2004 Yellowstone Earthquake Swarm
An earthquake is a sudden trembling or shaking movement of the Earth's surface caused by the abrupt release of slowly accumulated strain. Tectonic earthquakes are often caused by the movement of rocks along a fault, whereas volcanic earthquakes are often caused by the movement of gases or magma. A typical large earthquake will include a mainshock, the largest earthquake in a sequence, and may be preceded by smaller earthquakes known as foreshocks and followed by smaller earthquakes called aftershocks.
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.
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 the uppermost figure 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.
The largest recorded earthquake swarm in Yellowstone occurred in the fall of 1985 in the northwest rim of the caldera. The three-month period of increased earthquake activity included over 3000 events of magnitude 0 to 4.9 (about 1800 events with magnitudes between 1 and 4.9). Interestingly, the 1985 swarm was close in time to the change from caldera uplift to subsidence (see uppermost figure).
Another large swarm occurred in July of 1995 west of the Norris Geyser Basin (Figure 4). 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.
The swarm in April 2004, occurred in the northwest part of the park. It included about 400 events over a three-day period of time with magnitudes ranging from 0 to 2.7 (Figure 5). 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.