Tuesday 23 August 2005

G3
1000-1230 hours

125
Aseismic slip on the Hikurangi subduction interface, New Zealand
Beavan, John¹, Wallace, Laura¹, Douglas, Anne², Fletcher, Hilary¹, Townend, John²
1 GNS, Lower Hutt, New Zealand
2 Victoria University of Wellington, Wellington, New Zealand
Author email: j.beavan@gns.cri.nz
In common with other regions where continuous GPS networks have been established above subduction zones, a number of aseismic deformation episodes have been observed in New Zealand since 2002. We interpret these episodes as resulting from aseismic slip on the subduction interface, though with the current density of continuous GPS stations the details of the events recorded to date are not well resolved. We have observed events with accompanying surface displacements ranging from 5-30 mm magnitude, and several days to more than a year duration. Modelling suggests that the events are occurring near the down-dip end of the locked seismogenic part of the subduction zone, in the transition zone between the interseismically coupled and creeping portions of the interface. Comparison of event sizes, inter-event deformation rates, and long-term deformation rates suggest a repeat time of 2-4 years for an October 2002 event recorded near Gisborne in the northern Hikurangi margin. A similar-sized event recorded in November 2004 lends credence to this estimate. A longer duration, currently ongoing slow slip event beneath the central North Island may have triggered a series of small to moderate earthquakes earlier this year. There is preliminary indication of seismic tremor associated with the Gisborne events, as has been observed in Japan and Cascadia, but more work is needed to confirm this.

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