July 12, 1996 Auglaize County Earthquake

(from Fall 1987 Ohio Geology)

In contrast to the 5.0 Richter magnitude earthquake in Lake County on January 31, 1986, the quake that struck Auglaize County in western Ohio on July 12, 1986, drew comparatively little attention. Although the Auglaize County earthquake was smaller (4.5 Richter magnitude), it was strongly felt in most of Ohio and parts of Indiana, Michigan, Kentucky, and West Virginia.

It is probable that the early hour (4:19 a.m., EDT), the day (Saturday), and, by no small means, the fact that the July 12 event was not located near a nuclear power plant all contributed to the lesser attention given the July earthquake.

This earthquake was centered in western Auglaize County, near St. Marys. Damages were relatively minor. Electric power to about 30 homes in St. Marys was disrupted and the Joint Township District Memorial Hospital in this community suffered cracked plaster and dislodged floor tiles. Little additional damages were reported.

The 1986 Auglaize County earthquake occurred on the Anna-Champaign fault, a northwest-southeast-trending structure that has been interpreted to extend from Champaign County through Shelby, Auglaize, and Mercer Counties. This fault passes beneath the Shelby County community of Anna, the site of the most extensive earthquake damages in Ohio, in 1937.

Two earthquakes, on March 2 and March 9, 1937, had Richter magnitudes of 4.7 and 4.9, respectively, and caused considerable damage in the town of Anna. Although these events were reported at the time to have had epicenters at Anna and were later reported to have had Richter magnitudes as high as 5.5, recent analysis of the seismic records by seismologists at the U.S. Geological Survey have adjusted the magnitudes downward and relocated the epicenters. These newly assigned epicenters are in the general vicinity of the epicenter of the July 12, 1986, event, although the paucity of seismographs in 1937 makes such relocations somewhat imprecise.

The July 12, 1986, St. Marys earthquake casts some doubt on the relocated epicenters of the 1937 events. There has been speculation that Anna received the brunt of damages from the 1937 earthquakes because the community is located directly above the sediment-filled valley of the preglacial Teays River. This ancient river apparently followed the trace of the Anna-Champaign fault until it was obliterated by an early Pleistocene glacier and its valley was filled with nearly 500 feet of glacial sediment. The speculation is that ground motion is amplified by these unconsolidated sediments in the Teays River valley and that Anna, located over the buried valley, receives a greater degree of shaking than nearby communities located on shallow bedrock.

If this relationship was completely accurate, the 1986 earthquake at St. Marys in Auglaize County should have caused considerable shaking and perhaps some damages in Anna, similar to that of the 1937 events. This does not, however, appear to be the case and suggests that the 1937 earthquakes may have been closer to and perhaps southeast of Anna. Douglas Christensen of the University of Michigan has reached a similar conclusion corroborated by a seismic record made by a Jesuit priest in Wapakoneta (Auglaize County, north of Anna) on a three-component seismograph during the 1937 earthquakes. According to Christensen, first motion data from these events indicate that the 1937 quakes could not have been in the quadrant southwest of Wapakoneta (where the 1937 quakes were recently relocated by the U.S. Geological Survey) and they were most likely in the southeast quadrant, which would place them at or southeast of Anna.

Such a relocation of the 1937 earthquakes would appear to be more consistent with the felt reports and damages from these events and would fit the observed pattern of shaking during the 1986 earthquake. This exercise in speculation emphasizes the uncertainty involved in locating historic earthquakes from which there are no seismic records or, as in the case of the 1937 Anna events, only sparse instrumental records. The degree of uncertainty of epicenter location, or the "error ellipse" associated with a particular historic earthquake, becomes even greater with many older events for which there are only sparse newspaper accounts.

Interestingly, the July 12, 1986, earthquake was not accompanied by foreshocks or aftershocks. The epicenter was located within the Anna network, a series of nine seismic stations operated from 1977 to 1992 by the Seismological Observatory of the University of Michigan. This network had the capacity to record very small microearthquakes in the area and would have recorded even tiny aftershocks.

The highest Modified Mercalli intensity observed for the July earthquake was VI, and confined to a small area surrounding the epicenter. Intensities of V were generally observed in a broader area surrounding the epicenter; however, several isolated areas of intensity V were delineated from isoseismal studies by Carl Stover of the U.S. Geological Survey. These areas are in northern Clermont County and along the Adams-Brown County line near the Ohio River. A small area of intensity V was reported in northern Union County. The reasons for these isolated areas of intensity V are uncertain.

Data for July 12, 1986, Western Ohio Earthquake