Number of Earthquakes Increasing at Mount St. Helens
July 1, 1998
The number of well-located
earthquakes at Mt. St. Helens has increased again this month, this
time from 165 events in May to 318 in June. These quakes were so small,
however, that the total seismic energy release was about the same
as the month of May. A survey of volcanic gases showed that magmatic
carbon dioxide was present above the volcano. As magma rises towards
the surface, carbon dioxide bubbles form within the magma. This carbon
dioxide then rises faster than the magma and is released through cracks
in the rocks. Many areas on or near the crater floor may have large
concentrations of carbon dioxide, making these areas very dangerous
to approach, as asphyxiation could occur.
The increase in earthquakes and release
of carbon dioxide from St. Helens indicates that the magma chamber beneath
the crater is probably refilling. The refilling process leads the way
for renewed magmatic eruptions. The possibility of small steam explosions
like those between 1989 and 1991 has increased slightly over the past
month.
Spring, 1998
"The level of earthquake activity
at Mount St. Helens has been gradually increasing over the past several
months and accelerated during May. Rates of activity have gone from
an average of ~60 well located events per month last winter to 165 events
in May. Most of these recent earthquakes are very small with only 3
events larger than magnitude 2. The largest earthquake was on May 1
with a magnitude of 2.2. These earthquakes are occurring in two clusters
directly beneath the lava dome in the crater. One cluster is in the
depth range of 2 to 5 km below the dome and the other is in the depth
range of 7 to 9 km. Almost no events have been located in the very shallow
region of 0-2 km below the dome. None of these earthquakes are low-frequency,
volcanic events that typically occur as precursors to major eruptions.
This increased earthquake activity seems
to be similar to that which occurred in 1995, although the current activity
is now more energetic. The 1995 activity lasted for several months in
the summer and fall, had a maximum earthquake rate of 95 events per
month, and resulted in no volcanic activity. A similar increase in earthquake
activity in the Mount St Helens system occurred in 1989-91. However,
at that time there were also a number of very shallow earthquakes accompanied
by a series of sudden steam explosions. These explosions were small
eruptions of steam and gas that ejected rocks and dust (ash) from cracks
in the dome. Rocks were thrown up to 1 km (1/2 mile) from the dome and
ash clouds reached altitudes up to 6 km (20,000 feet) and locally deposited
a dusting of ash downwind in a few cases. Some explosions melted snow
in the crater and generated small lahars that flowed onto the Pumice
Plain.
Because increased earthquake activity
within the deep St. Helens system may reflect increased pressure at
depth, it is possible that the current earthquake activity may eventually
lead to renewed volcanic activity. However, it is unlikely to do so
without significant additional precursory activity."
January 1 - September 30, 1995
The U.S. Geological Survey's Cascade
Volcano Observatory reports that the number of small magnitude earthquakes
(less than magnitude 1) beneath Mount St. Helens has increased slowly
and steadily from less than 10 events per month in January to about
100 events per month in September. They point out that the volcano has
been quiet during this period and that no explosion or emission of gas
or ash has occurred from the lava dome. Similar seismic activity was
observed prior to and during a series of small gas explosions from the
dome in 1989-1991. Although these explosions were relatively small,
they did throw dome rocks 1 foot (30 cm) in diameter 0.5 miles (0.8
km) from the dome and sent ash plumes as high as 20,000 feet (6,100
m) above the sea level. Because these events can happen without warning
some trails in the area have been closed. |