How Do We Make This Map?

The 24 hour probability of strong earthquake shaking is updated every hour. It starts with a background probability based on geological and earthquake information. (see the USGS National Seismic Hazard Maps web site).

Usually this information is shown for 50 years; we convert it to show 24 hours. 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. The likelihood of that shaking is then added to the background probability on the map. For complete details, see the USGS Open File Report, Real-time Forecasts of Tomorrow's Earthquakes in California: A New Mapping Tool.

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.