Repeating earthquakes suggest volcanic and tectonic origins of Yellowstone seismic swarms
Yellowstone is located in a region of the United States where both volcanic and tectonic processes occur. Yellowstone is also at the edge of the Basin and Range Province and therefore experiences tectonic strain associated with slow spreading of the earth's crust. Earthquakes associated with this type of movement are considered "tectonic." In addition, magma accumulation and hot waters provide another source of stress that affects Yellowstone. In a study published in 2013, University of Utah seismologists studied patterns of earthquakes at Yellowstone, grouping events according to similarity of their ground motion records. They found that some earthquakes appear to repeat over time, while others are unique, without a clear precursor in the past 30 years (the period when seismic data have been collected). Repeating earthquakes are interpreted to be dominantly tectonic, whereas the more unique earthquakes may be induced by intrusion of molten rock or hot pressurized ground (hydrothermal) water along pre-existing fractures.
The senior author of the publication is postdoctoral fellow Fred Massin. He and his University of Utah colleagues carefully examined more than 33,000 earthquakes between 1984 and 2010. They found that over 15,000 were "repeating," meaning that they came from the same location with similar motions from the same seismic sources, and thus produced nearly identical shaking characteristics recorded at distant seismometers.
The Utah researchers then analyzed earthquakes from different earthquake swarms -- episodes where dozens to hundreds to thousands of earthquakes are closely clustered in space and time, but without a clear main-shock. They found that many of Yellowstone's earthquake swarms consist of families of repeating earthquakes called multiplets, and these events are generally more prevalent at times when the geodetically-measured caldera ground motion is transitioning from an episode of uplift to subsidence or vice versa.
Two major swarm sequences at the western edge of the caldera are interpreted as magmatic fluid induced: the 1985 swarm, and the relatively recent 2010 swarm. Both contained numerous repeating earthquakes. Also in both cases, modeling of fluid injection on vertical volcanic dikes was used to show that expansion of the swarm fronts was a result of flow along pre-existing fractures of hot pressurized waters (hydrothermal fluid) for the 2010 swarm, and of magmatic fluids for the 1985 swarm.
Another example of migrating Yellowstone swarm activity occurred in 2008-2009 in northern Yellowstone Lake. This energetic swarm exhibited an extraordinarily high migration rate of ~1 km/day and is interpreted to be associated with magma injection along a pre-existing vertical fracture at a volumetric rate of up to 5 m3/s. This process produced a succession of migrating swarm fronts (Figure 3). Very few repeating earthquakes were identified, and the authors concluded that the sequence was initiated by volcanic pressurization that caused the generation of new fractures.
This research was supported by the Brinson Foundation, the University of Utah, and private research sources. The digitally recorded earthquake data were from the Yellowstone Seismic Network, which is operated by the University of Utah and funded by the U.S. Geological Survey and the National Park Service as part of the Yellowstone Volcano Observatory. Online information on Yellowstone earthquakes can be found at the University of Utah Seismology and Active Tectonics Research Group's website.