Kekerengu Fault, New Zealand: Timing And Size Of Late Holocene Surface Ruptures

To test models of plate motion and seismic hazard through central New Zealand, similar to 10 months prior to the November 2016 Kaikoura earthquake, we excavated three trenches across the Kekerengu fault. Two of these were dextrally displaced similar to 9 m in the M-w 7.8 2016 Kaikoura earthquake. We analyze 13 radiocarbon samples and document paleoearthquakes at 249-108, 528-356, and 1249-903 cal. B.P. Including the 2016 rupture, the youngest three events indicate a mean recurrence interval (RI) for the Kekerengu fault of 376 +/- 32 yrs (1 sigma). This RI is within error of that of the Hope fault; moreover, using a 9 +/- 4.5 m single-event displacement for the Kekerengu fault, this RI implies a dextral-slip rate of 24 +/- 12 mm/yr (1 sigma), which overlaps with the late Quaternary slip rate of the Hope fault. Our data support the hypothesis-one encoded into the New Zealand National Seismic Hazard Model-that slip on the Hope fault is transferred predominantly northward onto the Jordan-Kekerengu-Needles fault to reach within 60 km of Wellington, rather than extending east-northeast along the offshore Hope fault.The last three paleoearthquakes on the Kekerengu fault were closely spaced in time or coeval with earthquakes on the Hope fault suggesting possible stress triggering. It is possible that this correspondence may reflect the previous occurrence of complex, multifault earthquakes that-like the 2016 earthquake-involved mutual surface rupturing of these two faults and perhaps also of the subduction interface below them. Two Holocene earthquakes on the Kekerengu fault have been closely followed by others on the Wairarapa fault, most recently in 1855. The latter two faults straddle an similar to 30 km width of Cook Strait. Since A.D. 1855, they have combined to rupture > 200 km in central New Zealand with unusually large surface displacements.