Paleoseismology of the 2010 Mw 7.1 Darfield (Canterbury) earthquake source, Greendale Fault, New Zealand

• Published in: Tectonophysics 2014-12, Vol.637, p.178-190
• Main Authors: Hornblow, S., Quigley, M., Nicol, A., Van Dissen, R., Wang, N.
• Format: Article
• Language: English
• Subjects: Darfield earthquake; Ground surface rupture; Paleoearthquakes; Strike slip displacements
• ISSN: 0040-1951; 1879-3266
• DOI: 10.1016/j.tecto.2014.10.004

The previously unknown Greendale Fault ruptured in the September 2010 moment magnitude (Mw) 7.1 Darfield Earthquake. Surface rupture fracture patterns and displacements along the fault were measured with high precision using real time kinematic (RTK) GPS, tape and compass, airborne light detection and ranging (lidar), and aerial photos. No geomorphic evidence of a penultimate surface rupture was revealed from pre-2010 imagery. The fault zone is up to 300m wide and comprises both distributed (folding) and discrete (faulting) deformation dominated by right-lateral displacement. Surface fracturing accommodates ~30% of the total right-lateral displacement in the central fault zone; the remainder is accommodated by distributed deformation. Ground penetrating radar and trenching investigations conducted across the central Greendale Fault reveal that most surface fractures are undetectable at depths exceeding 1m; however, large, discrete Riedel shears continue to depths exceeding 3m and displace interbedded gravels and sand-filled paleochannels. At one trench site, a Riedel shear displaces surface agricultural markers (e.g., fences and plow lines) and a subsurface (0.6m deep) paleo-channel by 60cm right-laterally and 10cm vertically, indicating the paleochannel has been displaced only in the Darfield earthquake. Optically stimulated luminescence (OSL) dating of the displaced paleochannel yields an age of 21.6±1.5ka. Two additional paleochannels at ~2.5m depth with OSL ages of 28.4±2.4ka and 33±2ka have been displaced ~120cm right-laterally and ~20cm vertically. The doubling of displacement at depth is interpreted to indicate that in the central section of the Greendale Fault the penultimate surface-rupturing event occurred between ca. 20 and 30ka. The Greendale Fault remained undetected prior to the Darfield earthquake because the penultimate fault scarp was eroded and buried during Late Pleistocene alluvial activity. Similar active faults with low slip rates (i.e. lower than sedimentation/erosion rates) are likely to be concealed in alluvial settings globally. •Comparison of displacement of buried paleochannels suggests multiple events.•OSL dating suggests a penultimate surface rupturing event between 20 and 30ka.•Discrete surface fractures accommodate ~30% of the right-lateral displacement.•The penultimate fault scarp was eroded and buried during alluvial aggradation.•Similar active faults are likely to be concealed in alluvial settings globally.