Some quakes give early warning
Shallow crustal earthquakes shake violently almost immediately. During these quakes, drop, cover, and hold on is likely your best choice of action to avoid getting injured from projectiles coming off walls and shelves, broken glass from windows, or furniture and appliances tipping over. Leaving a building during violent shaking can be risky.
Some quakes may give you a bit of warning before the shaking gets destructive. The most violent subduction zone quakes--where one tectonic plate is forced under another--initially produce light pressure waves (P-waves) that travel across the earth faster than the more destructive shaking waves. The further you are from the fault line, the more time you have between feeling the P-waves and the violent shaking waves. In other words, these light P-waves may give you a minute of warning. Depending on the building type and your location near an exit, it may make sense to evacuate immediately if you feel light P-wave shaking.
Older buildings pose greater risk
"In Oregon, we're concerned about any building that was built before the mid-1990s," said Erica Fischer, Professor of Structural Engineering at OSU. As Fischer explains, though we started to understand how to build structures in high seismic regions in the 1970s and 1980s, we didn't discover the Cascadia subduction zone until 1988. Excluding wood frame houses, 70 percent of the built infrastructure in West Coast US cities is not up to modern seismic standards, meaning that they may collapse.
According to Chris Goldfinger, if you are near an exit and start feeling the gentle P-waves of a quake, and you are in a non-ductile concrete building (built before mid-90s) or in a URM, you may have about a minute to evacuate the building before the destructive shaking occurs. If you don't leave the building, duck, cover, and hold on. Check out this video for more details.
Karen Ronning-Hall, Disaster Preparedness Evangelist, living in beautiful Portland, Oregon, with hubby Bill, daughter Geneva, Bean dog, Thumper kitty, and Terry the turtle.