Abstract

Contouring the intensity data for the northern Sonora earthquake of 1887 yields an area of perceptibility defined by the MM III isoseismal that is greatly elongated to the southeast of the epicenter along the Sierra Madre Occidental. North of the epicenter, the isoseismals are compressed due to the change in crustal structure between the Colorado Plateau and the southern Basin and Range province. The higher intensity isoseismals are more symmetric, although local structure strongly influences individual intensity values. To evaluate the attenuation of intensity with distance, three regression curves are calculated as below 
alldata:I=12.0+3.20.0015R1.5In(R)25500km:I=12.0+0.690.0069R0.82In(R)Isoseismal:I=12.0+8.10.0031R2.3In(R)
where R is the epicentral distance in kilometers. The “All data” curve is derived from a regression computed using all 171 intensity values. The “25-500 km” curve eliminates the near and far points, and the “Isoseismal” curve is a regression on the average radii of the area enclosed by each isoseismal contour. The latter two curves are similar to curves for the Western United States as a whole (Anderson, 1978) or the Cordilleran curve of Howell and Schultz (1975) for intensities less than or equal to MM X. The curve for “All data” indicates less attenuation at lower isoseismals than the other two 1887 regressions and falls between the Cordilleran and Eastern United States models of Howell and Schultz.

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