Why was the Christchurch earthquake so devastating?

A M6.3 earthquake hit Christchurch, New Zealand on 22 February (21 Feb in UTC), leaving at least 75 people dead and hundreds injured or missing. Hundreds of houses were destroyed, including the Christchurch Cathedral, and damages will probably sum up to some billion dollars. On 4 September 2010 (3 Sept in UTC), a M7.0 event struck Christchurch, but then no one was killed. So: what’s the difference between the two events?

The 2010 shallow event took place ~45 km west of Christchurch at 4:35 AM local time. The movement was almost entirely strike-slip. PGS reached about 30% g and intensities were in the order of VIII at the epicentre, in Christchurch only VI-VII. Due to the distance, the shaking was not that hard in the city itself, and because of the early morning, not many people were on the streets and hurt by collapsing buildings or falling boulders and bricks.  The building code applied seemed to have been sufficient, despite many buildings were damaged. However, aim of a seismic code is to prevent buildings from collapsing, you can’t avoid damages. This great video shows the surface rupture track:

The February, 2011 earthquake was reported to be an aftershock of the main event last year, but the epicentre location suggests that another part of the fault system ruptured like already stated by Ontario Geofish. It took place directly in Christchurch, media reports PGA of 1–2 g. This is enormous, former studies like Stirling et al, 2002 expected much lower values for a 475 yrs return period. Especially the old-fashioned masonry buildings like the Cathedral couldn’t withstand that and collapsed. The quake had an oblique thrust mechanism, leading to significant vertical offsets in the city. So the epicentre close to the city, the low depth (5 km), and the high PGA lead to the desaster. Additionally, site amplification effects made the shaking even worse. There is evidence of intense liquefaction in the epicentral area, typical for sedimentary basins with saturated, rather fine grained material. Witnesses report on rockfalls, liquefaction, primary and secondary ruptures in the city or the immediate vicinities.

Rockfalls in Summer:

Open cracks, surface ruptures and liquefaction:

Aerial view on primary and secondary surface ruptures, liquefaction, floodings (due to groundwater change and liquefaction?), rockfalls:

Another interesting effect is described here, the quake was reported to have shaken 30,000,000 tons of ice off a glacier into Tasman Glacier’s terminal lake, causing a 30 minute lasting tsunami with waves up to 3.5 m height in the Mt Cook area. This is an effect not described in the ESI scale, but might be involved for the future.

One major problem is that there is a lack of studies on the fault zone that ruptured, which makes it complicated to judge on the future hazard. Will there be more strong events along the fault line? Will the entire fault system rupture in an even stronger event? The paleoseismological studies (eg., Adams, 1980; Berryman et al, 1989; Wells et al., 1999; etc.) in New Zealand mainly focussed on the Alpine Fault…

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