When a quake registers as at least 6.0, it activates a site of monitoring station. When at least two stations are activated, they relay a signal to Mexico city, automatically triggering an alarm. In Mexico city, distance equals time. The city salvation lies in grade school Algebra and it's already worked.

September 14th, 1995, 8:04am, from the Pacific, a fierce quake founders a shore. In 13 seconds, it registers on station 11, four seconds later, on station 12. A quake measured 7.3 on the Richter scale and it's moving toward Mexico city at 12 times the speed of the world's fastest commerical jet. But the SAS alert beats the quake to the city by 72 seconds. Not one resident is killed.

When the alarm sounds, a carefully celerograph movement begins. Buildings empty, subways shut down. To avoid fires, power is cut and gas lines are closed. Rescue workers scramble. But Mexico city's entire civil defence depends on the thin red line of sentinels, the earthquake sensors on the coast. If they fail, a quake would hit with no warning at all.

To prevent failures, the sensors are routinely inspected. Today, it's a sensor at the Saint Peter station near Acapulco. For Rovato Islas, just getting there, is shaky.