Beneath the Surface: The Impact of Local Geology on Earthquakes
On October 17, 1989, tragedy struck California’s Bay Area when the Loma Prieta earthquake—triggered along the San Andreas Fault—shook the region. The deadliest event was the collapse of the Cypress Street Viaduct in Oakland, killing 42 people. That same quake caused a 50-foot section of the San Francisco–Oakland Bay Bridge to collapse, killing one motorist and shutting down a critical commuter route for a month—costing the region millions in lost productivity.
Remarkably, these failures occurred more than 60 miles from the earthquake’s epicenter. Both structures were built on deep, soft bay sediments, which amplified seismic shaking and intensified the damage. A measure of this shaking, which we use to monitor for earthquakes in our applications, is peak ground acceleration (PGA) —the maximum instantaneous acceleration experienced during an earthquake.
PGA is influenced by many factors, but local geology is one of the most important. In Loma Prieta, soft sediments beneath the Cypress Street Viaduct and Bay Bridge greatly increased the shaking compared to what would have been felt on solid bedrock. A bridge anchored in hard granite will respond very differently than one built on unconsolidated gravel when the ground begins to shake.
The figure above shows how the PGA varies with rock type for a theoretical earthquake along the San Andreas Fault. Horizontal lines mark the recommended threshold values for bridges and dams. As the underlying material becomes stronger and more consolidated, the PGA at the structure decreases.
The U.S. Geological Survey (USGS) factors local bedrock conditions into their PGA estimates, available through ShakeCast. Geology is not a direct input but should be considered when alerting based on seismic magnitude-distance policies or when running seismic simulations in our applications.
Knowing how the local geology attenuates seismic energy into infrastructure is important for maintaining the resiliency of our assets.
At USES Corporation (USES Corp.), we understand the value of real-time seismic monitoring. Our products continually monitor for earthquakes detected by the USGS ShakeCast product and will provide alert notifications to stakeholders if the PGA or magnitude is exceeded for a given bridge, dam, or other critical infrastructure.
Prepared by Michael Chojnacki, Geospatial Client Support Specialist, USES Corp.
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