Seismogenic processes on the subduction thrust - Part 1 - Hikurangi Subduction Zone, and Part 2 - Alpine Fault

The passive and active-source Seismic Array HiKurangi Experiment (SAHKE) investigated the structure of the forearc and subduction plate boundary beneath southern North Island along a 350 km transect. We used 92 onshore seismographs and 20 ocean-bottom seismographs to record >69,000 shots of a 400 km marine multichannel seismic-reflection survey, and then recorded 12 borehole explosive sources (350-500 kg) distributed approximately 8 km apart on 835 onshore seismographs. Tomographic inversion of first arrival travel times was used to derive a P-wave image of the crust that is resolved to 20-25 km depth, which was combined with refracted phases and migrated reflection events to image subducting slab geometry and crustal structure. Offshore, the subducted Pacific Plate oceanic crust is recognized from its high positive velocity gradient, but it becomes less distinct beneath the onshore Tararua Ranges, where the interface increases in dip at about 15 km depth from <5° to >15° farther west. This bend in the subducted plate is associated with vertical clusters in seismicity, splay fault branching above, and low-velocity high-attenuation material that we interpret to be underplated subduction channel material. We infer a step down in the decollément that transfers slip on the plate interface to the top of subducted oceanic crust. We suggest that this drives local uplift of the Tararua Range, and corresponds to an important lateral change in hydrological conditions at the plate interface. The change in dip of the Hikurangi subduction interface at 10–20 km depth is also spatially correlated with the transition from geodetically determined locked to unlocked areas of the plate interface and partitions stable and unstable slip regimes.

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Technical Abstract - Part 2

Tectonic tremor is characterized by persistent, low-frequency seismic energy seen at major plate boundaries. Although predominantly associated with subduction zones, tremor also occurs along the deep extension of the strike-slip San Andreas Fault. Here we present the first observations of tectonic tremor along New Zealand’s Alpine Fault, a major transform boundary that is late in its earthquake cycle. We report tectonic tremor that occurred on the central section of the Alpine Fault on 12 days between March 2009 and October 2011.

Tremor hypocenters concentrate in the lower crust at the downdip projection of the Alpine Fault; coincide with a zone of high P-wave attenuation (low Qp) and bright seismic reflections; occur in the 25–45 km depth range, below the seismogenic zone; and may define the deep plate boundary structure extending through the lower crust and into the upper mantle. We infer this tremor to represent slow slip on the deep extent of the Alpine Fault in a fluid-rich region marked by high attenuation and reflectivity.

These observations provide the first indication of present-day displacement on the lower crustal portion of the Australia–Pacific transform plate boundary.