A very large earthquake took place on 28 March 1787 along the Mexican subduction zone. A unique characteristic of the 1787 event is the large tsunami reported at various coastal locations in southern Mexico. The segment of the coast affected by the strong tsunami coincides with locations where high felt intensities (modified Mercalli intensity [MMI] >VIII) were reported. Assuming, as is generally the case for large and great earthquakes in Mexico, that the fault rupture of the 1787 earthquake encompasses the areas where intensities greater than VIII were reported and where a strong tsunami invaded land, the estimated length of the rupture is approximately 450 km long. This fault length would correspond to an earthquake of approximately Mw 8.6. In the instrumental record there is no evidence of earthquakes of this magnitude in the Middle American subduction zone. This evidence indicates that the rupture area of the 1787 earthquake was at least three times longer that those normally observed for earthquakes that take place in the Mexican subduction zone. The 1787 earthquake appears to have ruptured a long fault segment that more recently broke in earthquakes of more moderate magnitude (Mw 7–8) and relatively short recurrence times of about 30 to 40 yr. Examples of this mode of variable rupture length where great but infrequent earthquakes rupture the fault areas of relatively smaller and more frequent earthquakes have been observed in the Sumatra–Andaman and Colombia–Ecuador plate boundaries, among others.

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