Fifteen people injured after Magnitude 7.3 Earthquake off the coast of Fukushima Prefecture, Japan.

The Japan Meteorological Agency recorded a Magnitude 7.3 Earthquake at a depth of about 10 km, about 37 km off the coast of Fukashima Prefecture on Honshū Island, slightly before 6.00 am on Tuesday 22 November 2016 Japan Standard Time (slightly before 9.00 pm on Monday 21 November GMT). There are no reports of any serious damage following this event, however fifteen (mostly elderly) people were injured during evacuations triggered by the event and people have reported feeling it across most Honshū as well as on the northern part of Kyushu. The event triggered a small tsunami, with a maximum recorded wave-height of 1.4 m.

The approximate location of the 22 November 2016 Fukashima Earthquake. Google.

Japan has a complex tectonic situation, with parts of the country on four different tectonic plates. Eastern Honshū area lies on the boundary between the Pacific, Eurasian and Philippine Plates, where the Pacific Plate is passing beneath the Eurasian and Philippine Plates as it is subducted into the Earth. This is not a smooth process; the rocks of the two plates constantly stick together, only to break apart again as the pressure builds up, causing Earthquakes in the process. 

The movement of the Pacific and Philippine Plates beneath eastern Honshū. Laurent Jolivet/Institut des Sciences de la Terre d'Orléans/Sciences de la Terre et de l'Environnement.

Earthquakes along subductive margins are particularly prone to causing tsunamis, since these often occur when the overlying plate has stuck to the underlying plate, being pulled out of shape by its movement.. Eventually the pressure builds up to far and the overlying plate snaps back, causing an Earthquake and a tsunami.

Simplified graphic showing tsunami generation along a convergent margin.NASA/JPL/CalTech.

Witness accounts of Earthquakes can help geologists to understand these events, and the structures that cause them. The international non-profit organization Earthquake Report is interested in hearing from people who may have felt this event; if you felt this quake then you can report it to Earthquake Report here.

 

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Magnitude 5.8 Earthquake in Fukushima Prefecture, Japan.

The Japan Meteorological Agency recorded a Magnitude 5.8 Earthquake at a depth of 20 km in the southeast of Fukushima Prefecture on Honshū Island, Japan, at 2.25 am Japan Standard Time on Friday 20 September 2013 (5.25 pm on Thursday 19 September, GMT). There are no reports of any damage or casualties arising from this quake, however it was felt across much of northern and eastern Honshū.

The approximate location of the 20 September 2013 Fukushima Earthquake. Google Maps.

Japan has a complex tectonic situation, with parts of the country on four different tectonic plates. To the east of northern Honshū lies the Japan Trench, along which the Pacific Plate is being subducted beneath the Okhotsk Plate which underlies northern Japan, passing under the island as it sinks into the Earth. This is not a smooth process, the two plates continuously stuck together then broke apart as the pressure built up, causing Earthquakes in the process. 

The epicenter of the Earthquake lies about 50 km to the southwest of the Fukushima I Nuclear Power Plant, which was destroyed by a tsunami following a Magnitude 9.0 Earthquake off the Japanese coast on 11 March 2013. The site has been leaking radioactive water into the sea since 7 August 2013, a leak which Japanese authorities are currently struggling to halt. While this quake does not appear to have made matters at the sight any worse, it does serve to emphasize that any long term solution to the site's problems must be able to cope with further Earthquakes.

See also Magnitude 6.9 Earthquake in the Izu Islands to the south of Japan, Japanese city covered in ash by eruption on Sakurajima Volcano, Eruptions on Sakurajima, Magnitude 6.0 Earthquake east of the Oshika Peninsula, Japan and Eruption on Suwanosejima.

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2011 Tōhoku Tsunami debris reaches coast of British Columbia.

On 11 March 2011 the Tōhoku Coast of northeast Japan was devastated by an Earthquake with a magnitude of 11 on the Richter Scale, and the tsunami that it triggered (a tsunami is a huge wave caused by an Earthquake or landslide), which reached heights of over 40 m in places and travelled up to 10 km inland. This was the most devastating Earthquake ever to strike Japan, and one of the largest Earthquakes ever recorded.

Video footage of the 11 March Tsunami striking a Japanese coastal town.

In total the earthquake and tsunami are known to have killed 15 844 people, with another 3451 missing and 5890 injured. It devastated coastal towns across eighteen prefectures, destroyed a nuclear power station, a dam, around 125 000 buildings and the infrastructure of a large area of northeast Hokkaido.

As a consequence of this devastation, around 18 million tonnes of debris was washed into the Pacific Ocean, and has been drifting eastward (the prevalent current direction in the North Pacific) ever since. In October 2011 Jan Hafner and Nikolai Maximenko of the International Pacific Research Center at the University of Hawaii calculated that the debris would cover an area roughly 3200 km by 1600 km and would reach Hawaii in early 2013 and British Columbia in 2014, with debris eventually washing up from Alaska to Baja California. This model seemed to have been confirmed when the debris field was observed in the area predicted by the Russian tall ship STS Pallada.

The path of the debris from the 2011 Tsunami, as predicted by the University of Hawaii Team.

