Imaging the Hikurangi Plate interface region, with improved local‐earthquake tomography

• Published in: Geophysical journal international 2012-08, Vol.190 (2), p.1221-1242
• Main Authors: Eberhart‐Phillips, Donna, Reyners, Martin
• Format: Article
• Language: English
• Subjects: Computational fluid dynamics; Seismic tomography; Seismicity and tectonics; Subduction zone processes
• ISSN: 0956-540X; 1365-246X
• DOI: 10.1111/j.1365-246X.2012.05553.x

SUMMARY The properties of the plate interface region influence plate coupling and rupture behaviour during large earthquakes. Plate coupling varies greatly along the Hikurangi subduction zone in the southern half of the North Island, New Zealand, and heterogeneous material properties can be examined with the well‐recorded seismicity. For this study, we have used a modified velocity inversion to incorporate earthquake differential times from selected groups of distributed earthquakes, which improves the spatial resolution of features in the lower crust, and sharpens velocity gradients. Data are selected from temporary deployments and the permanent GeoNet network, which has expanded in the last decade. The resulting 3‐D velocity model shows that the overlying plate exhibits patterns related to geologic terranes. The Rakaia terrane has low Vp/Vs and high Vp, and is spatially related to the zone of strong plate coupling. Seismicity occurs throughout the overlying plate, and extends to the plate interface without a lower crustal aseismic zone. The Wairarapa–Waewaepa fault zone may form the updip limit of strong coupling in future plate interface earthquakes. The crust of the subducting plate is characterized by abundant seismicity and is bounded by strong velocity gradients. Low‐velocity zones above the plate interface are indicated from 30 to 50 km depth. Seismic velocities near the plate interface show an excellent correlation with the distribution of plate coupling, and provide insight into what controls such coupling. The plate interface has the highest Vp/Vs (>1.85) and sharpest Vp/Vs gradient in the region of strongest coupling, consistent with the suggestion that strong coupling is related to the inability of fluid to cross the plate interface. In regions of recurrent slow slip, Vp/Vs is still high, but the gradient of Vp/Vs near the plate interface is much broader, suggesting movement of fluid across the plate interface. The area of deep slow slip corresponds to the most extensive high Vp/Vs mantle region above the slab.