Derivation of direct on‐fault submarine paleoearthquake records from high‐resolution seismic reflection profiles: Wairau Fault, New Zealand

Abstract

Direct, on‐fault submarine paleoearthquake records can be derived from high‐resolution seismic reflection profiles of active fault growth sequences. Coseismic vertical increments of displacement are best preserved in the architecture of the growth sequence when the long‐term rate of sedimentation exceeds the rate of fault vertical displacement. Postseismic stratigraphic intervals can be recognized on vertical displacement history curves, from which estimates of the earthquake timing and vertical displacements can be made. The strike‐slip Wairau Fault is a major seismic hazard in central New Zealand and is partially submarine. Analysis of 10 high‐resolution seismic profiles spanning a 20 km section of the offshore fault trace reveals two types of postseismic growth sequences and a composite paleoearthquake record with up to eight surface‐rupturing earthquakes since 18 ka. The recurrence intervals range from ∼0.9 to 3.8 ka (mean is ∼2.2 ka), while the coseismic vertical displacements range from 0.5 to 5.3 m (mean is ∼2.5 m). These data conform to the variable slip model of earthquake behavior. The vertical coseismic displacement is not always predictable from the recurrence interval preceding the event. The data indicate that the seismic moment release has not been constant between earthquakes or that the ratio of horizontal to vertical coseismic displacement at a given site has varied over multiple earthquake cycles. The offshore data are consistent with onshore paleoearthquake records, long‐term dextral slip rate of >2 mm/yr (compared to 3–5 mm/yr, 40 km in land), and paleoearthquakes of >Mw7.5. The last earthquake occurred on the fault 2.0 ± 0.3 kyr ago, indicating that significant elastic strain has now accrued.