Seismic Structure Beneath the Wellington Region from Receiver Functions

<p>Seismic velocity structures, interpreted as being associated with the Hikurangi subduction system beneath the lower North Island of New Zealand, are imaged using stacked P wave receiver functions computed using teleseismic earthquakes. Receiver functions are a seismological technique that exploits the phenomenon of wave conversion. The upcoming P wave interacts with seismic velocity impedance contrasts below the receiving station to produce polarized P to SV converted phases. The time delay between the first arriving P wave and the SV converted phase is interpreted to infer the depth of interfaces and the velocity structure directly below the receiver, allowing estimates to be made of the physical properties of the interface. Passive seismic data were recorded at eighteen seismic stations deployed across a ~90km transect stretching across the breadth of lower North Island of New Zealand, from Kapiti Island, 5km off the west coast, to the eastern coast. The transect is oriented normal to the strike of the subducting Pacific Plate, as it dives beneath the overriding Australian Plate. Data were recorded at 10 broadband and 2 short period sensors, deployed as part of the Seismic Array Hikurangi Project (SAHKE 1 deployment), 3 Geonet (New Zealand Geonet Project) permanent short period stations, and 3 temporary stations from part of the 1991-1992 POMS project. Seismic data were recorded between November 2009 and March 2010 on the short period sensors and up to 18 months on the broadband sensor. Data recorded between November 2009 and November 2011 were utilised from the Geonet stations. P wave receiver functions are computed using the multi-taper correlation method using 389 > 6.0 Mw teleseismic earthquakes recorded at the individual seismic stations. A total of 1082 individual receiver functions from all the stations are stacked for both the individual stations and as a ‘super-stack’ across the complete transect, using the common conversion point (CCP) method. The CCP stack shows a distinct, thick low velocity layer (LVL), dipping to the west, from ~18km depth in the east to ~30km depth in the west. This is above a higher velocity layer, also dipping west, at depths of between ~22km and ~ 37km. The LVL is interpreted as being subducted sediments overlying the higher velocity plate interface. Structures towards the west indicate the presence of possibly imbricated features associated with the overriding plate. Deeper structures, down to a depth of 140km are evident, but have less clarity than the shallower features. Some of the deeper layers appear to be dipping towards the west, some to the east. The results of the CCP stack agree well with results from active source methods.</p>