Print

Mail

wiki links

St. Helens calms down after its latest steamy outburst

For the fifth straight day, Mount St. Helens let out a burst of steam and ash on Tuesday, similar to the earlier bursts that have risen out of the crater and perhaps larger.

But with that exhalation, it suddenly dozed off, and earthquakes that have shaken the area almost continually for two weeks stopped almost entirely.

"The seismicity dropped off quite dramatically," said Steve Malone, director of the seismology lab at the University of Washington.

The earthquakes began Sept. 23, when Mount St. Helens began to revive, and gradually increased in intensity and pace, culminating in an eruption a cloud of steam and ash on Friday, the first eruption since 1986.

The earthquakes tailed off briefly, but returned in an hour or two and became stronger. On Saturday, at the peak of seismic unrest, three to four quakes a minute shook St. Helens, with the larger ones, between magnitude 2 and magnitude 3, occurring every couple of minutes.

After Tuesday's steam burst, the larger earthquakes ceased completely, and the smaller ones, almost all smaller than magnitude 1, slowed to a pace of one per minute or two.

William Steele, another seismologist at the University of Washington, said, "That is an indication perhaps that it's running out of energy. Maybe it's accomplished what it's going to do."

So far, St. Helens has done relatively little compared to the months leading up to its cataclysmic 1980 eruption. A large bulge formed on the northern flank, rising 1.5m a day. By May 18, 1980, the bulge had been pushed upward and outward by more than 135m when a magnitude-5.1 earthquake hit, and the bulge collapsed in a landslide. That, in effect, took the lid off the magma underground, and later that day, Mount St. Helens erupted explosively.

This time, global positioning system sensors detect no movement on the volcano's outer flanks.
This story has been viewed 1820 times.