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

Abstract

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.

Seismic Hazard Analyses From Geologic and Geomorphic Data: Current and Future Challenges

“…Although paleoseismic trenching studies can provide interevent times important for slip rate calculations, the numerous ways that numerical ages can either overestimate or underestimate the true age of a unit can often lead to a mix of sample ages within a single trench unit or sample ages that appear perplexingly out of stratigraphic order. Samples that are obviously out of stratigraphic order are often excluded from OxCal models designed to constrain interevent times (Bennett et al, 2018; DuRoss et al, 2019; Hatem et al, 2019; Morell et al, 2018), but it can sometimes be difficult to decide which ages most reliably represent the depositional age of the deposit and which are outliers that should be excluded from further analysis. These issues can result in varying earthquake ages and interevent times depending on which numeric ages are included in the analysis, and it can be difficult to accurately specify the uncertainty presented by this problem.…”

“…Radiocarbon dating of detrital charcoal samples extracted from sedimentary units is one typical application. For example, paleoearthquake ages are commonly bracketed in paleoseismic trenches by radiocarbon dating of detrital charcoal extracted from preearthquake and postearthquake stratigraphy, and these data are used to calculate interevent times used in slip rate calculations (e.g., Hatem et al, 2019). However, there many ways that radiocarbon dating of detrital charcoal can either overestimate or underestimate the true depositional age of an important unit used in a slip rate calculation.…”

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

“…1.1 ka) through to the present, including the current ongoing open interval following the most recent Conway segment surface rupture at ca. 1731-1840 CE (Hatem et al, 2019). As long as this open interval continues, this 8.2 +2.7/−1.5 mm/yr rate will progressively decrease.…”

“…Although we did not record a paleoearthquake chronology at the Hossack Station site, previous work by Hatem et al (2019) determined the age ranges of the five most-recent events along the Conway segment at the Green Burn study site, ~40 km east of the Hossack Station site (pink star labeled GB on Fig. 1B; Fig.…”

“…H op e R. se g. 7.8 Kaikōura earthquake, the majority of the Conway segment did not exhibit any signs of surface rupture (Litchfield et al, 2018;Hatem et al, 2019). In contrast to the Conway segment, the Hurunui and Hope River segments of the Hope fault immediately west of the Hanmer Basin ruptured together in the 1888 CE M w ~7.1-7.3 Amuri earthquake (McKay, 1890;Cowan, 1990Cowan, , 1991Cowan and McGlone, 1991;Khajavi et al, 2016).…”