The USGS now offers a real-time Earthquake forecast for California. You can grab a cup of java, check the weather and scores and double check that your trip to San Francisco is still low probability for imminent death and destruction. Here’s the USGS page.
How it works
(from the site)

The background probability of shaking exceeding Modified Mercalli Intensity VI is taken from the National Hazard Maps. This background probability does not change with time. The highest probabilities in California are near San Francisco and Los Angeles and reach 1 in 10,000 per day.

The system then considers all the earthquakes, large and small, that are recorded by the California Integrated Seismic Network (CISN). For each event, the probability that it will be followed by an earthquake large enough to cause strong shaking is calculated from the known behavior of aftershocks. The shaking that would be produced by such an earthquake is then predicted from the known relations between earthquake size and shaking patterns and the likelihood of that shaking is added to the background probability on the map.