Myth 1: Tsunamis are just like ordinary waves, but bigger.
Here's precisely what makes surfers in Hawaii often fail to heed tsunami warnings only to end up in a 10-minute-long rush of white water: A normal North Shore wave is a 30-by-300-foot wave, which just crashes and dies. A tsunami, on the other hand, is a 30-by-633,600-foot (120-mile) wave. That long wavelength is key: instead of just crashing and dying, a tsunami will keep coming in as a 5-to-15-minute-long torrent of seawater. So, you may be fine riding it out in the ocean... ah, but once you reach land, the tsunami won't stop. It'll keep coming, and before you know it your surfboard is suddenly a life preserver that you end up holding onto for dear life as the water keeps rushing in.
Myth 2: All waves — including tsunamis — look the same
Don't let this myth fool you either. The tsunami that left Japan in 2011 reached Hawaii in about 5 hours, but it subsided into a series of 10-foot swells. Yet despite their low height, they still had enough energy to, just like a flood and/or storm surge, devastate coastal towns and damage many buildings beyond repair. An episode of "Weather Caught On Camera" on the Weather Channel includes a piece of supposedly viral video documenting the rush of seawater that turned out to be the tsunami, which looked nothing like a wave at all, but rather a sort of flash flood of seawater that just kept coming.
Myth 3: All undersea earthquakes trigger tsunamis
It's not the shaking that triggers a tsunami, despite how strong it may feel. The seismic waves are too short and low to displace water. In order for a quake to generate a tsunami, a quake needs to cause permanent vertical deformation of the ocean floor. Meaning, of course, that a strike-slip fault like the San Andreas, where the tectonic plates slip past each other, would only cause horizontal displacement of the ocean floor, and thus, no tsunami. Subduction zones and thrust faults, on the other hand, do indeed cause permanent deformation of the ocean floor, and thus, a tsunami can indeed be triggered.
Myth 4: Earthquakes are the only tsunami triggers that exist
Many people assume that because earthquakes are the most common triggers, they must be the only triggers. Not so. In 1958, a strike-slip Fairweather Fault quake, registering 8.3 on the MMS, caused 40 cubic yards of rock to detach itself from a mountain lining Lituya Bay. Alaska. The slab slid into the bay at over 100 miles per hour, displacing enough water to cause a 1,720-foot wave in the bay.
In geologic and Neolithic history, however, there were some tsunamigenic landslide events far bigger than that one. Landslide debris carbon-dated to roughly 6000 BC, for example, detached itself from Mount Etna in Sicily — roughly 8 cubic miles, or approximately 43.6 billion cubic yards, of it, that is. The resulting wave, when plugged into computer models, that is, would have had initial heights of 165 feet in the open ocean (Lituya Bay was far shallower, which is why the wave was so much higher), enough to swamp countless Neolithic villages. The modern threat posed by the Cumbre Vieja volcano in the Canary Islands, as another example, consists of a 120-cubic-mile, or 654.2-billion-cubic-yard, block of debris just waiting to slide into the Atlantic. The tsunami generated by that could have initial heights of 2000 feet and, remote from its generation location, it could break up into a series of about 20 165-foot waves, enough to devastate the entire Eastern United States.
As if landslide triggers don't sound scary enough, there's also impact events. The asteroids that created the Chicxulub and Burckle Craters, for example, were big enough, and their impact plumes wide enough, to displace hundreds of cubic miles of pure ocean, generating tsunamis with initial heights of close to 10,000 feet (especially in the case of Burckle Crater, which, if my calculations are correct in my other post on it, displaced, at the very least, about 600 cubic miles of pure seawater; in the case of Chicxulub, the impact was in a shallow [less than 200-foot-deep] sea, not a 12,500-foot-deep ocean), and still 1000+ feet as they approached landmasses — enough to wipe entire countries off the map should repeats of these impacts happen today.
Myth 5: Earthquakes can impact California, but tsunamis can't
While the San Andreas Fault is indeed a strike-slip (which, let's face it, is indeed incapable of triggering a tsunami), other California faults sure aren't. Take the Puente Hills Fault, which was responsible for April's 5.1 in La Habra and could trigger a 7.5 directly under downtown Los Angeles if it rips entirely, for example: it's a blind thrust fault. That means, yeah, it's thrusting the Puente Hills, east of Los Angeles, upwards... and oh, yeah, it would take a very long time to rupture due to the large, long, shallow underground rupture area. In the event of a similar fault off the coast, underwater instead of on land, rupturing, that could cause a tsunami in proportions far out of range for its magnitude, which for the SoCal coast could be devastating.
As for remote tsunami sources, while the tsunami of Japanese origin in 2011 obviously didn't do much damage to American shores despite traversing the entire Pacific ocean, there are indeed sources much closer to home that can still trigger far distant tsunami damage effects. In 1964, for example, a 9.2 quake on the eastern edge of the Aleutian Trench triggered a tsunami which devastated Northern California, and Crescent City in particular... but even that isn't the most significant megathrust tsunami threat to SoCal. In that case, we could be looking at a 50-foot-plus tsunami that could reach SoCal in as little as 2 hours from its source: the Cascadia subduction zone.
The last time Cascadia ripped, triggering a quake that could have easily been about as big as Alaska's monster, was on January 26, 1700. The tsunami from that one was big enough that, oh, yeah, even Japan got swamped... and when a tsunami is generated, it doesn't just propagate in one direction, it propagates in all directions. According to computer models, a repeat of that one could cause a tsunami to reach San Francisco in 1 hour, Los Angeles in 2 hours... oh, yeah, and could easily cause in excess of $70 billion in damage to the entire West Coast of the United States, including the portion in California not affected by the quake itself. Yeah, I certainly wouldn't want to be on the beach when that happens...