Characterizing earthquake recurrence parameters for offshore faults in the low‐strain, compressional Kapiti‐Manawatu Fault System, New Zealand

Abstract

[1] Seafloor fault scarps and near-surface deformation of late Quaternary seismic reflectors occur along the eastern margin of the Wanganui Basin, 200 km behind the active Hikurangi subduction front, southern North Island, New Zealand. The offshore scarps are associated with the low-strain, compressional Kapiti-Manawatu Fault System (KMFS), which comprises high-angle (>60°) reactivated reverse and normal faults oriented NE-SW, highly oblique to the coast. Seafloor scarps range from <10 to 50 km in length with vertical seafloor offsets of 2 to 30 m. The longest structure is the Mascarin Fault, with maximum late Quaternary vertical slip rates of 3 mm a À1 (where a is years). Other faults in the KMFS have typical rates of <1 mm a À1 , comparable to long-term estimates. Three zones of recent deformation are identified: faults in the north and south of the KMFS are characterized by high scarps (>10 m high) and moderate to long fault seafloor rupture lengths, and those in central parts of the fault system are characterized by low scarps (<5 m high), variable slip rates, and short to moderate fault lengths. Empirical equations indicate that KMFS faults may generate earthquakes with moment magnitudes (M w ) of 5.7-7.5 (mean 6.9 ± 0.3, ±1 standard deviation, for sources with M w ! 6.5). Estimated recurrence intervals that are generally >10,000 a, suggest that the seismic hazard of the Kapiti-Manawatu region is relatively low. Incorporation of these new geological data, however, is likely to increase slightly the expected seismic hazard in southern North Island. The method of determining the earthquake recurrence parameters of offshore faults has potentially wider applications elsewhere.

National Seismic Hazard Model for New Zealand: 2010 Update

“…The largest differences between this model and that of the earlier NSHM are the addition of seven offshore faults, which replace the earlier Wairau offshore fault source interpretation (Lamarche et al, 2005;Nodder et al, 2007;NIWA unpublished data). Onshore the fault distribution has been revised slightly, with many faults lengthened to include adjoining structures in order to calculate displacements that match observed field displacements.…”

“…Slip rates are well constrained for only a few faults (e.g., Pillans, 1990;Nodder et al, 2007) and in the absence of paleoseismic data were inferred for on-land faults by conversion of fold differential uplift rates to dip-slip rates. The offshore rates north of Kapiti Island (Fig.…”

The kinematics of a transition from subduction to strike‐slip: An example from the central New Zealand plate boundary

“…In contrast, faults in southern Cook Strait accommodate a combination of strike‐slip and contraction, which is often associated with localized areas of contemporary uplift. Off the western Wellington (Mana) coast, predominantly NE‐SW striking faults are continuous with the southern components of the Kapiti – Manawatu Fault System [ Lamarche et al , 2005; Nodder et al , 2007], and therefore may be reverse dextral structures. The continental slope of southern Wairarapa and Marlborough is dominated by NE‐SW striking thrust faults [e.g., Mountjoy et al , 2009; Barnes et al , 2010].…”

“…This gap in knowledge has hampered efforts to define the transition from subduction to strike‐slip in central New Zealand, and was largely due to a lack of high‐quality marine geophysical data in central Cook Strait. High‐resolution marine seismic reflection and bathymetric data were acquired in 2003 and 2005, and now provide a clear picture of the active faults in the central Cook Strait region [ Nodder et al , 2007; Barnes et al , 2008; Mountjoy et al , 2009; Barnes and Pondard , 2010; Pondard and Barnes , 2010]. …”

Recasting Historical Earthquakes in Coastal Areas (Gargano Promontory, Italy): Insights from Marine Paleoseismology

“…Whenever feasible, such locations should in fact be constrained through independent data, the ultimate goal being the identification of the true seismogenic source. One possibility is being disclosed by recent developments in high-resolution geophysical methods that allow detailed reconstruction of offshore active faults (Pratt et al, 2003;Polonia et al, 2004;Cormier et al, 2006;Nodder et al, 2007;Barnes and Pondard, 2010). When such faults displace shallow-recent sediments for which chronostratigraphic data are available, a deformation history can be reconstructed and significant physical/ quantitative parameters of the fault can be derived, allowing for an evaluation of the seismogenic potential in terms of recurrence interval and expected maximum magnitude.…”