Fires pose one of the greatest hazards in earthquakes. The shaking in an earthquake can knock over candles, water heaters, gas ranges and dryers, damage chimneys, break electrical wires, and other such possible ignition sources. Once a fire begins, it can be difficult to control if water mains have also been damaged in the earthquake.
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.)
Sometimes - about 6% of the time in California - an aftershock will be larger than its mainshock. We then change our terminology and call the first earthquake a foreshock and the aftershock is called the mainshock. The most notable difference between foreshock-mainshock pairs and mainshock-aftershock pairs is that foreshocks always occur very close in space to their mainshock. Although aftershocks often occur over the full length of the fault and sometimes tens of kilometers away from the fault, we have never seen a foreshock more than 10 km away from its mainshock.
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.
Earthquakes always occur on faults. Faults are places in the earth where the rocks are broken and the rocks on one side have moved in some direction relative to the other. Faults are planes, not lines. The line seen on a map is the intersection of two planes - the plane of the fault with the plane of the earth's surface. The fault plane can be in any orientation (vertical, horizontal or at some angle to the surface), and the slip direction can be at any angle. We can divide all that up into three basic cases - thrust, normal and strike-slip. A thrust occurs on a plane that is at an angle to the surface such that one side slides up over the other. Normal faults are also on angled fault planes, but one side moves down and away from the other. Strike-slip events occur on vertical faults and one side slides by the other.
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.