Jose Flores was driving with his family to see \u201cToy Story 5\u201d on Wednesday in Caracas, Venezuela, when a loud earthquake alert went off on his wife\u2019s Google Android phone. Six seconds later, he felt the earth starting to shake.<\/p>\n
Venezuela does not have a national early warning system of its own, but people with Android phones received alerts from Google\u2019s Earthquake Alerts system, which can pull data from more than two billion phones equipped with built-in accelerometers. The same sensor that detects rotation on the screen can also sense vibrations from seismic waves.<\/p>\n
Three seconds after the quake started underground <\/p>\n
Seismic waves reached the surface and were picked up by phones.<\/p>\n
Phones detecting seismic activity<\/p>\n
Google said the system, which is available in nearly 100 countries, sent warnings that reached 11.4 million people on Wednesday, giving users seconds or up to two minutes notice before back-to-back powerful earthquakes struck.<\/p>\n
Several countries, including Japan, Mexico, Canada and the United States<\/a>, have government-operated early warning systems<\/a>. These largely rely on widespread regional networks of underground sensors that detect earthquakes and can send alerts to most phones \u2014 iPhone or Android \u2014 via government alert settings that are often enabled by default.<\/p>\n When earthquakes hit, they send out two types of waves that travel at different speeds. The fast-moving and milder primary waves, or P-waves, travel at four miles per second and are less likely to cause destruction. The slower and stronger secondary waves, or S-waves, travel at about half that speed and produce shaking.<\/p>\n When P-waves start radiating from the epicenter, Android phones sense the vibrations, start collecting data and send it back to Google servers for processing. The servers use information from many phones to figure out if an earthquake is happening. The phones have to be stationary \u2014 on a tabletop or in a bag on the floor, for example, and not in the pocket of someone walking around \u2014 in order to sense an earthquake.<\/p>\n The system quickly estimates the earthquake\u2019s location and magnitude and then pushes alerts to phones. All Android phones in the affected region receive the alerts.<\/p>\n Nine seconds after the quake started underground <\/p>\n The system had crunched enough data from phones to identify an earthquake and send out the first alerts.<\/p>\n Phones detecting seismic activity<\/p>\n The epicenters of the quakes in Venezuela were beneath highly populated areas. Within three seconds, phones sensed the P-waves of the first quake, said Marc Stogaitis, a principal engineer at Google who works on the early warning system. Six seconds later, the system identified an earthquake and sent out the first alerts.<\/p>\n The system is continuously receiving and processing data, Mr. Stogaitis said. As an earthquake develops, the system often tweaks the magnitude, time, location and alert area. Google\u2019s system sensed the increasing magnitude of the Venezuela earthquakes, and \u201cthe alert region grew as the earthquake grew,\u201d Mr. Stogaitis said. Several seconds after the first quake, a second, stronger earthquake hit, and more alerts were sent.<\/p>\n \u201cAs the seismic waves from both events overlapped with each other, the system treated it as a single large event alerting those experiencing shaking from both events,\u201d he said.<\/p>\n Distance matters. The farther people are from the epicenter, the more likely they are to get an early warning alert, giving them more time to act before the shaking starts. It\u2019s more challenging to get timely warnings to people near the epicenter; those alerts usually arrive when the shaking has already started.<\/p>\n 15 seconds after the quake started underground <\/p>\n The system continued to collect data from phones and sent alerts to a broader area that included Caracas.<\/p>\n Phones detecting seismic activity<\/p>\n Google sends out alerts for earthquakes of magnitude 4.5 and above, which are considered powerful enough to cause some damage. People receive different types of alerts depending on the estimated magnitude where they are. One blares a loud sound and uses urgent messaging to tell people to take immediate action in areas with potentially extreme shaking, another beeps to tell people to be ready, and a third cautions people to be aware in areas where lighter shaking could occur.<\/p>\n 21 seconds after the quake started underground <\/p>\n Alerts eventually reached millions of phones across a large area.<\/p>\n Phones detecting seismic activity<\/p>\n The two quakes in Venezuela were powerful \u2014 the first had a magnitude of 7.2 and the second had a magnitude of 7.5. The second earthquake was the most powerful<\/a> to strike the country since 1900. Google said its full spectrum of alerts was sent out. Almost 1.4 million of its most severe warnings, which the company calls \u201cTake Action\u201d alerts, went to people in areas where the intensity of the shaking was strongest.<\/p>\n