Things are usually quiet near Oregon's version of Mount Rainier, Mount Hood. However today, September 24, 2018, something seems to have changed. A low frequency earthquake appeared near Mount Hood. Yes, you saw that correctly! A low frequency earthquake, possibly indicative of being involved with magma or magmatically derived volcanic gas, has occurred just 15 miles west of Mount Hood's summit. This is not something that is questionable. This really was a low frequency event. So what does this mean? Well this seems to have been an isolated incident, so far, and seismicity remains at low levels. But if swarms start to break out, it is probably a good idea to stay away and monitor the area. I am not saying an eruption is coming! Just that this was not glacier or tectonic activity and extremely likely to be involved with magma in some way due to the characteristics of the event. Could this be a sign that something is changing or is all as it should be? Of course low frequency events do occur from time to time at volcanoes worldwide, many times without eruptions occurring or being in danger of occurring, but it is always a good idea to pay attention to anything caused by magma. Please click this post title or "read more" to read more.
In the image directly above you can see the location of the earthquake (marked by a red star) and the locations of the two seismic stations I retrieved data for.
On September 24, 2018 at 22:01UTC (3:01pm Pacific Time) a magnitude 1.9 earthquake struck at 5.3km in depth just west of Mt. Hood. Now the earthquake did not occur under the stratovolcano itself, but just beyond its base to the west. I thought this was just a normal tectonic earthquake... Until I decided to download the data and review it for myself. First, I would like to show the spectral analysis of this event from two stations at Mount Hood: TIMB and VLL.
The following is a spectral analysis of this earthquake from two seismic stations at Mount Hood. This does NOT show horizontal time period. Vertical is power and horizontal is frequency range. Note the power of this earthquake weakens substantially after 4-5HZ.
You can tell there are dominant frequencies of about 1-4HZ therefore characterizing this event as a low frequency earthquake, sometimes called "long period" earthquakes. And as you can clearly see below, this event was definitely of the long period variety. Note length of coda (end tail of an earthquake) vs. amplitude and frequency. This was for sure an interesting event! Below are the seismogram/spectrogram analysis images of this particular earthquake. Data provided by IRIS and I used the seismic program SWARM.
This one is taken from seismic station VLL which is a little bit north of the northern base of Mount Hood. Once again, please note the long period, low frequency characteristics indicative of magma involvement. The very long coda (end tail of an earthquake) can be seen better on the spectrogram. Please note the dominant frequencies between 1-5HZ. For how close this earthquake was to these stations, and how large it was, this can officially be categorized as a long period, low frequency earthquake. The large majority of these types of earthquakes are almost always involved with magma somehow. There is nothing indicative of a coming eruption, yet. However this type of earthquake has not been seen very often lately at cascade volcanoes. What does this mean? Only time will tell. Please monitor this area with me. If you ever need any advice or any help, please feel free to email me. Be safe out there and God bless!
Ben Ferraiuolo is a fast learner and someone who will always stand for the truth. Visit "About Me" for more!