Rupert Sutherland, Principal Investigator, GNS Science
John Townend, Principal Investigator, Victoria University of Wellington
Virginia Toy, Principal Investigator, University of Otago
Simon Cox, Quaternary Team Leader, GNS Science
|The start of DFDP-2 drilling activities in the Whataroa Valley 1/9/2014.|
The aim of the DFDP-2 project is to examine a major fault before it ruptures in a large earthquake. What are the ambient conditions in the fault zone? What are the physical properties of the materials? What is the fault zone’s internal structure? Answers to these and many other questions will help us understand what happens when a fault ruptures in an earthquake and how slip accumulates to form a plate boundary. Have a look at our video for an introduction to the DFDP-2 project:
Our results will have implications for global earthquake science. This is why scientists from twelve countries have come together to gather information on fault geometry, temperature and pressure conditions, collect rock and fluid samples, and install long-term monitoring equipment. It is also why the international community, via the International Continental Scientific Drilling Program (ICDP) has funded more than half of the project, and why we have been successful in obtaining funding from the Marsden Fund.
The DFDP-2 project will have specific value to New Zealanders. It is surely important to understand the largest source of seismic hazard in South Island. What will happen during and after the next major earthquake? Does the hazard vary from day to day? Does anything unusual happen before an earthquake? Could we develop a warning system? Does the Alpine Fault offer benefits as well as hazards? For example, does the fault host a geothermal resource? Our project will help address these questions too.
|DFDP-2A during coring of sediments 17/9/2014.|
On 14 September, we started coring sediments. After moderate recovery between 125 and 160 m, we drilled open-hole with a stratopack bit and rapidly progressed to 206 m. On 17 September, we stopped coring glacial diamictites at 212 m depth.
|Rupert Sutherland and Simon Cox inspect core.|
The relatively young (<500 yr) cobbly river gravels are only a few metres thick. Below this, to a depth of about 50 m, there is a sequence of pebbly river gravels that are at least 12,000 yr old. Deposits from an ancient river delta are found between 50 and 77 m depth, and below this, we discovered a thick sequence of silts, likely deposited on the bed of an ancient lake that filled the space carved out by a glacier. Below 197 m, we find evidence for rocks that were rafted and dropped into a fairly deep lake from floating ice as the glacier retreated.
We infer that the lake-bed was at least 300 m below the sea level of the time, which itself was at least 100 m lower than today. We are examining samples to see if it was a marine fiord. We have recovered many wood samples suitable for radio-carbon dating the thick sequence of sediments.
The static water level in the upper gravel unit (29 m depth) is about 5 m below ground surface. Below 120 m, we encountered artesian pressures, as anticipated, but a shut-in pressure could not be determined. Low flow rates (<0.3 l/min) imply low permeability, consistent with high silt contents.
We measured the temperature at 120 m depth in DFDP-2A and calculated a geothermal gradient of 139°C/km. This result needs to be interpreted with caution. First, slight artesian flow of warm water from the open hole below 120 m depth may be significant. The corrected value may be closer to 80°C/km. Second, we anticipated an elevated geothermal gradient and artesian pressures based on hydrogeological modelling, and it may not persist to greater depth.
Scientific drilling often provides unexpected results. The new sediment thickness, temperature, and fluid pressure data are useful for planning deeper operations, but we also have to now re-examine models of landscape evolution, geothermal circulation, and ancient fault movements in the Southern Alps.