Multiyear study of earthquake early-warning system

California scientists are upgrading hundreds of seismic monitors throughout the state, installing new devices that will vastly improve the state's system for detecting and warning of major earthquakes.

The changes will allow first responders, scientists and eventually the public to be notified of an earthquake up to five seconds faster than is possible now. Those precious seconds could allow emergency officials to shut off gas and water lines, raise fire station doors, stop subway operations and possibly even warn the public of shaking to come. READ MORE

This process is possible because when an earthquake occurs there are two primary seismic waves that move through the earth. Compressional waves, also known as primary or P waves, travel fastest, at speeds between 1.5 and 8 kilometers per second in the Earth's crust. Shear waves, also known as secondary or S waves, travel more slowly, usually at 60% to 70% of the speed of P waves. The P waves are used to trigger the early-warning system.

USGS Press Release ...