Simulations show how earthquake early warning might be improved for magnitude-9 earthquakes

Earthquake warning times on the West Coast for a magnitude 9 earthquake — Earthquake early warning times for a magnitude-9 event with an epicenter in southern Oregon. With a lower alert threshold (left) some locations closest to the source feel the ground shake before the alert arrives (late alert, pictured in dark gray). For a higher alert threshold set only to warn of moderate shaking (right) a larger region close to the source feels the ground shake before the alert arrives (dark gray), and most of Washington state has either a missed alert or a late alert. Researchers suggest that lowering the alert threshold, from intensity-5 to intensity-3 or -4, would help to improve the alerts’ performance for offshore earthquakes. Black patches on the maps are highly populated areas, and red dots are seismic stations.

When the next major earthquake hits the Pacific Northwest, a system launched last spring should give some advance warning, as emergency alerts go out and cell phones buzz. But how well the system functions might depend on whether that quake is the so-called “really big one,” and where it starts.

The Pacific Northwest’s last magnitude-9 event from the offshore subduction zone was in 1700. Only a few clues remain about how it unfolded. But with the earthquake early warning system being built out and improved, seismologists want to know how ShakeAlert would do if the really big one were to happen today.

A research project by the University of Washington and the U.S. Geological Survey uses simulations of different magnitude-9 slips on the Cascadia fault to evaluate how the ShakeAlert system would perform in 30 possible scenarios. Results show the alerts generally work well, but suggests ways the system could be improved for some of these highest-risk events.

The research will be presented Dec. 13 as an online poster at the American Geophysical Union’s annual fall meeting, being held as a hybrid event based in New Orleans.

“I’ve experienced both the Loma Prieta and the Nisqually earthquakes, and both times my first thought was: ‘Is this really happening?’” said lead author Mika Thompson, a UW doctoral student in Earth and space sciences. “An early warning system gives people a moment to collect their thoughts and prepare to react. That’s especially important for a major earthquake.”

The work used detailed computer simulations of magnitude-9 earthquakes created for a previous study looking at how a big offshore event would play out, depending on where and how deep the Cascadia tectonic fault slipped. Thompson played those simulations through an off-line version of the ShakeAlert system and calculated the alerts that would go out across the region.

“The alerts are generally doing well, but they’re not perfect,” said co-author Renate Hartog, manager at the UW-based Pacific Northwest Seismic Network. “This project is trying to understand the system’s limitations so that we can make recommendations for future alerting strategies.”

The alerts performed well even though big offshore earthquakes are harder for the system to detect and locate. But there were cases in which a warning arrived too late to some areas.