A geodetic investigation of slow slip in the Hikurangi subduction zone beneath Raukumara Peninsula, New Zealand

Abstract

In October 2002, a slow slip event of 20-30 mm surface displacement was observed on two continuous Global Positioning (GPS) instruments near Gisborne, North Island, New Zealand. This event was the first of its kind to be documented with continuous GPS in New Zealand. We have processed and analysed nearly ten years of regional campaign GPS records (1995-2004), which we use, in conjunction with recent continuous GPS and broadband seismic data, to study the spatial extent of the Gisborne 2002 slow slip event and its effect on regional deformation. We find that infrequent sampling in the campaign GPS data set has aliased the Gisborne 2002 slow slip event and we cannot constrain the spatial extent and moment release of this event any further than with the continuous GPS data. We successfully use campaign and continuous GPS time series from neighbouring stations to make estimates of the recurrence interval for events of similar surface displacement to the 2002 slow slip occurring in the Gisborne region. Our calculations show that such events recur at 2-4 year intervals, which is in agreement with the recent November 2004 slow slip event in the Gisborne region. Campaign GPS time series and regional deformation fields show evidence for more than one slow slip event occurring in the Gisborne area prior to 2001. Forward modelling shows that the surface displacements recorded during the Gisborne 2002 slow slip event are the result of 18 cm of slip on a plane, ca. 60 km x 25 km, on the subduction interface offshore of Gisborne, though the fault length is poorly constrained. The Gisborne 2002 slow slip event is shown to have had no significant effect on inter-seismic coupling and slip deficit on the Hikurangi subduction margin, or on regional deformation patterns over the Raukumara Peninsula. Our model of slip suggests that slow slip beneath the Raukumara Peninsula currently occurs in episodic events involving local slip at the base of the seismogenic zone on the subduction interface offshore of Gisborne. Our studies using regional data sets also indicate that effects of the Gisborne 2002 slow slip event were localised. We have made a preliminary study into the possible association of seismic tremor with the slow slip events recorded on the Raukumara Peninsula. We find evidence for tremor recorded on multiple stations across the Raukumara Peninsula broadband seismic network during both the 2004 Gisborne slow slip event, but we have not rigorously examined the spatial and temporal relationship of this tremor to the slow slip event.