Mass Intrusion at Mount St. Helens (WA) Between 2010 and 2014 from Temporal Gravity Variations Mass Intrusion at Mount St. Helens (WA) Between 2010 and 2014 from Temporal Gravity Variations

Abstract:

Repeated high-precision gravity measurements made at Mount St. Helens (WA) have revealed systematic temporal variations in the gravity field several years after the 2004-2008 dome-building eruption. Changes in gravity with respect to a stable reference station 36 km NW of the volcano were measured at 10 sites in the summit region and at 4 sites far afield (10 to 36 km) from the summit in August 2010 and August 2012. After removing the gravity signal associated with changes in mass of the crater glacier and the local (perched) hydrothermal aquifer, the gravity field observed at sites near the volcano’s summit significantly increased with respect to sites far from the summit (maximum change 146 ±7 μgal). The pattern of gravity increase is radially symmetrical, with a half-width around 3 km and a point of maximum change centered 1.5 km NW of the 2004-2008 lava dome. Inversion of residual gravity data using the same source geometry, depth and location inferred from geodetic data (a spheroidal source centered 7.5 km beneath the 2004-2008 dome) indicates a mass increase of about 10¹² kg. For a reasonable magma density (~2250 kg/m³), the volume of magma intrusion beneath the summit region inferred from gravity exceeds the volume inferred from inversion of geodetic data, suggesting that magma compressibility and other processes are important aspects of magma storage at Mount St. Helens. A third survey will be completed in August 2014, and we will present results of those measurements in the context of the 2010-2012 gravity changes.