Thankfully no tsunami – this time

People living along the coast of Indonesia and indeed around the fringes of the Indian Ocean will have had a spine-tingling moment last night as they felt the vibrations from the magnitude 8.6 earthquake that struck off the west coast of Northern Sumatra.

Source: USGS
Source: USGS

Many will have had flashbacks to 2004, and the Boxing Day tsunami, which also had its epicentre of Sumatra’s west coast. The tsunamis resulting from that earthquake killed 230,000 people in 14 countries. It goes down in history as the 6th most deadly quake on record.

Thankfully, there was no repeat last night as tsunamis were not triggered by the seismic activity. Media reports indicate that authorities in Indonesia, Malaysia and coastal areas of other countries bordering the Indian Ocean responded efficiently, evacuating low-lying coastal areas and sounding the alarm.

Scientists were quick to provide analysis and we rounded up their initial observations at the Science Media Centre. Professor  Pennsylvania State University’s Kevin Furlong, who did such a good job helping the media with expert commentary in the wake of the Canterbury quakes when he was a visiting academic at the University of Canterbury had the following to say:

The 11 April 2012, Mw 8.7 earthquake west of Banda Aceh, Sumatra, Indonesia is a very large earthquake within the Indo-Australian plate. Although it is within the plate, its occurrence is almost certainly linked to the plate interactions between Indo-Australian plate and Indonesia (part of the Sunda segment of the Eurasian plate). This earthquake reflects a style of faulting (strike-slip) which involves principally horizontal motion, and thus is unlikely to generate a significant tsunami; although very strong ground shaking would be felt on Sumatra. This is also an extremely large magnitude earthquake for this style of faulting, meaning that it likely involved substantial fault movement, and the fault likely extends for 200+ km.

This earthquake is of the same style of faulting and in approximately the same location as a Mw 7.2 earthquake on January 10, 2012. Although this earthquake was within the Indo-Australian plate, any earthquake of this size will change the stress regimes acting on the nearby plate boundaries. The result is that stress conditions on the subduction plate boundary beneath Sumatra have changed, although the implications of that change are uncertain.

Hot on the heals of last night’s earthquake, came another this morning – a magnitude 7 quake off the coast of Mexico. It’s easy to believe that we are having more and stronger earthquakes than in the past, but as the British Geological Survey’s Dr Bruce Malamud told the SMC, that’s not supported by the science.

One of the questions that has been asked by many is whether there have been more frequent large earthquakes in the last few years. Let’s take as a ‘large’ earthquake one with moment magnitude 7. The number of earthquakes per year with moment magnitude greater than or equal to 7 varies certainly, year to year, but the average from 1900 to present is about 17 magnitude 7 or greater earthquakes per year (compared to about 1 magnitude 8 or greater earthquake). If we just look at 1990 to 2010, then the average was about 15 magnitude 7 or greater earthquakes per year. And if we look at the last three years, then the average is also 15 of this size earthquake per year. So, no, the actual number of very large earthquakes is not increasing over time. It fluctuates year to year, with some years less and some years more.

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