January 29, 2004

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

Pu`u `O`o throws yet another curve ball

As reported in the previous Volcano Watch, a new vent opened on the south flank of Pu`u `O`o two weeks ago, on Sunday, January 18. The behavior of the volcano during the previous few months mimicked the behavior that preceded the Mother¡s Day breakout on May 12, 2002. Consequently, volcanologists anticipated the new breakout, though they were uncertain about its exact timing and location. They expected lava flows from the new breakout to blaze a new trail to the coastal plain, as has happened so many other times during the Pu`u `O`o eruption.

At first, Pu`u `O`o followed expectations as lava flows moved south and east away from the cone. Perhaps sensing that volcanologists had anticipated her plans, capricious Pele changed her strategy.

At about noon on Friday, January 23, only five days after the breakout on the south flank of Pu`u `O`o, Kilauea¡s summit began a rapid deflation, and volcanic tremor at the summit increased. As usual, after a brief lag, the deflation of Kilauea¡s summit was mirrored by a deflation at Pu`u `O`o, located 18 km (11 miles) east-southeast of the summit on Kilauea’s east rift zone.

The deflation is caused by a reduction in magma pressure beneath Kilauea¡s summit. As the pressure drops, the eruption rate drops at Pu`u `O`o.

By Saturday night, January 24, the flow of lava from the new south-flank vent had stopped, and some minor collapses occurred within Pu`u `O`o¡s crater. Although the flow of lava from Pu`u `O`o slowed down a lot, it did not completely stop. Some lava continued to flow through the old Mother¡s Day lava-tube system, feeding a few small lava flows above Pulama pali, south of Pu`u `O`o.

By Sunday, January 25, Kilauea¡s summit had deflated about 8 microradians (0.0005 degrees), the most since January 1997. The summit began to re-inflate on Sunday evening, and Pu`u `O`o followed suit about noon the next day.

The flow of lava from Pu`u `O`o sometimes stops completely. These temporary interruptions--called pauses¢usually last only a few days or less. The shortest lasted two or three hours, the longest, 24 days.

Since the start of the Pu`u `O`o eruption 21 years ago, there have been 79 pauses. Twenty-one occurred between February 1992 and February 1993, and none since December 2000.

The instrumental signature of pauses¢the combination of deformation and seismic signals--can vary substantially. Nine pauses in 1990 followed rapid but small deflations of Kilauea summit and increasing summit tremor. Once a pause began, the summit rapidly inflated and summit tremor decreased.

This behavior contrasts with that observed during the 21 pauses mentioned above. Only about one-half of these were preceded by slight summit deflation, but many comparable instances of summit deflation were not followed by a pause. The most consistent feature of this group was summit inflation, which accompanied nearly all 21 pauses.

The differences in instrumental signature reflect the fact that the plumbing system connecting Kilauea¡s summit and Pu`u `O`o is dynamic rather than static. The location of blockages, the number of pipes, the dimensions of the pipes, and places where the pipes empty onto the earth’s surface all change from time to time.

As of January 29, the eruption output is still significantly below normal, presumably because of a partial blockage or diversion somewhere within the system. Like an old house, Kilauea has interesting plumbing.

Activity update

The condition of Kilauea¡s eruption is described above.

Two earthquakes were felt on the island during the week ending January 21. Residents of Volcano and the national park felt a magnitude 2.9 earthquake at 8:57 p.m. on January 23. The shallow earthquake was located about 2 km (1 mile) west of Volcano at a depth of 2 km (1 mile). Another earthquake was felt in the same area the next morning at 7:37. It had a magnitude of 3.1 and was centered 2 km (1 mile) northeast of HVO at a depth of 2 km (1 mile). These two earthquakes were probably related to the period of unusually strong volcanic tremor beneath Kilauea¡s caldera mentioned above.

Mauna Loa is not erupting. The summit region continues to inflate. Seismic activity remains very low, with two earthquakes located in the summit area during the last 7 days.

The small swarm of earthquakes at Lo`ihi active during the previous week has died away.

Visit our website (hvo.wr.usgs.gov) for daily volcano updates and nearly real-time earthquake information.