Seebeck H, Thrasher GP, Viskovic GPD, Macklin C, Bull S, Wang X, Nicol A, Holden C, Kaneko Y, Mouslopoulou V, Begg JG. 2021. Geologic, earthquake and tsunami modelling of the active Cape Egmont Fault Zone. Lower Hutt (NZ): GNS Science. 370 p. (GNS Science report; 2021/06). doi:10.21420/100K-VW73.
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Abstract:
The Cape Egmont Fault Zone in the southern Taranaki Basin, New Zealand, is a complex series of synthetic and antithetic dip-slip normal faults accommodating present-day extension. The fault zone comprises new and reactivated faults developed over multiple phases of plate boundary deformation during the last 100 Myrs. The fault zone is well imaged on petroleum industry seismic reflection data, with a number of faults exposed and studied onshore.
The Cape Egmont Fault Zone is seismically active, with damaging historic earthquakes of up to MW 5.4. Most earthquakes occur beneath the Late Cretaceous to Holocene sedimentary sequence at depths greater than 5–8 km. The maximum depth of fault rupture is c. 20 km, above which 90% of recorded earthquakes occur. Focal mechanisms from these earthquakes generally indicate strike-slip to oblique-normal faulting, which contrasts with the predominantly dip-slip faulting observed in the overlying sedimentary sequence and surface fault traces. Data from regional earthquake studies and petroleum well deformation show faults imaged in the sedimentary sequence to be preferentially oriented for slip in the present-day stress field.
The greatest earthquake risk is on major basement-penetrating crustal-scale faults greater than 20 km in length. Fault lengths and maximum vertical offsets of the sedimentary sequence, determined from a three-dimensional structural model, are consistent with global displacement-length scaling relationships. This validation permits fault lengths to be used to determine potential future earthquake magnitudes using global fault length-magnitude relationships. Fault lengths of post-Pliocene normal faults are typically ≤21 km, resulting in maximum predicted magnitudes MW 6.3. The most likely earthquake magnitude from the fault population sampled is MW 5.4 ± 0.5. The largest and most mature fault – the Cape Egmont Fault – is at least 53 km long and, depending on the number of segments ruptured during a future event, is capable of generating an earthquake between MW 7 and 7.3.
Numerous fault traces offset the seafloor and land surface. Surface and seafloor offsets range up to 6 m and record multiple earthquake events. The largest fault offsets and fault scarp heights are adjacent to the Maui Gas Field along the Cape Egmont Fault. The major faults of concern are the main trace of the Cape Egmont Fault offshore and the Oaonui-Kina faults onshore. Onshore, west of the Oaonui Fault, is a crustal-scale graben that was mainly active prior to 1.5 Ma. Some activity continues, and the basement-penetrating faults in this area are associated with seismic activity.
Four regions of distinct fault style are observed along the Cape Egmont Fault Zone. In contrast to the majority of planar (constant dip) Pliocene–Holocene normal faults, the main segment of the Cape Egmont Fault has a listric (decreasing dip with depth) geometry that is most likely inherited from a previous deformation episode. Slip calculations for the main segment of the Cape Egmont Fault over the last 1.5 Myrs show a maximum vertical displacement rate of c. 0.43 mm/yr and a horizontal extension rate of c. 0.15 mm/yr within the sedimentary sequence.
Earthquake fault source geometry and slip rates are updated from previous compilations and incorporated into the New Zealand Community Fault Model (NZCFM). The NZCFM forms the basis for the fault source model used in an upcoming revision of the National Seismic Hazard Model. A probabilistic seismic hazard assessment for the study will be generated during this nationwide update of the National Seismic Hazard Model using results from this study.
To examine ground motions and shaking intensities from different fault source geometries, numerical simulation models for a MW 7.1 earthquake scenario have been generated for the Cape Egmont Fault. Predicted ground motions (Peak Ground Velocity [PGV] and Peak Ground Acceleration [PGA]) at four locations (Maui-B platform, Maari platform, Oaonui gas processing station and New Plymouth city) have been estimated using deterministic and stochastic modelling methods. Low-frequency (<2 Hz) kinematic and high-frequency (10–20 Hz) stochastic earthquake simulations are compared to empirically based Ground Motion Prediction Equations (GMPEs), ShakeMapNZ and recorded observations from the MW 7.1 Darfield earthquake.
The geometric complexity of modelled fault source geometries shows little influence on the spatial distribution of ground motions from kinematic models. The location of the hypocentre and resulting rupture propagation direction has the greatest influence on spatial distribution of ground motions. Due to the limited spatial resolution of the kinematic model, ground motions will likely be higher than those presented. Stochastic earthquake models result in greater PGVs than GMPEs, particularly within 10 km of the source.
Stochastic PGAs are comparable to observed ground motions from the Darfield earthquake, despite differences in fault source parameters. Shaking intensities predicted by the models are strong to very strong across the southern Taranaki Peninsula.
Near-fault permanent vertical ground displacements using different fault source geometries were generated for two MWM 7.1 earthquake scenarios. Permanent single-event displacement of the seafloor in these scenarios is greater than 1 m. The ground displacement model and the calculated vertical slip rate on the Cape Egmont Fault indicate that the recurrence interval for large magnitude earthquakes is 2–3 kyrs. The recurrence interval could be longer (by a factor of 2–5) if single-event displacements are larger than those predicted. Recurrence intervals determined for onshore active faults with similar single-event displacements are typically 2–7 kyrs within the range of 0.5–12 kyrs. More-frequent smaller, but damaging, earthquakes and aftershock sequences associated with large earthquakes are also likely to occur.
Numerical tsunami models generated from two MW 7.1 earthquake scenarios indicate that a significant east-directed tsunami could sweep the coastlines around the South Taranaki Bight. Predicted wave height and current velocity calculations for a number of locations within the bight are presented. A focused maximum wave height of 2 m is predicted at the coast to the northwest of Whanganui. A tsunami potentially generated by a local event has been recorded at a number of locations around the bight at about 1500 AD (c. 700 years ago). A direct link to the Cape Egmont Fault Zone has not been established.
Several non-active, subsurface, igneous features of less than c. 2 Myrs of age are identified and documented. None are thought to present significant present-day hazard. (auth)
