Yellowknife Array

The Yellowknife array is located in the Northwest Territories of Canada (Figure 4.1a). The stations are situated in the direct vicinity of the Great Slave Lake. YKA was built in 1962 by the British Ministry of Defence and the United Kingdom Atomic Energy Administration (UKAEA) as one of four seismic arrays designed to detect and locate nuclear explosions. The array is operated by the Geological Survey of Canada (GSC).
The Yellowknife area was selected for the array because of its location with respect to known nuclear test sites, its remoteness from coastlines, urban areas and other sources of cultural seismic noise, as well as its location on the stable Canadian shield. YKA is located near the southern terminus of the Slave Province, an archaean granite-greenstone block located in the northwestern Canadian shield.

Figure 4.1: a) The location of the Yellowknife array in the Northwestern Territories (Canada). The array is indicated by the inverse triangle. YKA is located on the Canadian Shield at the northern shore of the Great Slave Lake.
b) Geology of the enviroment of the array. The array is located on the Anton Terrane, a part of the Slave Province. The geology beneath and near the array is very homogeneous. The array is $\sim$ 100 -  150 km away from the Great Slave Lake shear yone (GSLsz) and the Wopmay orogen. The different geologic feature are indicated by different gray scales. The backazimuths of the events used in this study are 200$^{\circ}$ to 300$^{\circ}$ and traverses mostly homogeneous crust.

The location of YKA is shown by the triangle in Figure 4.1. The geology around the array is displayed in Figure 4.1b) (after Bostock, 1998). YKA is located near the eastern boundary of the Anton terrane, which is interpreted as the remains of an Archean microcontinent. YKA lies some 100 - 150 km north of the Great Slave Lake shear zone (GSLsz) and a comparable distance east of the Wopmay orogen. The GSLsz is a 1300 km long intracontinental transform fault and the Wopmay orogen borders the western margin of the Slave Province. Neither structure disturbs the incident waves, because the teleseismic waves arrive steeply at the array. The structure of the crust beneath the array is highly homogeneous and horizontally layered (Weichert and Whitham, 1969; Corbishley, 1970), and the velocity structure is well studied by a multitude of local studies. The layering beneath the array is horizontal. Therefore, little to no azimuthal travel time variations exist (Hwang and Clayton, 1991). Due to the similar travel paths of PP and P$^d$P beneath the stations, a correction for the structure beneath the array must not be applied to the data. The slowness error due to the crustal structure is small (Manchee and Weichert, 1968) and is not corrected.
The original array had 19 seismometers installed in vaults. One station was later removed due to bad noise conditions as a result of the proximity to the City of Yellowknife. Today, the array consists of 18 one-component, short-period stations, four 3-component, broad-band stations and one 3-component, short period instrument with a high sampling rate (100Hz). All stations allow digital recording since 1989. The location of the stations is displayed in Figure 4.2.

Figure 4.2: The location of the YKA stations. The short period stations are indicated by inverse triangles and the combined location of a sp and a bb instrument by a square. The branches are oriented in W-E (R-branch) and N-S (B-branch) direction. The coordinates and the elevations of the stations are summarized in table A.1 in the appendix.

The location of the short-period (sp) vertical instruments is marked by the inverse triangles and the combined location of sp vertical and broad-band (bb) 3-component instruments is indicated by a square. The sp stations are equipped with Guralp S13 seismometers and the broad-band stations with Streckeisen STS-1 seismometers. All stations, except the high-frequency station YKW3, are operated with a sampling rate of 20 Hz. The array is built in a cross shaped configuration with one branch (red or R-branch) striking west-east and one branch (blue or B-branch) in north-south direction. The interstation spacing is 2.5 km and the aperture of the array 20 km x 20 km. The coordinates and elevations of the stations are summarised in Table A.1 in the appendix.
The location of Yellowknife far away from coastlines and sources of civilisation noise guarantees very low noise conditions. The response function of the sp vertical stations of YKA and the noise spectrum, together with the configuration of the array, defines a spectral window around 1 Hz best suited for the recording of teleseismic P-waves.
The data centre for the YKA data is situated in Ottawa (Canada). The GSC kindly provided the possibility to extract the selected events at the data centre in Ottawa.