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 Mw 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 strike-slip plate-boundary faults may vary between more periodic and more episodic end members, even on adjacent, geometrically defined segments.