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

摘要

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.