23 December 2011

The latest events in the Christchurch earthquake sequence

Posted by Dave Petley

This morning has seen the next series of events in the now 16-month long Christchurch Earthquake sequence in New Zealand.  The GeoNet map below shows the location of this series of events, which have occurred to the east of Christchurch:

Although one of these events were large earthquakes (the largest is M=6.0), the shallow depth (6 km for the largest), proximity to the city and vulnerable materials will have combined to make this a significant series of events once again.  Chris Rowan at High Allocthonous has en excellent post on the earthquake focal mechanisms, so I won’t repeat that here.  However, it is worth looking at the above data alongside this map of the early events, produced by the ever-impressive people at GNS Science:

Note that the first event (the Darfield earthquake) and aftershocks (in green on the map) occurred to the west of the city.  The Feb 2011 events (red) were further to the east, and the June 2011 to the east once again.  This most recent sequence is further east again.

For the most recent events, Geonet also have a map of provisional strong motion data online:


These are really high peak ground motions, suggesting that liquefaction, structural damage and rockfalls will again be a problem.  Yahoo News reports that rockfalls have occurred in the bluffs to the east of the city (this is the area that I visited in the summer – and indeed I will be back there in three weeks time) and has this image, which is almost certainly the dust generated by rockfall events (compare it with these images from the earlier events):



Aside from the very obvious short-term impacts of this event, I suspect that there are a number of longer term effects, which might include:

1. Increased short-term seismic activity. This brilliant GNS Science graph shows the decay of seismic activity with time since the initial event:

The effect of this most recent event is probably to induce another step in this graph and thus to extend the timeline over which seismic activity will continue.  It will reduce to background levels in due course, but it is hard to know exactly when this will occur.

2. Further challenges for risk assessment. A major issue in Christchurch is risk assessment for liquefaction and in particular rockfalls.  This continued earthquake activity will be complicating these calculations, which of course have major implications for the people who live in those areas;

3. Increased pressure on the Earthquake Commission (EQC). The Christchurch event has placed substantial strains on EQC, which is a government-backed residential earthquake insurance pool.  These most recent events may well mean additional claims, which will further deplete their resources, and will also have an impact on the reinsurers who provide some of the cover.  I am not in any way suggesting that EQC cannot meet these claims – they can of course – but rebuilding the fund in the aftermath of this sequence must be a key priority.

4. Psychological impacts on the local population. Last and by no means least, the earthquakes are likely to have at least some impact on the local population.  There has already been a net out-migration from the city.  These repeated events are making the return to normal life difficult, which places a strain on everyone.