Geologists use seismometers, specialized equipment designed to measure vibrations, to study earthquakes and how rocks move under great pressure. Distant from large earthquakes, everyday ground motion is very small and cannot be felt directly; instruments measure the motion in nanometers, or one billionth of a meter (a speck of dust is thousands of nanometers in size). Seismometers are sensitive enough to detect more than just earthquakes, and can register sources of ground motion closer to the surface as well, such as the regular vibrations of waves and wind producing a background seismic hum, like a steadily beating drum.
Inland in Tallahassee, 140 feet below the EOAS building on the FSU campus, the FSUO seismic station measures changes in ground motion (shown as the red line). Prior to the storm, the seismic hum was relatively quiet, allowing for the detection of a magnitude 7.1 earthquake in Indonesia, thousands of miles away.
As Idalia moved over the shallow continental shelf in the Gulf towards the Florida coast, the intensity of the storm drove the waves harder into the ground, increasing the amount of vibration in the seismic hum. Once the storm was close enough to the FSUo seismometer, the signal for motion increased to an amplitude of about 50 nanometers per second. This increase in ground motion was detected as the storm continued to pass over land, the wind shaking the trees and vibrating the ground for the following days, at a period of about 2-5 seconds. The motion can be seen both in the change in amplitude of the red line, as well as the increase in power shown in the inset boxes prior to (left) and during (right) the storm. While this motion is far below what can be felt by people, the sensitive instruments at FSU enabled the increase in ground vibrations to be detected.
The seismic station named FSUO (observatory) is part of the global seismic network. The location at EOAS was chosen as it was a void in the Global Seismic Network. Data is sent to the National Earthquake Information Center (NEIC), the Albuquerque Seismological Laboratory, and the Data Management Center at the Incorporated Research Institutions for Seismology, in Seattle. The data is used to locate earthquakes and image the Earth’s interior. The seismic signal is displayed in the foyer of EOAS.
The Streckeisen STS-5A seismometer was a gift from Dr. Bob Hutt, retired from the U.S. Geological Survey’s Albuquerque Seismological Laboratory. He received the station as a gift upon retirement from Quanterra, Inc. This location was chosen as it was a void in the Global Seismic Network.