The 29 September earthquake near Samoa, which caused a fatal tsunami would have been a notable seismic event even if it had not claimed so many lives (at least 149 according to recent figures). The quake measured 8.0 on the Richter scale, and on average the world experiences only one quake of magnitude 8.0 or above per year. Less than 17 hours later there was a magnitude 7.6 quake just offshore of Sumatra, which devastated the city of Padang. This did not cause a tsunami, probably because it was too deep, but many more lives have been lost (at least one thousand and rising, mainly in collapsed buildings).

In an average year, we would expect about 20 quakes exceeding magntiude 7.0, so naturally the news media got rather excited as to whether the two quakes were linked. Had the Samoan quake caused the Sumatra quake? I spent quite a while on Wednesday evening talking to various print and broadcast journalists by telephone and on Skype, and was chauffeured down to London at 4am on Thursday to do a live interview on GMTV just after 6am. Some journalists also cottoned on to a magnitude 5.8 quake in Peru nine hours after the Sumatra quake, and looked for a link between all three...

The answer is that the events are most unlikely to be linked. Look at the map below: the epicentres of the Samoan and Sumatra quakes are more than 7500 km apart, and they are not even on the same plate boundary. The Samoan quake occured in what is popularly called the 'Ring of Fire' This (almost) circles the Pacific ocean, and is a system of trenches where the Pacific ocean floor is being subducted (pushed down below) continents or island arcs, running from New Zealand, northwards past Tonga (where the ocean floor is being subducted westwards, and hence the Samoan quake), thence onwards to the Philippines, Japan, Kamchatka, Alaska and down the west coast of the Americas to Chile. The magnitude 5.8 Peru quake was on the 'Ring of Fire' too, where the ocean floor is being subducted east below South America) and was entirely unremarkable because we expect three or four quakes exceeding magitude 5.0 somewhere in the world every day. Melting processes in subductioin zones, not directly related to earthquakes, feed the volcanoes that give the 'Ring of Fire' its name. An earthquake happens when strain that has built up over decades or centuries is released by the sudden, violent, slippage of a fault. The most that can be said is that a distant quake might be capable of precipitating a quake that was going to happen soon anyway. It certainly cannot cause one that was not fairly imminent.

Strictly speaking, the Sumatran quake was not on the 'Ring of Fire', despite what you may read or hear in many new reports, but was caused by the same sort of process. Here, the Indian ocean floor is being subducted northeastwards below Sumatra and Java. It was a large quake further along this plate boundary that caused the 26 December 2004 Indian ocean tsunami that claimed nearly 300,000 lives, and a smaller (magnitude 7.7) quake just offshore of Java that caused a smaller tsunami in July 2006.  There was a very intelligent report about many of these issues on the BBC's Newsnight on Thursday.

The epicentres of the Sumatran (on the left) and Samoan (on the right) quakes.  They are more than 7500 km apart, and a very complex system of tectonic plate boundaries lies between. [Map made in Google Earth]

Attention now must focus on rescue efforts, but I hope it will subsequently switch to enquiries into why buildings in Padang were not better able to withstand the shaking caused by the earthquakes. It seems that hospitals and schools collapsed. These are buildings with large rooms, and hence large extents of unsupported roofs and ceilings, but it is well known how to make such structures earthquake resistant. All too often, it turns out that seismic building codes have been flouted. I have blogged previously about shoddy school buildings in regions at risk from earthquakes, for example in the magnitude 7.9 quake in Sichuan, China on 12 May 2008 . Fortunately the local time when the quake struck Padang was after 5 pm, so presumably the schools were fairly empty - but not so the hospitals.

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Volcanoes, earthquakes and tsunamis   ⁸

Teach Yourself Volcanoes, Earthquakes and Tsunamis by David Rothery
published by Hodder Education

About the author

Dave Rothery is a volcanologist and planetary scientist at the Open University. His current research includes studying volcanic eruptions on the Earth and characterising planetary surfaces, especially Mercury.

