Volcano dome dynamics at Mount St. Helens:Deformation and intermittent subsidence monitored by seismicity and camera imagery pixel offsets

The surface deformation field measured at volcanic domes provides insights into the effects of magmatic, gravity-, and gas-driven processes, as well as the development and distribution of internal dome structures. Here we study short term dome deformation associated with earthquakes at Mount St. Helens, recorded by a permanent optical camera and seismic monitoring network. We use Digital Image Correlation (DIC) to compute the displacement field between successive images, and compare the results to the occurrence and characteristics of seismic events during a 6-week period of dome growth in 2006. The results reveal that dome growth at Mount St. Helens was repeatedly interrupted by short term meter-scale downward displacements at the dome surface, which were associated in time with low frequency, large magnitude seismic events followed by a tremor-like signal. The tremor was only recorded by the seismic stations closest to the dome. We find a correlation between the magnitudes of the camera-derived displacements and the spectral amplitudes of the associated tremor. We use the DIC results from two cameras and a high resolution topographic model to derive full 3D displacement maps, revealing the internal dome architecture and the effect of the seismic activity on daily surface velocities. We postulate that the tremor is recording the gravity-driven response of the upper dome due to mechanical collapse or depressurization and fault-controlled slumping. Our results highlight the different scales and structural expressions during growth and disintegration of lava domes, and the relationships between seismic and deformation signals.