Variable Deformation Rates at Sierra Negra Volcano: Geodetic Observations from 2008 to Early 2014

Abstract:

Sierra Negra is a basaltic shield volcano located at the southern end of Isabela Island, Galápagos Islands, Ecuador. The volcano, along with the rest of the Galápagos Islands, has formed by hotspot volcanism. The volcano has erupted twice over the last 35 years, producing VEI-3 eruptions in November 1979 and October 2005. Previous studies suggest that deformation is due to a flat-topped chamber or sill centered 1.9-2.2 km beneath the caldera (Amelung et al. 2000; Yun et al. 2006; Geist et al. 2007). Ten continuous Global Positioning System (cGPS) stations were installed within and on the rim of the caldera of Sierra Negra (i.e., sites GV01 - GV10) in 2002, 2006, and 2009 to track ongoing deformation. In addition, there is one cGPS site on the nearby (90 km) island of Santa Cruz (GLPS), where no recent volcanism has been observed. We analyzed the cGPS data for the period 2008 to 2014 using GIPSY-OASIS II and produced daily position time series for each site. We then estimated yearly velocities for each site assuming linear deformation. We subtract the velocity vector for site GLPS from all Sierra Negra cGPS sites to remove the motion of the Nazca plate. The resulting horizontal and vertical velocity fields indicate significant upward and outward displacement of the volcanic edifice, which we interpret as caused by the influx of new magma or magma chamber pressurization. There are significant yearly variations in both the direction and magnitude of station velocities. For the period covering 2008 to 2011 there is accelerating inflation. Maximum rates of deformation (mm/yr) are seen at GV07 in 2008 (87.02), GV09 in 2009 (161.18), and GV03 in 2010 (191.03). Inflation continues for the period 2011-2013, but at a slower rate. Maximum rates are seen at GV03 in 2011 (78.53) and GV06 in 2012 (22.44). The rate then increases again for the period covering mid-2013 through early 2014. Maximum rates are seen at GV06 in both 2013 (318.27) and early 2014 (308.53). The maximum vertical uplift rates (564.92) are seen at the sites closest to the center of the caldera, GV02 and GV04. A preliminary model for the influx of magma into the system during 2013 suggests 0.015 km³ and a source depth of 2.2 km.