During the past 24 hours and continuing at present (6 P.M. PST), about 200 very small (less than magnitude 0) earthquakes have been recorded at Mount St. Helens. The earthquakes are occurring at shallow depths (less than 1 kilometer, or 1/2 mile) mostly in or under the north flank of the lava dome, which formed between 1980 and 1986. Such earthquakes are common at St. Helens, but we have not recorded a swarm with this many earthquakes for several years. The cause of the earthquakes is uncertain, but may reflect increased ground-water levels with the onset of autumn rain. Such an increase may cause slippage on fractures in and below the lava dome and crater floor. The probability of small landslides and debris flows in the crater is enhanced during these periods. Such events could affect areas several kilometers (miles) north of the crater on the Pumice Plain. The probability of small steam explosions that hurl rocks a few hundred meters (yards) may also be increased during periods with increased shallow earthquakes. Larger-scale eruptions are unlikely without significant additional precursory activity.
The last period of increased earthquake activity at Mount St. Helens occurred in the spring and summer of 1998 when hundreds of earthquakes per month, many larger than M=2, were detected at depths as great as 9 kilometers (6 miles). An intrusion of magma, or molten rock, deep under the volcano and resulting increased gas pressure in the conduit that leads to the lava dome likely caused this increase in earthquakes. The current swarm is different in that the events are typically much smaller and shallower. We see no evidence that an intrusion similar to that of 1998 is underway.
Officials with the Gifford Pinchot National Forest ask that recreationists in the Spirit Lake basin and North Fork Toutle River valley north of the crater remain alert for possible debris flows. Loud rumbling noises may indicate that a debris flow is approaching. People that hear such sounds should immediately get away from stream channels and to higher ground.
We continue to monitor the situation closely and will issue additional Information Statements only if conditions change significantly.
Daily updates of earthquake data and other information can be found on the WORLD WIDE WEB at URL:
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