[1] The GPS velocity field in the North Island of New Zealand is dominated by the long-term tectonic rotation of the eastern North Island and elastic strain from stress buildup on the subduction zone thrust fault. We simultaneously invert GPS velocities, earthquake slip vectors, and geological fault slip rates in the North Island for the angular velocities of elastic crustal blocks and the spatially variable degree of coupling on faults separating the blocks. This approach allows us to estimate the distribution of interseismic coupling on the subduction zone interface beneath the North Island and the kinematics of the tectonic block rotations. In agreement with previous studies we find that the subduction zone interface beneath the southern North Island has a high slip rate deficit during the interseismic period, and the slip rate deficit decreases northward along the margin. Much of the North Island is rotating as several, distinct tectonic blocks (clockwise at 0.5-3.8 deg Myr -1 ) about nearby axes relative to the Australian Plate. This rotation accommodates much of the margin-parallel component of motion between the Pacific and Australian plates. On the basis of our estimation of the block kinematics we suggest that rotation of the eastern North Island occurs because of the southward increasing thickness of the subducting Hikurangi Plateau. These results have implications for our understanding of convergent margin plate boundary zones around the world, particularly with regard to our knowledge of mechanisms for rapid tectonic block rotations at convergent margins and the role of block rotations in the slip partitioning process.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2004JB003241

Subduction zone coupling and tectonic block rotations in the North Island, New Zealand

[1] The installation of a continuous GPS (cGPS) network in New Zealand since 2002 has enabled the discovery of 15 slow slip events at the Hikurangi subduction margin. Our analysis and interpretation of the cGPS data reveal a marked diversity in characteristics of slow slip events (SSEs) in the North Island, with durations varying from 6 days to 1.5 years, equivalent moment release between Mw 6.3–7.2, and recurrence intervals of repeating SSEs on the order of 2 years to more than 5 years. The duration and magnitude characteristics of Hikurangi SSEs appear to be related to the depth where they occur. The deepest, longest duration, and largest SSEs occur at the southern Hikurangi margin, near the downdip limit of deep (down to ∼40 km depth), strong interseismic coupling. The shallowest, shortest duration, smallest, and most frequent SSEs occur at the northern and central Hikurangi margin near the downdip limit of unusually shallow (<10–15 km depth) interseismic coupling. The direction of slip on the interface in the Hikurangi SSEs is consistent with slip partitioning at the Hikurangi subduction margin. We also show that moment accumulation rates on the interface for the time periods between SSEs are ∼40% higher than for the period of more than a decade averaging through the SSEs, indicating that SSEs compose a major portion of the overall moment release budget of the Hikurangi subduction interface.

Diverse slow slip behavior at the Hikurangi subduction margin, New Zealand

SUMMARY

We present the first New Zealand-wide study of surface wave dispersion, using ambient noise observed at 42 broad-band stations in the national seismic network (GeoNet) and the Global Seismic Network (GSN). Year-long vertical-component time-series recorded between 2005 April 1 and 2006 March 31 have been correlated with one another to yield estimated fundamental mode Rayleigh wave Green's functions. We filter these Green's functions to compute Rayleigh wave group dispersion curves at periods of 5–50 s, using a phase-matched filter, frequency–time analysis technique. The uncertainties of the measurements are estimated based on the temporal variation of the dispersion curves revealed by 12 overlapping 3-month stacks. After selecting the highest quality dispersion curve measurements, we compute group velocity maps from 7 to 25 s period. These maps, and 1-D shear wave velocity models at four selected locations, exhibit clear correlations with major geological structures, including the Taranaki and Canterbury Basins, the Hikurangi accretionary prism, and previously reported basement terrane boundaries.

/pdf/ambient-noise-rayleigh-wave-tomography-of-new-zealand-zeuyxgqumb.pdf

Ambient noise Rayleigh wave tomography of New Zealand

SUMMARY We present the first New Zealand-wide study of surface wave dispersion, using ambient noise observed at 42 broad-band stations in the national seismic network (GeoNet) and the Global Seismic Network (GSN). Year-long vertical-component time-series recorded between 2005 April 1 and 2006 March 31 have been correlated with one another to yield estimated fundamental mode Rayleigh wave Green’s functions. We filter these Green’s functions to compute Rayleigh wave group dispersion curves at periods of 5‐50 s, using a phase-matched filter, frequency‐time analysis technique. The uncertainties of the measurements are estimated based on the temporal variation of the dispersion curves revealed by 12 overlapping 3-month stacks. After selecting the highest quality dispersion curve measurements, we compute group velocity maps from 7 to 25 s period. These maps, and 1-D shear wave velocity models at four selected locations, exhibit clear correlations with major geological structures, including the Taranaki and Canterbury Basins, the Hikurangi accretionary prism, and previously reported basement terrane boundaries.

