Early Result – Not London Hill Fault

Analysis of Global Positioning System (GPS) data by GNS Science shows that the rupture that occurred during last Sunday’s strong earthquake in Cook Strait was not along the London Hill Fault.

Thursday, 25th July 2013

In a media release last night, GNS Science said that movement caused by last Sunday’s magnitude 6.5 earthquake occurred between the London Hill and Hog Swamp faults in Marlborough.

The earthquake began about 18 km below the seabed, moving upward and outward vertically to within 6 km of the seafloor. An 18 kilometre rupture running northeast to southwest for a distance of 19km ended a short distance from the coastline at Clifford Bay. The earthquake ruptured along half of the length of the previously unknown 40 kilometre-long fault.

The London Hill and Hog Swamp faults are little-known on-land faults, and yesterday’s survey by NIWA’s research vessel Tangaroa attempted to identify any evidence of earthquake activity on the seabed in the area. The full results of NIWA’s work will not be available for a week.

This GPS analysis by GNS Science shows that the earthquake rupture was in an area between the two known faults and caused parts of Marlborough to move 5 cm to the east during the magnitude 6.5 earthquake. Horizontal movement on the seabed in southern Cook Strait above the earthquake epicentre was up to 8 cm.

GNS Science also announced revised aftershock probabilities for central New Zealand, although it is not clear that this was a result of the GPS data analysis. The method for calculating the probability of an aftershock was tested during the Canterbury earthquake episode.

The area covered by the calculations ranges from Rai Valley in the west to Riversdale Beach on the Wairarapa Coast (in the east), and from Masterton in the north to Clarence on the Marlborough coast in the south.

For the next seven days –
A 64% probability of a magnitude 5.0 to 5.9 aftershock (down from 87% on Monday)
A 10% probability of a magnitude 6.0 or higher aftershock (down from 19% on Monday)

For the next year –
A 97% probability of a magnitude 5.0 to 5.9 aftershock (down from 99% on Monday)
A 30% probability of a magnitude 6.0 or higher aftershock (down from 39% on Monday)

[Compiled from GNS Science media release, July 24th.]

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