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

Abstract The previously unknown Greendale Fault ruptured in the September 2010 moment magnitude (M w ) 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 300 m 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 1 m; however, large, discrete Riedel shears continue to depths exceeding 3 m 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 (0.6 m deep) paleo-channel by 60 cm right-laterally and 10 cm 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 ± 1.5 ka. Two additional paleochannels at ~ 2.5 m depth with OSL ages of 28.4 ± 2.4 ka and 33 ± 2 ka have been displaced ~ 120 cm right-laterally and ~ 20 cm 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 30 ka. 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.