Southern Cook Strait earthquake response
Authors: Martha Savage, John Townend, Katrina Jacobs, Tim Stern, Victoria University of Wellington; John Louis, University of Nevada, Reno
Contributing Students: Rachel Heckels, PhD; Ernestynne Walsh, MSc; Adrian Shelly, PhD; Jesse Dimech, PhD; Emily Warren-Smith, PhD; Hamish Hirschberg, Summer Undergraduate scholar.
Paper number: 3785 (EQC 13/U654)
Abstract
We deployed 21 seismometers along a 400-km line on the North and South Island, on either side of the Seddon Earthquake sequence, with the aim to image the Earth parallel to the subduction interface beneath the Lower North Island. The deployment lasted from 6 September 2013 to 21 January 2014. It consisted of two broadband seismometers borrowed from Massey University, 14 short period (2 Hz natural period) seismometers borrowed from the United States Passcal instrument center, and five seismometers borrowed from Y. Iio at Kyoto University.
The line runs parallel to the subduction zone, with the expectation of learning how the structures change as the subduction zone shallows to the south. The data has been archived and is available for other researchers worldwide. Our initial images are interpreted as imaging the subduction interface as deepening from 25 km under Wellington to 35 km under southern Marlborough.
These results are helping to build 3-D information about the plate interface. We hope that this will help us to understand future hazards posed by subduction thrust earthquakes in this region, and the feeding system for deep slow-slip earthquakes.
Technical Abstract
We have recorded data from 21 seismometers that we deployed along the North and South Island, on either side of the Seddon Earthquake sequence, with the aim to image the Earth parallel to the strike of the subduction interface beneath the Lower North Island. The deployment lasted from 6 September 2013 to 21 January 2014. It consisted of two broadband seismometers borrowed from Massey University, 14 short period (2 Hz natural period) seismometers borrowed from the United States Passcal instrument center, and five seismometers borrowed from Y. Iio at Kyoto University.The data has been compiled and archived at VUW and at the United States IRIS data center.
Using the data, we perform seismic migrations using aftershocks of two M>6 earthquakes as sources. The Southern Cook Strait earthquake sequence, beginning on 19 July 2013, included the 21 July M=6.5 and 16 August M=6.6 2013 earthquakes, which were the largest shallow earthquakes to strike the Wellington region since 1942. This line of seismometers ties into the SAHKE line, which was an array of up to 900 seismometers that recorded air gun and explosion shots in deployments from 2009-2011. The SAHKE project characterized the structures perpendicular to the strike of the subduction zone. Our results use the SAHKE line as a starting point and look for strike-parallel variations in the depth of the Moho and other structures. Previous studies have suggested potential changes along strike in this region, and deep slow slip events (> 35 km) are also observed north of Wellington, further indicating that variation in properties exists along slab strike.
We have used 246 M > 3 earthquakes that occurred from September 2013 through January 2014 to create common receiver gathers. Multicomponent prestack depth migration of these receiver gathers, with operator antialiasing control and prestack coherency filtering, produces reflectivity sections using a 1-D velocity model derived from the SAHKE project. Relocation of aftershocks of the Seddon earthquakes using the deployment of a temporary array by New Zealand GeoNet facilitates the migration. An initial P-P migration shows a north-dipping reflector at 15-25 km depth under the earthquake sequence, and suggests the Moho at 20-25 km depth. From Wellington, a reflector dips very gently south from 25-35 km depth, which is probably the slab interface. These results are helping to build 3-D information about the plate interface. We hope that this will help us to understand future hazards posed by subduction thrust earthquakes in this region, and the feeding system for deep slow-slip earthquakes.
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