Using lidar to elucidate the slip behavior of the southern Alpine Fault, New Zealand

Jozi K. Pearson, & Nicolas C. Barth

Submitted August 15, 2017, SCEC Contribution #7880, 2017 SCEC Annual Meeting Poster #144

We use a newly acquired airborne lidar swath to interpret geomorphic features associated with the southernmost 25 km of the onshore Alpine Fault (AF) in New Zealand. In this heavily vegetated environment a ground return point cloud density up to 4 pts/sq.m is sufficient to elucidate the expression of the AF including subtle fault scarps. We use our geomorphic interpretations to constrain the surface slip distribution, fault kinematics, and long- and short-term slip rates of the AF in this region. This includes a complete reevaluation of strike-slip offsets (<10 m single event displacements to 500 m post-Last Glacial Maximum previously documented in the field and from 20 m contours). We also find that the southernmost 15 km of the AF onshore exhibits greater surface rupture complexity than the 10 km to the north, correlating strongly with bedrock geology observations. Throughout the entire 25 km-long swath a primary fault trace accommodating the bulk of the slip correlates with a single fault core juxtaposing drastically different lithologies (the plate boundary sensu stricto). However, along the southernmost 15 km we find multiple fault splays occur within 700 m northwest of the primary trace, accommodated within a less competent and shallowly exhumed portion of a tectonic mélange, the Kaipo Mélange. To the north where the same mélange has greater competency and the clasts suggest deeper exhumation, the AF has a single well-defined surface trace that correlates with a 12m-wide saponite-rich gouge zone at the southeastern margin of the mélange indicating strong lithologic control on near-surface fault segmentation. Coupling these analyses and interpretations leads to a more holistic understanding of the AF in a region with a well-documented record of large magnitude regularly recurring earthquakes.

Key Words
Alpine Fault, New Zealand, lidar, surface slip, geomorphology

Citation
Pearson, J. K., & Barth, N. C. (2017, 08). Using lidar to elucidate the slip behavior of the southern Alpine Fault, New Zealand. Poster Presentation at 2017 SCEC Annual Meeting.


Related Projects & Working Groups
Earthquake Geology