A new U.S. Geological Survey study looks at how earthquake warnings can be calculated by how long it could take for a temblor to grow in magnitude instead of simply gauging how long it could take for the ripples of a quake to reach a certain neighborhood.
Researchers are trying to weigh how to best tailor warning systems to give potential victims enough time to take protective action, even issuing series of regional alerts depending on how the fault slip grows. If alerts are only saved for the worst seismic bombs, the timing of the alert will give people in harm’s way of the strong shaking less time to take action.
Publishing their findings in Science Advances, the USGS scientists note the benefit of using “simple seismological relationships to calculate the minimum time that must elapse before such ground motion can be expected at a distance from the earthquake, assuming that the earthquake magnitude is not predictable.”
“Earthquake early warning (EEW) systems are in operation or development for many regions around the world, with the goal of providing enough warning of incoming ground shaking to allow people and automated systems to take protective actions to mitigate losses,” the report states. “However, the question of how much warning time is physically possible for specified levels of ground motion has not been addressed. We consider a zero-latency EEW system to determine possible warning times a user could receive in an ideal case. In this case, the only limitation on warning time is the time required for the earthquake to evolve and the time for strong ground motion to arrive at a user’s location.”
“We find that users who wish to be alerted at lower ground motion thresholds will receive more robust warnings with longer average warning times than users who receive warnings for higher ground motion thresholds. EEW systems have the greatest potential benefit for users willing to take action at relatively low ground motion thresholds, whereas users who set relatively high thresholds for taking action are less likely to receive timely and actionable information.”
Early warning alerts, they stress, can be transmitted faster than the travel of the seismic wave, and extend beyond simply alerting a homeowner to move to a safe location — alerts mean that airports can freeze takeoffs and landings, hospitals can be on alert and pause delicate medical procedures, trains can slow down, workers can set down hazardous materials, and first responders can lift firehouse doors and prepare to go out into the field.
As a real-world example, the researchers used the northern segment of California’s infamous San Andreas fault. The hypothetical quake “initiates off the northern California coast near the Mendocino triple junction, rupturing south toward the San Francisco Bay Area, eventually becoming a large [magnitude 8] earthquake.”
“This is the type of scenario that is typically used to illustrate the potential usefulness of EEW: a major earthquake that begins distantly and then ruptures close to a major population center, creating a long delay between rupture initiation and the arrival of strong ground motion in the populous region,” states the report, detailing “the evolution of the expected ground motion at six cities in northern California (Oakland, San Francisco, San Jose, Santa Cruz, Santa Rosa, and Ukiah), assuming that the EEW system perfectly and instantaneously knows the evolving rupture extent and magnitude of the rupture.”
Under their scenario, four seconds into the quake it’s reached 6.0, and has hit 7.0 within 20 seconds. If San Franciscans got a warning 67 seconds into the shaking, they’d have 8 seconds of warning before strong shaking began. By adjusting the alert thresholds to tell people to take cover when shaking is light, warnings could be sent at 19, 33, or 48 seconds — yet “even in the most favorable case for our idealized, theoretical EEW system — a major city with strong local site amplification situated near a large earthquake whose rupture propagated unilaterally toward the city from a distant initiation point — long warning times for strong motion (for example, 20%g) are not possible.”
In Mexico City, where an earthquake warning system is used, “long-period resonance and amplification effects associated with sedimentary basins can amplify ground motions from distant earthquakes and increase warning times for potentially damaging levels of ground motion.”
“The conundrum is that little earthquakes are much more common than big ones,” said USGS seismologist and lead study author Sarah Minson. “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.”
Even if the alert is sounded for lower-grade shaking that doesn’t intensify into a major quake, disaster-response personnel and other stakeholders would get valuable training experience in putting quake-warning plans into action.
“Even given the timeliness limitations of source parameter-based EEW, false alert-tolerant users could still derive significant benefits from this type of system if they are willing to subscribe to alerts for a low threshold of ground motion and live with the resulting unnecessary alerts when most of these events do not go on to produce strong ground motion,” the report concludes. “This type of cost-benefit analysis for EEW should be developed in future studies.”
The USGS has been working to develop a U.S. early earthquake warning system since 2006, focusing on the highest-risk areas starting with California, Washington, and Oregon — ground zero for ShakeAlert system development and testing.