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September 5, 2022

Many people assume that earthquakes at Yellowstone are due to magma and the region “heating up.” It turns out that most Yellowstone seismicity is caused by water coming from crystallizing magma.

Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from David Shelly, seismologist, and Shaul Hurwitz, research hydrologist, both with the U.S. Geological Survey.

This is a plot showing the evolution of the 2017 Maple Creek earthquake swarm.
Evolution of the 2017 Maple Creek earthquake swarm. Plots show earthquake locations colored by time. a) Map view. b) West-east cross-section. c) Three-dimensional view, looking from the east-southeast, along the axis for much of the swarm activity.

It is well established that pressure variations associated with water flowing through the crust (the outermost layer of the Earth) can trigger earthquakes.  This is because pressurized fluids inside a fault zone can partially counter the force pressing the two blocks of rock on both sides of a fault together, making it easier for those blocks to slip along the fault.  In other words, water in faults can help to promote earthquake activity.

Water is known to be a cause of earthquakes at Yellowstone and may be particularly important in triggering earthquake swarms, where many earthquakes are clustered in space and time and are not preceded by a large earthquake, or mainshock.  Although moving magma can also trigger earthquakes, this process creates different patterns of seismicity, and triggering by water is thought to be much more common in Yellowstone.

There are two main sources of water in the crust at Yellowstone.  Water emerging from thermal springs and geysers in Yellowstone is dominantly meteoric water, originating as rainfall or snowmelt that percolates into the ground, is heated, and returns to the surface.  A second, perhaps less obvious source of water in the crust is from crystalizing magma beneath Yellowstone Caldera.  As the rhyolite magma underlying Yellowstone crystallizes, the minerals that form (mostly quartz and feldspar) do not incorporate water (H2O), carbon dioxide (CO2), chlorine, or sulfur in their structure. The concentration of these components (called “volatiles”) increases in the magma until the magma can no longer contain them (the magma becomes supersaturated with respect to these components), and they are released into surrounding rocks. In rhyolite, the concentration of water is much higher than the concentrations of the other volatile components. The rocks surrounding the magma into which the water is released are “ductile,” meaning that they flow or bend rather than break.

Cartoon showing differences between magma- and water-driven earthquake swarms
Simplified cartoon showing differences between patterns of earthquakes expected for a swarm driven by a magma-filled crack (left) versus one driven by pressurized water in a fault (right).

However, as the volatiles (mainly water) flow upward or outward, away from the magma beneath the caldera, they eventually encounter rocks that are cooler and more brittle (rocks that tend to break).  This transition from ductile to brittle rock is important for two reasons.  First, while ductile rocks tend to deform gradually, brittle rocks may accumulate stress until they suddenly break and are thus capable of generating earthquakes.  Second, the pressure applied by the water released from the magma tends to be much higher within ductile rocks compared to within brittle rocks. This means that when water flows from a ductile zone to a brittle zone, the fluid pressure in the brittle zone may suddenly increase, potentially triggering an earthquake.  Then the fault on which the blocks of rock slipped in the first earthquake becomes a permeable pathway for water-rich fluids, potentially causing other earthquakes to be triggered near the periphery of the first earthquake.  When the crust is stressed, as is usually the case in Yellowstone and other tectonically active areas, this process can lead to high rates of seismicity—in other words, an earthquake swarm.  This is exactly what is thought to have happened during the Maple Creek earthquake swarm of 2017—the second-largest/longest swarm ever recorded in Yellowstone!  It began as pressurized water caused faults to rupture, and the resulting pathways that opened allowed for more faults to break, almost like a runaway process.  But it is also the cause behind smaller swarms, like that which occurred during the past week or so.

Schematic cross section of the magmatic and hydrothermal systems underlying Yellowstone Caldera
Schematic cross section of the magmatic and hydrothermal systems underlying Yellowstone Caldera, showing magmatic volatiles emitted during crystallization of the rhyolitic magma and/or from basalt intrusions or convection, and the hypothesized relation with earthquake swarms on the caldera margins.  The exsolved fluids accumulate at lithostatic pressures in the ductile region of the subsurface and under very high strain rates are episodically injected into the brittle crust, where the fluid pressures are generally much lower.  The sudden increase in fluid pressure in the brittle region then triggers earthquake swarms.  Figure modified from Lowenstern and Hurwitz (2008), Shelly et al. (2013), and Hurwitz and Lowenstern (2014).

This brings us to another question:  What about the historic floods that occurred in Yellowstone in June 2022?  That was a lot of water!  Did that event trigger any earthquakes?  The answer is probably not.  In most places it takes a long time for water to percolate to depths where earthquakes are likely (usually at least a few miles deep), and it doesn’t all arrive at the same time. Sometimes it’s also possible to trigger earthquakes simply by the weight of the water on the surface stressing rocks below, as is occasionally observed when a new water reservoir is filled.  However, compared to a large reservoir that might collect deep water for many months or years, the 2022 floods were much more distributed in space and brief in time, despite their destructiveness at the surface.

In summary, although Yellowstone’s thermal features mostly discharge meteoric water (rainfall and snowmelt) that has percolated into the subsurface, earthquake swarms in Yellowstone are often triggered by a deeper source of water. Crystalizing magma deep beneath Yellowstone Caldera slowly releases water-rich fluids that accumulate in hot, ductile rocks.  These fluids episodically migrate upwards and/or outward into cooler, brittle rocks, sometimes triggering an earthquake swarm.  So, in effect, many earthquakes in Yellowstone are triggered by the cooling and crystallization of the underlying magmatic system, rather than by movement of magma!

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