Dusky Sound Quake Loads Nearby Fault

The Dusky Sound earthquake of July 15th has increased stress in the nearby part of the Alpine Fault.

Friday 24th July 2009

GeoNet has published a comprehensive report on the events surrounding the magnitude 7.6 earthquake in Dusky Sound, concluding that the earthquake has increased stress in the nearby off-shore part of the Alpine Fault. It is not known how close the southernmost section of the Alpine Fault is to rupturing, so the effect of the loading is uncertain.

However, on a more positive note, they believe that if the offshore section of the Alpine Fault was to rupture, the fault movement would not continue into the onshore section which runs the length of the Southern Alps and into Marlborough. GeoNet adds that from what is known about the geology of the Alpine Fault, earthquakes have not been closely clustered in time. But there are dissenting views that the Alpine Fault could rupture in sections over a short period of time measured in minutes.

Two notable periods of New Zealand history, when large earthquakes were clustered together in time but geographically dispersed, have been recorded. In the mid-1800s a burst of activity included magnitude 7 events near Wanganui in 1843, Marlborough in 1848 and the Wairarapa magnitude 8.2 quake of 1855. A magnitude 6.8 quake was recorded near New Plymouth in 1853 and a magnitude 7.5 quake struck Hawke’s Bay in 1863.

During the 13 years between 1929 and 1942, nine large earthquakes struck New Zealand. The cluster began with a magnitude 6.9 quake near Arthur’s Pass and the magnitude 7.8 Buller (Murchison) earthquake in 1929. Two magnitude 7 events struck Hawke’s Bay in 1931, another near Pahiatua in 1934 and another pair in Southern Wairarapa in 1942.

With clustering in mind, it is possible that the probability of another earthquake is higher after a large event. GeoNet points out that large earthquakes can strike many parts of New Zealand, not just Fiordland.

[Compiled from data provided by the GeoNet project and its sponsors EQC, GNS Science and FRST, and EQC media release.]

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