What is a Strong Motion Sensor?

At the Pacific Northwest Seismograph Network (PNSN) we have several different types of seismometers that are used for monitoring seismic activity in Washington and Oregon. All of our seismometers are integral to maintaining our comprehensive network. However, strong motion sensors perform a unique role in our system. Strong motion sensors are a type of seismometer used by the PNSN to record the strong ground motion typical of large earthquakes (> 5.0 in magnitude). These sensors are specifically designed to stay on-scale during the strongest ground motion possible.

Other seismometers in our network are designed to detect the small volcanic and tectonic earthquakes that frequently occur in Washington and Oregon. However, these seismometers "clip" and distort the signal generated by strong ground motion. When clipping occurs, critical data is lost that allows seismologists to correlate intensity with actual measurements of ground motion. Data from strong motion sensors allows seismologists to accurately interpret the way the ground moved during a large earthquake. This information is vital to the work of emergency managers and essential for civil engineers, citizens, policy makers, developers, and urban planners interested in mitigating earthquake hazards in their communities.