A paper written by Amy Parker in the Journal of Volcanology and Geothermal Research shows that Lassen Peak has been sinking since the mid-1990’s and it has scientists trying to figure out why. Lassen Peak erupted in 1915 causing all sorts of geothermal activity in the region but what’s baffling scientists is that the area started sinking over 70 years later.
Here is the information from an article in Wired:
The team examined satellite data from 1992 to 2010 and found that an area 30-40 kilometers (19-25 miles) across centered near Reading Peak (see below; just southeast of Lassen Peak) has been sinking at a rate of ~10 millimeters (~0.4 inches) per year. So, over that span the Lassen Peak area has subsided ~18 centimeters (~7 inches).
That measurement wouldn’t be possible without using satellites to detect very minute changes in the Earth’s surface over years to decades—in this case, with Interferometric Synthetic Aperture Radar, more commonly known as InSAR. This method uses precise measurements of the Earth’s surface performed by satellites that were captured some time apart and then compares them, looking for where the data (land surface elevation) no longer matches, producing interference in the images. Then these interferences are converted into values of up and down based on the extent of interference. Considering InSAR uses microwaves to determine the elevation, things like cloud cover and night don’t matter when the measurements are being taken. The only trick is you need to have the satellite pass over the same place multiple times to be able to compare the images.
Now, how long has this sinking been happening at the Lassen Volcanic Center? Parker and others looked at land-based geodetic data collected by leveling for the last 70 years and found no measurable evidence for subsidence prior to the early 1990s. Now, it could have been subsiding at a very low rate that wasn’t measured, although the total across that span should have been noticed if it was sinking. So, it seems that this sinking is a (geologically) recent event in the Lassen area.
So with the evidence showing that Lassen Peak has only been sinking for about 20 years, why did it take so long for the geothermal activity below the volcano to begin the sinking process? The most likely culprit is the cooling and crystallization of magma following the 1915 eruption, but one would assume that would begin immediately after the eruption. Here are a couple of theories presented in the article:
Could it be the change of flows of the magma possibly due to an earthquake?
You can also change the flow of hydrothermal fluids (water heated by magma at depth) underneath the area to prompt subsidence. There is some loose correlation between the times of greater subsidence between 2004-07 and more earthquakes within the area of the hydrothermal system, so there could be connection there. There is even the chance that the M7.3 Landers earthquake in 1992, centered about 840 kilometers (520 miles) away, might have started the ball rolling as that earthquake seems to have triggered a M3.5 earthquake at Lassen within 13 minutes.
Could it be related to the expanding of the fault line, thus sinking the volcano further into the fault?
That being said, some of the sinking might not be related to the magmatism at the Lassen Volcanic Center at all. Local faults related to the Basin and Range province, where North America is stretching, are causing a deepening basin around the Lassen Volcanic Center and the warmer nature of the crust in the area (thanks to Lassen and friends) might mean this area is susceptible to more sinking compared to cooler areas. Medicine Lake in northern California is one of the other volcanoes in the Cascades that is also sinking. Studies there point the finger at tectonic forces along with cooling of a magma body at depth, so this combination might be a common occurrence in the Cascades.
Certainly, in the coming decades we will learn much more about why volcanoes around the world are sinking. Scientists are gathering more information each day to learn about the causation of the complicated geothermal activity going on beneath the earth’s surface.
Until more answers come to light, enjoy this awesome video of the Lassen Eruption: