World's largest coseismic strike‐slip offset: The 1855 rupture of the Wairarapa Fault, New Zealand, and implications for displacement/length scaling of continental earthquakes

Abstract

We used detailed microtopographic surveys to measure fault offset along the southern trace of the Wairarapa fault, near Wellington, New Zealand, which most recently experienced a Mw> 8.1 earthquake in 1855. Our measurements at 16 localities support the inference that dextral slip in 1855 reached 18.7 m and averaged ∼16 m over the 16 km length that we studied. Five measurements were made where a single active strand comprises the fault zone, yielding “smallest” dextral offsets of 13.0–18.7 m. At Pigeon Bush, sequential beheading of a stream and new14C dating support the interpretation that its 18.7 ± 1.0 m of offset accumulated in 1855. We also measured three “next‐smallest” offsets on single‐strand faults of 26.3–32.7 m, evidence that dextral slip during the previous event was ∼14 m. Eight measurements were made where the Wairarapa fault includes two closely spaced strands, yielding smallest dextral offsets of 12.9–16.0 m. At Tauherenikau River,14C dating of postoffset mud yielded ages indistinguishable from A.D. 1855. Combining all single‐strand and two‐strand (minimum) estimates yields an average dextral slip of 15.5 ± 1.4 m in the study area. Historical observations and our data indicate that vertical slip reached ∼2.5 m. The large displacement and short (∼145 km) strike length yield an unusually high displacement/length ratio for the rupture. As suggested by previous dislocation modeling, we propose that the rupture extended tens of kilometers downdip (W) to merge with the underlying subduction interface. Alternatively, the rupture may have been strongly segmented at depth, yielding an earthquake with an unusually large static stress drop.