Displacement of the south flank of Kilauea Volcano; the result of forceful intrusion of magma into the rift zones

Seismic evidence has long indicated that the south flank of Kilauea Volcano is mobile. Examination of triangulation, trilateration, and leveling data obtained throughout the 20th century shows that the south flank of Kilauea has been displaced upward and away from the rift zones by as much as several metres. The amount of horizontal displacement approximates the probable amount of dilation that accompanies the intrusion of magma as dikes in the rift zones and is greatest for p~riods of most intense intrusive activity, as evidenced by the frequency of eruptions. Displacement and seismic events on the south flank take place soon after intrusive activity, indicating that the displacement is the result of forceful intrusion in the rift zones, not the cause of relatively passive intrusion. The dikes are thought to be nearly vertical or to dip steeply southward, on the basis of both interpretation of seismic and displacement data for Kilauea itself and comparison with dikes exposed in older, eroded Hawaiian shield volcanoes. In contrast to the south flank, seismic and geodetic data indicate that the north flank is virtually immobile. This contrast is believed to reflect the fact that Kilauea was built on the south slope of Mauna Loa and was consequently influenced by the gravitational stress system of Mauna Loa, which favors displacement away from the volcanic edifice. The north flank of Kilauea is effectively buttressed by Mauna Loa, whereas thr south flank is unbuttressed and free to move away from the edifice when prompted by forceful intrusion of magma. The active part of the east rift zone of Kilauea has apparently migrated several kilometres southward with time. This is shown by the location of recent vents and by the location of the axis of a positive gravity anomaly along the north edge of the active part of the rift zone. The southward migration helps explain several features of the geometry of the east rift zone, particularly its prominent bend near Kilauea Caldera. The southwest rift zone and the caldera also show some evidence of southward migration. The Hilina fault system is considered to be a gravity-controlled system not directly related to the rift zones. Gravitational instability resulting from uplift and seaward displacement is eventually relieved by normal faulting along the seaward part of the south flank. The Hilina faults are thought to bottom at shallow depth without interseeting magma reservoirs, except possibly along part of the lower east rift zone, where the fault system impinges upon the rift zone. Strains have been accumulating within the Hilina system throughout this century, and a high level of instability may have been reached. We anticipate a subsidence event in the not too distant future, possibly similar to the damaging events of 1823 and 1868. (While this paper was in press, such an event occurred on November 29, 1975.) INTRODUCTION Kilauea Volcano is the southeasternmost shield volcano in the Hawaiian-Emperor chain of islands and seamounts that extends some 6,000 km across the central Pacific Ocean. Kilauea and its giant neighbor Mauna Loa (fig. 1) are among the most active volcanoes on earth, presently erupting tholeiitic basalt at an average rate of 0.1 km/yr (Shaw, 1973; Swanson, 1972). Kilauea is built on the southeast flank of Mauna Loa, and flows from the two volcanoes probably interfinger extensively at depth. Much has been learned about the structure and magma-conduit system of Kilauea during the past 20 years. A wealth of geodetic, geologic, geophysical, and seismic data indicates that magma rises from the mantle through a nearly vertical conduit to a holding reservoir about 2 to 5 km beneath the summit caldera (fig. 2). This reservoir, almost certainly an intricate plexus of dikes, sills, and irregularly shaped chambers, swells as it fills, causing easily measurable tumescence at the surface. With continued magma supply, the strength of the reservoir rocks is eventually exceeded, and magma is leaked from the system, resulting in a shallow intrusion and usually, but not always, in an eruption. Virtually all eruptions and shallow intrusions occur either in the summit area or along the east or southwest rift zones, which extend outward from the caldera (table 1). Summit eruptions, most of which take place in Halemaumau Crater, are accompanied by numerous earthquakes as magma works its way toward the surface. Local uplift is generally observed very near the site of summit eruption or jntrusion, but the overall summit area may unde_rgo net exp~nsion (table 1, May 1970, August 1971, July and September 1974), contraction (table 1, 1~24, 1938, 1959, 1967, December 1974), or virtually no change (table 1, 1907-24, 1927, 1929, 1930, 1931, 1934, 1952, 1961), depending upon complex interrelations among rate of eruption, rate of magma recharge from the mantle, and degree of shallow intrusion versus extrusion. During rift eruptions, the summit area subsides (table 1, 1955, 1960, 1961, 1962, August and October 1963, March and December 1965, August and October