About every 300 years, the Alpine Fault on New Zealand’s South Island produces a large—approximately magnitude 8—earthquake. The fault last ruptured in 1717, and scientists anticipate another major earthquake in coming decades. Before this happens, researchers have jumped on the opportunity to study the properties of a fault before an anticipated large earthquake so that they can better understand the processes that control earthquake nucleation and rupture.
The Alpine Fault is located between the Pacific and Australian tectonic plates and extends for approximately 600 kilometers along the west coast of New Zealand’s South Island. By global standards, it is considered a “fast” fault: On millennial timescales, the fault slips horizontally at a rate of about 27 millimeters per year and moves vertically by up to 9 millimeters per year. Most of this motion occurs in large earthquakes, between which the fault reaccumulates stress ahead of the next earthquake.
The Deep Fault Drilling Project is an ambitious multinational effort to study the Alpine Fault’s structure and earthquake-generating processes late in its typical seismic cycle. Researchers have drilled into the Alpine Fault to collect samples of rocks and fluids and make in situ observations of the geologic and hydraulic properties of the fault zone.
In the second phase of the Deep Fault Drilling Project, in 2014, researchers drilled a 22-centimeter-diameter borehole (DFDP-2B) to a depth of 818 meters and to within roughly 200–400 meters of the Alpine Fault core beneath New Zealand’s Whataroa Valley.
Townend et al. recently published measurements made in the DFDP-2B borehole and studied the hydraulic properties of the fault and the damage zone surrounding it. Using downhole logging equipment and surface measurements, the scientists measured temperature, pressure, and a broad range of other geophysical parameters along the length of the borehole.
The science team discovered that the rocks surrounding the Alpine Fault are much hotter than anticipated and are extensively fractured. Mud level data revealed that the hanging wall portion of the fault—the upthrown (Pacific) side of the fault—contains an active hydraulic system in some areas. Scientists observed that mud levels equilibrated rapidly during the drilling, showing that some fractures in the rock surrounding the fault can transmit large volumes of water in a matter of hours.
The data also reveal a wide damage zone surrounding the fault core, which the authors suggest is caused by a combination of earthquake shaking and longer-term (“interseismic”) deformation. The researchers estimate that the hydrogeologically active damage zone surrounding the fault is at least 10 times as wide as previously thought. This active geothermal system likely modulates processes of fracture creation and sealing and spatial and temporal variations in fluid pressure.
Understanding how fluid pressures vary within a fault zone at different points in its seismic cycle may help scientists understand how earthquakes are triggered and the effects of near-field and far-field shaking on the fault zone itself. The findings from this new study of fault zone damage and hydrogeology will inform future models of changes in fluid pressure throughout the seismic cycle. (Geochemistry, Geophysics, and Geosystems, https://doi:10.1002/2017GC007202, 2017)
—Alexandra Branscombe, Freelance Writer