USGS Shakemap based on instrumentally recorded shaking intensity and site conditions. Note the pockets of stronger shaking (green) in sedimentary basins well north of the epicenter.
Want to see what happens when you can know an earthquake is coming? Mexico has footage that’s got you covered.
Update 6 April 2012: They also have an app that alerts you to earthquakes before they hit… in Mexico City. (También tiene una App que se alerta de sismos antes de tiembla!)
On March 20, 2012 a very large earthquake rocked southern Mexico. With a magnitude of 7.4 and a moderate depth of 20km this quake was widely felt, and resulted in damage and casualties near its epicenter in Oaxaca. But Oaxaca wasn’t the only place that got rocked. Mexico City, more than 200 miles north, felt the waves amplified as usual by the lakebed sediments it’s situated upon. The sedimentary basin upon which Mexico City lies is the seismological bane of the city, trapping and amplifying seismic waves like a swimming pool. This unfortunate setting has devastated the city in the past, but is at least widely recognized as a liability. Because the effects of distant earthquakes are amplified in the Distrito Federal, it’s actually an ideal candidate for an early warning system that relies on the time it takes damaging seismic waves to reach a sensitive area once they’ve already been detected. Along with Japan, Mexico is one of the few countries in the world with a functioning early warning system, and last week’s large quake exercised it.
Once large-amplitude shaking is detected among the nation’s seismometers, a signal is transmitted immediately to activate alarms in the capitol city that warn residents shaking may be imminent. Damaging seismic waves travel at a little over 7,000 mph, so at 200 miles from the epicenter, Mexico City would have upwards of a minute and a half of warning before they hit. Of course the seismometers couldn’t detect the seismic waves immediately as they’re initiated, and it would take even more precious seconds before they could reliably identify the quake as a dangerous magnitude, but once they did the alarm was sent.
Footage of a Mexican Senate hearing captured the whole event. A shrill alarm starts going off at the beginning of the video, and the man speaking is interrupted by concerned colleagues who immediately and orderly evacuate the room. The alarm shuts off 50 seconds into the video. Although mild shaking may have begun at this point, capturing the attention of a couple of senators by swinging ceiling fixtures, substantial shaking doesn’t begin until just after the 1:05 mark. From there it clearly continues, noisily rocking light and audio apparati suspended from the ceiling. The electricity flickers on and off a couple times, and the rest of the recording is just the earthquake dying down.
This video is a beautiful demonstration of an earthquake early warning system in action, complete with the successful evacuation of a crowded room. If only we all had the fortune of Mexico City to be hundreds of miles from the earthquakes that plague us.
Bonus video: an extremely fortunate camera crew was set up along the Paseo de la Reforma in Mexico City shooting HD video of a tall statue before the earthquake started. They realize shortly into this video that the ground is swaying, and they capture it all as the towering statue starts oscillating heavily and noisily.