A seismic investigation of the evolution and structure of the Kerepehi Fault, in the context of the Hauraki Rift, New Zealand

Abstract

The Hauraki Rift is a low strain rate structure, running ~N-S approximately 30 km to the east of the Auckland and Hamilton metropolitan areas. A significant tectonic feature in the North Island, but largely scientifically neglected since the 1980’s, this structure contains the known active Kerepehi Fault, capable of long recurrence interval (2.5 -9 kyr) M6-7 earthquakes. The geological age of the rift is circumstantial, but it was believed to have initiated about 10 Ma. This places the extension contemporaneous and transverse to the then-active Colville Arc, with the impact of the Hikurangi Plateau subduction a potential trigger for passive rifting. Seismic reflection and refraction data were acquired across a 9 km transect of the Hauraki Rift and the Kerepehi Fault near the foreshore of the Firth of Thames, using a vibroseis source. Crooked-line reflection processing was completed in Schlumberger Vista. Gas-rich near surface sediments limited frequencies to < 30 Hz. Signal to noise ratio was impacted by unconsolidated rift infill units, but a high fold survey design (> 200) resulted in a stack with reflectors to 1.5 s two-way travel time. Refraction arrival velocities are estimated to range between 600 – 2200 m/s. The reflection profile is interpreted to have imaged the uplifted fault block, five separate seismic sequences and a number of minor fault structures. Depth-conversion using refraction velocities indicates that the basin at this location extends to > 2.6 km in depth, and the meta-sedimentary basement fault block rises to < 100 m below the surface. In addition to the Kerepehi Fault, two recently active, shallow faults were identified. A temporal decrease in faulting density indicates that the Hauraki Rift is less active today than in the past. Additional available datasets including gravity profiles across the rift and airgun marine seismic lines were used to constrain the location and segmentation of the Kerepehi Fault offshore through the Firth of Thames to a three-segment interpretation. East of Waiheke Island the structure of the Hauraki Rift may change from a two-graben structure to a single depression, and the Kerepehi Fault is not evident in the basement or shallow sedimentary fill. The northern onshore Awaiti segment was extended further north to at least 38 km in length, with a potential rupture magnitude of M6.9.