John Beavan

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

[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.

Contemporary horizontal velocity and strain rate fields of the Pacific‐Australian plate boundary zone through New Zealand

We have inverted velocity solutions from nine geodetic networks distributed across New Zealand to derive present-day continuous horizontal velocity and strain rate fields at the Earths surface throughout the country. The nine networks contain a total of 362 Global Positioning System (GPS) stations that have been observed at least twice and at least a year apart between 1991 and 1998. The model velocity field is expanded as bicubic spline interpolation functions defined within a curvilinear grid that covers the country and extends into the assumed rigid Australian and Pacific plates to west and east. The inversion jointly minimizes the magnitudes of fitted strain rates and the misfit to the observed velocity data. The spline technique allows high spatial resolution of strain rate variations, especially in regions with spatially dense GPS data. Expansion of the model velocity field on the surface of a sphere allows arbitrarily large areas to be studied. Previously known aspects of New Zealand plate boundary deformation are highlighted with improved resolution, including back-arc extension in the Taupo Volcanic Zone, rotation of the Hikurangi forearc away from this zone in the North Island, and a band of high shear strain rate under the Southern Alps to the southeast of the Alpine fault. Our results reveal several new features, including a region of enhanced shear straining apparently associated with strike-slip faults in the southern North Island and a band of contractional straining subparallel and well east of the Alpine fault that is similar to features found in numerical and sandbox models of continental collision.

Crustal deformation during 1994–1998 due to oblique continental collision in the central Southern Alps, New Zealand, and implications for seismic potential of the Alpine fault

The positions of 115 ground marks in a 150 × 100 km area of oblique continental collision in the central Southern Alps, New Zealand, have been measured by Global Positioning System (GPS) two to four times between 1994 and 1998. Contemporary velocity and strain rate fields derived from these observations are largely invariant along the northeasterly strike of the mountains and Alpine fault. Across strike, more than 60% of the strain occurs within a band from 5 km NW to 20 km SE of the Alpine fault, but significant strain continues at least a further 60 km SE to near the edge of the Southern Alps foothills. Projections of the fault-parallel and fault-normal components of velocity onto an Alpine faultnormal profile show that about 85% of the NUVEL-1A model relative plate motion is observed within the GPS network. The surface displacements in the high strain rate region are well fit by a model in which stable slip or shearing is occurring at 50–70% of the relative plate rate in a region deeper than about 5–8 km on the down-dip extension of the SE dipping Alpine fault. Material shallower than this is behaving elastically and thus storing elastic strain in the region of the Alpine fault. The longer-wavelength displacements can be modeled either as distributed deformation beneath the Southern Alps, or by localization of elastic strain around the upper end of a discrete NW dipping fault or shear zone that is slipping stably below about 30 km depth and would outcrop near the SE boundary of the mountains if extrapolated to the surface. Strain determined from a small-scale survey network crossing the Alpine fault indicates no significant near-surface aseismic fault slip on the central Alpine fault over the past 25 years. Our results are consistent with independent geological evidence that the central section of the Alpine fault is capable of producing large to great earthquakes.

Plate boundary deformation in South Island, New Zealand, is related to inherited lithospheric structure

Abstract The Alpine Fault is the main active structure in the oblique continental collision zone of South Island, New Zealand. It is continuous at the surface for ∼800 km and accommodates ∼70% of current plate motion. A 460 km offset of basement rocks suggests it has accommodated >50% of plate displacement since 45 Ma. Geophysical, geological, and contemporary kinematic data are successfully modeled by slip on a narrow Alpine Fault zone extending into the lower crust. The data are consistent with plate boundary deformation occurring on either northwest- or southeast-dipping shear zones in the upper mantle, or with widely distributed strain in the upper mantle. We propose that the position of the Alpine Fault and any associated lithospheric discontinuity is controlled by an Eocene passive margin that separated Palaeozoic continental lithosphere of the Challenger Plateau from much younger oceanic lithosphere. Strike-slip motion since ∼25 Ma has translated the passive margin into the continental collision zone, resulting in subduction of oceanic lithosphere beneath South Island and progressive localisation of shear strain near the Alpine Fault. The Eocene rift boundary formed by exploiting Cretaceous oceanic transform faults and an older discontinuity within the New Zealand continent. We propose that a precursor to the Alpine Fault could be as old as Palaeozoic. Inherited structure has clearly controlled the first-order deformation pattern through New Zealand since at least 100 Ma and demonstrates that ancient faults can play an important role in determining lithospheric-scale patterns of deformation at continental plate boundaries.

The Mw 6.2 Christchurch earthquake of February 2011: preliminary report

Abstract A moment magnitude (Mw) 6.2 earthquake struck beneath the outer suburbs of Christchurch, New Zealands second largest city, on 22 February 2011 local time. The Christchurch earthquake was the deadliest in New Zealand since the 1931 Mw 7.8 Hawkes Bay earthquake and the most expensive in New Zealands recorded history. The effects of the earthquake on the regions population and infrastructure were severe including 181 fatalities, widespread building damage, liquefaction and landslides. The Christchurch earthquake was an aftershock of the Mw 7.1 Darfield Earthquake of September 2010, occurring towards the eastern edge of the aftershock zone. This was a low recurrence earthquake for New Zealand and occurred on a fault unrecognised prior to the Darfield event. Geodetic and seismological source models show that oblique-reverse slip occurred along a northeast–southwest-striking fault dipping southeast at c. 69°, with maximum slip at 3–4 km depth. Ground motions during the earthquake were unusually large at near-source distances for an earthquake of its size, registering up to 2.2 g (vertical) and 1.7 g (horizontal) near the epicentre and up to 0.8 g (vertical) and 0.7 g (horizontal) in the city centre. Acceleration response spectra exceeded 2500 yr building design codes and estimates based on standard New Zealand models. The earthquake was associated with high apparent stress indicative of a strong fault. Furthermore, rupture in an updip direction towards Christchurch likely led to strong directivity effects in the city. Site effects including long period amplification and near-surface effects also contributed to the severity of ground motions.

