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Earthquake rethink

fault

Last November's magnitude 7.8 Kaikoura earthquake was so complex and unusual that it is likely to lead to changes in the way scientists think about earthquake hazards in plate boundary zones worldwide, a new study says.

Not only was it a record-setter for its complexity, but it was also one of the best recorded large earthquakes anywhere in the world. This latter feature has enabled scientists to undertake analysis in an unprecedented level of detail in a co-authored research paper.

The rupture started in North Canterbury and propagated northward for more than 170km along some well-known, and some previously unknown faults. It straddled two distinct active fault domains, rupturing faults in both the North Canterbury Fault zone and the Marlborough Fault system.

The earthquake ruptured at least 12 major crustal faults plus another nine lesser faults and there was also evidence of slip along southern end of the Hikurangi subduction zone plate boundary, which lies about 20km below the North Canterbury and Marlborough coastlines.

The study shows the quake moved parts of the South Island more than 5 metres closer to the North Island in addition to being uplifted by up to 8m. The largest movement during the earthquake occurred on the Kekerengu Fault, where pieces of the Earth’s crust were displaced relative to each other up by to 25m at a depth of about 15km. Maximum rupture at the surface was measured at 12m of horizontal displacement.

"This complex earthquake defies many conventional assumptions about the degree to which earthquake ruptures are controlled by individual faults, and provides additional motivation to re-think these issues in seismic hazard models," says Dr Ian Hamling of GNS Science

The quake has underlined the importance of re-evaluating how rupture scenarios are defined for seismic hazard models in plate boundary zones worldwide.

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