Their modifications were mainly to degrees IV and V, with VI contingent on reports of damage to man-made structures, and VII considering only "damage to buildings or other man-made structures". See details at Stover & Coffman 1993, pp. 3–4. Stover, Carl W.; Coffman, Jerry L. (1993), "Seismicity of the United States, 1568 – 1989 (Revised)"(PDF), U.S. Geological Survey, Professional Paper 1527.