Results of scientific studies such as this can be used to design alerting strategies for earthquake early warning systems such as USGS’ ShakeAlert, now being developed for the U.S. West Coast.
This new study examines what the expected warning times could be for earthquake early warning systems by considering how long it takes an earthquake to grow in size (magnitude) compared to how long it takes earthquake waves (shaking) to arrive at a user’s location.
Modern earthquake early warning systems can monitor the evolving rupture, issuing alerts to regions expected to experience a certain level of shaking as the earthquake is occurring. If the earthquake rupture grows, and the region impacted by ground motion expands, alerts may be updated and extended to new locations. A person will experience very strong ground motions only if the earthquake grows to a large-enough magnitude and if the fault rupture breaks close to their location.
“Small and large earthquakes begin in similar ways, so we can’t know just after an earthquake starts how large it will ultimately become,” said Annemarie Baltay, a USGS seismologist and coauthor of the report. Consequently, earthquake early warning systems have the greatest potential benefit for people who can take protective action when warnings are issued for low levels of ground shaking. If alerts are only issued for very strong shaking, people will have less time to respond and take action.
When and if an advance warning is issued thus depends critically on the ground motion threshold set for alert notification. The time available to issue an alert depends on both a user’s distance to the rupturing fault and the minimum level of ground motion for which the user wants to be alerted. The amount of warning time depends most strongly on the ground-motion level that is used to trigger alerts. Longer warning times are possible if alerts are issued at lower thresholds, when only weak ground motion is expected from the earthquake.
“Using the example of an earthquake on the San Andreas Fault, that starts in northern California and ruptures toward San Francisco, alerts issued when just light shaking is expected in San Francisco could provide warning times as long as about 48 seconds in advance of when the earthquake shaking is felt there,” said Elizabeth Cochran, a USGS seismologist and coauthor of the report. “In contrast, if you wait to alert San Francisco until very strong shaking is anticipated, only 8 seconds of warning are possible.”
The authors noted that if users are willing to receive alerts and take safety actions even when it is unlikely the ground shaking will grow to become damaging, they are more likely to receive timely information they can act on. In contrast, if users prefer to limit alerts to events during which ground motion is expected to be very strong, then warning times will be short, or perhaps even arrive too late to act.
“The conundrum,” noted USGS seismologist and lead study author Sarah Minson, “is that little earthquakes are much more common than big ones. You could get much longer advance warning if you take action to protect yourself as soon as an earthquake begins and not wait to see if that earthquake happens to grow large enough to cause potentially damaging shaking.”
Alerts for weak shaking could help provide effective response training for when a bigger, damaging earthquake eventually occurs. “There are many low-cost actions that can be taken even when scientists can’t forecast the final strength of an earthquake. This includes drop-cover-hold-on; slowing down trains to prevent high-speed derailments; putting down hazardous work materials, such as chemicals or saws; backing away from warehouse racks and other fragile structures in ‘big box’ stores; and so on,” said Men-Andrin Meier, a Caltech seismologist and coauthor of the report.
The full report, “The Limits of Earthquake Early Warning: Timeliness of Ground Motion Estimates,” was published in “Science Advances,” and is available online.