7/16/2007 Honshu, Japan

Updated 9/22/2007

This 6.6 Magnitude quake hit at 10:13 AM local time (01:13 GMT), just off the west coast of the island of Honshu in central Japan. The epicenter was 45 miles west of the city of Nigata and 150 miles to the northwest of Tokyo. It was followed a couple of hours later by a 6.8 M quake nearby. Although the second quake was stronger, it was focused depper and thus cause less damage. According to the US Geological Service account of the quake, these quakes were different enough that the second one is not considered an aftershock of the first.

The earthquake caused damage to 875 buildings, especially older ones, 1,088 injuries and 11 deaths. The shaking was so strong that people said they couldn't stand during the quake. A nuclear power plant remains shut down two months after the quake due to damage that resulted in the release of radiation. This accident follows several other incidents at nuclear power plants in Japan that have called into question the safety of Japan's extensive nuclear power program.

Japan is located at the western edge of the Pacific Plate, where it interacts with the Eurasian Plate and some small tectonic plates inbetween. This results in many earthquakes as these plates interact. In 2004 a 6.6 M quake in the same general area killed 40 people and injured 3,000. The US Geological Service account of the quake describes the geology in more detail. (See the Plate Tectonics page for more information on these processes.) The US Geological Survey (USGS) is an invaluable resource in understanding and tracking earthquakes.

See also the following news stories:

• Nuclear scare after Japan quake (BBC 7/16/07)
• In pictures: Japan earthquake(BBC 7/16/07)
• Japan post-quake power shake-up(BBC 7/20/07)
• Japanese fears over nuclear power(BBC 7/17/07)
• Fire at Japan's quake-hit plant(BBC 9/20/07)

9/12/2007 Sumatra Earthquake

Updated 9/20/2007

An 8.4 Magnitude earthquake hit southern Sumatra at 11:10GMT on September 12, 2007. 6:10pm local time. The epicenter was 80 miles SW of the city of Bengkulu. This is the same area as the June 11, 2000 7.9 magnitude quake. This is the third quake over 8.0 magnitude this year, following last month's Peruvian quake and the April Solomon Islands quake (see below). 23 deaths have been reported so far. This is quite low for a major earthquake in this area. Over 45,000 buildings have been damaged but casualties have been surprisingly low for a quake of this size. There have been numerous strong aftershocks, including one 7.9 magnitude and one 6.8. Both of these are strong enough to be considered major quakes on their own. In all, there have been 23 aftershocks greater than 5.0 Magnitude in the week following the quake.

There were fears of a tsunami throughout the Indian Ocean area. However, aside from a 3 ft wave in the local Sumatra area that didn't cause any damage, there hasn't been one. After each of the larger quakes, warnings were issued and then withdrawn when no tsunami occured. This nervousness is understandable with memories still fresh from the disastrous 2004 tsunami, which was caused by a larger (8.9 M) quake further north along the southern Sumatra coast. Tsunami predictions are difficult because the size of a tsunami following a large quake is dependent on conditions on the seabed at the epicenter. This is often not known. What we do know is that a large undersea earthquake could produce a tsunami. The larger the quake, the greater the chance of a tsunami. With a limited amount of time to get the word out, governments have been quick to issue a warning since the 2004 tsunami, which hit many areas without warning. This quake was large, although considerably smaller than the 2004 one. In fact, it was larger than any quake to hit the the US outside of Alaska in the last 300 years.

The quake is caused by the subduction of the ocean beneath the Sumatra coast. The subduction zone stretches over 2800 miles along the southern coast of Sumatra and Java. This movement created strain in the crust that is relieved by quakes in different areas of the subduction zone. That is why there have been so many large quakes along the southern edge of the Sumatra and Java coasts. Each quake relieves pressure in one section of the zone but in the process builds pressure in neighboring sections. Over a period of 150-300 years the whole zone will experience earthquakes. The process continues as the Australian Plate is being forced under the smaller plates to the north, between the Eurasian Plate and the Pacific Plate. This is certainly the most seismically active area of the world. The smaller quake 7 years ago relieved some strain but apparently not enough, especially given the other quakes on other segments of the subduction zones in the interim.

US Geological Survey Paghttp://theearthquakemuseum.weebly.com/plate-tectonics.html"> Plate Tectonics page for more information on these processes.) The US Geological Survey (USGS) is an invaluable resource in understanding and tracking earthquakes.

See also the following news stories:

• Huge earthquake shakes Indonesia (BBC 9/12/07)
• Fresh Earthquakes rock Indonesia(BBC 9/13/07)
• Rescuers assess quake-hit Sumatra(BBC 9/13/07)
• Sumatra Quake Raises Risk of Temblors Off Indonesia(Bloomberg 9/18/07)