How Can I Use This Map?

The primary use of this map is educational. Watching the fluctuations in the probabilities will help you understand the nature of earthquake clustering and how the patterns change with time. In almost all cases, the probabilities are very low. The background probability of MMI VI shaking in most of California is between 1 in 10,000 and 1 in 100,000. By comparison, the average American's risk of being in a car accident in any 24 hours is about 1 in 2,500 - much higher than the risk of strong earthquake shaking even in California. Immediately after a M4.0 earthquake, the probability of MMI VI shaking in the next 24 hours rises to about 1 in 100 and then drops to 2 in 1000 within 6 hours, so that the risk is greater than the chance of a car accident - but not much more. There would be no reason to change any of your plans because of this small increase.

The only time that the probabilities become large enough to affect you is after a significant earthquake that may have already caused damage. You already knew that aftershocks are likely in this situation. The map is showing you where those aftershocks are most likely to be felt and how the risk changes with time. Remember that this probability is for shaking large enough to knock objects off shelves. The probability of damaging shaking is significantly less.