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The largest recorded earthquake swarm in Yellowstone occurred in the fall of 1985 in the northwest rim of the caldera (Figure 3). 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 Figure 1).
Other noticeable swarms occurred in 1977, 1995, 1999, and 2004.
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.