Collapsed adobe homesteads in Sonora, Mexico.

Intensity map for 3 May 1887 earthquake.

“On May 3, 1887, a major earthquake shook much of the southwest United States and Mexico, an area of nearly two million square kilometers.”  That’s the lead sentence in Susan Dubois’ and Ann Smith’s historic account of the effects of one the largest historical earthquakes to rock northern Mexico and the southwestern U.S.

Dubois and Smith used newspapers, oral histories, pioneer journals, military reports, and personal correspondence to reconstruct the impact of the earthquake on communities throughout Sonora, Mexico, Arizona, New Mexico and Texas.  See the isoseismic map for a distribution of intensity on the Modified Mercalli Intensity Scale.

The epicenter of the estimated 7.4 M event was about 65 km south of Douglas, Arizona, on the Pitaycachi Fault.  The resulting fault scarp broke the Earth’s surface for about 100 km, with the northern tip exposed just 8 km south of Douglas.  The average vertical offset on the fault was a whopping 3 meters (9 feet); maximum displacement was between 4.5- and 5.1 meters (16 feet)!   It is the largest earthquake to have caused historical damage in Arizona – either as a territory or state.

The community of Tepic, Sonora, about 190 kilometers south of Tombstone, Arizona, was flattened by the event; most homes were destroyed, irrigation ditches broken, and the town plaza and environs “ripped up” by ground fissures.  In northern Sonora, 51 lives were lost.

Fault scarp with 2 meter vertical offset.

Ground shaking in southeastern Arizona lasted 1- to 3-minutes.  Throughout the impacted area, homes were damaged, roads were disrupted, and rock avalanches occurred in nearby mountain ranges.

In 1887, fewer than 90,000 people lived in the Arizona Territory.  Now over five million people inhabit the metropolitan areas of Phoenix and Tucson combined.  While events such as the 7.4 M 1887 earthquake (7.2 M) are rare in the southwestern U.S., excluding California, they do occur.  (Geomorphic evidence points to a recurrence interval of more than 100,000 years on the Pitaycachi Fault).   Nonetheless, civil authorities, geologists, and the broader public must ask, “are we ready?”.

Resources

The 1887 Earthquake in San Bernardino Valley, Sonora: Historic Accounts and Intensity Patterns in Arizona, by S.M. DuBois and A.W. Smith, 1980, 112 p, Bureau of Geology and Mineral Technology, Special Paper No.3.  Printed copies are available through the Arizona Geological Survey for $8.75 plus shipping.

The 1887 Sonoran Earthquake:  It wasn’t our fault, by Tom G. McGarvin, 1987,  Fieldnotes, p. 1-2.

Historic Earthquakes, Northern Sonora, Mexico, 1887, May 03, U.S. Geological Survey, http://earthquake.usgs.gov/earthquakes/world/events/1887_05_03.php

Active Tectonics of Northeastern Sonora, Mexico (Southern Basin and Range Province) and the 3 May 1887 Mw 7.4 Earthquake, by Max Suter and Juan Contreras, 2002, Bulletin of the Seismological Society of America, v. 92, no. 2, p. 581-589.