USGS Watching Mount St. Helen's Volcano Following Earthquake Swarm
The United States Geological Survey is watchting the Mount St. Helen's volcano closely after a series of earthquakes struck on Monday. The area around Mount St. Helen's in Washington state has been experiencing minor earthquakes since an initial quake measuring 4.3 in magnitude struck around 10:35 a.m. local time, according to the USGS. Since that first quake, several smaller aftershocks were registered ranging between 1.0 and 2.8 in magnitude.
All of these earthquakes were centered in an area approximately five to six miles north of the Mount St. Helen's crater near the Johnston Ridge Observatory. The quakes were registered at depths between 1.7 and 3.7 miles. In total, at least 12 small earthquakes were registered in the area since the first quake on Monday.
This series of small earthquakes occurring in approximately the same location over a short period of time is known as an earthquake swarm. According to the Global Volcanic Earthquake Swarm Database earthquake swarms are especially common around volcanoes and are often reliable methods of predicting an eruption but the Alaska Volcano Observatory website offered that there may be no need for alarm. The AVO website explained that while earthquake swarms may offer information that a volcano is becoming restless, they are not necessarily indicators of a pending eruption. According to the AVO, "Most seismic swarms are not precursors to eruptions."
The AVO website states that a great deal of seismic data from satellite imagery, deformation, gas measurements and the history of the volcano's geologic past are all taken into consideration when determining an eruption risk.
According to the USGS, seismic activity is common around volcanoes as volcanoes can produce various types of earthquakes--tectonic-type or volcanic-type. Tectonic-type earthquakes around a volcano occur when rocks break along faults or fractures around the volcano. Seismologists must determine if an earthquake near a volcano is tectonic or volcanic as the differences are very subtle.
Earthquakes are different from volcanic tremors, which are vibrations that last longer in duration (lasting from minutes up to days) and are caused by magma movement, movement of pressurized fluids or the escape of pressurized steam or gas.
According to the USGS, the most recent earthquakes are in the same location as another small swarm that happened in late January. In addition, the most recent swarm of quakes is similar to a swarm that began in August 1980. That swarm culminated in a 5.5 magnitude earthquake in February 1981.
The USGS website stated that studies conducted after the 1980 eruption of Mount St. Helen's "suggested that the magma removed during the May 1980 eruption and subsequent lava-dome building caused faults along the seismic zone to slip in response to the magma withdrawal. Similar interaction of volcanic activity and tectonic fault movement is possible in the case of" recent earthquakes,
The USGS also stated that "at present there appears to be no signs of unrest in the volcanic system" and that "The USGS and the Pacific Northwest Seismic Network at University of Washington continue to watch conditions at Mount St. Helens closely."
The current volcano alert level for Mount St. Helen's is still set at normal.
Tamara L. Morris developed a special interest in weather issues and natural disasters after a tornado swept through her hometown in 1982. She is certified as a Community Emergency Response Team (CERT) member and has served locally in this capacity after a rare derecho (inland hurricane) struck her area in 2009. She researches and writes about earthquakes, volcanoes, tornadoes and other natural phenomena.