/pdf/ambient-noise-rayleigh-wave-tomography-of-new-zealand-34o0minmyc.pdf

Ambient Noise Rayleigh Wave Tomography of New Zealand

Taupor volcanic zone (TVZ) is the currently active volcanic arc and back-arc basin of the Taupo-Hikurangi arc-trench system, North Island, New Zealand. The volcanic arc is best developed at the southern (Tongariro volcanic centre) end of the TVZ, while on the eastern side of the TVZ it is represented mainly by dacite volcanoes, and in the Bay of Plenty andesite/dacite volcanoes occur on either side of the Whakatane graben. The back-arc basin is best developed in the central part of the TVZ and comprises bimodal rhyolite and high-alumina basalt volcanism. Widespread ignimbrite eruptions have occurred from this area in the past 0.6 Ma. Normal faults occur in both arc and back-arc basin. They are generally steeply dipping (>40°) and strike between 040° and 080°. In the back-arc basin, fault zones are en echelon and have the same trend as alignments of rhyolite domes and basalt vents. Open fissures have formed during historic earthquakes along some of the faults, and geodetic measurements on the north side of Lake Taupo suggest extension of 14±4 mm/year. In the Bay of Plenty and ML=6.3 earthquake occurred on 2 March 1987. Modelling of known structure in the area together with data derived from this earthquake suggests block faulting with faults dipping 45°±10° NW and a similar dip is suggested by seismic profiling of faults offshore of the Bay of Plenty where extension is estimated to be 5±2 mm/year. To the east of the TVZ, the North Island shear belt (NISB) is a zone of reverse-dextral, strike-slip faults, the surface expression of which terminates at the eastern end of the TVZ. On the opposite side of the TVZ in the offshore western Bay of Plenty and on line with the NISB is the Mayor Island fault belt. If the two fault belts were once continuous, as seems likely, strike-slip faults probably extend through the basement of the TVZ. When extension associated with the arc and back-arc basin is combined with these strike-slip faults, the resulting transtension provides a suitable tectonic environment for caldera formation and voluminous ignimbrite eruptions in the back-arc basin. The types of volcano in the TVZ are considered to be related to the source of magma and overlying crustal structure. Lavas of the arc are probably formed by a multistage process involving (1) subsolidus slab dehydration, (2) anatexis of the mantle wedge, (3) fractionation and minor crustal assimilation and (4) magma mixing. High-alumina basalts of the back-arc basin may be derived by partial melting of peridotite at the top of the mantle wedge, while rhyolitic magmas are thought to come from partial melting of lavas and subvolcanic reservoirs associated with the southern end of the Coromandel volcanic zone. Extreme thinning associated with transtension in the back-arc basin will favour the eruption of large-volume, gas-rich ignimbrites accompanied by caldera formation.

Structural control and origin of volcanism in the Taupo volcanic zone, New Zealand

Shallow morphology of the subducted Pacific plate along the Hikurangi margin, New Zealand

Unfolding the Wadati-Benioff zone in the Kermadec-New Zealand region

The nature of faulting that took place during the M L 5.9 Tikokino earthquake of 11 April 1993 has been determined using data from six temporary seismographs installed immediately after the event. The rupture initiated at 25 km depth, within the thrust zone between the subducted Pacific and overlying Australian plates. The earthquake had surprisingly few aftershocks, but those that did occur define a rupture zone which parallels the plate interface. When combined with the focal mechanism of the mainshock, this rupture zone indicates that the earthquake involved thrusting at the plate interface. The earthquake ruptured unilaterally to the south, and this explains the strong directivity seen in both strong motion accelerograph and seismograph records. Movement of the plate interface during the mainshock did not lead to significant triggering of other earthquakes in the subducted or overlying plates. A plausible explanation for the very few aftershocks is that the rupture initiated at an asperity at the plate interface and then propagated into subducted sediment lying in the conditionally stable frictional field. The nearby M L 6.1 Ashley Clinton earthquake of 1958, which also had a conspicuous absence of aftershocks, may have involved a similar process.

/pdf/the-tikokino-earthquake-of-11-april-1993-qsxd71i4hr.pdf

J.H. Ansell

Papers

Shallow morphology of the subducted Pacific plate along the Hikurangi margin, New Zealand

Unfolding the Wadati-Benioff zone in the Kermadec-New Zealand region

The Tikokino earthquake of 11 April 1993

The location and mislocation of subcrustal earthquakes

The decoupling of P and S waves in inhomogeneous media