Everyone knows the story of California's rather serious seismic threat. The disaster movies department in Hollywood has even exaggerated it time and time again, with (absolutely unrealistic, scientifically speaking) movies such as "Escape from Los Angeles", "10.5", and, now, one of the biggest movies to come out in the new year effective midnight tonight, "San Andreas" (which is the most realistic of the three, for the most part ― then again, there are some aspects of the trailer, such as A, the scientific impossibility of a quake on the real San Andreas Fault being felt on the East Coast, and B, the depiction of a gaping hole where in reality there's pulverized rock instead, that certainly are criticism points among seismologists as, among other things, unnecessary scaremongering), and of course, time and time again, people actually believe what is supposed to be fiction. Unfortunately, what they don't realize is that the San Andreas isn't California's worst seismic threat, at least from the standpoint of proximity to populated areas. There are other faults, of far more dangerous types besides, that lie not way out in the Mojave Desert and/or Coachella Valley but, instead, either directly underneath densely populated areas, or, worse yet, offshore.
First, for those who are wondering why films such as 10.5 are scientifically improbable: The Moment Magnitude Scale is a base-30 logarithmic one. That is, a magnitude-9 quake is 30 times more powerful than an 8, which is in turn 30 times more powerful than a 7, which is, likewise, 30 times more powerful than a 6, and so on. Based on that scale, even a fault that completely circled Earth would only produce a 10.4 quake. What's more, seismic waves are a result of friction, NOT of mere splitting. Friction does NOT create islands. Rifting, or spreading, does, and rifting, like what is seen in Africa's Great Rift Valley, the Mid-Atlantic Ridge (Iceland), the Mediterranean Ridge (Sicily), and the Red Sea, creates volcanoes, not quakes. That's because continental drift occurs on magma, not water... so, when continents spread apart, that magma becomes tempted to rise up and gush out, where it then cools and adds more rock to the tectonic plate(s) in question. Instead of water in the San Andreas Fault, there, once again, is fault gouge... and whenever it slips, more rock is ground up into that flour-like consistency to replace it, and the crack itself continues to remain a hairline while the visible scar on the surface is only visible from either A, the air, or B, space.
Also, the San Andreas is, should I say it again, a strike-slip fault. A strike-slip fault is a kind of fault that slips horizontally, one in which one plate slides one horizontal direction and the other plate slides in the exact opposite horizontal direction relative to it. In the case of the San Andreas, what that means is that the Pacific Plate is sliding northwest and the North American Plate is blindsiding it, drifting toward the southwest. So, in the event of the real "ShakeOut" scenario, Los Angeles would find itself 20 feet to the northwest of where it was prior to the quake in question. Even in the portions where it dives below the sea surface (such as north of San Francisco), it only displaces the sea floor horizontally, never vertically. Therefore, there's only one way the San Andreas can possibly displace even a small portion of the ocean column: by first inducing landslides. That has happened before, in 1906, when the quake that was most notorious for inducing firestorms that, quite literally, burned San Francisco to the ground, also sent Mussel Rock tumbling into the Pacific... but by the time the resulting wave got to San Francisco, it was so small ― I'm talking only a few inches high ― that only tide gauges could detect it. Even then, however, that's an anomaly, not the norm. Most quake-induced landslides happen inland, not on the coast.
Secondary faults, however, are a completely different story. Unlike the San Andreas, most of these auxiliary faults ― those that actually underlie heavily populated areas and are responsible for seismic events of the likes of the Chino Hills, Whittier Narrows, and Northridge quakes ― are buried fault structures with vertical, not horizontal, movement. Such faults are called blind thrust faults by seismologists, and some of them, such as the Puente Hills Fault that the Whittier Narrows quake is now seen by seismologists as having been a partial rupture of, are capable of rivaling the San Andreas in terms of magnitude (7.5 vs. 7.8), and, according to seismologists, some of them can be a whopping 15 times more dangerous than the San Andreas itself, should they rupture in their entirety. The Pico Fault, which set off the Northridge quake, as another example, also only partially ruptured back in 1994 (a full rupture would have put the Northridge quake in the range of 7.2 or higher), and it too is capable of, naturally, causing a great deal of damage. However, that's missing the bottom line: where there's thrust faulting, there's vertical movement. While vertical movement on land is, indeed, bad enough (any buildings, highways, or other structures that happen to lie directly on top of a blind thrust fault, regardless of how well they're built, will find their ground floors, and, by extension, entire structural support dangerously twisted out of proportion), it becomes even worse if that vertical movement happens to occur underwater.
That's because vertical ocean floor displacement happens to result in vertical ocean column displacement, and, ultimately, vertical sea surface displacement. Initially, in the deep ocean, this displacement is far more pronounced on the ocean floor than at the surface. While surface displacement may only initially be a few feet, it spreads out over hundreds of miles in length, because it actually has room to spread out. Because of its length, it can travel at, quite literally, jet speeds: between 400 and 600 miles per hour, capable of traversing the entire Pacific in less than a day. As this hundred-plus-mile-long wave approaches enters shallow water, however, it no longer has the room to spread out that it once had. Consequently, the landmass forces this once fast-moving wave to slow down and grow taller. Ultimately, it ends up manifesting itself as a 100-foot bore wave with a 50-mile-long, seemingly permanent plateau of water on its tail, capable of knocking over every structure in its path and, perhaps most significantly, inundating even 10-mile-inland structures with seawater. Or, to put it in layman terms, the exact same phenomenon that ravaged Indonesia in 2004 and Japan's Sanriku Coast in 2011: a tsunami.
