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Caldera records: What causes earthquake swarms in Yellowstone? | wild montana
The evolution of the 2017 Maple Creek earthquake swarm is shown in these graphs. The plots show earthquake locations colored by time. a) View the map. b) The cross section is between west and east. c) 3D view, east to southeast, along the axis of much of the swarm activity.
David Shelley, USGS
Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators at the Yellowstone Volcano Observatory. This week’s contribution is from David Shelley, a USGS seismologist.
Earthquake swarms – sequences of high seismic activity with no apparent main shock – are common in Yellowstone and many other places.
Swarms occur in a variety of volcanic and tectonic environments and have several potential causes. Some swarms are pushed by slow slips into the fault which causes earthquakes to occur on a few sticky patches of the fault. Other swarms are generated when fissures filled with magma work its way through the crust. Perhaps most commonly, swarms can be generated when aqueous fluids (water) enter and interact with pre-existing defects in the crust. Sometimes, combinations of these mechanisms may be active in a particular swarm.
But how can we distinguish between these processes? Such identification is not straightforward and remains an important topic of ongoing research, but we can examine many aspects of swarms to guide our interpretations.
At the volcano, everyone wants to know, “Was the swarm moved by magma?” This question is particularly urgent if the swarm occurs at shallow depths (within two miles from the surface). A shallow swarm driven by rocky formation is likely a precursor to a volcanic eruption.
The primary distinction between magma and water as a driving mechanism is the crack width required for the fluid to move through the crust. While water can travel through very small pre-existing cracks or defects within the crust, magma requires a thicker crack to allow the magma to continue spreading without cooling and solidifying quickly.
We are likely to observe several seismic activity characteristics associated with this difference in crack widths. For example, earthquakes caused by water usually occur as cracks in the fissure(s)/fault(s) along which fluids propagate. That is, the two sides of the crack hosting the fluid are still in contact with each other, but the fluid reduces clamping forces and lubricates the fault enough to allow it to slide.
By contrast, the magma walls that host magma are not in direct contact with each other – they are separated by the magma itself. Therefore, instead of slipping between crack walls, earthquakes will occur near the tip of the crack (the point in front of the magma where the crack begins to open) or on either side of the crack (due to the crack opening pressing on the surrounding rock).
In the event of a swarm due to slow fault slip, earthquakes frequently occur on the same small sticky spots within a dominant creeping fault. There may be no earthquake progression at all – just the same small patches that constantly generate earthquakes.
This simplified cartoon shows the differences between the expected earthquake patterns for a swarm driven by a magma-filled fault, left, versus an earthquake driven by pressurized water in a fault, right.
photo courtesy
In addition to seismic observations and the pattern of earthquake locations, we can also use deformation measurements to examine changes in surface shape over an earthquake swarm. The opening of the thick cracks needed for magma to spread results in deformation of the Earth’s surface, which becomes increasingly large and easily noticeable for shallow cracks which may be a major concern. By contrast, swarms driven by aqueous fluids pressing on pre-existing fissures would only cause very small surface deformation, often due to slip in the fissures themselves, and are usually too small to be observed. Slow fault slip, if large and shallow enough, can be observed on the surface, but the deformation pattern is quite different from that of a magma-filled crack and is therefore easy to discern.
When interpreting the process(s) that may cause a swarm of earthquakes, it is also important to consider context. In Yellowstone, for example, the last eruption of igneous rocks was a lava flow that occurred about 70,000 years ago, but the area is home to one of the most powerful hydrothermal systems on Earth. So although volcanic eruptions are rare, we do observe many small earthquakes in Yellowstone every year, and relatively large seismic swarms every few years. The characteristics of the swarms, and their context, indicate that the vast majority are driven by water moving underground.
As an example of putting all of this together, let’s consider the Maple Creek swarm in June and September 2017 – the second largest earthquake swarm ever recorded in Yellowstone. About 2,400 earthquakes, the largest of which was M4.4, occurred over a three-month period in the northwestern part of Yellowstone National Park, between the Norris Geyser Basin and Hebegin Lake. The earthquakes moved over time but were occurring in an area with many existing faults. Earthquake migration was rapid, and no deformation of the earth occurred. Observations like this indicate that water was the cause of the swarm. Water was also the most likely cause of a heavy swarm that occurred near Madison Junction in the western part of Yellowstone in 2010.
The most complex case was the 2008-2009 Yellowstone Lake Squadron. The rapid migration of earthquakes in this swarm suggests that it may have been driven by a low-viscosity fluid (such as water or carbon dioxide) that could easily move through small cracks in the Earth’s interior. But unlike other modern swarms in the park, this swarm was accompanied by a small amount of observed surface deformation, making it difficult to completely rule out the source of the molten rock. Fortunately, with improvements in the Yellowstone monitoring network since 2009, we can learn more similar swarms that may occur in the future.
Each swarm of Yellowstone earthquakes is unique, lasts from minutes to weeks and includes a few to thousands of earthquakes that may or may not move over time. Through long-term investments in earthquake and deformation monitoring, we continue to learn more about the Yellowstone system by studying this common form of seismic activity.
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