currently overdue.
The tsunami height may have exceeded expectations
With so much infrastructure damaged or destroyed, manual observations will be necessary to determine the location of the maximum wave heights that occurred during the most recent event. This data will be gathered over the coming weeks and months as field teams are deployed during the earthquake recovery phase.
The extensive list of wave heights published by the Japan Meteorological Agency has several entries for “unknown” in Iwate Prefecture, Kesennuma, Fukushima, and other locations. The report suggests that an off-shore measurement of 6.8 metres was made at Kamaishi.
The Fukushima Dai-ichi nuclear power station may have been subjected to ground acceleration beyond its design specification
Reactor No. 1 at Fukushima Dai-ichi was designed to withstand a peak ground acceleration (PGA) of 0.18 gravities, and survived the 1978 Miyagi earthquake when PGA reached 0.125 g. Initial reports for the latest event include PGA values of 0.35 g, twice the design limit of the reactor.
The Fukushima Dai-ichi reactor number 1 was scheduled for de-commissioning this year but a ten-year extension was granted by regulators in February
There have been comments in the media that some of the reactors at Fukushima were getting old, and this is the first concrete evidence that I have seen indicating that this might be the case. The No. 1 reactor was commissioned in 1970, No. 2 in 1973, No. 3 in 1974, No. 4 in 1978, No. 5 in 1977, and No 6 reactor was commissioned in 1979. Generally the reactors took four or five years to build, adding to the age of the technology.
The four troubled reactors use technology that is forty years old.
Japan has been struck by a catastrophe with three major components (massive earthquake, major tsunami, and a nuclear crisis), any one of which would constitute an emergency on its own. The aggravating factors of cold, snowy weather and serious disruption to transport infrastructure are hampering their efforts at getting to grips with the real emergencies.
[Compiled from data supplied by the U.S. Geological Survey, Japan Meteorological Agency, Japanese government agencies, Wikipedia and media reports.]