The following is an excerpt from The Big Ones: How Natural Disasters Have Shaped Us (and What We Can Do About Them) by Lucy Jones.

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The Big Ones: How Natural Disasters Have Shaped Us (and What We Can Do About Them)

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Earthquakes are happening constantly around the world. The seismic network that measures earthquakes in southern California, where I live and spent my career as a seismologist, has an alarm built into it that goes off if no earthquake has been recorded for 12 hours–because that must mean there’s a malfunction in the recording system. Since it was put into effect in the 1990s, Southern California has never gone more than 12 hours without an earthquake.

The smallest earthquakes are the most common. Magnitude 2s are so small they are only felt if someone is very nearby their epicenter, and one happens somewhere in the world every minute. Magnitude 5s are big enough to throw objects off shelves and damage some buildings; most days a few of these strike somewhere. The magnitude 7s, which can destroy a city, occur more than once a month on average, but luckily for humanity, most take place underwater, and even those on land are often far from people.

But for more than 300 years, none of these, not even the tiniest, has occurred on the southernmost part of the San Andreas fault.

Someday that will change. Big earthquakes have happened on the southern San Andreas in the past. Plate tectonics hasn’t suddenly stopped; it is still pushing Los Angeles towards San Francisco at the same rate your fingernails grow–about 1.5 inches each year. Even though the two cities are in the same state and on the same continent, they are on different tectonic plates. Los Angeles is on the Pacific plate, the largest of the world’s tectonic plates, stretching from California to Japan, from the Aleutian Arc of Alaska to New Zealand. San Francisco is on the North American plate, which extends east to the Mid-Atlantic Ridge and Iceland. The boundary between them is the San Andreas fault. It is there that the two plates get carried slowly past each other; their motion cannot be stopped any more than we could turn off the sun.

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In a strange paradox, the San Andreas produces only big earthquakes because it is what seismologists consider a “weak” fault. It has been ground so smooth, across millions of years of earthquakes, that it no longer has rough spots to stop a rupture from continuing to slip.

To understand the mechanics of it, imagine you’ve laid a large rug on the floor of a room that has wall-to-wall carpeting. After placing it, you decide that, on second thought, you want to move it one foot closer to the fireplace. If it had been laid on a hardwood floor, it would be easy enough to move: you could simply grab the side nearer to the fireplace and pull. But it’s on carpeting, so the friction between the carpet and the rug makes that impossible. So what could you do? You could go to the far side of the rug, pick it up off the carpeting and put the edge of the rug where you want it, a foot closer to the fireplace. You now have a big ripple, which you could push across the rug until you’ve reached the end, at which point the entire rug would be one foot closer to the fireplace.

In an earthquake, a seismologist sees not a ripple but a rupture front. The motion of that ripple across the “rug” of the San Andreas fault creates the seismic energy that we experience as an earthquake. It is a temporary local reduction in friction, allowing a fault to move at lower stress. In the same way that the rug couldn’t move all at once, an earthquake too must begin at one particular spot on its surface, its epicenter, and the ripple must roll across it for some distance.

The distance the rupture front travels determines is one of the chief determinants of an earthquake’s size. If it moves a yard and stops, it is a magnitude 1.5 earthquake, too small to be felt. If it goes for a mile down the fault and stops, it’s a magnitude 5, causing a little damage nearby. If it goes on for a hundred miles, it is now a magnitude 7.5, causing widespread disruption.

The San Andreas fault has been smoothed to such a degree that now, when an earthquake begins, there is nothing left to keep it small. The ripple will continue to move down the fault, radiating energy from each spot it crosses creating an earthquake that lasts for a minute or more and a magnitude that grows to 7 or even 8. Only after such an earthquake has broken the fault and made new jagged edges can it begin to produce smaller, less damaging earthquakes.

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So we wait for that big earthquake. And wait.

The southernmost part of the fault had its last earthquake sometime around 1680. We know this because it offset the edges of Lake Cahuilla, a prehistoric lake in much of what is now the Coachella Valley, filling with water the flats where the Coachella music festival now meets each year. It left behind geologic markers, as did previous earthquakes, so we know that there were 6 earthquakes between 800 and 1700 AD. That means the 330 years since the last earthquake on this part of the San Andreas is about twice the average time between its previous earthquakes. We don’t know why we are seeing such a long interval. We just know that plate tectonics keeps on its slow, steady grind, accumulating more offset and energy to be released the next time. Since the last earthquake in Southern California, about 26 feet of relative motion has been built up, held in place by friction on the fault, waiting to be released in one great jolt.

Someday, maybe tomorrow, maybe in a decade, probably in the lifetimes of many people reading this book, some point on the fault will lose its frictional grip and start to move. Once it does, the weak fault, with all that stored energy, will have no way of holding it back. The rupture will run down the fault at 2 miles per second, its passage creating seismic waves that will pass through the earth to shake the megalopolis that is southern California. Maybe we will be lucky and the fault will hit something that can stop it after only a hundred miles or so—a magnitude 7.5. Given how much energy is already stored, however, many seismologists think it will go at least 200 miles, and thus register 7.8, or even 350 miles and reach 8.2.

If it ruptures as far as central California, all the way to the section of the fault near Paso Robles and San Luis Obispo, it will hit a part of the San Andreas that behaves differently. This part accumulates a fingernail-growth rate of tectonic offset, just like the rest of the fault. But it’s what known as a “creeping section.” Instead of storing energy to release in one big earthquake, the energy here oozes in small motions, sometimes with little earthquakes, sometimes with no seismic energy at all. We think, we hope, that the creeping section will act as a pressure valve of sorts, keeping the earthquake from growing any bigger than 8.2.

Excerpted from The Big Ones: How Natural Disasters Have Shaped Us (and What We Can Do About Them) by Lucy Jones (2018).