THE SOURCE OF EXPLOSIVE ERUPTIONS OF SAKURAJIMA VOLCANO, JAPAN

To understand the nature of explosive eruptions, in particular the role played by exsolution of magmatic gases, we studied the seismic waveforms produced by two such eruptions of Sakurajima volcano on October 30 and November 14, 1986. An explosion begins when a mixture of magmatic gases and fragmented magma flows upward through a narrow vertical channel to the surface. The source region is a reservoir of gas-rich magma, where the pressure drops as gases flow out. The form of low-frequency seismic waves can be explained by a source that consists of two components. One component is a single force directed downward that represents the reaction to ejection of gas and magma. The other, more important component is a moment tenser that represents expansion or contraction of rock around a source. The first motion of seismic waves produced by an explosion is directed away from the volcano. Tilt and strain measurements show that the initial motion directed outward is soon followed by subsidence around the volcano. The eigenvectors of the seismic moment tenser are either nearly horizontal or vertical. The ratio of horizontal to vertical eigenvalues was 1.8:1 for the October 30 explosion. This suggests the geometry of the explosive source can be approximated as a buried vertical cylinder. The volume of contraction, 2 x 10(3) m(3), can be produced by a pressure change of 0.3 MPa over a cylinder with a radius of 160 m and a height of 300 m. We propose that an explosive eruption occurs after a small shallow source of high pressure forms within a vertical conduit, possibly exsolution of magmatic gases. Eventually, the small source bursts and pushes open a pathway to the surface, providing an upward escape for trapped gases. It is this burst that causes the initial outwardly directed motion at the surface. A deeper reservoir of gas-rich magma is then subjected to a lower confining pressure and also bursts, causing the explosion.