Strain offsets with monotonous damped oscillations during the 1986 Izu‐Oshima volcano eruption

Abstract

The sequential process of the 1986 Izu‐Oshima volcano eruption, Japan, was recorded by a volumetric strain meter near the erupted summit. The records show occasional strain offsets accompanying long‐period damped oscillations, which we interpret as responses of the volcanic system to perturbations in magmatic pressure. The polarities of strain offsets were contractive for all events before the enormous ground deformation of November 21 due presumably to the intensive dike intrusion. On the contrary, all events after that had expansive polarities. The characteristic frequencies of oscillations were ∼42.6 mHz for the events before, changing abruptly to half that after the ground deformation. In our model, which is a one‐dimensional magma‐filled conduit, the boundary condition at the top of the conduit changed from a closed to an open end upon a void formation at the top of the magma head due to its descent immediately after the dike intrusion. Before the magma escape from the reservoir as a dike, magma was episodically supplied from the reservoir to the conduit, and thus contraction events were generated. After the magma escape, magma left in the conduit was episodically drained back to the reservoir to generate expansion events. This episodic magma supply or drainage was modeled by a prescribed pressure change at the bottom of the conduit. We solved the free oscillation problem of the conduit pipe with an elastic sidewall to calculate the resultant volumetric strain outside the pipe. Physical properties of the magma and the excitation mechanism are inferred from the application of our formulation to the observation. The conduit length and fluid acoustic velocity are estimated to be ∼10 km and 430 m/s, respectively, suggesting that the fluid is composed of a gas‐liquid mixture. If the conduit radius is taken to be 40 m and if the damping of oscillation occurred by viscous dissipation, the fluid viscosity is 108–109 Pa s, a reasonable estimate for the basaltic magma. The applied pressure changes at thé bottom of the conduit are in a range from 105 to 106 Pa with risetimes of the order of 4 to 100 s.