Across the metropolitan area of Los Angeles, as seismic waves extended from outside, warnings began to appear on computers, mobile phones and other electronic devices connected to the ShakeAlert early warning system.
an earthquake of magnitude 5+ in front of Santa Cruz Island, and that its energy waves, radiating outward, would reach Los Angeles in less than a minute.
And for that, experts praised the budding network of sensors and processors to test how far it has come in the four years since Southern California last suffered an earthquake.
"The success of the system," said Doug Given, the coordinator of the ShakeAlert System developed by the United States Geological Survey.
"It worked like it was supposed to," said Josh Bashioum, founder of Early Warning Labs, which has developed distribution applications using ShakeAlert data.
It may be a good point, but there was less agreement about when the earthquake would occur. rive.
About 20 seconds later, the Southern California Earthquake Center at USC received its first warning. At Caltech in Pasadena, he was close to 10 seconds. In the Hollywood area, the cell phone with the beta version of a mobile application sent a notification that the weak tremor was at 38 seconds.
Seismic waves take longer to reach places farther from the epicenter. But experts said that only would not account for such differences between these three areas, and suspect that there are other factors involved.
Thursday after the earthquake began south of Santa Cruz Island on the southern coast of California, ShakeAlert sent its data, including the preliminary magnitude, location of the epicenter and start time to one tenth of a second, before the First S waves will reach the continent, said Given.
The data took different paths for its recipients, and once they arrived, they were processed by different systems to generate the estimated time of arrival, two factors that could lead to differences of a few seconds, said Given.
Believe that some of the discrepancy problems will be resolved next year, when USGS plans to include the arrival time calculation with its ShakeAlert product. That will be an essential element when delivering ShakeAlerts through the Wireless Emergency Alert System, since currently the Amber and weather alerts are.
For now, recipients of ShakeAlert data must have their own systems for calculating the arrival time. This requires considering not only the distance, but how the speed of the seismic waves depends on the variations in the earth's crust.
The wireless transmission of cellular data to mobile devices brings another variable, but that alone would not account for that second atypical value of 38 seconds. a notice that caught my attention because online journalist Alissa Walker had tweeted it. She did not really feel that the earthquake was coming, but she felt her close colleagues with whom she was in a conference call, and in retrospect, it seemed to be less than 38 seconds, Walker said Friday.
Bashioum's team investigated, and concluded that the significant factor may have been the privacy setting on Walker's phone. It was configured to allow access to location data only when the application is open, not all the time. Because the application was not open when the notification was sent to your screen, the application was based on the recorded location of the phone the last time the application was opened, farther from the epicenter and therefore, the estimated time of arrival of the earthquake .
"It's a difficult balance between privacy and precision," Bashioum said.
When Walker heard and saw the notification and opened the application, he gained access to his actual location, the arrival time would have been updated, Bashioum said. He said that, in general, the calculated arrival times were "very precise".
Early Warning Labs will continue to respect location privacy preferences, but looks for ways to deal with the accuracy of push notifications, Bashioum said, as he continues to prepare the application for release to the public. That will require a green light from the USGS when it is satisfied that its sensor network is sufficiently developed, and is expected for the summer.
Bashioum's company has already received approval to proceed with certain commercial applications, including a fixed installation last year. the Regatta Seaside condominium community in Marina del Rey.
Integrate the earthquake warning with the fire alarm system to alert residents of the 224 units. Future additions will include triggers to open doors and stop elevators and open doors so that residents are not trapped during an earthquake.
For Metro and Light Rail, Early Warning Labs installed a system that will close trains when a major earthquake is anticipated.
The threshold for firing is set at an anticipated level of tremor that could cause damage or injury. The shaking of Wednesday's high seas earthquake was too light to reach that threshold.