The data shown on this map and listed in table 1 are for earthquakes that were originally included in a study of seismic risk in the United States (Algermissen, 1969). This data file has been recompiled and updated through 1981. Some revisions of epicenters and intensities in the original file have been made, and intensities have been assigned to earthquakes that previously had none assigned. Only earthquakes that had epicenters within the boundary of the state are listed in table 1, even though earthquakes that had epicenters in bordering states or countries may have been felt or may have caused damage in the state. Intensity values were updated from data sources that were not available at the time or original compilation. Some epicenters were relocated on the basis of new information. The data shown in table 1 represent best estimates of the epicenter, magnitude, and intensity of each earthquake on the basis of historical and current information. Some of the aftershocks from large earthquakes are listed, but not all, especially for the earthquakes that occurred before seismic instruments were universally used.

The data in table 1 were used to compile the seismicity map. The latitude and longitude coordinates of each epicenter were rounded to the nearest tenth of a degree. The triangles on the map represent epicenters plotted to the nearest tenth of a degree. The number of earthquakes at each location is shown on the map by the number to the right of the triangle. A Roman numeral to the left of a triangle is the maximum Modified Mercalli (MM) intensity (Wood and Neumann, 1931) of al earthquakes at that geographic location. The absence of an intensity value indicates that no intensities have been assigned to earthquakes at that location. The four-digit number shown below each triangle is the latest year for which the maximum intensity was recorded.