Mystery Solved? New Research Sheds Light on 1937 Anna Earthquake Damage
By Matt Bayman
Since 1875, more than 40 earthquakes have been cataloged in the Anna Seismic Zone in western Ohio, including the largest earthquake in Ohio’s recorded history, which took place on March 9, 1937 near the village of Anna in Shelby County.
The earthquake registered at magnitude 5.4 on the Richter scale and was strong enough in Anna to topple chimneys, knock over shelves of books at the library and food at the grocery store, damage two churches, rotate gravestones on their bases at nearby cemeteries and do enough damage to the high school that it needed to be razed and rebuilt.
The earthquake was felt in eight states in the Midwest, as well as parts of Canada, and it affected the flows of water, oil and gas wells near the area of the epicenter.
The March 9 earthquake was the stronger of two quakes that hit Anna that week. The first occurred on the morning of March 2 and registered at 4.9 on the Richter scale. Residents were cleaning up and repairing damage from this first quake when the second one hit. Thankfully, there were no deaths from either incident.
In the aftermath of these events, it didn’t take long to notice that while Anna was moderately damaged, other towns within a 10-mile radius experienced little to no damage, including the much larger community of Sidney. Even Botkins, located just five miles from Anna, came out relatively unscathed.
The reasons for this discrepancy remain uncertain, but a 2018 study by two geologists from the Ohio Department of Natural Resources, Daniel R. Blake and Andrew Nash, may shed new light on an answer.
Using new advancements in the field of passive seismic research, Blake and Nash mapped the subsurface of what is known as the Botkins and New Knoxville quadrangles, an area in the Anna Seismic Zone that covers northwestern Shelby County and small parts of Auglaize County and that includes Anna, Botkins, Kettlersville and New Knoxville. The epicenter of the 1937 earthquakes was in this area.
Using what is known as the H-to-V Spectral Ratio method, as well as a pair of MoHo Trominos (a geophysical instrument), Blake and Nash took hundreds of measurements in the quadrangles and were able to update a series of old bedrock and glacial drift thickness maps of the region. This process allowed them to visualize the subsurface like never before, which in turn led to new conclusions not only about the 1937 earthquakes but also about the size and shape of an ancient river valley that’s buried beneath western Ohio. (Article continues below)
Above: The epicenter
of the 1937
earthquakes in Anna.
Right: A local grocer cleans up after the earthquake. Below: Anna High School was damaged so badly that it had to be razed and rebuilt. Left: Individual homes also experienced damage.
(Images courtesy of the Shelby County Historical Society)
The Deepest Part of the Valley
In their Open File Report titled, “Mapping Bedrock Topography and Drift Thickness of the Preglacial Teays River within the Anna Seismic Zone, Ohio,” Blake and Nash acknowledge that geologists have long speculated that the reason Anna experienced more shaking than other nearby communities during the earthquakes is because it was built directly above a preglacial valley known as the Teays River Valley, which itself sits on top of faults that date back more than a billion years.
The Teays River, which had its headwaters in North Carolina and drained into what eventually became the Gulf of Mexico, flowed through Ohio from the south to the northwest and carved a deep valley through the preglacial landscape, including through parts of Champaign, Shelby, Auglaize and Mercer counties.
However, when the Ice Age came, and a series of massive glaciers covered western Ohio and then retreated, this valley was buried by hundreds of feet of glacial till (dirt and rocks left behind by the glaciers) and smoothed out until any sign of it disappeared and the landscape came to resemble the one we know today.
Older geological maps of the area (which mostly used well records) indicated that there was indeed a valley below the glacial till in Anna, which they estimated to be about 400 feet deep. However, Blake and Nash’s updated data not only shows that the valley is deeper than first interpreted, but also more gorge-like. (see below)
Even more, the two men found that Anna sits on top of the deepest part of the valley, some 651 feet below Main Street. For reference, that is taller than the Gateway Arch in St. Louis.
This means that Anna rests on top of 650 feet of gravel and dirt. By contrast, Botkins, Ketterlsville and New Knoxville were built over shallow bedrock (meaning much less glacial till), and in this case on top of flat ridges that once surrounded the Teays River Valley. Just by chance, the founders of Anna happened to build their town over the deepest part of the gorge, and the deepest accumulation of glacial till in the area. This may have led to the increased damage in the 1937 earthquakes.
The reasons for this have to do with how dense the subsurface is in the area of an earthquake.
Blake and Nash note: “Geologists widely accept that in regions where thick glacial materials overlie bedrock, there is a greater risk for destructive shaking during a large earthquake.” Adding, “The behavior of seismic waves is dependent on the density of the material through which the waves travel. Bedrock has high relative density and facilitates the transfer of seismic energy efficiently with low amplification, meaning the perceived shaking is typically low. In contrast, unconsolidated sediments like those associated with the Teays River Valley, amplify seismic waves and result in more intense shaking during earthquakes.”
Therefore, they conclude, it is likely that the ground-motion experienced in Anna was amplified in the buried valley, which is not only deeper and more gorge-like, but infilled with hundreds of feet of unconsolidated sediments that made the shockwaves even more powerful compared to nearby communities where the bedrock is closer to the surface, such as Sidney, Botkins and New Knoxville.
Additionally, Blake and Nash note that Anna’s close proximity to the earthquake epicenter was “also certainly linked” to the shaking and damage experienced in 1937. (Article continues below)
A new bedrock topography map shows Anna’s location on top of the deepest part of the ancient Teays River Valley, meaning that the town sits on top of more than 650 feet of dirt and rocks. By contrast, Botkins, Kettlersville and Knew Knoxville sit on the sides of the old valley, where there’s less glacial till between the towns and the bedrock below. (Images courtesy of the Ohio Department of Natural Resources)
New data indicates that the buried valley below Anna is more gorge-like than first interpreted.
New Insights for the Future
This new information, along with future passive seismic measurements in the region, will help the Ohio Department of Natural Resources better understand and prepare for any future earthquakes that may occur in the Anna Seismic Zone.
According to ODNR Geologist and Seismologist, Sara Kowalke in her 2017 article “Historic Anna Earthquakes May Hold Insights to Future Seismic Risk,” such findings will help the Ohio Department of Natural Resources to better visualize the bedrock surface in the Anna region and throughout the state and help identify regions of particularly thick glacial sediment that may be associated with relatively high-intensity shaking during earthquakes.”
Blake and Nash add that this, in turn, will “lead to increased understanding of the likelihood and behavior of seismic amplification from earthquakes throughout the region.”
There is no way to predict if or when an earthquake will take place in the Anna Seismic Zone, but thanks to this new research, efforts are being made to understand what can be done if they do occur, and hopefully before they occur.
To read the full article by Blake and Nash, click HERE.