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July 2, 1998

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


Is Mount St. Helens about to wake up?

It has been more than 18 years since Mount St. Helens had its powerful eruption, almost 12 years since its latest quiet dome-building eruption, and 8 years since its latest small explosions. But this length of time is just a wink of the eye to a volcano. Recent developments give our colleagues at the U.S. Geological Survey's Cascades Volcano Observatory (CVO) reason to believe that St. Helens is only dozing fitfully and may be slowly waking up.

Earthquake activity at Mount St. Helens, as monitored by the University of Washington's Geophysics Program, has been gradually increasing over the past several months. Before May, about 60 earthquakes were located each month. In May this number rose to 165 and in June, to 318; more earthquakes were detected but could not be located. These numbers are low by Hawaiian standards but well above background levels for St. Helens. The earthquakes are small; the largest recorded so far had a magnitude of only 2.2. June's earthquakes were even smaller, the largest being 1.8. The earthquakes are directly beneath the lava dome in the crater and cluster at depths of 2-5 km (1.2-3 miles) and 7-9 km (4.2-5.4 miles). Almost none came from shallower than 2 km (1.2 miles).

Two other times in the past 10 years Mount St. Helens has had elevated seismicity. In 1989-91, seismicity resembling today's but with even shallower earthquakes was associated with small steam explosions. The explosions threw small rocks as far as 1 km (0.6 mile) from the dome, and ash clouds rose to altitudes of 6 km (20,000 feet).

Another period of increased earthquake activity took place in 1995. It was not as strong as the present one and resulted in no steam explosions or lava eruption.

Recent airborne measurements show that a lot of carbon dioxide gas is being emitted by Mount St. Helens. Carbon dioxide is dissolved in magma at great depth but forms bubbles that separate from the molten rock as it rises toward the surface and encounters less pressure. The carbon dioxide then escapes into fractures in the rocks and moves upward, eventually into the atmosphere. The high output of carbon dioxide measured at St. Helens strongly suggests the presence of new magma beneath the volcano. Most likely this means that the magma reservoir, whose top is about 7 km (4 miles) below the crater, is being replenished. Such refilling is a necessary first step that will eventually lead to renewed eruptions.

There is no evidence that an eruption is imminent, however. A network of surveying targets was established on the dome, crater floor, and lower flanks of the volcano in order to detect ground movements of the kind that foretold each of the eruptions of the 1980s. Remeasurements to these targets showed no significant movement between June 5 and June 29.

HVO scientists have a more than casual interest in the workings of Mount St. Helens. Six of HVO's staff scientists were once stationed at CVO and played important roles in studying the volcano during the 1980s and early 1990s. Several others have, over the years, applied their expertise to research at St. Helens. Volcanology is a small science, and its members form an extended `ohana and help each other when needed. Surely if Mount St. Helens were to threaten or actually have a big eruption again, HVO would speed both people and equipment to the scene.

Earthquake activity at Mount St. Helens can be viewed on the web at http://vulcan.wr.usgs.gov/Volcanoes/MSH/CurrentActivity and http://www.geophys.washington.edu/SEIS/PNSN/HELENS

Eruption and Earthquake Update

The east rift zone eruption of Kilauea Volcano from the Pu`u `O`o vent continued unabated during the past week. Lava flows through a network of tubes from the vent to the coast, but occasional surface outbreaks are observed upslope of the ocean entry areas at Waha`ula and Kamokuna. The public is again reminded that these two areas are extremely dangerous. The National Park Service has restricted access to them because of frequent explosions that accompany collapses of the growing lava bench.

Eleven minutes after midnight on Saturday, June 27, residents of the Big Island and Maui felt an earthquake that originated 54 km (32 mi) offshore (west) of Kawaihae at a depth of 52 km (31 mi). The earthquake had a magnitude of 5.0 and was the largest local earthquake recorded by the HVO seismic network since the magnitude 5.2 south flank earthquake on June 30, 1997.

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Updated: 2 July 1998

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