Mw 7.8 2016 Kaikoura, New Zealand Earthquake: Hundalee Fault Paleoseismology

Date: 4/24/2019

Time: 02:45 PM

Room: Vashon

The Hundalee Fault was one of the 20+ faults that ruptured during the 2016 Kaikoura earthquake. It was previously mapped as a major bedrock fault of approximately 30 km in length, and with a late Cenozoic throw of over 1 km. About 23 km of the fault ruptured during the Kaikoura earthquake, and ruptures were diffuse and distributed along and away from the main bedrock fault. The 2016 ruptures showed reverse-dextral motion on east-northeast-striking strands of the fault, and reverse-sinistral on north-northeast striking strands, consistent with the regional principal strain axes. Our surface rupture mapping identified several locations where scarps were present prior to the 2016 earthquake. The most prospective site identified for trenching was an approximately 2 m high pre-existing scarp at Okarahia Stream that experienced reactivation in 2016. This involved about 0.5 m of reverse-sinistral motion. This scarp was trenched in early 2018, revealing approximately 2 m reverse separation of lensoidal-bedded alluvial gravel and silt across a fault plane with a 30 degree dip. The deformation comprised discrete offset as well as broad folding, but it is unclear how much of the folding was produced by the 2016 rupture. Only about 0.5 m of the reverse separation can be attributed to the 2016 event, based on the displacement of near-surface units. There is no evidence of more than one pre-2016 event in the trench exposure. Radiocarbon dating of detrital charcoal from the middle of the faulted sediment package yielded ages of 3550+/-83 and 3499+/-68 calendar years BP. These provide maximum ages for the penultimate earthquake on the Hundalee Fault, an earthquake that produced more surface rupture deformation at this site than occurred in the 2016 event.

Presenting Author: Mark W. Stirling

Authors