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Kerepehi Fault

A story appeared in the NZH on 12 May by Jamie Morton that bears repeating because I don't think it was picked up by many people here at the time. It was in the context of a general story about the major earthquake threats facing the country. The information was generall y supplied by Dr John Ristau and is available in detail throiugh the Geonet Website.

Here is the patrticular information about the Kerepehi Fault:

"Meanwhile, scientists have just revealed new insights into the Kerepehi Fault, which runs for about 80km between Matamata and north into the Firth of Thames.

Research just shared by GNS Science today showed that the fault is more complex than previously thought.

Previously, the fault was thought to consist of up to five loosely connected single strands or segments, but the new findings show it actually consists of a belt of many faults, in a wider zone suggesting a more complicated arrangement of segments.

Scientists had earlier understood the average interval between ruptures of the Kerepehi Fault was between 6000 and 8000 years, based on the better studied segments.

Although there is one period several thousand years ago when three quakes appear to have clustered relatively closely in time.

However, the new research combined the rupture history of all fault segments studied so far, and found the average rupture interval to be about 1000 years.

In other words, the rupture interval on a single segment of the fault was still likely to be several thousand years, but the region may experience a large earthquake every 1000 years.

Previous ruptures have involved up to 2m of vertical displacement of the ground surface per event, which suggests associated earthquakes were between magnitude 6.3 and 7.0 in size.

Geologist Dr Pilar Villamor, who co-led the study, said earthquakes of this size would result in significant damage to weaker buildings, disrupt services, threaten flood defence structures, and cause liquefaction in weak soils where the water table is close to the ground surface.

While the Hauraki Plains is considered a region of only moderate recent earthquake activity - one notable 5.1 quake resulted in 1300 claims to the Earthquake Commission for property damage when it struck near Te Aroha in 1972 - Dr Villamor said further work was needed to estimate the impacts of a major earthquake in the region."





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