Elsevier

Tectonophysics

Volume 637, 10 December 2014, Pages 178-190
Tectonophysics

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

https://doi.org/10.1016/j.tecto.2014.10.004Get rights and content

Highlights

  • Comparison of displacement of buried paleochannels suggests multiple events.

  • OSL dating suggests a penultimate surface rupturing event between 20 and 30 ka.

  • 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.

Abstract

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 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.

Introduction

Despite significant scientific advances in the detection and mapping of active faults worldwide, many historical earthquakes have caused surface rupture on faults that were previously unknown due to a paucity or absence of evidence of prior surface rupture. Recent examples include the 2001 Bhuj (India) Mw 7.7 (McCalpin and Thakkar, 2003), 2010 El Cucapah Mw 7.2 (Oskin et al., 2012), and the 2010 Darfield (Canterbury) Mw 7.1 earthquakes (Quigley et al., 2010a, Quigley et al., 2010b). Characterizing the earthquake history of previously undetected faults and understanding why they evaded detection is important for assessing the completeness of active fault catalogues contributing to seismic hazard models. It is also important for understanding the maximum earthquake Mw potential for areas where surface rupturing faults have not been identified (e.g., Stirling et al., 2012).

Sedimentation or erosion may obscure or remove evidence for surface faulting in alluvial settings and increase the challenge of detecting active faults. Strike-slip faults with typically low-relief rupture traces are particularly susceptible to burial or erosion. Undersampling of active faults at the ground surface is exacerbated when fault slip rates are slow relative to the rates of surface processes (e.g., Gold et al., 2013). Fault detection is likely to be most difficult at the peripheries of active plate boundary zones, where rapid rates of surface processes due to proximal orogenic activity may overlap with areas of lower strain rates and longer earthquake recurrence intervals. Furthermore, when rupture occurs through thick packages of unconsolidated sediments, the total displacement may be expressed as a combination of discrete surface faulting and broad wavelength folding (Quigley et al., 2012, Van Dissen et al., 2011), with the latter typically difficult to recognize in the geologic record (Bray and Kelson, 2006, Fielding et al., 2009, Oskin et al., 2012, Rockwell and Klinger, 2013, Rockwell et al., 2002, Wesnousky, 2008). For this reason, the use of displaced geomorphic features to estimate the slip and Mw of paleoearthquakes relies upon the careful documentation of single-event coseismic slip and slip variability from historic earthquakes for which slip and Mw were recorded (e.g., Wells and Coppersmith, 1994, Wesnousky, 2008). Discrete surface ruptures typically account for 50–60% of the slip of their subsurface equivalent (e.g., Dolan and Haravitch, 2014).

The 2010 Mw 7.1 Darfield (Canterbury) earthquake triggered the 2010–2011 Canterbury earthquake sequence, which includes three earthquakes of Mw 6 or greater (Bannister and Gledhill, 2012). The 22 February 2011 Mw 6.2 Christchurch earthquake caused 185 fatalities and the greatest damage (e.g., Bradley et al., 2014, Kaiser et al., 2012) (Fig. 1). Of the faults that accrued slip during the Canterbury earthquake sequence only the Greendale Fault generated ground-surface rupture (Fig. 1, Fig. 2A) (Beavan et al., 2012, Elliott et al., 2012, Quigley et al., 2010a, Quigley et al., 2010b). The Greendale Fault surface rupture morphology and associated coseismic displacements have been extensively studied using combined field, lidar, InSAR, and geodetic techniques (Barrell et al., 2011, Duffy et al., 2013, Elliott et al., 2012, Litchfield et al., 2014a, Quigley et al., 2012, Van Dissen et al., 2011, Van Dissen et al., 2013, Villamor et al., 2011, Villamor et al., 2012). Abandoned river meanders and terrace patterns have been tentatively interpreted to suggest fault-related pre-2010 Holocene uplift at the western end of the Greendale Fault (Campbell et al., 2012). However, neither interpretation of ortho-photographs predating the Darfield earthquake nor analysis of post-Darfield imagery provides unequivocal evidence that the Greendale Fault ruptured the ground surface prior to 2010 (Villamor et al., 2012). In the absence of a clear pre-2010 surface trace, sub-surface information is required to constrain the paleoearthquake history of the fault. The Greendale Fault paleoseismic history has not been studied prior to this investigation.

