Study of seismic and geodetic strains along the Pacific-Australian plate boundary through the South Island of New Zealand
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Orientations of the principal axes of strain determined from historical seismic activity are consistent with those determined from triangulation data for most of the South Island. In both data sets the principal axis of relative contraction trends at 105°-125° and has a shallow plunge. A notable exception to this is the Central Otago region, in the south central part of the South Island where the trend of principal axis of relative contraction determined using geodetic techniques changes to about 060°, while the existing data suggests that the seismic strains are unaffected.
The orientation of the principal strains calculated from geodetic techniques over most of the South Island is compatible with oblique (dextral strike-slip and reverse) movement on the Alpine Fault and reverse movement on the northeast trending faults east of the main divide. In Central Otago, the principal axis of relative contraction is oriented at 45° to the major structures, which is consistent with strike slip movement along these structures. This can be reconciled with the apparently reverse nature of the faults by recognising that reverse movement on a series of orthogonal northeast and northwest striking thrust fault can produce a uniform dilatation which would not be detected with the geodetic techniques available in the South Island. The model also involves some combination of dextral strike-slip movement on the northeast trending faults and sinistral strike-slip movement on the northwest trending faults. If all of the strains observed in central Otago are to be explained by this model, the rate of strike-slip movement on the faults would have to be 4-8mm/yr. There is currently no evidence for strike-slip rates of this magnitude in central Otago, suggesting that some of the strain is ultimately released on structures outside the region.
Dislocation models of strains near the Alpine Fault near Okarito require at least 24 mm/yr strike-slip and 10 mm/yr dip-slip movement on the fault to obtain a satisfactory match with the observed strains. The best-fit locking depth is 8 km, in satisfactory agreement with estimates of the brittle-ductile transition in the area. At Haast Pass, the best-fit dislocation model suggests about 16-18 mm/yr of strike-slip and 4-6 mm/yr of dip-slip movement on the fault. The locking depth is 3-4 km. This model is unable to account for high shear strain rates on both sides of the fault, suggesting that about half of the deformation in the Haast area takes place off the Alpine Fault. Possible locations for the extra-fault deformation are the coastal planes immediately west of the fault and the Haast Pass area 30 km east of the fault both of which have comparatively high strain rates that are difficult to reconcile with a single dislocation model for the Alpine Fault.
Geodetic strain measurements along the Moonlight Fault from western Southland to Haast Pass are consistent with dextral strike-slip combined with a variable amount of reverse movement on the fault. If the measured geodetic shear strains are all associated with movement on the Moonlight Fault, then strike-slip rates would range between 4 mm/yr and 8 mm/yr combined with a variable amount of reverse movement.
In the northern half of the South Island, the strain rates associated with seismic activity over the last 150 years are comparable to geodetic strain rates and are in reasonable balance with relative plate velocities for the Pacific-Australian plate pair using either the NUVAL 1 or P071 plate models. In contrast, in the central South Island, historic seismicity represents only a small fraction ( <10%) of geodetic strain rates or relative plate velocities.
Throughout the Macquarie Ridge, the orientation of the principal axes of strain calculated from the last 13 years of seismic activity are in excellent agreement with predictions from the NUVAL-l plate model. The agreement with the P071 plate model is significantly poorer. In the South Island of New Zealand, the situation is reversed with the P071 model being supported by both the seismic and geodetic data.
All of the crustal seismic activity with magnitudes greater than 5.5 in the South Island of New Zealand is consistent with a principal axis of compressive stress oriented at 120°-140°. In contrast recent seismic activity in the Macquarie Ridge area, excluding the aftershocks of the 1989 magnitude 8.2 Macquarie Ridge earthquake, is consistent with a principal axis of compressive stress oriented at 80°.
The orientation of the principal strains calculated from geodetic techniques over most of the South Island is compatible with oblique (dextral strike-slip and reverse) movement on the Alpine Fault and reverse movement on the northeast trending faults east of the main divide. In Central Otago, the principal axis of relative contraction is oriented at 45° to the major structures, which is consistent with strike slip movement along these structures. This can be reconciled with the apparently reverse nature of the faults by recognising that reverse movement on a series of orthogonal northeast and northwest striking thrust fault can produce a uniform dilatation which would not be detected with the geodetic techniques available in the South Island. The model also involves some combination of dextral strike-slip movement on the northeast trending faults and sinistral strike-slip movement on the northwest trending faults. If all of the strains observed in central Otago are to be explained by this model, the rate of strike-slip movement on the faults would have to be 4-8mm/yr. There is currently no evidence for strike-slip rates of this magnitude in central Otago, suggesting that some of the strain is ultimately released on structures outside the region.
Dislocation models of strains near the Alpine Fault near Okarito require at least 24 mm/yr strike-slip and 10 mm/yr dip-slip movement on the fault to obtain a satisfactory match with the observed strains. The best-fit locking depth is 8 km, in satisfactory agreement with estimates of the brittle-ductile transition in the area. At Haast Pass, the best-fit dislocation model suggests about 16-18 mm/yr of strike-slip and 4-6 mm/yr of dip-slip movement on the fault. The locking depth is 3-4 km. This model is unable to account for high shear strain rates on both sides of the fault, suggesting that about half of the deformation in the Haast area takes place off the Alpine Fault. Possible locations for the extra-fault deformation are the coastal planes immediately west of the fault and the Haast Pass area 30 km east of the fault both of which have comparatively high strain rates that are difficult to reconcile with a single dislocation model for the Alpine Fault.
Geodetic strain measurements along the Moonlight Fault from western Southland to Haast Pass are consistent with dextral strike-slip combined with a variable amount of reverse movement on the fault. If the measured geodetic shear strains are all associated with movement on the Moonlight Fault, then strike-slip rates would range between 4 mm/yr and 8 mm/yr combined with a variable amount of reverse movement.
In the northern half of the South Island, the strain rates associated with seismic activity over the last 150 years are comparable to geodetic strain rates and are in reasonable balance with relative plate velocities for the Pacific-Australian plate pair using either the NUVAL 1 or P071 plate models. In contrast, in the central South Island, historic seismicity represents only a small fraction ( <10%) of geodetic strain rates or relative plate velocities.
Throughout the Macquarie Ridge, the orientation of the principal axes of strain calculated from the last 13 years of seismic activity are in excellent agreement with predictions from the NUVAL-l plate model. The agreement with the P071 plate model is significantly poorer. In the South Island of New Zealand, the situation is reversed with the P071 model being supported by both the seismic and geodetic data.
All of the crustal seismic activity with magnitudes greater than 5.5 in the South Island of New Zealand is consistent with a principal axis of compressive stress oriented at 120°-140°. In contrast recent seismic activity in the Macquarie Ridge area, excluding the aftershocks of the 1989 magnitude 8.2 Macquarie Ridge earthquake, is consistent with a principal axis of compressive stress oriented at 80°.
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351 p. ; ill., maps (some col.) ; 30 cm.
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1991Pearson
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Citation
Pearson, Christopher, 1951-, “Study of seismic and geodetic strains along the Pacific-Australian plate boundary through the South Island of New Zealand,” Otago Geology Theses, accessed February 7, 2025, https://theses.otagogeology.org.nz/items/show/255.