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Are small magnitude earthquakes in N. Sonora aftershocks of the 1887 M7.5 earthquake?

Magnitude (M) 3.9 and M3.5 earthquakes occurred in rapid succession in the early evening of Thursday, 10/10/19, causing minor shaking in Douglas, AZ. The M3.9 earthquake occurred at 6:11 pm local time, followed 26 minutes later by the M 3.5 event. Both earthquakes were located in northernmost Sonora south of the Arizona – New Mexico border, near the northern end of surface ruptures associated with the M7.5 1887 Sonoran earthquake.

The 1887 earthquake is by far the largest historical earthquake in the southern Basin and Range province, and is about as large as any historical earthquake in the Intermountain region of western North America. The general area where yesterday’s earthquakes occurred has experienced intermittent bursts of small to moderate earthquakes in the subsequent ~130 years since the 1887 event, including in 1988-89 and 2008. The largest earthquake of these fairly recent events was M 4.2 in 1989. We don’t know for sure what is causing this seismic activity, but is possible that it reflects continuing stress adjustments following the 1887 earthquake – essentially a very long aftershock sequence.

Seismograms of M3.9 event
Seismograms from the three Arizona broadband seismic stations nearest to the earthquake event of 10/10/19. Provided by Jeri Young Ben-Huron (AZGS).

For the location of faults and historic earthquake epicenters, please visit the interactive Natural Hazards in Arizona viewer.

Some other online resources.

Phil Pearthree, Arizona State Geologist