Earthquake Swarms at Yellowstone

Figure 1. A plot of recorded earthquake activity at Yellowstone from 1974 through 2004 (blue bars) along with caldera uplift and subsidence (red dotted line). The green line shows the cumulative number of earthquakes for the thirty-year period (From Smith, et. al., 2004).

There are often 1 to 20 earthquakes recorded in Yellowstone National Park every day. For a few days in April, however, the earthquake activity increased and 400 earthquakes were recorded over a three-day period. The earthquakes were small and occurred in a limited area over a limited period of time. This type of increase in earthquake activity is called a swarm. Earthquake swarms are generally defined as clusters of earthquakes closely spaced in time and area that do not have a defined mainshock. Defining the length of time of an earthquake swarm is often difficult and there are many scientific papers that discuss commonly used methods (for example Waite, G. P. and R. B. Smith, 2002). Swarms are not uncommon on volcanoes and also happen in regions without volcanic activity, e.g., January 2004 near Livermore, California. In this article we present information on two large historic Yellowstone earthquake swarms from the past twenty years and then compare them to the recent April 2004 swarm.

Figure 2. The largest recorded earthquake swarm at Yellowstone occurred during the fall of 1985. There were about 3000 earthquakes over a three-month period. Over 200 events were recorded on two separate days during this time period. The red line shows the cumulative number of earthquakes over the three-month period.

There are many ways to identify swarms — here we use the simplest method of looking for increases in earthquake activity above a baseline. A graph of earthquake activity (blue bars) from 1974 through 2004 is shown in Figure 1 along with information on caldera uplift and subsidence. Swarms can be identified on the graph by looking for the tallest blue bars. The largest spike in activity shown on the graph occurred in late 1985 with almost 1000 earthquakes per quarter. Other noticeable swarms occurred in 1977, 1995, 1999, and 2004. We plot earthquake activity and ground deformation information together to search for correlations between the two.

Figure 3. The July 1995 swarm included about 480 events over a seven-day period. Over 200 events were recorded on July 4th 1995. The red line shows the cumulative number of earthquakes over the one-month period.

Another large swarm occurred in July of 1995 west of the Norris Geyser Basin (Figure 3). There were approximately 480 events over a seven-day period ranging in magnitude from 0.2 to 3.1. In this swarm there was only one day with over 200 events, July 4th. A few days later, from July 12th to the 14th, there were an additional 92 earthquakes ranging in magnitude from 1.0 to 2.9. If we were to include these earthquakes with the earlier group, there would be approximately 590 earthquakes from July 1 through July 14 ranging in magnitude from 0.2 to 3.1. This swarm was also close in time to a change from caldera subsidence to uplift.

Figure 4. The most recent swarm from April 2004 included about 400 events over the period of three days. On April 14th there were over 200 events. The red line shows the cumulative number of earthquakes over the one-month period.

The most recent swarm, in April 2004, occurred in the northwest part of the caldera. It included about 400 events over a three-day period of time with magnitudes ranging from 0 to 2.7 (Figure 4). On April 14th there were over 200 events, however, none of the events were as large as the larger 1985 or 1995 earthquakes.

There are many interesting questions that the Yellowstone swarms raise including, "What causes the swarms?" and "Is there a correlation between the swarms and the inflation and subsidence of the caldera?". At this point we are confident that the swarms are not caused by magma, but they may be related to migration of hot water and gas (hydrothermal fluids). Scientists continue to study the Yellowstone swarms and we look forward to learning more as we compare seismic and ground deformation data.

Additional Information

• Yearly plots of earthquake activity at Yellowstone National Park for the past ten years.
• General earthquake information from the U. S. Geological Survey Earthquake program.
• Information on seismicity and earthquakes from the Cascades Volcano Observatory website.
• An article on the 1985 Yellowstone swarm as a PDF. Waite, G.P. and Smith, R.B., 2002, Seismic evidence for fluid migration accompanying subsidence of the Yellowstone caldera, JGR, v. 107, pp 2177-2192.
• An article that discusses a change from caldera subsidence to uplift. Wicks Jr., C., Thatcher, W., and Dzurisin, D., 16 October 1998, Migration of Fluids Beneath Yellowstone Caldera Inferred from Satellite Radar Interferometry, SCIENCE, VOL 282, pp 458-462.
• Animation of earthquakes at Yellowstone for the year 1998. Animations for other years can be obtained by clicking on the link at the bottom of the page.