EEW: Earthquake Early Warning
What is EEW?
Earthquake early warning, akin to warnings for other natural disasters, is an alert that seismic activity is imminent. We are used to hurricane warnings, which can come days in advance of severe weather. Tsunamis can build over the course of a few minutes to a few hours before making landfall. For earthquakes, however, the lead-time is much smaller. Earthquake Early Warning (EEW) is a system that can provide a few to tens of seconds warning prior to ground shaking at a given location. The purpose of such a system is to provide warnings in California or other earthquake prone regions around the world. The warning messages provided by EEW can be valuable to reducing the damage, costs, and casualties resulting from an earthquake. EEW through the Berkeley Seismological Laboratory is currently being tested as part of the realtime seismic system in California. This demonstration system will be used to assess the timeliness and accuracy of warnings that can be expected if early warnings are implemented.
How does EEW work?
To answer that question, you have to look at what we are actually measuring (these are warnings, after all, not earthquake predictions). Suppose an earthquake begins at the source depicted by the blue star, which causes seismic waves (the black line) to radiate outward. The fast moving primary wavefront (in yellow) hits a nearby seismometer first. This triggers the EEW system's triad of algorithms to determine the magnitude and source location of the quake. Some time later, the damaging secondary wavefront (in red) makes it to the station. It is this time lag that makes the whole system work. We can analyze the character of the wave and send an alert to a distant location ahead of the seismic disturbance. Perhaps this gives the business time to open their doors and enact their hazard plan.
Now, communications do move at the speed of light, and significantly faster than the seismic wave, but there are still lots of time sinks involved. P waves travel at roughly 5 mi/s (or 18,000 mph), so if the closest seismic station is 10 mi away, you have already lost two seconds before anything is detected. Some of the EW algorithms require triggering on more than one station, which is one of the reasons building a dense seismic network, especially along known faults is critical. Data Transfer between the stations, the decision module, and the end-user runs at the speed of light, which is quite fast, but not infinite. Those fractions of seconds add up. Finally, each algorithm needs to collect a certain amount of data before it can start crunching numbers. We are working on minimizing all time sinks in this area, but there will always be a minimum.
Even with the inevitable time sinks, the method can provide warning before the arrival of the S-wave, which brings the strong shaking that usually causes most of the damage. Similar EEW systems are already in place, or are being developed, throughout the world. Warning systems currently exist in Japan, Taiwan, Mexico, Turkey and Romania. For a look at EEW around the world, click on the map to the right and see which countries have a high earthquake risk, and which countries have or are testing EEW systems.
For more on Earthquake Early Warning basics, please visit the Berkeley Seismo Lab EEW research group pages. For more in-depth, technical information about how EEW works, visit http://www.shakealert.org/eew-research/ or take a look at some of our technical publications.
How Could I Benefit from EEW?
Feasibility studies of the EEW methodology show that the amount of warning time would range from a few seconds to a few tens of seconds depending on your distance from the epicenter of an earthquake. This is enough time to slow and stop transportation such as trains, taxiing planes, and cars entering bridges and tunnels; to move away from dangerous machines or chemicals at work, and take cover under a desk; as well as to automatically shut down and isolate industrial systems. It can also prevent cascading failures in the aftermath of an event, like fires indirectly caused by the earthquake. Taking these actions and more before strong shaking begins can reduce damage and casualties during an earthquake.
ShakeAlert is the name of the public earthquake early warning system now being implemented in California, Oregon and Washington. ShakeAlert has been running as a demonstration system since 2012 issuing alerts to test users. More on ShakeAlert.
Timeline: When with the United States get its public earthquake early warning system?