Spatial and Temporal Clustering of Large Earthquakes on Upper-Plate and Subduction Thrust Faults Along the Southern Hikurangi Subduction Margin, Aotearoa-New Zealand

ABSTRACT Large (≥Mw 6.5) earthquakes recorded in active fault systems are commonly clustered in space and time, which presents challenges for time-dependent seismic hazard modeling. We investigate the spatial and temporal clustering of earthquakes in the last 5500 yr on upper-plate faults (Wairarapa, Wellington, and Ōhāriu) and the subduction interface in the southern Hikurangi margin in Aotearoa-New Zealand. We recalibrated radiocarbon ages and reinterpreted some earthquake timing interpretations from 37 on-land sites (trenches) to produce revised earthquake timings and recurrence intervals on three upper-plate faults. We compare these ages with the timings of great earthquakes (≥Mw 8) on the Hikurangi subduction interface and the 1848 Marlborough and 1855 Wairarapa historical surface-rupturing earthquakes. Temporally clustered surface-rupturing earthquakes occurred on two or more upper-plate faults at 270–90, 880–520, 2300–1825, 3640–2810, and 5170–4855 cal. B.P. The youngest four of these earthquakes overlap in age with the timing of ruptures on the southern Hikurangi subduction interface. A further two subduction interface earthquakes at 515–475 and 1505–1250 cal. B.P. do not temporally overlap with the upper-plate earthquakes studied. Over half of the earthquakes sampled on the subduction interface are clustered in time with upper-plate earthquakes on the Wairarapa, Wellington and/or Ōhāriu faults. The observed spatial and temporal clustering of large earthquakes could reflect co-rupture of multiple faults and/or sequences of earthquakes closely spaced in time. The clustering is consistent with geometric intersection and/or stress interactions between upper-plate faults and the subduction interface.