The unique seismicity of Mount St Helens

Most people know Mount St. Helens from the explosive eruptions it produced during 1980-1986. To this date, that period was the deadliest volcanic event in American history. But this volcano also has some very interesting seismicity resembling a drumbeat pattern because of its extremely repetitive nature, which is why I am featuring it in this post.

But first, some background. Mount St. Helens is a volcano which is a part of the Cascade Range, located in southwestern Washington State, USA. The Cascade mountain range extends all the way from British Columbia to California, housing many volcanoes alongside the mountains. The reason for this is that it is a part of the Ring of Fire: an area encompassing the Pacific Ocean which is where the majority of all the earthquakes and volcanic eruptions in the world happen.

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Mount St Helens (Lyn Topinka – CVO Photo Archive)

Mount St. Helens is still an active volcano to this day, with several recorded major explosive eruptions and many smaller eruptions in its history. 1980-1986 was one of these periods where the volcano exhibited eruptive activity, experiencing increased seismicity and explosive activity, resulting in 57 deaths.

In 1989-2001, Mount St. Helens again had periods of increased seismicity as a result of hydrothermal gas explosions. After this, it returned to a state of rest until 2004, when it was reawakened.

From 2004-2008, Mount St Helens exhibited increased seismicity again. This was unlike the previous awake periods as it didn’t actually have that many explosive events (only two! The 1980-1986 period had 17 lava dome-building episodes and hundreds of small gas and steam explosions). The other interesting quality of this reawakened period was the type of seismicity that was occurring. Small regularly-spaced earthquakes were repeatedly occurring during the eruptions. They are nicknamed “drumbeats” due to their resemblance of the sound pattern that is produced from the beating of a drum.

A days worth of seismicity during this period can be seen below, whereby each horizontal line represents 90 minutes.

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Mount St Helens day seismicity – one horizontal line is 90 minutes long.

We can even zoom into this and look at a 4 hour block (each horizontal line this time is only 30 minutes).

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Mount St Helens 4 hour seismicity – one horizontal line is 30 minutes long.

The repetitiveness of these small earthquakes is very clear to see in these images. This led scientists to wonder, what is causing these drumbeats?

Theory one (Iverson at al., 2006; Iverson, 2008; Anderson et al., 2010)
The drumbeats were due to a stick-slip motion of a piece of hardened magma (a conduit plug) being forced up through the vent which carries the magma from the magma chamber to the surface (the conduit) by ascending magma. The forcing of the plug up through the conduit causes it to interact with the sides. This happening repeatedly could then be what is causing the drumbeats.

Theory two (Waite et al., 2008)
The volcano is essentially acting like a steam engine. This would be due to there being a complicated crack system (think like those plumber games where you want to connect up all the pipes for the flow of water to begin) and a steady supply of heat and fluid from the magma chamber. This would then also cause the drumbeats to occur, similar to a train choo-chooing.

Some great analysis has been done on the similarity of these seismic signals (see References), as if the drumbeats are similar, it means that they have come from effectively the same source. This is where methods such as my correlation matrix become handy, as this measures how well correlated events are with one another. With this analysis, we can then see which events are true repeating events.

Mount St Helens is a great case study for building up any algorithm that focuses on finding any sort of pattern in seismic data, which is why I have been looking into it. This can then go towards our analysis for repeating events in earthquakes, although I doubt we will ever get as clean a signal at these drumbeats!

–Roseanne

References
Anderson, K., Lisowski, M., and Segall, P. (2010). Cyclic ground tilt associated with the 2004-2008 eruption of Mount St. Helens. Journal of Geophysical Research: Solid Earth, 115(11):1–29.

Iverson, R. M. (2008). Dynamics of Seismogenic Volcanic Extrusion Resisted by a Solid Surface Plug , Mount St . Helens , 2004 2005. In Sherrod, D., Scott, W., and Stauffer, P., editors, A Volcano Rekindled: The Renewed Eruption of Mount St. Helens 2004-2006, U.S. Geological Survey Professional Paper 1750, chapter 21, pages 425–460. USGS.

Iverson, R. M., Dzurisin, D., Gardner, C. A., Gerlach, T. M., LaHusen, R. G., Lisowski, M., Major, J. J., Malone, S. D., Messerich, J. A., Moran, S. C., Pallister, J. S., Qamar, A. I., Schilling, S. P., and Vallance, J. W. (2006). Dynamics of seismogenic volcanic extrusion at Mount St Helens in 200405. Nature, 444(7118):439–443.

Waite, G. P., Chouet, B. A., and Dawson, P. B. (2008). Eruption dynamics at Mount St. Helens imaged from broadband seismic waveforms: Interaction of the shallow magmatic and hydrothermal systems. Journal of Geophysical Research: Solid Earth, 113(2):1–22.

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