Laura M. Wallace

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.

National Seismic Hazard Model for New Zealand: 2010 Update

A team of earthquake geologists, seismologists, and engineering seis- mologists has collectively produced an update of the national probabilistic seismic hazard (PSH) model for New Zealand (National Seismic Hazard Model, or NSHM). The new NSHM supersedes the earlier NSHM published in 2002 and used as the hazard basis for the New Zealand Loadings Standard and numerous other end-user applica- tions. The new NSHM incorporates a fault source model that has been updated with over 200 new onshore and offshore fault sources and utilizes new New Zealand-based and international scaling relationships for the parameterization of the faults. The dis- tributed seismicity model has also been updated to include post-1997 seismicity data, a new seismicity regionalization, and improved methodology for calculation of the seismicity parameters. Probabilistic seismic hazard maps produced from the new NSHM show a similar pattern of hazard to the earlier model at the national scale, but there are some significant reductions and increases in hazard at the regional scale. The national-scale differences between the new and earlier NSHM appear less than those seen between much earlier national models, indicating that some degree of consis- tency has been achieved in the national-scale pattern of hazard estimates, at least for return periods of 475 years and greater. Online Material: Table of fault source parameters for the 2010 national seismic- hazard model.

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

[1]xa0The 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.

Slow slip on the northern Hikurangi subduction interface, New Zealand

[1]xa0In October 2002, a surface displacement episode of 20–30 mm magnitude was observed over a ∼10 day period on two continuous Global Positioning System (GPS) instruments near Gisborne, North Island, New Zealand. We interpret this to result from slow slip on the northern Hikurangi subduction interface. Using ten years of regional campaign GPS (1995–2004) and recent continuous GPS data, we estimate the recurrence interval for similar events to be 2–3 yrs. In November 2004, a similar slow slip event occurred within this recurrence period. The 2002 event can be modeled by ∼18 cm of slow slip near the down-dip end of the seismogenic zone on the subduction interface offshore of Gisborne. The campaign GPS data show that the 2002 slow slip event had little effect on regional strain patterns.

GPS and seismological constraints on active tectonics and arc-continent collision in Papua New Guinea: Implications for mechanics of microplate rotations in a plate boundary zone

[1]xa0The island of New Guinea is located within the deforming zone between the Pacific and Australian plates that converge obliquely at ∼110 mm/yr. New Guinea has been fragmented into a complex array of microplates, some of which rotate rapidly about nearby vertical axes. We present velocities from a network of 38 Global Positioning System (GPS) sites spanning much of the nation of Papua New Guinea (PNG). The GPS-derived velocities are used to explain the kinematics of major tectonic blocks in the region and the nature of strain accumulation on major faults in PNG. We simultaneously invert GPS velocities, earthquake slip vectors on faults, and transform orientations in the Woodlark Basin for the poles of rotation of the tectonic blocks and the degree of elastic strain accumulation on faults in the region. The data are best explained by six distinct tectonic blocks: the Australian, Pacific, South Bismarck, North Bismarck, and Woodlark plates and a previously unrecognized New Guinea Highlands Block. Significant portions of the Ramu-Markham Fault appear to be locked, which has implications for seismic hazard determination in the Markham Valley region. We also propose that rapid clockwise rotation of the South Bismarck plate is controlled by edge forces initiated by the collision between the Finisterre arc and the New Guinea Highlands.

Do Great Earthquakes Occur on the Alpine Fault in Central South Island, New Zealand?

Geological observations require that episodic slip on the Alpine fault averages to a long-term displacement rate of 2-3 cm/yr. Patterns of seismicity and geodetic strain suggest the fault is locked above a depth of 6-12 km and will probably fail during an earthquake. High pore-fluid pressures in the deeper fault zone are inferred from low seismic P-wave velocity and high electrical conductivity in central South Island, and may limit the seismogenic zone east of the Alpine fault to depths as shallow as 6 km. A simplified dynamic rupture model suggests an episode of aseismic slip at depth may not inhibit later propagation of a fully developed earthquake rupture. Although it is difficult to resolve surface displacement during an ancient earthquake from displacements that occurred in the months and years that immediately surround the event, sufficient data exist to evaluate the extent of the last three Alpine fault ruptures: the 1717 AD event is inferred to have ruptured a 300-500 km length of fault; the 1620 AD event ruptured 200-300 km; and the 1430 AD event ruptured 350-600 km. The geologically estimated moment magnitudes are 7.9 ± 0.3, 7.6 ± 0.3, and 7.9 ± 0.4, respectively. We conclude that large earthquakes (Mw >7) on the Alpine fault will almost certainly occur in future, and it is realistic to expect some great earthquakes (Mw ≥8).