Fault slip models of the 2010–2011 Canterbury, New Zealand, earthquakes from geodetic data and observations of postseismic ground deformation

Abstract We present source models derived from geodetic data for the four major Canterbury earthquakes of 2010–2011. The September 2010 Darfield earthquake was largely right-lateral, but with several other fault segments active. The February 2011 Christchurch earthquake was mixed right-lateral and reverse with a left-stepping offset interrupting an ENE-striking rupture. The June 2011 earthquake included left-lateral slip on a NNW-striking fault. The December 2011 earthquakes were characterised by offshore reverse slip on an ENE-striking plane. Displacements of GPS sites define small but clearly detectable postseismic deformation east of the September 2010 earthquake, near the February 2011 earthquake and following the June 2011 earthquake. There has been no major moment release in a 15-km-long region between the eastern end of the September 2010 faulting and the western end of the February 2011 faulting. We recommend careful monitoring of this region for the next several years.

Simultaneous long‐term and short‐term slow slip events at the Hikurangi subduction margin, New Zealand: Implications for processes that control slow slip event occurrence, duration, and migration

[1] We document a sequence of simultaneous short-term and long-term slow slip events (SSEs) at the Hikurangi subduction zone during the 2010/2011 period. The sequence of short-term events (each 2–3 weeks in duration) ruptured much of the shallow plate interface (<15 km) at central and northern Hikurangi over a 6-month period, was accompanied by microseismicity and involved patchy, irregular migration of SSE slip. We suggest that the patchy migration of the short-term SSE is due to large-scale ( 100–3500 km) heterogeneities on the plate interface related to seamount subduction and sediment subduction and/or underplating. This is in contrast to a 2010/2011 long-term SSE at the central Hikurangi margin, which evolved steadily over 1.5 years and ruptured much of the plate interface between 20 and 70 km depth. We suggest that the occurrence of long-term versus short-term SSEs at Hikurangi is related to differences in effective normal stresses and relative heterogeneity of the subduction interface. The long-term SSE sequence began 1 year before the short-term sequence. Coulomb stress change models suggest that the long-term SSE may have triggered initiation of the subsequent short-term SSE sequence. Initiation of the short-term sequence occurred in a region just updip of or within an interseismically locked portion of the plate interface and may be located within the updip transition from seismic to aseismic behavior. Alternatively, it could be characteristic of a region undergoing partial interseismic coupling. This is in contrast to SSEs observed elsewhere in the world that typically occur within the downdip transition from seismic to aseismic behavior.

Rapid microplate rotations and backarc rifting at the transition between collision and subduction

Using global positioning system velocities from convergent plate boundaries in Papua New Guinea, New Zealand, Tonga, Vanuatu, and the Marianas, we note a spatial correlation between rapid tectonic block rotations and the transition from subduction to collision. We present a mechanism for the block rotations, in which the change from collision of a buoyant indentor to normal subduction exerts a torque on the upper-plate microplate. This work improves our understanding of the causes of rapid vertical axis rotations, often observed in paleomagnetic studies. We also show how collision-induced rotations may lead to backarc rifting.

Crustal deformation of the Marlborough Fault Zone in the South Island of New Zealand: Geodetic constraints over the interval 1982–1994

Crustal deformation across the Marlborough fault zone in the South Island of New Zealand has been investigated by resurveying with the Global Positioning System (GPS) a triangulation and trilateration network across part of the zone. The principal strain rates, which mostly have errors between 5% and 10%, vary systematically across the region. The principal axis of horizontal contraction gradually swings from SE-NW southeast of the Hope fault to almost east-west in the west. The component of velocity parallel to the strike of the faults can account for 95% of the relative motion between the Australian and Pacific plates. This component exhibits little variation west of the Alpine (Wairau) fault, and shows an almost linear variation from the Wairau fault to the east coast. The horizontal strain rate northwest of the Wairau fault is a small east-west uniaxial contraction (∼0.1 ppm yr−1). If it is assumed that the crustal blocks bounded by the major faults are primarily driven by basal shear tractions, with stresses on the faults being relatively insignificant, then the slip rates expected on the faults may be derived from the observed velocity variations across the fault zone. These predicted slip rates are in good agreement with geologically observed slip rates. The larger slip rates on the Hope fault are related to the observation that the strain field extends southeast of the fault over a distance large compared with the spacing between the faults. The agreement between predicted and geological slip rates lends support to the idea that the observed pattern of deformation reflects that in the lower lithosphere.

Enigmatic, highly active left-lateral shear zone in southwest Japan explained by aseismic ridge collision

Global positioning system (GPS) site velocities and earthquake focal mechanisms reveal an active left-lateral shear zone cutting across Kyushu in southwest Japan. Surprisingly, no active faults have been identified in association with this zone of rapid contemporary deformation. To explain the existence of this shear zone, we propose a model comprising subduction of an aseismic ridge (Kyushu-Palau Ridge) at the southwest end of the Nankai Trough. Because of rapid (~40 mm/yr) along-strike migration of the ridge, we suggest that the ridge subduction point (and resulting left-lateral shear zone) is never in one place long enough to enable the development of a through-going fault zone that can be identified at the ground surface, reconciling the mismatch between the GPS, seismological, and geological data in this region. Our conceptual model is supported by numerical modeling results. We also suggest that the along-strike change in subducting plate buoyancy explains the recent counterclockwise rotation of the Kyushu forearc documented in paleomagnetic studies, as is found in many other western Pacific subduction margins.