A 2000 yr paleoearthquake record along the Conway segment of the Hope fault : implications for patterns of earthquake occurrence in northern South Island and southern North Island, New Zealand

Paleoseismic trenches excavated at two sites reveal ages of late Holocene earthquakes along the Conway segment of the Hope fault, the fastest‐slipping fault within the Marlborough fault system in northern South Island, New Zealand. At the Green Burn East (GBE) site, a fault‐perpendicular trench exposed gravel colluvial wedges, fissure fills, and upward fault terminations associated with five paleo‐surface ruptures. Radiocarbon age constraints indicate that these five earthquakes occurred after 36 B.C.E., with the four most recent surface ruptures occurring during a relatively brief period (550 yr) between about 1290 C.E. and the beginning of the historical earthquake record about 1840 C.E. Additional trenches at the Green Burn West (GBW) site 1.4 km west of GBE reveal four likely coseismically generated landslides that occurred at approximately the same times as the four most recent GBE paleoearthquakes, independently overlapping with age ranges of events GB1, GB2, and GB3 from GBE. Combining age constraints from both trench sites indicates that the most recent event (GB1) occurred between 1731 and 1840 C.E., the penultimate event GB2 occurred between 1657 and 1797 C.E., GB3 occurred between 1495 and 1611 C.E., GB4 occurred between 1290 and 1420 C.E., and GB5 occurred between 36 B.C.E. and 1275 C.E. These new data facilitate comparisons with similar paleoearthquake records from other faults within the Alpine–Hope–Jordan–Kekerengu–Needles–Wairarapa (Al‐Hp‐JKN‐Wr) fault system of throughgoing, fast‐slip‐rate (⁠≥10  mm/yr⁠) reverse‐dextral faults that accommodate a majority of Pacific–Australia relative plate boundary motion. These comparisons indicate that combinations of the faults of the Al‐Hp‐JKN‐Wr system may commonly rupture within relatively brief, ≤100‐year‐long sequences, but that full “wall‐to‐wall” rupture sequences involving all faults in the system are rare over the span of our paleoearthquake data. Rather, the data suggest that the Al‐Hp‐JKN‐Wr system may commonly rupture in subsequences that do not involve the entire system, and potentially, at least sometimes, in isolated events

A method to evaluate the degree of bleaching of IRSL signals in feldspar: The 3ET method

In addition to dating, IRSL luminescence signals can preserve information about erosional, transport, and depositional histories of a population of grains. Knowledge of the degree of bleaching can be useful in understanding the processes that occurred during previous depositional events, as certain transport conditions result in a well bleached signal, while others result in grains retaining an inherited signal from prior events. This information can be accessed by making single-grain IRSL measurements across successively increasing temperatures, thereby isolating signals from traps of different bleachabilities. A new approach offers a way to evaluate the completeness of bleaching of a grain by testing patterns of equivalent dose (DE) values measured at three elevated temperatures (3ET), 50, 125, and 225 °C. Consistent DE estimates across two or more temperatures suggest a single bleaching event of sufficient duration to fully depopulate the traps involved. Incompletely bleached grains with inconsistent DE values across temperatures will lack a 3ET “plateau.” Modes in the distribution of DE values for fully bleached grains can suggest depositional ages, subject to assessment of fading. We developed a Python code in a Jupyter Notebook environment for data analysis and visualization to expedite processing the large data sets produced by the 3ET protocol. The 3ET protocol was tested on a radiocarbon dated sequence of playa samples from California, USA and on a set of fluvial terraces in the Marlborough region of New Zealand as part of a larger project to reconstruct regional seismic history. Where standard pIRIR apparent ages can be inconsistent or ambiguous, 3ET age estimates produce generally consistent apparent ages. Modes of 3ET plateaus can be used to infer the most recent and prior events that resulted in a sub-population of grains being fully bleached. These initial results suggest that the 3ET method can be useful to characterize both the age and degree of bleaching of depositional events

Holocene to latest Pleistocene incremental slip rates from the east-central Hope fault (Conway segment) at Hossack Station, Marlborough fault system, South Island, New Zealand: Towards a dated path of earthquake slip along a plate boundary fault

