Berkeley blog - Mount St. Helens and earthquakes

Flying back from Germany a week ago Saturday, our plane was diverted over Iceland to get north of the ash cloud blowing from the Eyjafjallajökull volcano towards Spain. This caused a delay of two hours, not bad compared to what others have experienced, and I got to see the ash cloud from the south-facing windows. First time I have ever seen an erupting volcano. Pretty cool, even though it was a volcanic eruption. The dark cloud was nearly vertical, before it topped out, and then headed off towards Spain.

Anyway, it is now thirty years since the last large volcanic eruption in the conterminous United States at Mount St. Helens. The Berkeley Seismo Blog has a nice description of the harmonic tremors that reflected the magma moving within the volcano, and then the 5.1 magnitude earthquake that triggered slope failure of the mountain and the explosive volcanism.

Small earthquakes continue to occur. Check out the graphics and info at the Pacific Northwest Seismic Network.

Mount St. Helens seismogram, May 18, 1980

From the USGS Cascades Volcano Observatory

Note the harmonic tremors in the bottom half of the record, which are characteristic signals accompanying magma rising through a volcano.

But the event at 15:32 GMT (8:32 am Pacific time) is the quake that accompanied the main blast.

From the U.S. Geological Survey:

Within 15 to 20 seconds of a magnitude 5.1 earthquake at 8:32 a.m., the volcano's bulge and summit slid away in a huge landslide - the largest on Earth in recorded history. The landslide depressurized the volcano's magma system, triggering powerful explosions that ripped through the sliding debris. Rocks, ash, volcanic gas, and steam were blasted upward and outward to the north. This lateral blast of hot material accelerated to at least 300 miles per hour, then slowed as the rocks and ash fell to the ground and spread away from the volcano; several people escaping the blast on its western edge were able to keep ahead of the advancing cloud by driving 65 to 100 miles an hour! The blast cloud traveled as far as 17 miles northward from the volcano and the landslide traveled about 14 miles west down the North Fork Toutle River.

The lateral blast produced a column of ash and gas (eruption column) that rose more than 15 miles into the atmosphere in only 15 minutes. Less than an hour later, a second eruption column formed as magma erupted explosively from the new crater. Then, beginning just after noon, swift avalanches of hot ash, pumice, and gas (pyroclastic flows) poured out of the crater at 50 to 80 miles per hour and spread as far as 5 miles to the north. Based on the eruption rate of these pyroclastic flows, scientists estimate that the eruption reached its peak between 3:00 and 5:00 p.m. Over the course of the day, prevailing winds blew 520 million tons of ash eastward across the United States and caused complete darkness in Spokane, Washington, 250 miles from the volcano.

During the first few minutes of this eruption, parts of the blast cloud surged over the newly formed crater rim and down the west, south, and east sides of the volcano. The hot rocks and gas quickly melted some of the snow and ice capping the volcano, creating surges of water that eroded and mixed with loose rock debris to form volcanic mudflows (lahars). Several lahars poured down the volcano into river valleys, ripping trees from their roots and destroying roads and bridges.

The largest and most destructive lahar was formed by water seeping from inside the huge landslide deposit through most of the day. This sustained flow of water eroded material from both the landslide deposit and channel of the North Fork Toutle River. The lahar increased in size as it traveled downstream, destroying bridges and homes and eventually flowing into the Cowlitz River. It reached its maximum size at about midnight in the Cowlitz River about 50 miles downstream from the volcano.

Note the sequence of events: thge earthquake first, the landslide next, and then the uncorked volcano blew its stack.

Mount St. Helens, May 18, 1980

Twenty-nine years - how time flies!

from The Pacific Northwest Seismic Network

"Graphs show average number of events occurring per day (blue) and average seismic strain energy release (square root of seismic energy) per day (red). The times and types of eruptions are shown at the top of each figure with the length of the symbol roughly proportional to the size of the eruption. Magmatic explosive eruptions are those which had a significant amount of new, juvenile magma ejected into an ash cloud. Phreatic explosions are steam or gas explosions involving little or no magmatic material. Dome growth eruptions are relatively quiet, non-explosive eruptions of lava onto the lava dome.

"This plot is for the explosive eruption period of 1980. The first phreatic eruption occurred on Mar. 27, 1980 and the cataclysmic avalanche, lateral blast and plinian ash eruption occurred on May 18 1980... In the 1980s the sensitivity of the seismic network was much less than it is today and thus only larger earthquakes were located. During the pre-cataclysmic eruption period before May 18 over 600 earthquake greater than magnitude 3 were located and 12 around magnitude 5. During this same period, both because of network sensitivity but also because the seismic noise level was so high that small earthquakes could not be well recorded, there were only 6 events located with a magnitude less than 2.

"After the major eruption on May 18 the seismic noise level was usually much less and smaller earthquake could be more easily located, thus the energy plot after May 18 is very small while the number of events continues to have bursts of activity at moderate levels. "

I like the multidimensional data on the graphs.

Half a lifetime ago (for me).