California’s ‘big one’ might be a megastorm, rather than an earthquake:
In the scenario — powerful back-to-back storms — floods could require about 1 1/2 million people to evacuate [by twosies, twosies] and cause more than $300 billion in property damage. The economic loss would be four times that of a very large earthquake.
The simulation was based on the most severe storm event on record in California, a 45-day series of storms that started in December 1861 and, according to the Geological Survey, caused such extensive flooding that the Sacramento Valley was turned into “an inland sea, forcing the state Capitol to be moved temporarily from Sacramento to San Francisco, and requiring Gov. Leland Stanford to take a rowboat to his inauguration.”
Geologists studying prehistoric flood deposits found evidence of even larger storms that occurred about every 300 years.