This paper summarizes the tectonic, geologic and geomorphic setting of the Greendale Fault together with the surface rupture morphology and displacements obtained from the fault trace following the Darfield earthquake. New ground penetrating radar (GPR) and trenching data from two sites on the central Greendale Fault constrain the subsurface fault geometry and displacements. The timing of the penultimate event in the trenches has been constrained by new optically stimulated luminescence (OSL) dating of faulted stratigraphic units. Our results illuminate some of the challenges of detecting and studying active faults in alluvial landscapes at the comparably low strain rate fringes of tectonic plate boundaries. We illustrate how robust paleoseismic information for long-recurrence interval faults with diffuse and complicated patterns of surface rupture can be obtained by combining subsurface displacement measurements with multi-method high resolution surface displacement measurements.

Section snippets

Tectonic, geologic and geomorphic settings

The Greendale Fault sits at the eastern periphery of the Pacific–Australian plate boundary deformation zone in New Zealand's South Island (Fig. 1). The Pacific and Australian plates converge obliquely in a west to southwest direction at ~ 35–44 mm/year (e.g., Beavan et al., 2002, DeMets et al., 2010, Wallace et al., 2007). In the central South Island, slip on the Alpine Fault accommodates ~ 75% of the plate convergence and produces uplift of the Southern Alps, with the remainder of the convergence

Geometry and slip of the Darfield earthquake rupture

InSAR imagery, GPS measurements and seismicity data indicate that the 4 September 2010 Mw7.1 Darfield earthquake was sourced from a complex rupture comprising multiple faults and fault segments. These structures included E-W striking right-lateral, NE-striking reverse, NNW-striking left-lateral, and NW-striking normal right-lateral faults (Beavan et al., 2010, Beavan et al., 2012, Elliott et al., 2012, Holden et al., 2011). Because the earthquake initiated on a steep reverse fault, the first

Pre-trenching site investigations

Prior to trenching, the surface and sub-surface structure of small sections of the central Greendale Fault were studied using terrestrial lidar and GPR to aid trench-site selection. Given the abundance of offset cultural features, the Highfield Road site (Figs. 2B, Fig. 3, Fig. 4) was the first targeted for paleoseismic investigation. Acquisition of terrestrial lidar data at this site was undertaken immediately following the earthquake (see Litchfield et al., 2014a for data capture and

Greendale fault penultimate event and recurrence intervals

Following the Darfield earthquake, questions were raised about how many destructive earthquake sources remain undetected close to New Zealand's main cities and how often these sources generate large magnitude earthquakes. The Canterbury earthquake sequence revealed a number of previously unrecognized active faults, but the slip rates and recurrence intervals on these faults remain largely unresolved. For the Greendale Fault the absence of a clear pre-2010 trace on the Canterbury Plains mapped

Discussion

The five dated OSL samples collected from the gravel-dominated alluvial sediments exposed in the Highfield and Clintons Road trenches range in age from 21.6 ± 1.5 ka to 33.3 ± 2.0 ka (Table 1, see Supplementary Information), and provide the first numerical age constraints for this portion of the Waimakariari fan and Burnham Formation. To test the repeatability of these ages, we dated two OSL samples from a nearby gravel quarry (Fig. 2). These samples were collected at 1 m and 4 m depth from sand lenses

Conclusions

  • 1.

    The previously unknown Greendale Fault ruptured the ground surface in the September 2010 moment magnitude (Mw) 7.1 Darfield Earthquake producing a fault zone up to 300 m wide that comprised both distributed (folding) and discrete (faulting) deformations dominated by right-lateral displacement.

  • 2.

    Discrete surface fracturing accommodates an average of ~ 30% of the total right-lateral displacement with the remainder taken up by broad wavelength folding about vertical hinges accompanied by a slip

Acknowledgments

This research was funded by the New Zealand Earthquake Commission (EQC) (grant number 6/4/1BIE12/624) and by a GNS Science Sarah Beanland Scholarship (2011) to SH. We thank the landowner Mr Fitzgerald for unlimited access to the Highfield Road paddock, and all the workers and managers of the Clintons Road dairy farm for site access.

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