The magnitude 8.9 earthquake off the east coast of Honshu, Japan has triggered a tsunami warning for the coastal areas of California north of Point Concepcion north to the border between Oregon and Washington. Hawaii is due to be hit first; at this point, I believe it's supposed to happen at around 3 AM Hawaii (5 AM PST). The tsunami will reach San Francisco Bay at 8:07 AM PST.
Tsunamis are generated when large earthquakes send shock waves through ocean waters. Those shock waves travel at some 500 MPH in deep water, but are barely noticeable in mid-ocean. The trouble occurs when the shocked water approaches gradually shallower coastlines: waves slow down, allowing following waves to catch up with them. This produces new waves of greater height. When those larger, slower waves actually reach shore they may only be traveling in the tens of MPH, but the amount of water makes those slower waves push farther onto land than usual. Moreover, tsunamis are a series of those larger, slower waves.
The National Oceanic and Atmospheric Administration (NOAA) has four levels of message it issues, ranging from information, the lowest, to warning, the highest. The warning issued to northern and central California and Oregon (and parts of Alaska) signifies "inundating wave possible." (A lower-level advisory is in place for the rest of California.)
The trouble is, the actual damage from a tsunami depends on how high the waves are when they hit shore. Tsunami height is not nearly as easy to forecast as tsunami propagation speed, and the best indicator, unfortunately, is its impact on other areas. As such, mainlanders will be watching what the waves look like in Hawaii.
For myself, I await the tsunami with some confusion and a little dread. The news (Weather Channel and CNN) has been reporting the tsunami will reach "San Francisco Bay" at 8:07, but San Francisco Bay is somewhat insulated from such large waves by virtue of the Golden Gate. The western part of San Francisco, however, like the rest of the California coastline, is pretty much open to the ocean. Being just a few miles from the beach, in theory I ought to be preparing to move to higher ground. However, the beach is a bit lower than most of the rest of the city, as you can see from a 1998 USGS elevation map of San Francisco. The question therefore is, how far will a tsunami travel inland when it has to confront an upward slope (or a series of them, as the paved streets have sculpted the landscape)?
Nobody knows. So I sit, watching cable news instead of sleeping (this is one of those times I'll violate my usual rule of staying the hell away from cable news), wondering if I'll hear the air-raid sirens in a few hours (we don't have dedicated tsunami alarms as far as I know).