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SAN FRANCISCO — Imagine picking up your baggage after a long flight when, over the public address system, a voice warns that a major earthquake is about to hit. 

Welcome to San Francisco International Airport of the not-so-distant future. 

Airport officials say that within a month they will install early warning sensors that go off seconds before an earthquake rumbles through. The system could be used to abort some plane landings, to shut off the airport’s jet fuel pipes, or to give passengers a chance to duck for cover, said airport emergency planner Dale Dunham. 

Dunham said it would be some time before features such as the public warning feature were in place. 

Such systems work because earthquakes send out two shock waves from their origin. The faster wave, which generally does little damage, tips off the second wave – the one that can level buildings to a single mound. 

The destructive 1989 Loma Prieta quake, for instance, rolled some 80 kilometers north from Watsonville. 

The second wave took about 40 seconds to arrive at the airport, according to Mike Blanpied of the U.S. Geological Service. But the first wave would have arrived in about 20 seconds – that 20 second difference could give airport officials valuable time to save lives and property. 

Some schools, hospitals, and nuclear plants have installed such early warning systems. The airport’s three sensors cost about $35,000, said spokesman Ron Wilson.