Late Holocene rupture behavior and earthquake chronology on the Hope fault, New Zealand

Abstract

The Hope fault is the most active and southernmost splay of the Marlborough fault system in the northern South Island of New Zealand. The fault consists of five geometrically defined segments. We used trenching to acquire paleoseismic data and radiocarbon dating of faulted late Holocene sediments on the Hurunui segment of the Hope fault to derive an earthquake chronology that extends from the historic 1888 M w 7.1 Amuri earthquake to ca. 300 C.E., thereby providing the longest chronologic record of earthquakes on the Hope fault to date. Earthquake event horizons were identified by upward fault terminations, colluvial wedges, unconformities, and/or progressive folding of shutter basin deposits. Six earthquakes identified at C.E. 1888, 1740-1840, 1479-1623, 819-1092, 439-551, and 373-419 indicate a mean recurrence interval of ~298 ± 88 yr, with successive median interevent times ranging from 98 to 595 yr. The large variance in interevent times with respect to mean recurrence interval is explained by (1) possible coalescence of rupture overlap from the adjacent Hope River segment onto the Hurunui segment at our study site (including the 1888 Mw 7.1 Amuri earthquake, sourced primarily from the Hope River segment), which results in apparently shorter interevent times at the study site compared to mean recurrence intervals from adjacent fault segments, (2) possible earthquake temporal clustering on the Hurunui segment, which could result in interevent times that are significantly shorter or longer than interevent times and mean recurrence intervals predicted by a periodic earthquake rupture model, and/or (3) "missing" events, which could result in interevent times and mean recurrence intervals at the study site that are longer than the actual mean recurrence interval. While we cannot exclude option 3 as a possibility, we prefer options 1 and 2 to explain earthquake chronologies and rupture behavior on the Hurunui segment of the Hope fault, given the detailed nature of our geologic and chronologic investigations. By demonstrating that the 1888 Amuri earthquake propagated through a proposed segment boundary, we provide the first evidence for coseismic multisegment ruptures on the Hope fault. In contrast, the penultimate earthquake ruptured the Hurunui segment at 1740-1840 C.E. with no known rupture of the Hope River segment. Paleoearthquake records near geometrically complex segment structural boundaries on major strike-slip faults may show temporal recurrence distributions resulting from earthquake ruptures that variably arrest or propagate through proposed segment boundaries. We note that earthquake recurrence along major strikeslip plate-boundary faults may vary between more periodic and more episodic end members, even on adjacent, geometrically defined segments.

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

“…In contrast to evidence for historical surface rupture of the Hope fault farther west during the 1888 Mw~7-7.3 Amuri earthquake [McKay, 1890;Cowan, 1991;Khajavi et al, 2016] Surface rupture GB2 is younger than the ages of the detrital charcoal samples collected from CW2 itself, as the material contained in the colluvial wedge must have existed higher on the scarp prior to deposition of the colluvial wedge. Thus, event GB2 is younger than samples SF-15 (1462-1627 CE) and SF-16 (1505-1643 CE), which were collected from within unit CW2.…”

“…Specifically, paleoseismologic records are now available from the Hope fault along the Hurunui and Hope River segment from the Matagouri Flats (MF) [Langridge et al, 2013] and Hope Shelter (HS) [Khajavi et al, 2016] sites ~100 km west of the GBE site, from the Kekerengu (EK) fault at a site ~ 100 km northeast of the Green Burn sites [Little et al, 2018], from the Cross Creek (CC) site on the Wairarapa fault, an extension of this fault system northward into southern North Island [Little et al, 2009], and from multiple sites along the central Alpine fault (A) with dendrochronologically dated records of tree disturbance [Wells et al, 1999] and records of strong ground shaking from paleo-seismite records in lakes in the footwall of the Alpine fault on the coastal plain of the Southern Alps [Howarth et al, 2012[Howarth et al, , 2014[Howarth et al, , 2016. It is worth pointing out, in contrast to paleoseismic results from trenches of the active fault traces, the dendrochronology and paleo-seismite data record strong ground shaking at the site off of the active fault traces, and thus could record earthquakes generated by other faults.…”

“…The 1730-1840 CE age range of the GB1 is similar to the 1700-1840 CE time range of the most-recent surface rupture documented on the Kekerengu fault by Little et al [2018], indicating that the Conway segment and the Kekerengu fault likely both ruptured within a <~100-year-long time window just prior to the beginning of European settlement. Subsequently, the historical 1855 Mw~8.1 Wairarapa earthquake ruptured a ~160-km-long section of the Wairarapa fault extending into Cook Strait [Grapes and Downes, 1997;Rodgers and Little, 2006], and the 1888 Mw~7-7.3 Amuri earthquake ruptured the Hurunui and Hope River sections of the central Hope fault [McKay, 1890;Cowan, 1990Cowan, , 1991Cowan and McGlone, 1991;Khajavi et al, 2016]. Thus, if the most recent events on the Conway segment (GB1) and the Kekerengu fault (EK1) occurred relatively late during their allowable time ranges, the events observed at all four sites could record a temporally brief sequence of large-magnitude earthquakes that ruptured the entire fault system northeast of the Alpine fault during the late th and 19 th centuries [Little et al, 2018].…”

“…Our preferred interpretation, however, is that events A3 and GB4 represent isolated events because of the lack of spatial continuity and paucity of faults that ruptured during this time period. Alternatively, it is possible that both A3 and GB4 were part of a brief, continuous sequence of events that involved rupture along the southern Kakapo strand of the central Hope fault system, and potentially bypassing the Matagouri Flats and Hurunui Shelter paleoseismic sites of Langridge et al [2013] and Khajavi et al [2016], which are located on the northern Hurunui and Hope River segments of the Hope fault. Currently, there are no paleoseismic data available for the Kakapo strand with which to constrain this possibility.…”

“…To complete plate boundary system earthquake behavior analysis, we study the fastestslipping strike-slip faults of the Australian-Pacific plate boundary of the northern South Island and southern North Island of New Zealand, where previous studies have documented patterns of earthquake recurrence on many of these major faults [e.g., Cooper and Norris, 1990;Wells et al, 1999;Langridge et al, 2003Langridge et al, , 2013Mason et al, 2006;Sutherland et al, 2007;Little et al, 2009;Van Dissen and Nicol, 2009;Berryman et al, 2012aBerryman et al, , 2012bDe Pascale and Langridge, 2012;Howarth et al, 2012Howarth et al, , 2014Howarth et al, , 2016Clark et al, 2013Clark et al, , 2015Nicol et al, 2016;Khajavi et al, 2016;Cochran et al, 2017;Nicol and Dissen, 2018]. The Hope fault, the subject of this manuscript, is the central link between high slip-rate faults to the southwest (Alpine fault) and northeast (Jordan-Kekerengu-Needles fault and the Wairarapa fault in southern North Island).…”