Holocene rupture of the Repongaere fault, Gisborne: Implications for Raukumara Peninsula deformation and impact on the Waipaoa Sedimentary System

Abstract

The Repongaere Fault is one of a series of active normal faults within the Raukumara Peninsula, eastern North island, New Zealand. These faults appear to form in response to rapid uplift of the Raukumara Range and related extensional strain. however, the activity of these normal faults is poorly constrained. This paper presents new mapping of the active surface trace of the Repongaere Fault, c. 18 km northwest of Gisborne, and the results of two paleoseismic trenches. These results are then used to assess the seismic hazard posed by this fault and impacts on the Waipaoa Sedimentary System in which the fault is situated. active traces can be mapped for c. 4.5 km, but we infer the surface rupture length to be at least 9 km. Tephras within the trenches constrain the timing of the most recent surface rupture event to have occurred during deposition of the Waimihia Tephra (c. 3400 cal. yr BP), and at least one event in the period c. 13 800-5470 cal. yr BP, with single-event displacements of ≥0.4-1.1 m. From these data a mean dip-slip rate of c. 0.1 mm/yr and a maximum recurrence interval of 4490-6900 yr, can be calculated. if the Repongaere Fault is representative of other Raukumara Peninsula normal faults, then this relatively low rate of activity supports the interpretation that these faults are not contributing significantly to the deformation of the Raukumara Peninsula. The low rate of activity is also consistent with the very localised evidence for landscape impacts, a calculated moderate M w of 6.3-6.7, and the fault's location within the lower part of the Waipaoa River catchment. Together, these observations suggest that Repongaere Fault earthquakes have minimal, localised impact on the Waipaoa Sedimentary System.

Marine Forearc Extension in the Hikurangi Margin: New Insights From High‐Resolution 3‐D Seismic Data

“…Active faults on the onshore Raukumara Peninsula are predominantly extensional structures that are responding to rapid landscape uplift and are not thought to play a significant role in crustal deformation (Berryman et al, ). Active extension is also mapped onshore in Hawke's Bay to the south of our study area (Cashman & Kelsey, ).…”

National Seismic Hazard Model for New Zealand: 2010 Update

“…The majority of the Raukumara Peninsula ( Fig. 3a) normal faults were assigned low (< 1 mm=yr) slip rates in accordance with the Repongaere fault (217, Berryman et al, 2009). Slip rates for many of the faults beneath the continental shelf were constrained by postglacial (< 20 ka) seismic-reflection markers.…”

“…Slip rates are well constrained for only a few faults (e.g., Kelsey et al, 1998;Barnes, Nicol, and Harrison, 2002;Litchfield et al, 2007;Berryman et al, 2009;Mountjoy and Barnes, 2011), but where paleoseismic data are absent were inferred by several methods. The majority of the Raukumara Peninsula ( Fig.…”

New Zealand‐Wide Geodetic Strain Rates Using a Physics‐Based Approach

“…High rates of extension (up to 10 −7 /yr) are observed in the northeastern portion of the Raukumara Peninsula and in the Cape Kidnappers area of southern Hawkes Bay, near the coast (Figure a). These are regions with recognized normal faulting in the forearc and/or deep seated landslides (Berryman et al, ; Pettinga, ) related to possible gravitational collapse of the forearc due to rapid uplift (Reyners et al, ). We expect that the large, short‐wavelength extensional strain and large VDoHS rates near the coast are due to shallow‐seated crustal sources (such as normal faulting and/or landsliding) and are not related to deeper subduction interface sources.…”