A magnitude 3.9 earthquake was observed near Mount Hood late in the day on June 5. Minor shaking was reported by people living around the volcano, but earthquakes this small do not typically produce damage. It has produced many aftershocks in what geologists are calling an earthquake swarm. The USGS defines an earthquake swarm as “a sequence of earthquakes in a small geographic area that are of similar size.”
As of the June 11 update from the Cascades Volcano Observatory, over 100 aftershocks of the June 5 earthquake had been observed by seismologists. The largest thus far were magnitude 2.8 and 2.6 earthquakes which struck on the evening of June 10. Seismologists and volcanologists at the volcano observatory noted in their weekly update that they “do not believe that this swarm signifies a change in volcanic hazard at Mount Hood.”
Earthquakes are not an uncommon occurrence at volcanoes, even when they are not erupting. Many things are going on under the surface of any active volcano that can cause the earth to shake. This includes moving magma and groundwater as well as surface sources such as rockfalls. On top of these hazards, geologists recently discovered a series of fault lines leading from around Mount Hood north to the Columbia River. These appear unrelated to the volcano itself, but do have active movement along them.
The geologists who discovered the fault lines note that they are capable of producing earthquakes as large as magnitude 7.2. Unlike the risk the Pacific Northwest faces from the Cascadia subduction zone, these would be crustal in origin. Rather than looking at a situation like the 2011 Japan earthquake and tsunami, this would be more similar to some of the larger earthquakes that are possible in the Columbia Basin.
Still, like all active Cascade volcanoes, Mount Hood is likely to erupt eventually. Hazards such as pyroclastic flows, deep ashfall, and large rocks falling are limited to within a few miles of the volcano itself and are more likely on the south side of the summit. Lahars, which are rapid mudflows resulting from volcanic debris mixing with melted glacial ice, may occur on the Sandy River, Hood River, and Deschutes River depending on how violent the eruption is and how much ice is present. A strong lahar traveling down the Sandy River has the potential to reach Troutdale and the Columbia River in about four hours.
Lava flows from related vents located near, but not on, the volcano itself are possible. Some other volcanoes that pose this hazard include Mount Adams and Indian Heaven in Washington as well as Newberry in Central Oregon and Medicine Lake in Northern California. Depending on weather patterns and the type of eruption, lighter ash may travel hundreds or thousands of miles downwind.
Some hazards are present even without a volcanic eruption. Earthquakes, of course, are one of these. Debris flows and small lahars can also occur without an eruption. This would most likely result from some sort of landslide as the acidic groundwater system slowly erodes some of the rock and can be triggered by heavy rain or snow. One such event happened in 1980 (this was unrelated to the Mount St. Helens eruption in the same year).
All of that said, people living near Mount Hood or any Cascade volcano have no reason to live in constant fear of its hazards. These volcanoes are closely monitored by scientists at volcano observatories. The Cascades Volcano Observatory in Vancouver, Washington monitors volcanoes in Washington, Oregon, and Idaho and publishes weekly updates on activity in the region. The California Volcano Observatory keeps tabs on volcanic hazards in California and Nevada with monthly updates. In total, the USGS operates five volcano observatories. With the amount of monitoring equipment on the Cascade volcanoes, it is extremely unlikely that an eruption will occur without warning.
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