The Ohio Seismic Network (OhioSeis) continuously seeks to better serve Ohioans through seismic monitoring and risk mitigation. To that end, it is currently installing the next phase of seismic stations across Ohio. The new Gen-3 seismic stations differ from previous monitoring sites in the way they’re constructed, leading to greater sensitivity.
The first generation of Ohio’s seismic monitoring stations required the instrumentation to be placed indoors, often in areas with numerous sources of human noise, making earthquake detection difficult. In 2016, the second generation of OhioSeis saw instruments placed in remote areas of the state but just below the ground surface in shallow barrel vaults, including one in Richland County. The earthquake magnitude minimum detection threshold dropped from about M3.0 to M2.0 with the second-generation installations. However, many earthquakes in the state remained undetected.
Enter the new, Gen-3 phase of earthquake monitoring: borehole seismometers. In a typical Gen-3 site, seismometers are now placed 20 feet below the ground surface in water-tight PVC casings and cemented into place. In late 2019, the Ohio Seismic Network began planning upgrades for the future of the network and promptly decided to select borehole-style installations, to keep pace with modern advancements in seismic detection. Seismologists have known for some time that the deeper an instrument is below the ground surface, the better the data quality. More and more, seismometer manufacturers are building borehole instruments to combat an increasingly noisy world.
In order to “hear” the vibrations seismologists are interested in, namely earthquakes, vibration-sensitive instruments must compete with natural and human-made seismic noise that is transferred into the ground. However, wind, water, storms, cars, trucks, trains, HVAC machinery, and even walking can transfer seismic energy into the ground over great distances. Fortunately for seismologists, the amplitude of this “cultural” noise falls off to near zero below about 10 feet deep, depending on soil and rock types present. By burying Ohio’s seismic sleuths from 15 to 20 feet deep, OhioSeis is better positioned to detect the vibrations of interest, microearthquakes.
Beginning this spring, nine new sites are being drilled for the emplacement of borehole seismometers to fill gaps in the OhioSeis network, including areas of research interest to better enhance our understanding of seismic risk within the state. Sites were chosen in southern Ohio to listen for microearthquakes deep within the boundaries of the Rome Trough on the border of Ohio and Kentucky. Earthquakes in this region of Ohio are 4–5 times deeper than in other parts of the state, and we don’t know why. Other sites were chosen in the Anna Seismic Zone to investigate if and why a formerly active region has gone “quiet.” Finally, additional sites in the Northeast Ohio Seismic Zone, location of several large M4.0–M5.0 earthquakes, are being installed to monitor activity there.
By continually increasing the network’s capability to capture ephemeral natural processes inside Earth, OhioSeis stands to gain a more complete understanding of the entire geologic realm beneath our feet, where there is still much to be discovered.
Source: Ohio Department of Natural Resources