[sperry]

Plus, earthquakes in earthquake prone areas don't do too much damage since the ground tends to be pretty broken up which dampens the quake and limits the travel of energy. A 7.5 hitting St. Louis would probably damage buildings in Boston, meanwhile that same 7.5 in San Jose wouldn't even be felt in Sacramento.

In addition, the earthquake prone areas of the west have very strict building codes to prevent catastrophic building failures. Earthquakes simply don't level cities in the west because they're all required to be well built. Usually the danger from earthquakes is from being buried in your own home (such as the tens and hundreds of thousands of people that die when a major quake hits in Mexico or that somewhat recent quake in Iran where the population tends to live in mud/earthen houses), but that threat simply doesn't exist in the western US. It would take a 10+ earthquake to do that sort of damage here.

Whereas the means for protecting a large area from catastrophic flooding... it's just not effective. You can't water-proof houses, and a single point failure in a levy results in massive flooding for many structures. A single point failure in a building during an earthquake will at worst damage/destroy a single building. Frankly the only real danger from earthquakes in a modern city is fire.