Velocities derived from the original time series

In this section we calculate the velocities, assuming REYK is fixed, using data from the beginning of measurements until December 31, 2001. The offsets due to the radome installation are removed from the vertical components of the data assuming an offset of -20 mm at the times of radome installation and an offset of 20 mm when the radome is removed. The offsets due to the SISZ June 2000 earthquakes are kept in the data and the formal errors are scaled by 4.0 in east and north and 2.5 in vertical before the velocity estimation. We note that since the coseismic displacements have not been removed from the data before velocity estimation the velocities at the stations including the offsets (HLID, HOFN, HVER, HVOL, OLKE, SOHO, THEY and VOGS) are not neccesarily representing the average plate velocities. Furthermore, not all stations were recording at the time of the earthquake sequence so the velocities have different physical interpretation depending upon if the station was recording data or not (see Figure 21).

 

Table 6: Calculated velocities of the stations in east, north and up, relative to REYK. The velocities are calculated applying equation 7 to data from the beginning of measurements at each station until December 31, 2001. Offsets due to radome installation have been removed from the data, but offsets due to the June 2000 SISZ earthquakes are included in the data set. The uncertainties dV_i are calculated according to equation 10, at the 2$\sigma$level. WSTD is the weighted standard deviation of the residuals (data minus best line fit), calculated according to equation 2. N is the number of data used in the velocity calculations and $\chi ^{2}_{\nu n}$ is from equation 13.

		Velocities [mm/yr]			Uncertainties [mm/yr]			WSTD [mm]			Chi squared
Station	N	Ve	Vn	Vu	dVe	dVn	dVu	e	n	u	$\chi^{2}_{\nu e}$	$\chi ^{2}_{\nu n}$	$\chi^{2}_{\nu u}$
AKUR	102	-5.5	4.9	3.0	2.5	3.1	12.8	1.3	1.7	7.0	1.7	1.2	1.8
HLID	460	22.0	-2.5	11.5	0.4	0.2	0.7	3.1	1.3	5.5	10.1	0.7	0.9
HOFN	873	24.4	-9.2	8.9	0.3	0.2	0.6	3.5	2.6	7.0	5.2	2.7	1.4
HVER	914	8.2	0.6	3.4	0.2	0.1	0.5	1.8	1.8	5.6	3.4	1.3	0.9
HVOL	635	23.5	-12.6	9.2	0.4	0.5	0.9	3.2	3.7	7.0	7.7	5.2	1.4
KIDJ	330	10.2	2.5	5.4	0.4	0.5	2.0	1.0	1.1	4.8	1.3	0.7	0.9
OLKE	814	-10.7	7.0	4.7	0.6	0.4	0.6	6.3	4.1	5.6	43.3	7.4	1.0
RHOF	128	18.0	0.1	-6.2	2.5	2.8	10.5	1.6	1.8	7.1	1.8	1.1	1.6
SKRO	416	2.7	1.1	16.1	0.5	0.5	2.4	1.8	1.6	8.4	3.1	1.4	2.7
SOHO	696	24.2	-15.3	5.6	0.6	0.6	0.9	4.4	4.2	6.8	15.0	6.8	1.3
THEY	500	22.6	-6.8	3.2	0.7	0.8	1.3	3.0	3.6	6.4	8.0	5.8	1.5
VMEY	497	20.0	-6.1	3.8	0.3	0.3	1.1	1.3	1.3	5.1	1.8	0.9	0.9
VOGS	925	30.0	-8.4	7.0	0.6	0.3	0.4	7.1	3.0	5.0	51.3	3.7	0.7

 

The results are shown in Table 6. To visualize the data in Table 6, a constant velocity vector representing the half-spread NUVEL-1A velocity [DeMets et al. (1990)], [DeMets et al. (1994)], at REYK, v_r = 9.6 mm/yr west and 2.1 mm/yr north, is added to the velocity vectors in Table 6. The vector v_r is obtained from the NUVEL-1A NNR model [DeMets et al. (1994)] by assuming that the plate velocities at REYK and HOFN are equal, but in opposite directions. This modification emphasizes the rifting in Iceland showing relative plate motions, i.e. on which tectonic plate the stations are located. The modified velocities are shown in Figure 22. Figure 23 is a close-up of the Hengill area.

Figure 22 shows quite well the plate spreading although the earthquake offsets are in the data for stations HLID, HOFN, HVER, HVOL, OLKE, SOHO, THEY and VOGS. The stations moving towards WNW (REYK, OLKE, SKRO and AKUR) are on the North-American plate whereas the other stations moving towards ESE or SE (HLID, VOGS, VMEY, THEY, SOHO, HVOL, HOFN and RHOF) are on the Eurasian plate. The chi-squared values in Table 6 vary highly. Large $\chi^{2}_{\nu}$ values are observed, up to $\chi^{2}_{\nu}=51$ for the east component of VOGS. This is to be expected since a straight line is a poor model for time series that include the offsets due to the June 2000 earthquakes (stations HOFN, HVER, HVOL, OLKE, SOHO, THEY and VOGS). This is also reflected in the WSTD values in Table 6. Vertical movements will be discussed in Section 4.2.5.

  

Figure 22: Velocities for the permanent GPS stations, as in Table 6, assuming REYK is moving at velocity 9.6 mm/yr west and 2.1 mm/yr north (black arrows) compared to the NUVEL-1A plate motion model velocities (orange arrows). The velocities are based on data that include the June 2000 SISZ earthquakes. Uncertainties are as in Table 6, scaled by a factor 2.

  

Figure 23: As in Figure 22, for the Hengill area.