S U M M A R YTwo types of seismic velocity heterogeneity are often observed in a given region: pronounced patterns of anisotropy shown by local earthquake shear wave splitting and seismic velocity structure imaged by local earthquake traveltimes. We seek to combine these types of observations by including anisotropy in 3-D velocity inversion. The fast polarization directions from local earthquake shear wave splitting are used to define an initial anisotropy model. The ray paths are estimated for each of the shear wave splitting observations and the 3-D initial model is determined via an averaging process and a coherence analysis. The initial magnitude of anisotropy at a node is dependent on the number of adjacent ray paths and the consistency of the shear wave splitting anisotropy measurements. The traveltime inversion is parametrized with an isotropic component and two azimuthal anisotropy parameters for each node. The ray paths are defined with approximate 3-D ray tracing, and for each point along a ray path, the velocity is defined as a function of the ray path azimuth at that point. A blended approach is used to damp the perturbations to the magnitude of anisotropy and/or perturbations to the fast direction.Tests with synthetic traveltime data show that the anisotropic velocity inversion method is reliable. Application to the Marlborough region shows variations in anisotropy in the brittle crust, ductile lower crust, mantle and subducted slab. The patterns are consistent with the shear wave splitting observations, but show some additional features. The maximum anisotropy is approximately 12 per cent oriented northeast, between the Awatere and Wairau faults. This high anisotropy may reflect deformation and extensive fracturing in a region of high total shear strain. In the region of interaction of the ductile lower crust with the shallow subducting slab, moderate anisotropy with east-west orientation is imaged. This is consistent with ductile deformation roughly parallel to the plate velocity.

No abstract

[1] Long-and short-term strain variations along the Australian-Pacific plate boundary through the South Island of New Zealand, including a 300% increase in orogen width, coexistence of oblique thrusting on orthogonal structures, and variability in the locus of orogenic gold deposits, coincide with rheologically relevant geological variation. Our model investigates the consequences of thin, strong lower crust in the north and thick, weak lower crust in the south. Solution of the full 3-D mechanical equations reproduces the larger wavelength strain patterns of the orogen. A 3-D perturbation-based analytical solution leads to the identification of the sensitivity of displacement type to minor stress changes. Transition from boundary-normal thrusting to boundary-parallel thrusting occurs at the transition from strong to weak lower crust and is related to an increase in either t yz (shear stress in the yz plane) or the ratio of the coordinate normal stresses, (s yy /s xx ), where x and y are in the horizontal and z is vertical. Both mechanisms are compatible with the geologically dependent rheological variation employed in our model.

Determining the rates and distributions of microseismicity near major faults at different points in the seismic cycle is a crucial step toward understanding plate boundary seismogenesis. We analyze data from temporary seismic arrays spanning the central section of the Alpine Fault, New Zealand, using doubledifference seismic tomography. This portion of the fault last ruptured in a large earthquake in 1717 AD and is now late in its typical 330 year cycle of Mw8 earthquakes. Seismicity varies systematically with distance from the Alpine Fault: (1) directly beneath the fault trace, earthquakes are sparse and largely confined to the footwall at depths of 4-11 km; (2) at distances of 0-9 km southeast of the trace, seismicity is similarly sparse and shallower than 8 km; (3) at distances of 9-20 km southeast of the fault trace, earthquakes are much more prevalent and shallower than 7 km. Hypocenter lineations here are subparallel to faults mapped near the Main Divide of the Southern Alps, confirming that those faults are active. The region of enhanced seismicity is associated with the highest topography and a high-velocity tongue doming at 3-5 km depth. The low-seismicity zone adjacent to the Alpine Fault trace is associated with Vp and Vs values at midcrustal depths about 8 and 6% lower than further southeast. We interpret lateral variations in seismicity rate to reflect patterns of horizontal strain rate superimposed on heterogeneous crustal structure, and the variations in seismicity cutoff depth to be controlled by temperature and permeability structure variations.

The Afar Depression, Ethiopia, offers unique opportunities to study the transition from continental rifting to oceanic spreading because the process is occurring onland. Using traveltime tomography and data from a temporary seismic deployment, we describe the first regional study of uppermost mantle P-wave velocities (V Pn). We find two separate low V Pn zones (as low as 7.2 km s −1) beneath regions of localized thinned crust in northern Afar, indicating the existence of high temperatures and, potentially, partial melt. The zones are beneath and off-axis from, contemporary crustal magma intrusions in active magmatic segments, the Dabbahu-Manda-Hararo and Erta'Ale segments. This suggests that these intrusions can be fed by off-axis delivery of melt in the uppermost mantle and that discrete areas of mantle upwelling and partial melting, thought to characterize segmentation of the uppermost mantle at seafloor spreading centres, are initiated during the final stages of break-up.