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

Abstract

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−1 (where a is years). Other faults in the KMFS have typical rates of <1 mm a−1, 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 (Mw) of 5.7–7.5 (mean 6.9 ± 0.3, ±1 standard deviation, for sources with Mw ≥ 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.