Three-dimensional P- and S-wave velocity structure along the central Alpine Fault, South Island, New Zealand

Wednesday, 17 December 2014: 8:00 AM
Bin Guo1, Clifford H Thurber2, Steven W Roecker3, John Townend4, Chris Rawles5, Calum John Chamberlain4, Carolin Morag Boese6 and Stephen C Bannister7, (1)University of Wisconsin Madison, Madison, WI, United States, (2)Univ Wisconsin-Madison, Madison, WI, United States, (3)Rensselaer Polytechnic Inst, Troy, NY, United States, (4)Victoria University of Wellington, Wellington, New Zealand, (5)Univ of Wisconsin- Madison, Madison, WI, United States, (6)The University of Auckland, Auckland, New Zealand, (7)GNS Science-Institute of Geological and Nuclear Sciences Ltd, Lower Hutt, New Zealand
The Deep Fault Drilling Project (DFDP) on the central Alpine Fault, South Island, New Zealand, has motivated a broad range of geophysical and geological studies aiming to characterize the fault system in the locality of the drill site at various scales. We have been developing three-dimensional P- and S-wave velocity models of the region by double-difference tomography utilizing datasets from multiple seismic networks (WIZARD, SAMBA, ALFA, GeoNet, and others). In our previous work, the quality of the S-wave model has been poor due to the small number of available S-wave picks. We have utilized a new high-accuracy automatic S-wave picker to increase the number of usable S arrivals by an order of magnitude, thereby dramatically improving the S-wave velocity model. Compared to previous studies, e.g. Eberhart-Phillips and Bannister (2002) and Feenstra et al (2013), our updated P-wave model shows a clear high Vp body (Vp > 6km/s) at depths of 5–15 km near the drill site. With our better resolved S-wave velocity model, we can see a sharp high Vs body (Vs > 3.7 km/s) in the same region. The newly added S-picks help to improve the accuracy of the relocations of the earthquakes . This in turn has highlighted the presence of earthquake swarms within the low velocity zone. Together with the updated earthquake relocations, the P- and S-wave tomography results reveal the Alpine Fault to be marked by a velocity contrast throughout most of the study region. The fault dips steeply from 5 to 20 km depth with an average dip of 50-60° SE, as inferred from the velocity structure and the seismicity.