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.
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.