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Research shows Kerepehi Fault more complex than previously thought - 12/05/2015

New research on the Kerepehi fault, which runs for about 80km between Matamata and north into the Firth of Thames, has revealed the fault is more complex than previously thought.

Prior to this research the fault was thought to consist of up to five loosely connected single strands or segments. The new research has shown it consists of a belt of many faults, in a wider zone suggesting a more complicated arrangement of segments.

The research took place over several field seasons and involved topographic mapping and excavations across many of the major onshore strands of the fault.

Kerepehi Fault

Prior to the research, scientists 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 is still likely to be several thousand years, but the region may experience a large earthquake every 1000 years.  

The research was undertaken by a team from GNS Science led by geologist Dr Pilar Villamor and  post-doctoral collaborator, Dr Mira Persaud, who had funding from the Swiss National Science Foundation.

The investigations revealed a pre-historic record of larger earthquakes than those that have occurred in the past 1000 years. The 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

Dr Villamor said earthquakes of this size will 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.

Results of this investigation have been discussed with both Waikato Regional Council and Hauraki District Council. Hauraki District Council has proactively engaged with the local business and civil defence community in a November 2015 workshop to consider how the new understanding of earthquake hazard in the district can be managed.

While the new research has added considerably to the knowledge of the Kerepehi Fault, there are several strands of the fault that have received little scientific attention.

Dr Villamor said further work was needed to estimate the impacts of a major earthquake in the region to assess options for strengthening local infrastructure and to evaluate business resilience.

The Hauraki Plains is a region of moderate recent earthquake activity with significant earthquakes in 1920 (near Te Aroha), 1926 (Morrinsville), and 1972 (Te Aroha). The 1972 magnitude 5.1 quake was centred 4km south of Te Aroha and 1km east of Waihou, and resulted in 1300 claims to the Earthquake Commission for property damage.  

Q&A

How was this work funded?

GNS Science core funding and funding from the Swiss National Science Foundation (Post-doctoral funding for Mira Persaud). In addition, some early research was funded by the Electricity Corporation  of New Zealand.

What are the next step(s) for any ongoing research?

Scientists are keen to gain funding to continue characterising the fault, especially the fault strands that have had little detailed investigation so far, to incorporate new results into the National Seismic Hazard Model, and to investigate the impacts on infrastructure, business and community in the region.

Many of the recently identified strands are only a few kilometres long and therefore their maximum earthquake potential is relatively small. Should people be concerned about such small fault strands?

The concern is that some of the small strands could potentially ‘join up’ at depth and produce a larger and longer rupture even if this is not reflected in the surface strands mapped. A rupture such as this could potentially produce a magnitude 6.3 to 7.0 quake. With further investigation, scientists hope to shed light on the probability of this type of rupture occurring.    

How do geologists properly assess the threat from conglomerate type faults such as this?

It needs detailed investigation of the many fault strands to assess how they interact with each other. Geologists need to determine if the faults join at depth or if several strands can rupture together, or within a short time period. 

Does this change the overall quake risk of the Waikato region?

It may change hazard values at long return periods (those important for critical facilities). However, it is hard to say if it will change the risk (the consequences)as it has not yet been appropriately assessed.  

How does the Waikato region compare to other parts of NZ for quake risk?

Moderate to low. Lower than Christchurch, slightly higher than Auckland, and similar to Invercargill.