Special caution should be taken after a major earthquake to not start a fire or create a fire hazard that could be triggered by an aftershock. For example, use flashlights rather than candles and turn off gas at the meter if you smell a leak. (You should only turn the gas off if you smell a leak. It may take days to get it reconnected by the gas company.)
Because of the possibility that any earthquake could be a foreshock, the state of California has issued earthquake advisories when an earthquake has occurred within 10 km of a fault long enough to produce a major earthquake such as the San Andreas fault. Just like most aftershocks occur right after their mainshock, most mainshocks occur very soon after their foreshock. The most likely time for a mainshock is within the first hour (one-quarter of all mainshocks happen within an hour of their foreshock) and after three days the risk of a larger event is almost gone. This is important to remember if you ever hear an earthquake advisory. The most likely time for the potential mainshock is immediately and any action you take in response to the advisory should be something you are willing to be doing in a big earthquake.
Strike-slip and thrust are the most common types in southern California. Every point on the fault plane releases energy so bigger faults produce bigger earthquakes. We can thus estimate how big an earthquake to expect by mapping the length of the fault. The longest fault in southern California is the San Andreas fault that is long enough to produce a magnitude 8 earthquake.
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