Geomorphic field and aerial lidar mapping, coupled with fault-parallel trenching, reveals four progressive offsets of a stream channel and an older offset of the channel headwaters and associ­ated fill terrace–bedrock contact at Hossack Station along the Conway segment of the Hope fault, the fastest-slipping fault within the Marlborough fault system in northern South Island, New Zealand. Radiocarbon and luminescence dating of aggra­dational surface deposition and channel initiation and abandonment event horizons yields not only an average dextral rate of ~15 mm/yr since ca. 14 ka, but also incremental slip rates for five different time periods (spanning hundreds to thousands of years) during Holocene to latest Pleistocene time. These incremental rates vary through time and are, from youngest to oldest: 8.2 +2.7/−1.5 mm/yr averaged since 1.1 ka; 32.7 +~124.9/−10.1 mm/yr averaged over 1.61–1.0 ka; 19.1 ± 0.8 mm/yr between 5.4 and 1.6 ka; 12.0 ± 0.9 mm/yr between 9.4 and 5.4 ka, and 13.7 +4.0/−3.4 mm/yr from 13.8 to 9.4 ka, with generally faster rates in the mid- to late Holocene relative to slower rates prior to ca. 5.4 ka. The most pronounced variation in rates occurs between the two youngest intervals, which are averaged over shorter time spans (≤1700 yr) than the three older incremental rates (3700–4500 yr). This suggests that the factor of ~1.5× variations in Hope fault slip rate observed in the three older, longer- duration incremental rates may mask even greater temporal variations in rate over shorter time scales

One tune, many tempos: faults trade off slip in time and space to accommodate relative plate motions

Analysis of incremental slip rates from the four major strike-slip faults of the Marlborough fault system (MFS) of northern South Island, New Zealand, provides a first-ever record at the scale of an entire plate-boundary fault system of how relative plate motions are accommodated in time and space. This record, which spans the past 350–450 m of relative plate motion and ca. 12–14 ky, demonstrates that the fault system as a whole accommodates a steady plate-boundary slip rate, with the MFS faults “keeping up” with the overall rate of relative Pacific-Australia plate motion at relatively short displacement (10 s of meters) and time (102–103 yr) scales. These results affirm the often-assumed but until now unproven assumption that the relative plate-motion rate provides a robust basic constraint on both geodynamical models and analyses of system-level seismic hazard at these scales. In marked contrast, the incremental slip rates of each of the four main Marlborough faults are highly variable through time, marked by coordinated accelerations and decelerations spanning 4–6 earthquakes and several millennia as the faults trade off slip to accommodate a steady relative plate motion rate. These results suggest that (a) the weakest fault in the system will slip faster than average while adjacent mechanically complementary faults slip more slowly, and (b) that these patterns switch back and forth through time, likely reflecting reversible changes in the strength (i.e., resistance to shear) of the individual faults as they collectively accommodate relative plate motion. Interestingly, the periods of fast slip on the MFS faults exhibit ∼20–25 m of displacement, suggesting that these may record periods of fast slip on a weakened fault/ductile shear zone that continues until it uses up all locally stored elastic strain energy, thus potentially approaching local complete stress drop, albeit during a few tens of meters of rapid fault slip during multiple earthquakes, rather than during a single event. This hypothesis is consistent with typical earthquake stress drops of ∼1–10 MPa and estimates of depth-averaged crustal shear stress of a few 10 s of MPa, such as might be released in clusters of 4–6 earthquakes. These results emphasize the need to analyze the collective behavior of the entire fault systems, rather than just individual faults, to understand the mechanics of the system. Moreover, these patterns suggest a potential path forward for more accurate estimation of time-dependent seismic hazard, with the possible incorporation of current position of a fault within a fast- or slow/no-slip period into the probability analysis, as well as a means of potentially estimating crustal shear stress

Strike-slip ground-surface rupture (Greendale Fault) associated with the 4th September 2010 Darfield Earthquake, Canterbury, New Zealand

This paper provides a photographic tour of the ground-surface rupture features of the Greendale Fault, formed during the 4th September 2010 Darfield Earthquake. The fault, previously unknown, produced at least 29.5 km of strike-slip surface deformation of right-lateral (dextral) sense. Deformation, spread over a zone between 30 and 300 m wide, consisted mostly of horizontal flexure with subsidiary discrete shears, the latter only prominent where overall displacement across the zone exceeded about 1.5 m. A remarkable feature of this event was its location in an intensively farmed landscape, where a multitude of straight markers, such as fences, roads and ditches, allowed precise measurements of offsets, and permitted well-defined limits to be placed on the length and widths of the surface rupture deformation