Characterizing the seismogenic zone of a major plate boundary subduction thrust: Hikurangi Margin, New Zealand

The Hikurangi subduction margin, New Zealand, has not experienced any significant (>Mw 7.2) subduction interface earthquakes since historical records began ∼170 years ago. Geological data in parts of the North Island provide evidence for possible prehistoric great subduction earthquakes. Determining the seismogenic potential of the subduction interface, and possible resulting tsunami, is critical for estimating seismic hazard in the North Island of New Zealand. Despite the lack of confirmed historical interface events, recent geodetic and seismological results reveal that a large area of the interface is interseismically coupled, along which stress could be released in great earthquakes. We review existing geophysical and geological data in order to characterize the seismogenic zone of the Hikurangi subduction interface. Deep interseismic coupling of the southern portion of the Hikurangi interface is well defined by interpretation of GPS velocities, the locations of slow slip events, and the hypocenters of moderate to large historical earthquakes. Interseismic coupling is shallower on the northern and central portion of the Hikurangi subduction thrust. The spatial extent of the likely seismogenic zone at the Hikurangi margin cannot be easily explained by one or two simple parameters. Instead, a complex interplay between upper and lower plate structure, subducting sediment, thermal effects, regional tectonic stress regime, and fluid pressures probably controls the extent of the subduction thrusts seismogenic zone.

The kinematics of a transition from subduction to strike‐slip: An example from the central New Zealand plate boundary

[1]xa0We develop a kinematic model for the transition from subduction beneath the North Island, New Zealand, to strike-slip in the South Island, constrained by GPS velocities and active fault slip data. To interpret these data, we use an approach that inverts the kinematic data for poles of rotation of tectonic blocks and the degree of interseismic coupling on faults in the region. Convergence related to the Hikurangi subduction margin becomes very low offshore of the northern South Island, indicating that in this region the majority of the relative plate motion has been transferred onto faults within the upper plate, as suggested by previous studies. This result has implications for understanding the likely extent of subduction interface earthquake rupture in central New Zealand. Easterly trending strike slip faults (such as the Boo Boo fault) are the key features that facilitate the transfer of strike-slip motion from the northern South Island faults further north into the southern North Island and onto the Hikurangi subduction thrust. Our results also indicate that the transition from rapid forearc rotation adjacent to the Hikurangi subduction margin to a strike-slip dominated plate boundary (with negligible vertical-axis rotation) in the South Island occurs via a crustal-scale hinge or kink in the upper plate, compatible with paleomagnetic and structural geological data. Despite the ongoing tectonic evolution of the central New Zealand region, our study highlights a remarkable consistency between data sets spanning decades (GPS), thousands of years (active faulting data), and millions of years (paleomagnetic data and bedrock structure).

Tectonic evolution of the active Hikurangi subduction margin, New Zealand, since the Oligocene

[1]xa0Deformation across the active Hikurangi subduction margin, New Zealand, including shortening, extension, vertical-axis rotations, and strike-slip faulting in the upper plate, has been estimated for the last ∼24 Myr using margin-normal seismic reflection lines and cross sections, strike-slip fault displacements, paleomagnetic declinations, bending of Mesozoic terranes, and seafloor spreading information. Post-Oligocene shortening in the upper plate increased southward, reaching a maximum rate of 3–8 mm/year in the southern North Island. Upper plate shortening is a small proportion of the rate of plate convergence, most of which (>80%) accrued on the subduction thrust. The uniformity of these shortening rates is consistent with the near-constant rate of displacement transfer (averaged over ≥5 Myr) from the subduction thrust into the upper plate. In contrast, the rates of clockwise vertical-axis rotations of the eastern Hikurangi Margin were temporally variable, with ∼3°/Myr since 10 Ma and ∼0°–1°/Myr prior to 10 Ma. Post 10 Ma, the rates of rotation decreased westward from the subduction thrust, which resulted in the bending of the North Island about an axis at the southern termination of subduction. With rotation of the margin and southward migration of the Pacific Plate Euler poles, the component of the margin-parallel relative plate motion increased to the present. Plate convergence dominated the Hikurangi Margin before ca. 15 Ma, with the rate of margin-parallel motion increasing markedly since 10 Ma. Vertical-axis rotations could accommodate all margin-parallel motion before 1–2 Ma, eliminating the requirement for large strike-slip displacements (for example, >50 km) in the upper plate since the Oligocene.

A large slow slip event on the central Hikurangi subduction interface beneath the Manawatu region, North Island, New Zealand

[1]xa0We present continuous GPS observations of a surface deformation episode in the Manawatu region of the North Island, New Zealand, lasting 18 months from January 2004 to June 2005. We invert vertical and horizontal displacements of GPS sites during the event for slow slip on the Hikurangi subduction thrust. We find that up to 350 mm of slip occurred on the thrust in the transition zone between strong interseismic coupling and aseismic creep. If the entire slip in the Manawatu slow slip event had occurred instantaneously, it would have resulted in an Mw 7.0 earthquake. Given the large amount of slip, we expect that the recurrence interval for similar slow slip events may be ten years or more.