On August 24, 2015, a warning horn blared in a lab at UC Berkeley 10 seconds before the shaking started. The ShakeAlert system had picked up the Napa Valley 6.0 earthquake 10 seconds before arrival of the main shock. The lab’s instruments accurately displayed the arrival time and strength of the coming quake. But is 10 seconds enough time to react and seek safety? Maybe enough time to scramble under a table to avoid falling objects. Or not step into an elevator.
The amount of advance warning depends on distance from the epicenter. The farther from the epicenter, the more warning time given. Berkeley is about 25 miles (42km) from the Napa Valley fault line rupture site.
The ShakeAlert system is currently under development by a team of seismologists from UC Berkeley, Caltech, U. of Washington, and the USGS. It is undergoing testing in California, where 400 sensors have been installed along the state’s fault lines. The developers have requested funding to add more sensors before the system is put into full operation.
ShakeAlert is based on an earthquake early warning system that has been operating in Japan since 2007, employing automatic warning signals for mobile phones, radio, TV, and the internet. When the magnitude 9.0 earthquake and tsunami hit Japan in 2011, a teacher in a school in Sendai, the largest city near the epicenter, received a 32-second advance warning signal on his mobile phone. He immediately had the class take shelter under their desks. After the severe shaking was over, he evacuated the class to safer ground and no one was injured. Unfortunately, there is not yet an early warning for a tsunami, and that is what did most of the damage.
Earthquake early warning systems are based on the lag time between three seismic waves that occur when a fault line ruptures. The first is a compression wave called the Primary Wave or P Wave. The P Wave is fast, traveling through granite at 5,000 meters per second. The second wave or S Wave is slower, traveling at about half the speed of the P Wave. The even slower Surface Wave arrives last. The S Wave and the Surface Wave are the big shakers that do all the damage.
To quote a USGS article posted on Sept. 11, 2014, “When an earthquake occurs, both compressional P Waves and transverse S Waves radiate outward from the epicenter. The P Wave, which travels fastest, trips sensors causing alert signals to be sent ahead, giving people and automated electronic systems time (seconds to minutes) to take precautionary actions before damage can begin with the arrival of the slower but stronger S Waves and late-arriving surface waves. Computers and mobile phones receiving the alert message calculate the expected arrival time and intensity of the shaking at your location.”
Once the ShakeAlert system is operating successfully in California, the USGS plans to install the system in all earthquake-prone areas across the US. Since this author lives less than 5 miles (8km) from the southern San Andreas Fault where the Big One is expected, the sooner the system is rolled out the better.