11 March 2011

First reports of the M = 8.9 earthquake in Japan

Posted by Dave Petley

Updated 08:30 UT with aftershock information at the end the post

At 05:46 local time this morning a very large earthquake struck off the east coast of Japan.  When such events occur the first port of call for information is always the USGS earthquake site, which is reporting that the provisional assessment of the event is that it had a magnitude of  8.9 (but this will be refined over the next few hours). The depth is comparatively shallow for such a large earthquake at about 24 km.  The nearest large city is Sendai, 130 km to the west.  The USGS shakemap, shown below, suggests that a large area of northern Japan will have suffered significant shaking:

This is a very  large event and potentially extremely destructive event – indeed it could well be one of the more expensive earthquakes in terms of financial losses once the tsunami is taken into consideration.  PAGER data suggests over 2 million people in the intensity = VIII zone, which suggests that the damage from shaking  alone will be significant, and large numbers of landslides in the mountains are likely.  Fortunately, with the tight building codes in Japan the loss of life from building collapse is unlikely to replicate the events in places like Haiti, but is unlikely to be negligable.

Of course, the real threat here is the tsunami, and once again we are seeing remarkable images of the destructive power of these waves.  For information on tsunami events a good starting point is the GDACS site, which already has a page dedicated to the event.  This is forecasting tsunami heights in excess of 5 metres.  As I write the tsunami is propagating across the ocean, a chilling thought, though the lack of major inhabited areas to the east is fortunate in terms of evacuation times.  Hopefully potentially affected communities will receive the warnings in time:

Thanks to Anthony Miner in Australia for sending a link to this remarkable footage of the event.  There is a gallery of some images here, from which this one is taken:

An earthquake of this magnitude generates very substantial sftershocks of course,  In the first 150 minutes after the earthquake the USGS has recorded nine events with a magnitude of greater than 6, including one with a magnitude of 7.1.  This latter aftershock was at least hypothetically sufficiently large to generate a tsunami in its own right, albeit much smaller than the one from the main shock.  The normal rule of thumb is that the largest aftershock will typically be one unit down on the magnitude scale, so an aftershco of >M= 7.5 should be expected at some point in the next few months.


Updates and comments welcome.  I am out of my office today, but can be contacted on my mobile: +44 7983 702717 if anyone wants to talk to me directly.