The earthquake that struck Japan 11 March is the largest on record in the country’s history. It is also one of the largest recorded earthquakes in the world.
Director of Hazards research in IHRR, Dr Alex Densmore, was interviewed by The Guardian about the aftershocks that have taken place since the 9.0 magnitude earthquake first hit Japan. Much of Densmore’s research has focused on large earthquakes and their secondary hazards, including the 2008 Sichuan earthquake in China. This is an excerpt from the article:
But tremors can continue for months after an earthquake. The earthquake that sparked the 2004 Asian tsunami, estimated at magnitude 9.1 or more, was followed three months later by an 8.6 tremor that Densmore said was almost certainly related, although some seismologists argue it was not a true aftershock.
He said: “The bigger the earthquake, the bigger the change in stress and the more aftershocks. There are many more aftershocks than after the Sichuan earthquake in China [in 2008], but that was magnitude 7.9 and this was 8.9. You have 30 times more energy released.
“It’s not about how much the ground shakes, but how much energy is released, because that’s what gives other parts of the fault increased stress.
“That’s what aftershocks are: areas around [the location of the original shock] relieving stress. But those also affect other parts of the fault: you get a cascade effect.”
Shocks can also have a cumulative effect on the risk of landslides, he said. Hill slopes are weakened and rock is loosened, allowing subsequent tremors, even if smaller, to trigger slips.
But Japan’s buildings – unlike many buildings elsewhere – should stand up well to aftershocks, he said.
“[They] are designed to move: they have expansion joints and shock absorbers … You are not creating cracks which get bigger and bigger.
“If they can survive the main shock they should be okay.
“The important thing is that they don’t collapse, even if they are damaged; as long as they don’t actually fall down, that’s 95% of the battle.”
‘Japan earthquake aftershocks threaten survivors.’ The Guardian
A large number of aftershocks have taken place around the epicentre of the initial earthquake as shown in this map produced by USGS on Google Earth:
A live version of this map is on the website of the Public Broadcasting Service (PBS) in the US: http://www.pbs.org/newshour/multimedia/japan-earthquake/
Another live map developed by Texas Tech University (US) is also available online: http://mapserver.gis.ttu.edu/japanquake/
The damage left by the tsunami is massive as you can see from these before/after images from Google Earth:
More photos of the tsunami impact are available on Picasa.
For those concerned about friends, family and loved ones who may have been affected by the earthquake/tsunami Google Crisis Response has posted phone numbers to enquire about missing persons: http://www.google.com/crisisresponse/japanquake2011.html