Palaeoseismic constraints on Holocene surface ruptures along the Ostler Fault, southern New Zealand

Abstract

Palaeoseismic trenching along the central Ostler fault zone reveals the nature and timing of past surface-rupturing earthquakes. A 26 m long trench excavated into a last-glacial (26.5 ka) outwash surface cut by the Ruataniwha strand of the North Central Ostler fault reveals evidence for at least two metre-scale surface displacements in the last c. 8 ka. Detailed logging of colluvial wedge and alluvial stratigraphy, combined with optically stimulated luminescence dating of loess within colluvial packages, provides a maximum bound on the most recent earthquake (MRE) of 2.3Á/4.5 ka. The MRE resulted in a surface displacement of at least 1.8 m, consistent with an estimated moment magnitude (M) 6.9Á/7.1 earthquake based on the total 60 km length of the Ostler fault zone. The penultimate event occurred sometime before 4.1Á/8.4 ka and resulted in a comparable or larger surface displacement. Similar ages from samples collected in previous trenches on the Ostler fault suggest that surface ruptures may persist across kilometre-scale stepovers defined by the active surface trace of the fault. At the trench scale, comparison between dip slip calculated from topographic fault-scarp profiling (7.8 m) and the total offset of exposed outwash gravels (6.0 m) suggests that surface folding immediately adjacent to the fault scarp accommodates roughly 25% of the total slip. Given a previously reported recurrence interval of c. 2Á/5 ka for the Ostler fault, the presence of only 2Á/3 palaeoearthquakes on the 26.5 ka outwash surface indicates that additional events likely occurred on nearby fault scarps in an overall complex zone of surface faulting.

Surface Rupture and Distributed Deformation Revealed by Optical Satellite Imagery: The Intraplate 2016 Mw6.0 Petermann Ranges Earthquake, Australia

“…This result is consistent with observations from paleoseismic observations made across reverse faults, where displacement at the emergent fault or shallowly buried fault tip (<100 m below surface) is often less than slip recorded by strain markers measured at length scales of tens to hundreds of meters from the emergent fault tip (Amos et al, 2011;Kelson et al, 1996;Streig et al, 2007). These relationships lead to ambiguity as to the whether the larger far-field displacements result solely from coseismic slip or from some combination of coseismic, interseismic, and/or aseismic slip.…”

Modeling Earthquake Moment Magnitudes on Imbricate Reverse Faults from Paleoseismic Data: Fox Peak and Forest Creek Faults, South Island, New Zealand

“…4). Vertical offset at the fault is less than 0.5 m; a large percentage of the total deformation is accommodated by coseismic folding accompanying faulting (e.g., Gold et al, 2006;Amos et al, 2011). The net slip at this location, determined from a survey transect over the length of the terrace, is ∼1:8 m (T. Stahl et al, unpublished manuscript).…”

Paleoseismology of the 2016 MW 6.1 Petermann earthquake source: Implications for intraplate earthquake behaviour and the geomorphic longevity of bedrock fault scarps in a low strain‐rate cratonic region

“…Commonly, surface rupture traces and/or related surface deformation features (e.g., fault scarps, folds) produced in contemporary and/or pre‐historic (‘paleo’) earthquakes are excavated by hand or machine to expose subsurface structural–stratigraphic relationships, from which earthquake displacements and chronologies may be characterized (e.g., Khajavi et al, 2016; Sieh, 1978; Stahl et al, 2016). Near‐surface fault exposures in natural settings (e.g., stream cut‐banks; Quigley et al, 2006; Sandiford, 2003) and progressive offsets of surface features (e.g., stream channels and terraces; Amos et al, 2011; Gold et al, 2017; Little et al, 2010) may also be utilized for this purpose. Determination of an earthquake recurrence interval requires estimates of the timing of multiple discrete seismic displacements, although other approaches that combine geodetic and seismologic data, slip rates, and earthquake scaling parameters are also used (e.g., Atwater et al, 2003; Dixon et al, 2003; Leonard & Clark, 2011; Nicol et al, 2016; Zielke, 2018).…”