When a 7.3-magnitude earthquake struck off Japan’s eastern coast early Friday morning, we all feared a tsunami. But San Francisco gets earthquakes all the time, and we’re not scared of a tsunami there. Why?
When news broke of a 7.3-magnitude earthquake off the eastern coast of Japan early this morning, our first reaction was to fear a tsunami. The devastating earthquake that hit Japan last March and left 15,000 dead was in large part so damaging because of the ensuing tsunami, massive waves of ocean water which crashed up to six miles inland and over a hundred feet high. Luckily, today’s earthquake and its aftershocks seem to have had minimal adverse effect, and the waves are not high enough to be damaging.
But it got us wondering. The San Francisco Bay Area, perhaps the most famous earthquake zone in the continental United States, is hit by dozens of small earthquakes every year — and only a century ago, pretty much the entire city of San Francisco was flattened by an earthquake. Yet we never worry about a San Francisco tsunami. Is that lack of foresight, or is something else going on entirely?
Tsunamis — the word derives from the Japanese characters meaning “harbour” and “wave” — are not like regular waves. Though they can be triggered by underwater landslides or even meteorological conditions, typically they’re the result of immense energy being transferred by displaced ocean water. And what displaces ocean water? Earthquakes.
So, plate boundaries! There are three main types: convergent, divergent and transform. Convergent is when two plates smack into each other, divergent is when they move apart from each other, and transform is when they rub laterally against each other. We’re putting aside divergent for now, because neither San Francisco nor Japan have to worry about them.
The movement of plates isn’t smooth and it isn’t clean; the plates stick to each other, pop loose, bounce backward of forward like a rubber band. (Yep, when you’re talking about this much rock, it has a tendency to act elastic.) The most important term you need to know about convergent plates is subduction. Subduction is when an oceanic plate smashes into a land plate (or “continental” plate). Oceanic plates are made of heavier rock, so when the Pacific Plate smashed into Japan, it slipped under the Japanese land plate. That’s not how all convergent boundaries work; sometimes, two continental plates will smash into each other and built a mountain range at the smack-point, which is what’s happening between India and Asia right now. But subduction doesn’t give us the Himalayas — just lots of trouble.
Subduction does crazy things to the sea floor. Remember that earthquakes aren’t smooth; these two plates have been smashing together for aeons, sticking in places, being jammed down into the Earth’s mantle, and all of a sudden, POP! Parts of the sea floor behind the contact point are forced up, in weird, non-uniform spots. If you had a piece of cardboard hanging off the edge of a table, and folded the edge down, it’d force some of the cardboard still on the table upwards. That’s kind of what’s happening here; it’s not only where the two plates smash together that sees the impact. But a whole mess of the sea floor very suddenly pops up.
That’s what happens in Japan. But it’s not what happens in San Francisco.
San Francisco and Japan are both at risk of earthquakes because they lie close to plate boundaries. But they’re not the same kind of plate boundaries. Remember those three types, convergent, divergent and transform? Japan’s dealing with a single convergent boundary, but San Francisco is staring down multiple faults, and the ones that matter are transform faults.
San Francisco’s seismic situation is incredibly complicated and frankly kind of a mess. We’re not dealing with one fault line and two plates here, even though it’s commonly referred to simply as the San Andreas Fault. The San Andreas Fault is the line between the Pacific Plate (which is oceanic) and the North American Plate (which is a land or continental plate), and it’s a transform boundary. That means instead of smashing into each other, those two plates are sliding past each other, violently scraping and getting stuck and popping free. The last time the San Andreas really let loose was in 1906, which any Northern Californian will know as the Big One. The 1906 earthquake destroyed 80 per cent of San Francisco and killed thousands.
So, San Andreas and Cascadia, that’s two of the three. The last one, where the Gorda and Pacific plates meet, is called the Mendocino fault, which is also a transform (slipping and sliding) boundary; jolts along this fault regularly cause earthquakes in northern California.
Of the three main fault lines that would affect San Francisco, two out of the three are transform faults, and one is currently (but ominously) inactive. Transform boundaries are just as dangerous to people on land as convergent boundaries; they still shake the hell out of the land, which can lead to fires and floods and all kinds of disaster. But one thing they don’t do is abruptly displace ocean water, because they’re moving laterally rather than up and down. So, no displaced water, no tsunami. And that’s why nobody’s worried about a giant wave turning Golden Gate Park into a swamp.
Emily Elert contributed a LOT to this article.
Image: Tsunami Alarm System, Long Island University, Google Earth and Wikimedia Commons