14 June 2011

Rockfalls from M=6.3 aftershock in Christchurch, New Zealand

Posted by Dave Petley

Christchurch was again affected by two large aftershocks yesterday.  At 1 pm local time a M=5.6 earthquake occurred, followed at 2:20 pm by a M=6.3 event.  Even though these are technically aftershocks from the Darfield event last year, the latter was of course a substantive earhquake in its own right.  The map below, from Geonet, shows the distribution of the aftershocks that have occurred in the Christchurch area over the last 60 days:

The map below is a Google Earth image of the aftershocks of the last 24 hours:

A few interesting things to note here i.  First, this cluster of very shallow events is continuing to occur to the east of the main Darfield shock last September, which was well to the west of the city.  Second, this cluster us occurring beneath the Sumner peninsular, which is an area that is characterised by steep slopes, as a zoom in of the Google Earth image (still with the aftershocks displayed) demonstrates:


But most remarkable is the exceptionally high ground accelerations that have again been associated with these events.  The following is the Geonet map of the peak ground accelerations associated with the larger of the two events:

Note that in the Sumner area a peak ground acceleration of over 2 g was recorded!This is really very high indeed for an earthquake of this magnitude.  Initial reports suggest that the event may have been associated with movement on another unmapped fault.  The same article has some images of the damage associated with rockfalls. The first shows the immediate aftermath of a coastal rockfall event:

Whilst the second shows the aftermath in terms of the impact on a clifftop home:

There is also quite a nice article about the impacts of the rockfalls on people at the foot of the cliffs here.

Finally, the magnitude of the problems in the Christchurch area is calling for some novel solution to rockfall problems.  NZ Herald has a piece on the use of shipping containers as a temporary but highly effective rockfall protection measure:



Comments and/or observations from anyone in the area would be very welcome!  I will heading down to New Zealand next month.