Scientists pool expertise to obtain deep image of Alpine Fault
Scientists from four countries have converged on the Whataroa Valley to build an image of the Alpine Fault beneath the 900m-deep borehole drilled in late 2014.
Scientists and equipment from four countries have converged on the Whataroa Valley in South Westland this week to build an image of the Alpine Fault beneath the 900m-deep borehole that was drilled there in late 2014.
Information obtained from the three-week-long project will be used to study the fault’s geometry and structure below the valley, and help in determining where future scientific drill holes could be sited.
The existing drill hole, which was completed in January 2015 as part of the Deep Fault Drilling Project, is 893m deep and is thought to be 200m to 300m short of the actual fault interface, where the Australian and Pacific tectonic plates meet. For the past year, equipment installed inside the borehole has been measuring temperature and earthquake activity.
Science team leader and geophysicist, Associate Professor John Townend of Victoria University of Wellington, said the Alpine Fault produces large earthquakes about every 330 years, and last broke in 1717.
“The seismic survey we’re doing this month is part of a much larger project addressing the temperatures, pressures, and stresses in the core of the Alpine Fault as it is being primed for the next big earthquake.
“What we’re interested in is how the Alpine Fault works, and what factors control its cycle of large earthquakes,” Assoc Prof Townend said.
“As time goes by, the Alpine Fault is being loaded by motion between the Australian and Pacific tectonic plates. What we’re trying to do is understand the processes that eventually break the camel’s back and cause the fault to slip.”
The Whataroa Valley is recognised internationally as a key site for studying earthquake processes, and the Deep Fault Drilling Project involves 120 researchers and technicians from more than a dozen countries. About 30 of these researchers will be on site for the next three weeks.
A key finding from the drilling in 2014 was that the fault zone is very hot – about 140 degrees Celsius - due to the combined effects of uplift of the Southern Alps and local groundwater flow. Scientists involved in the project are currently working to understand the implications this has for temperature-controlled processes occurring in the earthquake generation zone deep inside the fault.
The plan for this month’s seismic survey is to build a two dimensional seismic image, similar to a medical X-ray or a CAT scan, of the valley sub-surface to a depth of about 1.5km to get a clear picture of the Alpine Fault at depth.
To do this, the researchers will generate artificial seismic waves at known points on the surface and record these using a wide range of sensors.
The equipment being used includes a truck owned and operated by the University of Calgary that sends vibrations into the ground, and a 60m-long seismic profiling tool that will be lowered into the 900m-deep borehole on a special cable to take measurements.
The survey will also make use of the fiber-optic cable installed permanently in the borehole. By measuring how pulses of light emitted by a laser are scattered along the length of the cable, this equipment can be used to record seismic waves.
“Optical systems like this provide new opportunities for measuring conditions underground. We’ve been using the fiber installed in the Whataroa borehole to measure subsurface temperatures every couple of months for more than a year. Over the next few weeks, we’ll be attaching different equipment to the top of the fiber so that we can record seismic waves instead.”
In addition, scientists from Canada and Germany have brought several hundred geophones to the site. They will be placed in a grid pattern throughout the valley to record the way the vibrations from the truck are reflected back from different sub-surface rock layers.
The truck – called a vibroseis – was used in 2011 in Canterbury to produce seismic images of geological structures beneath Christchurch after the magnitude 6.3 quake on 22 February 2011. It has been brought back to New Zealand from Canada especially for this project and another geophysical investigation of the Kerepehi Fault in north Waikato.
The 9 tonne vehicle generates seismic waves by vibrating a steel plate on the ground.
An existing seismic image of the valley compiled several years ago reveals geological structures of interest. However, the equipment being used this summer is expected to produce a superior image that should provide new insights on the anatomy of the fault.
“Using an array of seismometers spread out across the valley as well as sensors lowered into the borehole and the fiber optic cable, we can record each vibroseis shot from a large number of different locations, on and below the ground surface. This will enhance our ability to determine the geometry and position of the Alpine Fault beneath the valley,” said GNS Science team member Dr Richard Kellett.
The large amount of data produced during the project will be pooled so that all the participants are able to work collaboratively on different aspects of the subsurface image.
Scientists participating in the project are from Victoria, Auckland, and Otago universities, GNS Science, the universities of Alberta and Calgary in Canada, TU Bergakademie Freiberg in Germany, and from Schlumberger in the United Kingdom. It is being funded by the Earthquake Commission, Victoria University of Wellington, and the German Science Foundation.