Crustal Structure and Seismicity Around the Deep Fault Drilling Project Site, Alpine Fault, South Island, New Zealand

Cliff Thurber, University of Wisconsin

Monday, August 12, 2013 at 10:30 AM

Building 3, Room 3240 (main USGS conference room)
Dave Shelly

In support of the Deep Fault Drilling Project on the Alpine Fault, in January 2012 UW-Madison (UW) and RPI, with assistance from Victoria University of Wellington (VUW) and GNS Science (GNS), deployed a 20-station seismic array around the planned drill site for the DFDP-2 boreholes in Whataroa Valley on New Zealand's South Island. Half of the stations are shortperiod PASSCAL instruments deployed by helicopter in remote sites, the other half are UW broadband instruments deployed in the lowland areas. All are being continuously recorded at 100 samples per second. In combination with the SAMBA array operated by VUW, ALFA-12 stations deployed by GNS, and regional GeoNet stations, there is now excellent seismic coverage of a > 100 km stretch of the Alpine Fault. The earthquakes that are detected will (1) help to define the geometry of the Alpine Fault and other active faults at depth, (2) provide data for tomographic imaging, focal mechanisms, and shear-wave splitting analysis, and (3) enable the assessment of possible changes in seismic activity induced by future fault zone drilling.

At present, data are in hand from the first 15 months of WIZARD array operation. As expected based on previous studies in the region, most of the local seismic activity lies outside the perimeter of the WIZARD array, mainly to the north and east. Included in this activity are swarms of similar earthquakes near the northeastern edge of the array. Earthquake swarms have also been reported by Boese (2012) within the SAMBA array to the southwest. Our array provides a lower detection threshold than previous studies in the area, however, so we have successfully detected a number of earthquake within our array. We have also identified explosions from tunneling operations from a hydropower tunnel construction project in the northern part of the WIZARD array. Arrival-time data from these explosions are proving useful for constraining the P-wave velocity structure.

For our initial tomographic analysis, we merged new P-wave data from the WIZARD, SAMBA, and ALFA-12 arrays with the regional dataset of Eberhart-Phillips and Bannister (2002) for the Southern Alps region of the South Island. The latter dataset includes active-source data from the SIGHT offshore-onshore project as well as arrival times from about 300 earthquakes. I will present an updated regional 3D P-wave velocity model, discuss some implications of our initial results and describe on-going efforts related to the drilling project.

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