Response of a volcanic conduit to step‐like change in magma pressure

Abstract

Freely damped oscillations associated with strain offsets were observed by a nearby volumetric strainmeter during the 1986 Izu‐Oshima Volcano eruption, Japan. These events are interpreted as a response of the magma‐filled conduit to step‐like changes in magma pressure. There were dramatic changes in the characteristic frequency of oscillation and the polarity of initial motion before and after the 6‐hour interval of the enormous ground deformation due presumably to magma intrusion. The frequency changed from 42.6 to 21.3 mHz and the polarity changed from contraction to expansion. We propose a conceptual conduit model to explain this polarity reversal, where magma is episodically supplied to the conduit before but is episodically drained back to the reservoir after an extensive magma escape away from the reservoir as dike intrusion. Stress caused by the dike intrusion is large enough to break the lid of the conduit, thus to change a boundary condition. This change in boundary condition reduces the frequency of oscillation by a factor of 2. In this model longitudinal wave speed of the magma fluid is 340 m/s if the conduit length is taken to be 4 km.