Seismological Characterization of the DFDP-2 Drill-Site and Surroundings, Whataroa Valley, Central Alpine Fault, New Zealand

Abstract:

Detailed analysis of the seismicity and seismic velocity structure of the crust surrounding the central Alpine Fault is being undertaken in conjunction with a broad range of other scientific activities under the auspices of the Deep Fault Drilling Project (DFDP). Drilling of the 1.3 km DFDP-2 borehole is scheduled to commence in the Whataroa Valley in October 2014. In preparation, several seismometers in the temporary network surrounding the valley have been equipped with cellular modems to enable telemetered data acquisition and distribution via the national geophysical monitoring network (GeoNet). Real-time analysis of seismicity within c. 20 km of the DFDP-2 drill-site will be performed prior to and during the drilling operations using RTQuake. Additionally, records obtained since 2008 have been re-analysed and supplemented with new data using standard earthquake detection techniques (sta/lta) and manual picking, and automatic detection via cross-correlation waveform matching. The results of this analysis are being integrated with newly developed P- and S-wave tomographic velocity models to refine hypocenters and earthquake magnitudes in the vicinity of the DFDP-2 borehole and elucidate spatiotemporal patterns of seismicity. Seismicity near DFDP-2 is sparse and of low magnitude: analysis of seven months’ data recorded by an extensive network in 2013 including four shallow borehole seismometers within 1.5 km of the drill-site revealed fewer than 40 earthquakes within 10 km of the drill-site. Of these, all but two earthquakes (M=2.6 located c. 6 km northwest of DFDP-2, and M=1.7 located c. 3.5 km north of DFDP-2) were deeper than 2 km and all but five were deeper than 3 km. Preliminary cross-correlation detection work using 14 template events and one year’s continuous data reveals on-going clustered activity. In particular, many newly detected events are observed close to a c. 10 km-deep cluster near Gaunt Creek known to produce fault zone guided waves (FZGWs). This demonstrates that waveform matching works well even when waveform complexity is high and highlights the potential for using repeated earthquakes occurring on or near the Alpine Fault to study temporal changes in fault zone properties late in the earthquake cycle.