Characterising Active Fault Earthquake Sources Beneath the Coastal Environments of Christchurch and Wellington Cities, New Zealand, Using Seismic Reflection Profiles and Fault Displacement Analysis Techniques

Abstract:

The coastal cities of Christchurch and Wellington, New Zealand, lie in different tectonic settings within the obliquely convergent Pacific-Australian plate boundary zone. Both cities have experienced damaging earthquakes in the last three years, which highlight the importance of locating and characterising hidden active faults close to urban areas.

The devastating and geologically complex Canterbury earthquake sequence of 2010-2012 developed on the periphery of the plate boundary, and reactivated several previously unidentified strike-slip and reverse faults. Major aftershocks initially beneath land, generally migrated eastward over time, and finally advanced offshore into Pegasus Bay. A study of active submarine faulting beneath the bay highlights the role of inherited crustal structure and inversion tectonics. Marine seismic reflection data reveals that faults have very low slip rate and negligible post-glacial (<15 ka) deformation, which is consistent with inferred long recurrence intervals between large magnitude (M_w>6) earthquakes.

Wellington City is surrounded by numerous high-slip rate strike-slip faults overlying the Hikurangi subduction zone. A dense network of secondary basement structures previously recognised throughout the region, mainly from tectonic geomorphology, have, until recently, been considered mostly inactive and excluded from seismic hazard models. We used high-resolution geophysical, bathymetric and sediment-core data to determine the structure, earthquake history and earthquake potential of a newly discovered active reverse fault beneath the inner reaches of Wellington Harbour. The fault has a slip rate of ~0.6 ± 0.3 mm/y, and a vertical displacement history indicating at least two large magnitude (M_w 6.3-7.1), surface-rupturing earthquakes in the last 10 ka. We infer that the fault extends southwards onshore beneath the city and potentially into Cook Strait, and represents a significant previously unrecognised seismic hazard.