The great Sonoran earthquake (M>7.2) ruptured the Pitaycachi fault on May 3,1887 and is the largest historic earthquake in the southern Basin and Range Province.
With the dawn of the atomic age seismic instruments became a premiere watch dog of world wide nuclear explosion testing.
This earthquake is significant for two reasons: First, just one year prior to this earthquake, Chinese scientists sucessfully predicted a major earthquake in this region. The evacuation saved many lives. Second, the Chinese scientists were not sucessful in predicting the Tangshan earthquake which resulted in between 250,000 and 500,000 deaths.
Although this earthquake occurred 200 km from Mexico City, the shaking of loose sediments in the capitol city was much stronger than at the epicenter. Nearly 10,000 people died and central Mexico City was heavily damaged. Liquifaction of the lake bed sediments was a critical factor.
The 1887 Pitaycachi Earthquake
Historical Event
The great Sonoran earthquake (M>7.2) ruptured the Pitaycachi fault on May 3, 1887 and is the largest historic earthquake in the southern Basin and Range Province. The Pitaycachi fault ruptures infrequently and the recurrence interval between large earthquake is at least 100,000 years (Bull and Pearthree, 1980). Large earthquakes with long recurrence intervals typically have protracted aftershocks. It is possible that earthquakes occurring in the 1980s are aftershocks from the great Sonoran earthquake; however, the size (magnitudes >4.0) and number of events since March 1987 are very unusual.
Recent Earthquakes in Northern Sonora Mexico
On May 25, 1989 a 4.2 magnitude earthquake shook Agua Prieta, Sonora, Mexico and Douglas, Arizona. The epicenter of the earthquake, which occurred at 12:43 am local time was in the San Bernardino Valley (Figure 1 and 2). Although no damage was reported, at least three ranches noted significant changes in water-well levels. The earthquakes was followed by numerous smaller events, the largest of which had a magnitude of 3.4 and occurred the following day (Table 1) during the night hours of May 26-27 and June 8-9, the seismic station at Tucson recorded more than 50 microseisms (M>2.0). The recent earthquakes appear to be a continuation of seismic activity that began in 1987 near the Pitaycachi fault in Sonora, Mexico.
Figure 1 Location map of Pitaycachi Fault region, showing epicenter of March-June 1989 earthquakes. Solid line indicates surface rupture due to 1887 earthquake. Note discontinuity near the northern part of El Tigre Valley.
Figure 2 Elevations for the San Bernardino and El Tigre Valleys projected onto a north-south profile. The vertical displacement from the 1887 great Sonoran earthquake is also shown. Near Colonia Morelos, the 1887 fault trace takes an abrupt bend and crosses into the El Tigre Valley. It is also at this point that the fault displacement from the 1887 earthquake drops to less than 1 meter. Between this elevation and ground-breakage profiles, the location of recent seismicity is projected onto the north-south profile. Note that the activity is concentrated near Colonia Morelos.
Discussion and Analysis
The May 25th event was large enough to determine the type of faulting from the seismograms. The fault apparently slipped in an oblique normal faulting episode. The seismic data is consistent with a fault that strikes N36E, dips 65 W and has a slip direction of vertical normal slip and left lateral. . This type of oblique slip is very common in the Basin and Range Province. The events of May 25-27 occurred near a bend in the Pitaycachi fault. in the southern part of the San Bernardino Valley. The fault appears to have a major structural discontinuity in the region of these earthquakes. The northern segment of the 1887 surface rupture is along the east side off the San Bernardino Valley and strikes approximately north-south. At 30 degrees 50 minutes N latitude, the fault makes an abrupt bend to the southwest and runs along the eastern margin of the El Tigre Valley. this bend is near the confluence of the Rio San Bernardino, which flows from the north and the Rio Bavispe, which flows from the southeast' beyond their confluence, these streams drain to the south through the El Tigre Valley.
Figures and summary adapted from articles by Terry Wallace and Philip Pearthree originally published in Arizona Geology, vol. 19, no.3, Fall 1989 pp. 6-7.