At 16:55 on November 22, 2014, a magnitude 6.3 earthquake occurred near Kangding County (30.3 degrees north latitude and 101.7 degrees east longitude) in Ganzi Tibetan Autonomous Prefecture, Sichuan Province. It is learned from Chengdu Hi Tech Institute of disaster reduction that the earthquake early warning system jointly built by Ganzi Prefecture Bureau of earthquake prevention and disaster reduction and Chengdu Hi Tech Institute of disaster reduction provides 7 seconds of early warning time for the urban area of Kangding County and 53 seconds for Chengdu.
In fact, this earthquake warning is not what we usually call earthquake prediction. "Earthquake early warning" and "earthquake prediction" do not belong to the same concept. Earthquake prediction is to give advance notice of earthquakes that have not yet occurred but are likely to occur; The "earthquake warning" is a kind of early warning for the earthquake that has occurred before it has spread to other areas.
So, how do people do earthquake early warning? In fact, when an earthquake occurs, the energy generated by the earthquake propagates in the form of seismic waves, which has a certain speed, and not all places shake at the same time. It is like throwing a stone into a calm water surface, and the water waves aroused by the stone will gradually disperse from the center to the surrounding. Therefore, when an earthquake occurs, we will not immediately feel the ground vibration, but wait for the corresponding seismic wave to spread to the person's location. This time difference leaves a space for earthquake early warning.
Generally speaking, the propagation speed of seismic waves is thousands of meters per second, while the speed of radio waves is 300000 km / s. Therefore, after an earthquake, people can use the characteristics that the propagation speed of seismic wave is less than that of radio wave to warn the places where the seismic wave has not yet spread through radio wave in advance.