Geologists to Probe Quake History of Northern Alpine Fault

Geologists to Probe Quake History of the Northern Alpine Fault

Geologists are planning to dig two trenches across the Alpine Fault at Springs Junction next week to study the rupture behaviour of the northern section of the fault.

They hope to identify and date the last three, or possibly four, ruptures on the fault to understand the timing and frequency of past surface rupturing earthquakes at the site. This will provide a pointer to the size and style of earthquake that can be expected in the future.

The investigation is part of an ongoing programme at GNS Science to better understand the rupture behaviour
along the entire length of the Alpine Fault, which features at least three distinct sections, each with different rupture characteristics.

At the Springs Junction site, next to the Maruia River, the long-term rate of movement on the Alpine Fault is 10mm-a-year, compared to 28mm-a-year further south in Westland. This means that some of the sting of the fault has dissipated by the time it moves north past Hokitika, as it splinters into several large faults that run northeast-southwest through Marlborough.

These include the Clarence, Awatere and Hope Faults, all of which take some of the seismic load off the northern part of the Alpine Fault and transfer it through Marlborough.

The trenches, each of about 20m-long will allow the geologists to take photos and make detailed drawings of the recently deposited strata to a depth of 2m.

They will also collect organic material from each horizon for radiocarbon dating. They anticipate the record of sediments and faulting evident in the walls of the trench will span 1000 to 1200 years.

Project leader Dr Rob Langridge of GNS Science said the aim was to get a clearer picture of the earthquake potential of the northern part of the Alpine Fault.

“The southern and central portions of the fault have been reasonably well studied over the past decade, and now we are including the northern section as the knowledge of its rupture history is fairly sketchy,” Dr Langridge said.

As a first step, late last year Dr Langridge acquired a total of 90km of airborne Lidar images of the section of the Alpine Fault that runs between Hokitika and Springs Junction. The images are made up of four separate ‘strips’ each of about 22km long and 1.5km wide.

Lidar images strip away the vegetation and provide a high-resolution image of the ground surface, often revealing subtle geological features not visible from surface observation.

Lidar images have helped to revolutionise the study of active faults, Dr Langridge said.

“Thanks to Lidar we have found that in places the Alpine Fault is several hundred meters away from where the fault appears on maps and has a more complex pattern of surface markings. Lidar enables us to focus very precisely on the parts of the fault that are most likely to yield useful information in trench studies.”

The Lidar images have also revealed ancient landslides that were previously unrecognised and were almost certainly triggered by Alpine Fault ruptures.

From previous ground studies, geologists have worked out that the old river terraces they plan to trench near Springs Junction have been subject to 11m of horizontal movement and about 2m of vertical movement in earthquakes.

But it is unclear how many fault ruptures produced that movement. It could be anywhere between two and five. Dr Langridge says the trench studies will resolve that.

The trenches will be filled in when the investigation is completed and the land returned to its previous state. The project has the approval of the Department of Conservation, which owns the land.

The longterm aim is to trench across the fault in all four places where Lidar has recently been obtained, and compare the findings to studies of lake sediment records along the West Coast.

The lake investigations, being led by geologist Jamie Howarth of GNS Science, have identified evidence for very strong shaking within several lakes, including the nearby Lake Christabel.

“The ultimate goal is to correlate data from the on-fault trenches and off-fault lake based work to get agreement on the dates and sizes of past Alpine Fault ruptures.”

The trenching work is being funded by the Natural Hazard Research Platform and findings are scheduled to be published on the Platform’s website in 2017.

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