GPS geodetic constraints on the November 21, 2004 Mw 6.3 earthquake off the northwest coast of Dominica: implications for in situ volatile solubilities and eruption dynamics



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Campaign GPS geodesy was initiated in Dominica in 2001. Observations collected by several REU cohorts in 2003, 2004, and 2006 were used to constrain the surface deformation field at 18 sites in 2006. The Mw 6.3 normal faulting event of November 21, 2004, which occurred between the islands of Dominica and Guadeloupe, affected our GPS interseismic time series. In order to remove the coseismic displacement, we used the Harvard CMT focal mechanism and aftershock hypocentral locations from the USGS NEIC to define a model fault plane. Our analysis yields a 35 km fault centered at 15.617 N and 61.709 W with a N33W strike and 55E dip. The rupture area is 60 by 35 km and the plane projected to the surface is 35 by 34.4 km. From the magnitude of the main event, the seismic moment was calculated (3.5075x1025 dyne-cm). We assumed constant slip over the entire fault plane to derive a total finite slip of 9.74 cm. Initial fault parameters were used to drive an elastic half space displacement model (DISL) to calculate the surface displacement from the main shock over a model domain of ~150 km x 150 km. The final slip in the model was adjusted from 9.7 to 14 cm in order to match an observed coseismic offset in the time series from the HOUE cGPS site on Guadeloupe. The displacement field from our elastic model indicates northwest motion over the island of Dominica (south of the fault) and southeasterly motion for Guadeloupe (north of the fault). As expected, the magnitudes of these displacement vectors increase as distance to the fault decreases. In order to assess the effect of such a seismic event on nearby magmatic systems in northern Dominica, a dilatation model was created using Coulomb 2.5. At a depth of 6 km, the model predicts volumetric strain increase (compression) below the islands of Dominica (Morne Au Diables and Diablotins volcanoes) as well as Basse-Terre Guadeloupe (Soufriere volcano), with a decrease beneath the fault plane. I conclude that the dynamic dilatation associated with Mw 6.3 seismic event, would increase in situ magmatic volatile solubilities and thus would lower the probability of an immediate eruption from a shallow source.