Coseismic offsets

Since there are no GPS measurements at the campaign sites in the few days preceding the earthquake and since the last survey occurred 2 years before the earthquake, we have to take into account the interseismic site displacements to calculate the coseismic offsets. We calculate the coseismic offset at each site by first fitting a straight line through the 1999 and 2001 survey data and extrapolating it to the day of the earthquake. This gives us the (expected) site position before the earthquake. We then average the positions for the October 2003 survey. This gives us the site positions after the earthquake. This is valid because the survey was performed less than a month after the event, interseismic displacements are negligible during that time interval. The position offsets are then given by the position difference between before and after the earthquake. Note that the estimated positions right before and after the earthquake are shown by a small red dot on the vertical dashed line on Figure 6.3. The corresponding offsets are given in Table 6.3, together with the associated uncertainties, and plotted on a map on Figure 6.6.

Table 6.3: Coseismic displacements. Sites in italics are continuous. For those sites, the coseismic offset is the difference between the average position 2 weeks before and 2 weeks after the earthquake.

Site	dE	dN	dU
ARRO	-5.0 $\pm$ 12.3 mm	-21.8 $\pm$ 7.4 mm	31.1 $\pm$ 38.4 mm
CAMP	-1.1 $\pm$ 9.0 mm	21.8 $\pm$ 7.5 mm	-160.3 $\pm$126.4 mm
CAPT	-3.7 $\pm$ 8.4 mm	4.5 $\pm$ 4.7 mm	18.1 $\pm$ 22.0 mm
CASX	-6.5 $\pm$ 9.8 mm	0.9 $\pm$ 5.9 mm	11.1 $\pm$ 21.5 mm
CRIS	not suitable	for coseismic	measurements
HIDA	15.0 $\pm$ 9.5 mm	9.0 $\pm$ 8.1 mm	2.6 $\pm$ 25.5 mm
LAVE	-23.3 $\pm$ 8.3 mm	0.4 $\pm$ 4.7 mm	-7.4 $\pm$ 21.2 mm
MONC	6.9 $\pm$ 8.5 mm	-1.5 $\pm$ 5.5 mm	-1.0 $\pm$ 22.6 mm
PEPE	not suitable	for coseismic	measurements
REUN	67.0 $\pm$ 9.3 mm	-10.3 $\pm$ 5.1 mm	79.6 $\pm$ 26.0 mm
SAGO	-2.8 $\pm$ 8.4 mm	7.7 $\pm$ 4.9 mm	8.6 $\pm$ 25.3 mm
TERR	-1.2 $\pm$ 8.3 mm	-8.3 $\pm$ 4.8 mm	-27.7 $\pm$ 23.6 mm
BAR_	4.4 $\pm$ 8.5 mm	0.7 $\pm$ 4.9 mm	3.8 $\pm$ 22.4 mm
LVEG	7.7 $\pm$ 8.3 mm	3.9 $\pm$ 4.9 mm	0.7 $\pm$ 22.7 mm
SPED	3.3 $\pm$ 9.2 mm	0.5 $\pm$ 5.0 mm	2.3 $\pm$ 24.0 mm
SROD	7.2 $\pm$ 8.7 mm	2.7 $\pm$ 5.1 mm	1.0 $\pm$ 23.3 mm

Figure 6.6: Map of horizontal offsets. Vectors show the change in position between before and after the Sept. 22 earthquake. Number by the site name is the offset in mm. Error ellipses are 95% confidence. Yellow stars: continuous GPS stations. Grey circles: Sept. 22 earthquake (largest circle) and main aftershocks, as located by INDRHI (locations courtesy L.O. Gomez).