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The tsunami that struck Samoa yesterday has the potential to be the biggest tsunami disaster since the Boxing Day (26 December) 2004 tsunami that devastated coasts around the Indian ocean and took nearly 300,000 lives. The death toll this time will be much less than that, but it seems likely to rise as more reports are gathered and it may exceed the 550 killed on 17 July 2006 when a tsunami hit southern Java and will certainly be worse than one that hit Sumatra in September 2007.

Like those, this tsunami was caused by an undersea earthquake, at a subduction zone where one tectonic plate is being pushed down below another. In this example, the floor of the Pacific ocean is being pushed westwards below the Tonga island arc. Plates do not slide past each other smoothly. Instead, strain builds up until the deformation is relieved in a major jerk. In this case, the 'jerk' began at a relatively shallow depth of about 18 km, and the seafloor above it was probably jolted upwards by several metres. This sudden displacement caused a series of waves on the sea surface, which became higher and steeper when they ran ashore. Local reports speak of waves reaching more than 5 metres above sealevel and rushing 100 metres inland.

Unlike the situation in the Indian ocean back in 2004, the Pacific ocean has a pretty good tsunami warning system, and evacuation of Samoa's capital, Apia, did occur although I am not sure whether this was achieved before any tsunami waves were likely to hit it, because the earthquake was so close by that the waves would arrive in less than an hour. Also the earthquake happened so early in the day, just before 7am local time, that many people may not have been aware of the situation.

Students of the Open University short course Volcanoes, earthquakes and tsunamis , which is supported by the book Teach Yourself Volcanoes, Earthquakes and Tsunamis will doubtless soon be discussing the issues and implications raised by this event. There are many current news reports, and already a rather good entry on Wikipedia.

Samoan region of the Pacific ocean
[data courtesy of US Geological Survey]

Earthquakes in the Samoa region in the 24 hours before the time stated at the top (there were none in the previous week). The largest blue square locates the epicentre of the magnitude 8.0 quake that caused the tsunami. The others are smaller aftershocks.  Samoa is the group of islands north of the earthquake swarm, Tonga lies to the south, and the outlying islands of the Fiji group are visible near the western edge of the map. The map covers a 10 by 10 degree block, approximately 1000 km across. Data courtesy of USGS.

Take it further

Volcanoes, earthquakes and tsunamis

Teach Yourself Volcanoes, Earthquakes and Tsunamis by David Rothery
published by Hodder Education

About the author

Dave Rothery is a volcanologist and planetary scientist at the Open University. His current research includes studying volcanic eruptions on the Earth and characterising planetary surfaces, especially Mercury.

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Following this post Dave wrote an excellent piece for Platform, our sister site, on preventing earthquake deaths - admin

"There has been a magnitude 6.3 earthquake to the north of Rome, but no news yet of casualties," says the BBC World Service radio news at 03:30 as I drive to Luton airport for a flight to Amsterdam. By the time I am in a Dutch taxi heading for my meeting at the European Space Agency my phone is ringing demanding a blog.

Earthquakes in Italy are not unusual. The whole region is riddled with faults as a result of the tectonic collision between Africa and Europe.

Today's quake at 01:32 GMT was moderately large but more significantly shallow - its depth is preliminarily estimated at only 10 km - so the shaking that it caused at the surface was large. I see from the news that 27 are confirmed dead, and I expect the death toll will rise.

Italian colleagues who arrived today from Rome and Padova for the same meeting as me felt the quake for themselves. My colleague from Rome was already awake, and according to him his house shook for 20 seconds. His wife was still in bed, but was awoken.
Here's a map of the location with the intensity of ground shaking.

Thousands of homes have been damaged. I wait to learn how many schools have collapsed, which is a common cause of tragedy because classrooms have large and often inadequately supported roofs. Fortunately this time the quake struck at night while the schools were empty.

About the author

Dave Rothery is a volcanologist and planetary scientist at the Open University. His current research includes studying volcanic eruptions on the Earth and characterising planetary surfaces, especially Mercury.

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