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Drilling reveals fault rock architecture in New Zealand’s central alpine fault

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Posted February 9, 2015

Rocks within plate boundary scale fault zones become fragmented and altered over the earthquake cycle. They both record and influence the earthquake process. In this new open-access study published in Lithosphere on 4 Feb., Virginia Toy and colleagues document fault rocks surrounding New Zealand’s active Alpine Fault, which has very high probability of generating a magnitude 8 or greater earthquake in the near future.

(A) Key tectonic elements of the Pacific-Australian plate boundary, including the Alpine fault through the continental South Island of New Zealand. Topography is after Sandwell and Smith (1997). White box illustrates location of B. (B) More detailed map of the Alpine fault (red line), illustrating locations mentioned in the text. Orange lines are roads; gray lines are topographic contours. (C) Composite schematic section through a typical Alpine fault oblique thrust segment, illustrating the sequence of fault rocks exposed in the hanging wall, modified after Norris and Cooper (2007). Outcrops at Stoney Creek, Hare Mare Creek/Waikukupa River, and Havelock Creek are particularly characteristic. DFDP--Deep Fault Drilling Project. Image credit: V. Toy et al., and Lithosphere

(A) Key tectonic elements of the Pacific-Australian plate boundary, including the Alpine fault through the continental South Island of New Zealand. Topography is after Sandwell and Smith (1997). White box illustrates location of B. (B) More detailed map of the Alpine fault (red line), illustrating locations mentioned in the text. Orange lines are roads; gray lines are topographic contours. (C) Composite schematic section through a typical Alpine fault oblique thrust segment, illustrating the sequence of fault rocks exposed in the hanging wall, modified after Norris and Cooper (2007). Outcrops at Stoney Creek, Hare Mare Creek/Waikukupa River, and Havelock Creek are particularly characteristic. DFDP–Deep Fault Drilling Project. Image credit: V. Toy et al., and Lithosphere

Descriptions already suggest that the complex fault rock sequence results from slip at varying rates during multiple past earthquakes, and even sometimes during aseismic slip. They also characterize this fault before rupture; Toy and colleagues anticipate that repeat observations after the next event will provide a previously undescribed link between changes in fault rocks and the ground shaking response. They write that in the future this sort of data might allow realistic ground shaking predictions based on observations of other “dormant” faults.

Source: GSA

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