Shaking loose mushy magma: the effect of seismic waves on magma mush bodies and the potential for triggering an eruption

Abstract

A central goal within volcanology is understanding how eruptions are triggered. Statistical analyses of earthquake and eruption data indicates that volcanoes show heightened activity after major earthquakes within ~750 km of the source, caused by transient (dynamic) and permanent (static) stress, particularly on gases within the reservoir, such as the accumulation of bubbles, and crustal extension and relaxation. Refinement of the volcanic plumbing structure via geophysical imaging reveals reservoirs are largely comprised of crystal mush, however the effect of earthquakes on crystal movement within this mush is unknown. This thesis explores whether seismic shaking encourages compaction and melt expulsion within mush, and whether energy produced by seismic waves is sufficient to form melt ‘caps’ at the top of mush columns, like in crystal-poor rhyolitic melts. Building on previous studies using saturated particle “packs” as synthesised mush, particle movement under oscillation is analysed using Stokes’ Law, combined with the acceleration of waves via Γ=A⁄g, where Γ is the effective wave acceleration, A is the shaking parameters (amplitude and frequency) and g=9.81 m/s^2. Within a few hundred kilometres, accelerations (PGA) produced by seismic waves are sufficient to encourage compaction (i.e. Γ>1), as applied to six case studies from locations such as Chile and Indonesia. However, not all volcanic bodies within these case studies fall within this effective distance, as waves decay over distance via an inverse square law. Γ at the volcanoes is <1, but above Γ=0.2, meaning minor compaction and expulsion from the mush may occur, but is not of significant volume. Hence, shaking alone may not be responsible for triggering volcanic activity, and melt segregation and dynamic stress work with other triggering mechanisms. Reservoirs must already be at a critical state of instability (within 99% of the maximum overpressure) if any process, including seismic forcing, is to affect the activity.