|Map of felt effects of the Christchurch earthquake|
Mid-day, Tuesday, a major earthquake struck 10 km south-east of Christchurch, New Zealand. The focal depth was shallow, only 5 km. The USGS called it a magnitude 5.5; the press and Geonet are saying that it is a 6.3. 65 people have been reported killed at this time, the toll is likely to go higher, and damage is extensive. There has been at least one strong aftershock.
The graph is a preliminary summary of damage, keyed to the Modified Mercalli intensity scale. MM8 (dark orange) is heavily damaged, light orange is damaging, yellow is slightly damaging, and green is strong. The red square marks the epicenter. Click on the Geonet link in the caption to get an updated version of this map.
In an earlier post, I discussed the damage caused by the September 3, 2010, M7.1 earthquake, also near Christchurch. A lot of the damage then was caused by liquefaction, and that is likely to be a problem this time as well. Many buildings had been damaged by the earlier earthquake, repairs had not been completed, and so they were vulnerable to the intense shaking that occurred.
There is more information on Dave Petley's Landslide Blog, and a good discussion of the geologic setting on Chris Rowan's Highly Allocthonous blog. Information, including some videos, is starting to come in to CNN.COM. This one shows a bit of footage taken during the earthquake at the very beginning of the video.
Our thoughts and sympathies go out to our Kiwi friends.