October 17, 1997

A weekly feature provided by scientists at the Hawaiian Volcano Observatory.

Current eruption of Kilauea produces little ground deformation

A volcano is a complex system. During periods of sustained eruption, such as the present time, Kilauea Volcano undergoes little internal change. Paradoxically, the subtle ground movements that volcanologists track to provide clues about the volcano's plumbing system show little change from day-to-day. This pattern indicates that, for the most part, that magma moves freely to the surface and that the inflow of magma is about equal to the outflow of lava. When the eruption pauses, the volcano's plumbing system backs up, and ground deformation increases.

Volcano watchers become uneasy during an eruption pause, or following a large earthquake or an earthquake swarm, because these are times when Kilauea's eruptive behavior is most likely to change. At these times, measurements of ground deformation with instruments such as tiltmeters and GPS provide clues about the changes that have occurred in the volcano's plumbing system.

Large earthquakes change the force balance within a volcano, which can produce a change in the eruptive behavior. Because they release accumulated strain energy, earthquakes relax the volcanic edifice. The most recent earthquakes on Kilauea's south flank, a M 5.3 event on June 30 and a M 4.5 event on August 14, 1997, did not noticeably result in changes in the current eruption.

The M 7.2 earthquake of November 29, 1975, however, was a pivotal event. Before this earthquake, Kilauea's summit was growing, and episodic eruptions occurred at the summit and rift zones. After the earthquake, the summit gradually subsided, and the current sustained eruption from the Pu`u `O`o-Kupaianaha vents on Kilauea's east rift zone became established. For Kilauea, earthquakes are considered to be the greater hazard to life and property than lava flows.

Sometimes Kilauea's eruption changes suddenly, without any obvious cause. An example is the episode 54 eruption from vents in Napau Crater, 5 km uprift of Pu`u `O`o, on January 30, 1997. The eruption resulted from the intrusion of a dike into the east rift zone.

The intrusion occurred during an ongoing eruption from Pu`u `O`o and was not preceded by any large earthquakes. Most likely, it resulted from slow stretching of the volcano by ongoing seaward movement of its south flank.

The current eruption of Kilauea is accompanied by little ground deformation. Surface flows are currently limited to unpopulated areas and do not pose imminent hazard to people or property.

Kilauea Eruption Status--October 24, 1997

During the past week, there was no change in the eruption from Pu`u `O`o. Lava continued to flow through a network of tubes down to the seacoast where it entered the ocean at two locations - Waha`ula and Kamokuna. The public is reminded that the ocean entry areas are extremely hazardous, with explosions accompanying frequent collapses of the lava delta. The steam cloud is highly acidic and laced with glass particles.

Recent Big Island Earthquakes

An earthquake at 7:11 a.m. on the morning of October 16 was reported felt by residents from Hilo, Puna, Ka`u, and Kona. The magnitude 3.8 earthquake was located 17 km (10 mi) north of Pahala at a depth of 12 km (7 mi).