Preliminary dextral slip and slip repetition during recent central Alpine fault earthquakes, based on lidar and field mapping, South Island, New Zealand
Abstract
In the South Island of New Zealand, the dextral-reverse Alpine fault forms the major plate boundary structure between the Pacific and Australian plates and thought to fail in large to great earthquakes approximately every 100 to 400 years, with the most recent faulting event (MRE) in A.D. 1717. The fault has a dextral slip rate of approximately 2.7 cm/yr based on late-Pleistocene displacements. To assess the timing, surface rupture lengths, and magnitudes of Alpine fault earthquakes, understanding single- and multi-event slip patterns are of critical importance. Slip-per-event for the MRE and previous recent earthquakes is unknown for the central "high-uplift" section of the fault. This is partially due to high precipitation rates (up to 15 m/yr) which leads to dense rainforest covering the majority of the fault. We reviewed all previous documentation of dextral slip and compiled a database of slip distribution, and then used a 2-meter light detection and ranging (lidar) digital elevation model (DEM) from 34 km along the central Alpine fault to measure single event displacements (SED) and other short (< 30 m cumulative) displacements, and then measured these same displacements in the field. Preliminary lidar measurements from south of Gaunt Creek show that dextral displacements range between 9 m ± 2 m for the MRE in A.D. 1717 (the smallest observable) displacement, with successive displacements on nearby higher surfaces recording displacements of up to 20 m ± 3 m. However, preliminary field measurements at the same locations show cumulative displacements from three distinct events (as opposed to the two visible in the lidar) with MRE measurements yielding mean displacements of 7 m ± 0.5 m, with older displacements averaging around 12.5 m ± 2 m, and 23 m ± 2. By plotting our preliminary mean SED from field measurments of ~ 7 m ± 0.5 m against the 2.7 cm/yr Quaternary dextral slip-rate, the timings of events prior to the MRE can be estimated. If ~ 7 m ± 0.5 m of slip occurred in A.D. 1717, then based on the slip-rate, the penultimate event was 259 yr ± 20 yr before 1717, or c. 1458 ± 20 yr. Using the same slip versus slip-rate relationship, the third event happened c. 1228 ± 40 yr. These timings are consistent with existing central Alpine fault paleoseismic surface ruptures at c. A.D. 1717, c. 1460, and c. 1230. Our preliminary results describe the repetition of recent slip along the central Alpine fault for the first time. Importantly, the 2-m lidar DEM was critical in identifying dextral slip at several locations from one or more surface rupture events, however the DEM resolution in this dense rainforest environment is coarse and favoured longer slip measurements than what we later measured in the field. Thus, the lidar data alone in this environment may ignore events or overestimate slip per event. Field checking and measurements show multiple events and accumulation of slip in landforms that were difficult to observe in the lidar data alone. Through a comparison of all recent slip measurements and recent mapping, preliminary segment boundaries can be proposed along with estimates of previous earthquake magnitudes.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFM.T21H..01D
- Keywords:
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- 1242 GEODESY AND GRAVITY / Seismic cycle related deformations;
- 7209 SEISMOLOGY / Earthquake dynamics;
- 8036 STRUCTURAL GEOLOGY / Paleoseismology;
- 8118 TECTONOPHYSICS / Dynamics and mechanics of faulting