And a new Natural Hazards Viewer
The recent 5.2 magnitude earthquake in southeastern Arizona shook things up and peaked interest in natural hazards. (See ADI story). The largest earthquake recorded in Arizona occurred more than a century ago.
On May 3, 1887, Southern Arizona and Sonora, Mexico were shaken by a 7.2 to 7.5 magnitude earthquake. The epicenter was located along the Pitaycachi fault in the San Bernardino Valley in Sonora, about 40 miles south of Douglas, Arizona, but shaking was felt and rumblings were heard as far north as Phoenix. Ground motion lasted up to 10 minutes.
It is reported that the earthquake caused church bells to ring as far south as Mexico City and many buildings in Mexico collapsed resulting in about a dozen deaths.
The 30-mile-long fault scarp shows displacements of up to 15 feet. According to McGarvin (1987), “Numerous rockfalls were reported in the mountain ranges of southeastern Arizona and northern Sonora. Sparks from the crashing boulders ignited dry brush and grass, and fires quickly spread to the forests.” But the shaking was not enough to dislodge many balanced rocks in Southern Arizona (see: Precariously Balanced Rocks and earthquakes).
McGarvin goes on to say, “Nearly all the valleys experienced changes in water conditions. Wells that had been excellent sources of water went dry, whereas artesian conditions and temporary lakes were created in other areas. One of the more colorful descriptions of the event came from Charleston, Arizona (near Sierra Vista), where ‘the walls of the saloon did a two-step and the floor did a shimmey’”
This fault and resulting earthquake were the result of on-going crustal extension that formed the Basin and Range province of Sonora, Arizona, Nevada, and Idaho. The Arizona Geological Survey (AZGS) considers another earthquake of this magnitude unlikely on the Pitaycachi fault. Based on analysis of soils and morphological analysis of fault scarps, AZGS estimates that such earthquakes would occur along a particular fault once every 10,000 years in Southeastern Arizona. “However, the major earthquake that occurred in 1887 in northeastern Sonora is evidently part of a series of 5 or 6 surface-rupturing earthquakes that have occurred since 20,000 years ago in a N-S-trending zone straddling the Arizona-New Mexico border with surface ruptures averaging 3,000 to 4,000 years apart”. Pearthree (1986), notes that such faulting has occurred from Tucson, east to the border during the last 120,000 years.
AZGS speculates that the most likely place for future earthquakes in Arizona (other than near Yuma and the San Andreas system) is in northwestern Arizona where the Basin and Range extension is more active.
You can download a special paper on the earthquake from the AZGS depository here (18Mb). It is over 100 pages long and contains many historical photos. The paper is referenced below: DuBois.
To find out more about when and where earthquakes occur in Arizona, check out the new Natural Hazard Viewer of Arizona website, http://data.azgs.az.gov/hazard-viewer. The viewer was developed by the Arizona Geological Survey (AZGS) in partnership with the Arizona Division of Emergency Management (ADEM). It covers earth fissures, active faults, earthquakes, flood potential, and fire risk. Click on the hazard that interests you. The screen shot below shows where earthquakes of magnitude 4 and greater have occurred in Arizona. On the website, you can “mouse over” each circle to get the date and magnitude. (By the way, the circle that includes Tucson was an earthquake that occurred March 13, 1965, magnitude 4.4.)
Dubois, S.M. and Smith, A.W., 1980, The 1887 Earthquake in San Bernardino Valley, Sonora: Historic accounts and intensity patterns in Arizona, Arizona Bureau of Geology and Mineral Technology, Special Paper 3 (Download, 18 Mb).
McGarvin, T.G., 1987, The 1887 Sonoran Earthquake: It wasn’t our fault, Fieldnotes, Arizona Bureau of Geology and Mineral Technology (now the Arizona Geological Survey), Vol. 17, no.2.
Pearthree, P.A., 1986, Late Quaternary Faulting and Seismic Hazard in Southeastern Arizona and Adjacent Portions Of New Mexico and Sonora, Mexico, Arizona Geological Survey, Open-File Report 86-8.
Sumner, J.R., 1977, The Sonoran Earthquake of 1887, Bulletin of the Seismological Society of America. Vol. 67, No. 4, pp. 1219-1223.