Seismic gap of Michoacan, Mexico

A 150 km segment of subduction plate boundary along the Pacific coast of Mexico between the aftershock areas of the Colima earthquake (Jan. 10, 1973; M s = 7.5) and the recent Petatlán earthquake (March 14, 1979; M s = 7.6) has not experienced a major earthquake since 1911 and, thus, has been designated as a seismic gap. There has been considerable discussion in the scientific community about instrumenting this gap for intensive observation. An examination of the 1911 earthquake (M = 7 3/4), however, provides strong evidence that its location was about 280 km NNW of the epicenter reported by Gutenberg and Richter. Study of seismicity of Mexico in the past century gives some additional evidence that no major earthquake (M ≳ 7.5) occurred in the area. Thus, presently available evidence suggests that no large earthquake has occurred in this gap for at least the past 78 years and perhaps for as long as 178 years.