Around the beginning of December 2011 reports started to come in of debris from the Tsunami being washed up on the shore of Vancouver Island, around the town of Tofino. At first the local authorities dismissed this, floating rubbish from Japan and China is a regular occurrence on the coast of British Columbia, but by the end of month it was becoming clear that there was a lot more being washed up than usual, and that the items arriving were different from the usual waste; personal items rather than bottles and packaging, and a large amount of timber.

Timber bearing markings identifying it as coming from Japan on beach near Tofino.

Local Mayor Perry Schmunk has been in contact with a number of Canadian governmental agencies in order to develop a plan to deal with any further increase in the debris arriving. Tofino residents have expressed concern at the possibility of radioactive waste from the Fukushima Nuclear Power Plant or even human remains arriving on the local beaches, though the United States National Oceanic and Atmospheric Administration have described this as unlikely.

See also Rethinking Energy, Magnitude 7.1 Earthquake off the Sanriko Coast, Magnitude 6.4 Earthquake off the coast of Honshu, Earthquake off the coast of Vancouver Island and Earthquakes on Sciency Thoughts YouTube.

The Nagano Earthquake of 30 June 2011

At 8.16 am on the 30th June 2011 the Japanese Prefecture of Nagano was hit by an earthquake with a magnitude of 5.5 on the Richter scale, at a depth of about 4 km. This was followed by aftershocks at 8.20, 13.54 and 14.11. Earthquakes are surprisingly common in Japan, on the same day an earthquake with a magnitude of 3.7 hit the Iwate Prefecture and one of 5.3 hit Fukushima. However the Nagano earthquake and its aftershocks were very close to the surface (by contrast the Iwate earthquake was at a depth of 10 km and that in Fukushima was 20 km deep), so that much of the power of the quake was felt by the inhabitants of the prefecture. This lead to at least eight injuries and some damage to the historic Matsumoto Castle.

Map of Japan, showing the epicentre of the earthquake (red cross) and the intensity felt at the surface, measured on the JMA intensity scale.

Japan uses the Japan Meteorological Agency (JMA) scale to measure the effects of Earthquakes. This is similar to the Mercalli Scale, in that it records ground acceleration at the surface rather that worrying about the energy output at the actual hypocenter of the quake. This enables then to map the effects on the surface, as shown in the map above, where the area around the epicenter experienced a JMA scale 5 quake, and an area to the north a scale 3 quake. It also enables comparison of how earthquakes actually impact on people. Thus on the 30th June Japan had two earthquakes with a magnitude of greater then 5 on the Richter scale, in Nagano and Fukushima, but while the shallow Nagano earthquake achieved a JMA scale of 5, the 20 km deep earthquake at Fukushima only produced a JMA reading of 3.

Map of Japan showing the epicenter of the June 30 Fukushima Earthquake, and intensity experienced on the shore.

Japan lies on the Pacific Ring of Fire, that is to say the ring of subduction zones that surounds the Pacific Ocean causing earthquakes an volcanicity. Two subdution zones pass under Japan, as the Philippine Plate to the south and the Pacific Plate to the east are forced under the Asian Plate to the northwest. Japan has over a hundred active volcanoes, caused by melted material from the subducting plates rising to the surface, and, as noted above, is extremely prone to earthquakes.

There have been five major earthquakes in Japan so far this year.

On the 9th of March an earthquake with a magnitude of 7.2 on the Richter Scale hit Tōhoku, on the northeast of Honshu Island, at a depth of 32 km. This is 12 km deeper than the Fukushima earthquake of 30 June, but the Richter Scale is logarithmic, each point on the scale being ten times as great as the previous point, to a magnitude 7 quake is a hundred times as strong as a magnitude 5 quake - more than enough to offset a 50% increase in depth.

At 14.46 local time on the 11th of March a second earthquake (though they were clearly connected) hit Tōhoku, this time with a magnitude of 9.0 on the Richter Scale and at a depth of only 24.4 km. This is one of the largest earthquakes ever recorded, and caused a tsunami which lead to widespread devastation. Several towns were completely destroyed, 15 500 people are known to have been killed, and at least 7000 more are missing. The earthquake and tsunami also caused a meltdown at the Fukushima Nuclear Reactor, which is still causing Japan serious problems.

The aftermath of the deadly Tōhoku Earthquake & Tsunami.

45 minutes later a second earthquake (though again this was clearly related) hit the same area, with a magnitude of 7.1 0n the Richer Scale, and at a depth of 19.7 km. This clearly contributed to the chaos and devastation caused by the earlier quake, though the two were to close together to produce accurate separate casualty statistics.

On April the 7th an Earthquake hit Miyagi (southeast Tōhoku) at a depth of 49 km and with a magnitude of 7.1 on the Richter Scale. This was almost certainly related to the earlier Tōhoku earthquakes. This quake caused the loss of four lives and 141 minor injuries. It also caused some damage to roads and houses, and a power outage which in places lasted over two days.

On the 11th of April an Earthquake measuring 6.6 on the Richter Scale hit Fukushima (also southeast Tōhoku) at a depth of 10 km, causing the loss of three lives, with another three people being critically injured, and seven more sustaining minor injuries. The quake also caused several fires, widespread power outages, and a mudslide that closed several roads. Again this was almost certainly connected to the earlier Tōhoku quakes.

See also The Christchurch Earthquakes,

Rethinking Energy and Earthquakes on Sciency Thoughts YouTube.