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February 11, 1999

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


Stand by for an eruption that may never occur

Imagine this scene: the crew at the Hilo Fire Department receives a call telling them to stand by because an arsonist is setting a fire. If the blaze warrants a response, the fire crew will be notified. In the volcano-monitoring business, we receive standby calls about four to six times a year. Our alerts come from equipment that tracks the earthquake and surface changes around Kilauea and Mauna Loa Volcanoes. Some alarms are truly false, the consequence of equipment failure or adverse weather, such as high wind, which may send a seismometer shaking for no geologic reason.

Many calls, however, are alerts to stand by. Here's how it works. Three of our seismometers are linked to our alarm system. Bells ring loudly if any of these seismometers records six minutes of sustained tremor. The alarm system is also monitored by a security company that calls us within a minute of the alarm. The security company can reach observatory staff even on weekends or nights. A staff member then hightails it to the observatory to determine the cause of the alarm. We also have tiltmeters positioned around the volcanoes to monitor changes in the ground surface. Eruptive events at Kilauea are invariably preceded by swelling or shrinking of the summit magma reservoir, which leads to ground tilt we call inflation or deflation. The tiltmeters warn us of incipient eruptions.

Many alarms result from seismic tremor or ground deformation that fades away without leading to an eruption. For example, in February 1997, Kilauea's summit inflated abruptly. In four hours the tiltmeter had recorded four microradians of inflation and was still tracking a substantial rate of deformation, owing to magma injection into the summit region.

If deformation had continued through six or seven microradians of tilt, it would have raised concern about the safety of the National Park's Crater Rim Drive and Chain of Craters Road, both of which have been severed by eruptions in past decades. During this 1997 episode, we had Park officers standing by with us as we sought a prudent decision about closing access to these areas.

As quickly as it began, however, the inflation ceased. We could step down from our alert, expecting that calm would prevail again across the volcano. Last weekend a similar event occurred--this time soon after midnight Saturday. The summit inflated abruptly following a pause in the eruption. It then deflated equally abruptly. The deflation resulted when magma moved down the east rift from the summit chamber. About two hours later this magmatic surge reached the vent area at Pu`u `O`o, where it overwhelmed the tube system. Throughout Sunday, lava upwelled from skylights along the tube to feed new surface flows that reached to the coastal plain. Park visitors received a spectacular show of surface lava flows visible from the end of the Chain of Craters Road. Fortunately for residents on the Big Island, the flows were confined to areas previously covered by lava during the past year.

Eruption Update

The eruption of Kilauea Volcano returned to normal following the events described above. Lava reoccupy the old tube system and flow from the Pu`u `O`o vent to the ocean near Kamokuna. Breakouts from the tube system continue to occur occasionally. The public is reminded that the ocean entry areas are extremely hazardous, with explosions accompanying frequent collapses of the new land. The steam clouds are highly acidic and laced with glass particles.

No felt earthquakes were reported during the week ending on February 11.

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Updated: 18 Feb 1999