Photograph courtesy USGS
The 24-Hour Aftershock Forecast provided by the USGS indicates the probability of strong earthquake shaking (Modified Mercalli Intensity VI or greater—roughly, shaking strong enough to cause objects to fall off shelves) in the next 24 hours. Not all aftershocks are smaller than the quake that precedes them. If there was a minimal quake in recent hours, there may be a bigger shock coming, so check out this map to know what to expect.
Scenario shake maps describe the expected ground motions and effects of specific hypothetical large earthquakes. To find out how a big earthquake might hit your neighborhood take a look at scenario maps created for Southern California regions.
For the basics in earthquake cartography—learn how hazard maps are made and what they mean.