Japanese for "harbor wave", this term was, to be fair, one that, with the exception of some sporadic communities that have used it to describe this wall of water having hit them before, pretty much unused outside of the scientific community, to say the least, for the longest time. That is, until 2004. When people learned that it was the tsunami, not the Indonesian quake itself, that was responsible for the majority of those 220,000 deaths, suddenly more and more of the public began to take notice, and suddenly "tsunami" became a far more popular buzzword than it was before. That public lexicon was repeated in 2011, when Japan's Sanriku coast, including the Tohoku province that includes the major port cities of Sendai and Kessennuma, was also rattled by a 9.0 quake and, only 20 minutes later, bashed by a 130-foot tsunami. What's more, it also has become a point of discussion among Seattle, Portland, Vancouver, and Pacific Northwest coast residents after American geologist Brian Atwater, Japanese geologist and historian Kenji Satake, and dendrochronologist David Yamaguchi all worked together to uncover evidence that a massive quake and tsunami (possibly one that ruptured the entire Cascadia megathrust from one end to the other) had struck the region at 9PM Japanese time (4AM Pacific time) on January 26, 1700. The fact that there are blind thrust faults right here in California, however, pose an even more ominous question: Could an offshore blind thrust fault displace the ocean floor enough off SoCal enough to unleash a killer tsunami within minutes of home?
In fact, that very offshore blind thrust scenario has happened before. The date was the winter solstice, December 21, 1812. At the time, Spanish missionaries were busy building a colony in California, while on the east coast, conflict between the US, Britain, and France was, merely 40 years after the American Revolution, brewing once again. It was business as usual for those Franciscan friars working in La Misión de la Señora Barbara, Virgen y Martír, better known as the twin-steepled Mission Santa Barbara, when, suddenly, the ground began to rumble. The source of the shaking was the Santa Monica Mountains-Santa Cruz Island segment of the Channel Islands blind thrust system, which, until fairly recently, was mostly unknown to seismologists. After the quake, which in itself did a lot of damage to several Spanish missions and presidios, the Native Americans that were being "missioned" out to ― the Chumash ― knew better than to stay put. They gave the Spaniards word that they would drown if they remained in low-lying land, and since the Chumash were there for hundreds, if not thousands, of years longer than the Spaniards, the Spanish missionaries agreed to climb uphill with them to get out of the way.
Then, according to military general and Franciscan friar Luis Gil Taboada, who was commander of the Santa Barbara Presidio at the time, "the sea receded and rose like a high mountain," and then remained that way for several minutes before receding again. Hundreds of miles to the west, boats floated a mile and a half up Gaviota Canyon (that's almost exactly how far inland the No. 18 Kyotoku Maru tuna fishing vessel floated in 2011 ― go figure). Even as far north as San Francisco, Spanish accounts of this killer wave were ominous, where according to presidio commanders up there, several galleons capsized and sank in a harbor where they were supposed to be protected. To the south, in San Diego, damage to galleons and other ships was, likewise, just as severe, and according to reports, ships down there also found themselves beached.
That isn't the only segment of the Channel Islands thrust system either. The Santa Rosa segment hasn't ruptured in almost 250 years, if carbon dating is any guide, and the San Miguel segment, according to seismologists, has gone almost 300 years without a rupture. This, of course, makes those segments, thanks to stress build-up, even more overdue than the Santa Cruz segment. Then, as if that's not enough, there's also the Palos Verdes-Catalina segment of the Compton blind thrust system, which also has gone hundreds of years without a rupture, and perhaps most alarming, the Thirtymile Bank blind thrust fault, which is capable of causing a magnitude-7.6 quake offshore and setting off a tsunami that could threaten downtown San Diego. All of those are easily capable of causing a repeat of the 1812 disaster, at the very least.
So, now we have done an almost complete 180 from Hollywood's depiction. Although the notion of California falling into the ocean is definitely the work of fiction, to the utmost degree, this new evidence builds the case for these auxilary faults being capable of causing the exact opposite problem: the Pacific invading California in the form of a tsunami. In modern times, especially in California, however, "tsunami hazard zone" signs are indeed posted all over the beaches, unlike in 1812, complete with the caption "In case of earthquake, go to high ground or inland," as a stern warning to those who may be tempted to get back in the water after they feel the ground shake. What's more, there are also lifeguards and harbor police that will actually escort people out of harm's way and barricade off the hazard zone until "all clear" is given. Combine that with California's rugged coastline, unlike the coastlines in Japan, Indonesia, and Thailand, where there are a boatload of hills right up next to the coastline that people can easily run to, and loss of life should be minimal.
However, while loss of life shouldn't be too big of a problem, the same cannot be said when it comes to loss of property. The real estate along the coast of California, in places like Newport Beach, Dana Point, Laguna Beach, Ventura, and, yes, Santa Barbara is certainly some of the most expensive real estate in the entire country. Santa Barbara County is the #1 most expensive county ― that's in nationwide terms ― to live in, and Orange County comes in a close second place, with, again, the bulk of the wealth concentrated right on the coast and in the tsunami hazard zone. Add up all those pieces of expensive real estate and send a tsunami into them to wipe that real estate out, and you're looking at, easily, damages in the tens, if not hundreds, of billions of dollars ― in fact, I wouldn't be too surprised if the damage caused by an event like this ends up totalling higher than the damages caused by Hurricane Katrina, which has since eclipsed the Northridge quake as the costliest natural disaster in US history. Here's hoping people actually heed the warnings so that, at the very least, loss of life can be prevented in an event like this...