Ohio is situated deep within the North American continental interior, far from the dynamic edges where tectonic plates meet. The question of whether Ohio has a fault line often arises from the perception that seismic activity is limited to states along major plate boundaries, such as California. Despite this stable location, Ohio has a history of earthquakes and a complex geological structure beneath its surface. The state is not actively splitting apart or sliding along a visible rift, but its basement rocks retain the scars of ancient, immense geological events, which can occasionally be reactivated. Understanding Ohio’s seismic potential requires looking beneath the surface layers to these deep-seated structures.
The Buried Faults of Ohio
Ohio’s geological profile includes numerous faults, but they are not the surface-breaking structures commonly associated with major seismic events. The state is underlain by thick layers of younger Paleozoic sedimentary rock, which conceal the much older, crystalline Precambrian basement rock below. These sedimentary layers, consisting of limestone, shale, and sandstone, vary in thickness from approximately 750 to 4,500 meters across the state.
The faults responsible for Ohio’s seismicity are primarily located within this buried Precambrian basement, which dates back over a billion years. These ancient structures are remnants of a massive, failed continental split known as the East Continent Rift Basin. This rifting ceased before a new ocean basin could form, leaving behind a deep-seated zone of weakness and faulting.
Another major geological feature affecting the basement is the Grenville Front, a zone of intensely deformed rock resulting from a continental collision that created the Grenville Mountains. While the mountains have long since eroded, the fault zones created during this era remain preserved in the crust. These faults are not active in the sense of constant movement, but they represent pre-existing fractures that can be exploited by modern tectonic stresses.
The Anna Seismic Zone
The most consistently active seismic area in Ohio is the Anna Seismic Zone, located in the western part of the state, primarily centered around Shelby County. This zone is directly associated with ancient weaknesses in the Precambrian basement, specifically the Fort Wayne Rift and the Auglaize fault. Since 1875, the region has cataloged more than 40 earthquakes, making it the most seismically prone area in Ohio.
The largest earthquake recorded in Ohio’s history occurred here on March 9, 1937, registering a magnitude of 5.4. This event caused significant damage in the village of Anna, including toppled chimneys and structural damage to buildings. The high school notably had to be razed and rebuilt following the event.
The damage in Anna was intensified by the local geology of the overlying sedimentary layers. The village is situated above the buried Teays River Valley, which is infilled with hundreds of feet of loose glacial sediments. When an earthquake occurs, these soft sediments amplify the seismic waves, leading to more intense ground shaking compared to nearby areas.
Ohio’s Tectonic Setting and Earthquake History
The underlying mechanism driving Ohio’s earthquakes involves the constant, subtle stresses acting on the North American Plate. Ohio’s seismicity is caused by the reactivation of billion-year-old faults within the stable continental interior, unlike plate boundary earthquakes. The entire plate is subjected to large-scale compressive forces, primarily driven by the expansion of the mid-Atlantic Ridge and the push of the Pacific Plate.
These forces create a regional mid-continent stress field that constantly strains the crust. Because the crust is not a uniform block, the weakest points—the ancient faults and rifts in the Precambrian basement—become the failure points where this stress is released. Earthquakes in this region typically occur at depths within this crystalline basement rock, often many miles below the surface.
Ohio has experienced several other notable historical earthquakes besides the 1937 Anna events. In northeastern Ohio, an earthquake with a magnitude of 5.0 struck near Painesville in 1986, causing minor to moderate damage. This event was felt over a wide area, including parts of Canada and ten other states.
Another event occurred in St. Marys in 1986, within the Anna Seismic Zone, registering a magnitude 4.5. This earthquake indicated a strike-slip motion along a fault plane that aligns with the proposed Anna-Champaign Fault.
A distinguishing characteristic of earthquakes in the central and eastern United States is that their seismic waves travel much more efficiently through the older, more rigid crust compared to the western US. As a result, an earthquake of a given magnitude in Ohio can be felt over an area up to ten times larger than a comparable event on the West Coast. While the frequency of large earthquakes in Ohio is low, the potential for widespread effects from a moderate event remains a factor in seismic risk assessment.