The earthquake activity in the Yellowstone region is part of a belt of seismicity extending from southern Utah to northwestern Montana. The 1959 magnitude 7.3 Hebgen Lake earthquake is the largest historical event along this trend. Yellowstone National Park is also the focus of extensive Quaternary volcanic activity that relates to a long lived volcanic system that has progressed from southwest to northeast along the trend of the Snake River Plain, ending in a 40 by 70 km caldera formed in a volcanic eruption 600,000 years ago. Thus the seismicity shown on these maps is part of a major tectonic feature of western North America and may also be influenced by or influence the volcanic processes occurring in the region. A more complete description of the geology and geophysics of the Yellowstone region can be found in Smith and Christiansen (1980), Christiansen (1984), and Smith and Brail (1984).

The seismicity displayed on these maps occurred over a 17 year period and was recorded at a variety of seismograph stations, which has resulted in much variation in the reliability and completness of the data set. The earthquake epicenter data are presented on 2 maps. Symbols on map 1 identify events by year of occurrence with the symbol size indicating the magnitude range. Symbols on map 2 indicate the reliability of the earthquake epicenters. Variations in the level of seismicity and strain release with time are shown (Fig. 1), as well as earthquake focal-depth cross sections and representative earthquake focal mechanisms (Fig.2).