Structure and activity of the Hauraki Rift

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. 

Seismic reflection and refraction data were acquired across a 9 km long transect of the Hauraki Rift and the Kerepehi Fault along the Firth of Thames stopbank, using the University of Calgary Envirovibe tractor tire mounted vibroseis source and 600 ch ARIES recording equipment. 

Crooked-line reflection processing was completed. Gas-rich near surface sediments limited frequencies to < 30 Hz. The 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 m/s and 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 apparent fault density may indicate that the Hauraki Rift is less active today than in the past. 

Seismic reflection and refraction analysis was complemented by co-incident gravity data acquisition to resolve ambiguity in the deepest part of the seismic section where signal attenuation masquerades as the geological basement. 3D context for the transect was provided by re-interpretation of existing marine seismic profiles in the Firth of Thames and analysis of industry acquired aeromagnetic data from the Hauraki Plains. Engineering geophysics site period measurements on the Hauraki Plains have been made, enabling calibration of the method and providing information about how ground shaking at different sites may manifest in an earthquake.