GPS ground deformation patterns at Mount St. Helens (Washington, USA) from 2004 to 2010

GPS measurements on Mount St. Helens spanning the 2004.00-2010.85 time interval, encompassing the 2004-2008 dome-building eruption, were analysed. First, a local reference frame was derived to isolate the volcanic ground deformation from the background tectonic pattern related to the interactions between the Pacific, Juan de Fuca and North American plates. Secondly, the resulting ground deformation patterns were modelled to constrain magmatic sources. Results suggest a vertically elongated magma chamber centred at 89 km depth beneath the summit area that deflated during the beginning of the dome growth and inflated since January 2008, indicating that the same primary magma reservoir was active both during and after the eruption in a magmatic system that is persistent over the time.