1. Department of Earth and Environmental Science, New Mexico Tech, Socorro, New Mexico 87801, USA
2. Geological Sciences, University of North Carolina, Chapel Hill, North Carolina 27599, USA
3. Institute for Geophysics, University of Hamburg, Hamburg 20146, Germany
4. Earth and Environmental Sciences, Lehigh University, Bethlehem, Pennsylvania 18015, USA
5. Facultad de Ciencias, University of Colima, Colima 28045, Mexico

Correspondence to: Jeffrey B. Johnson¹ Correspondence and requests for materials should be addressed to J.B.J. (Email: jeff.johnson@ees.nmt.edu).

Dome growth and explosive degassing are fundamental processes in the cycle of continental arc volcanism^{1, 2}. Because both processes generate seismic energy, geophysical field studies of volcanic processes are often grounded in the interpretation of volcanic earthquakes³. Although previous seismic studies have provided important constraints on volcano dynamics^{4, 5, 6}, such inversion results do not uniquely constrain magma source dimension and material properties. Here we report combined optical geodetic and seismic observations that robustly constrain the sources of long-period volcanic earthquakes coincident with frequent explosive eruptions at the volcano Santiaguito, in Guatemala. The acceleration of dome deformation, extracted from high-resolution optical image processing, is shown to be associated with recorded long-period seismic sources and the frequency content of seismic signals measured across a broadband network. These earthquake sources are observed as abrupt subvertical surface displacements of the dome, in which 20–50-cm uplift originates at the central vent and propagates at 50 m s^-1 towards the 200-m-diameter periphery. Episodic shifts of the 20–80-m thick dome induce peak forces greater than 10⁹ N and reflect surface manifestations of the volcanic long-period earthquakes, a broad class of volcano seismic activity that is poorly understood and observed at many volcanic centres worldwide⁷. On the basis of these observations, the abrupt mass shift of solidified domes, conduit magma or magma pads may play a part in generating long-period earthquakes at silicic volcanic systems.

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