According to the U.S. Geological Survey (USGS), roughly 500,000 detectable earthquakes occur each year—meaning at least a few will have hit by the time you’ve finished reading this article. Of that gigantic number, however, only about 100,000 are intense enough for humans to feel the effects, and just 100 or so of those actually cause any destruction. In other words, the Earth quakes a lot, whether we realize it or not. So why do earthquakes happen, when do they happen, and can you avoid them by moving to the moon? Those questions and more, addressed below.
1. You can blame earthquakes on Earth’s inner core.
Understanding earthquakes requires a brief journey to the center of the Earth, which is a solid ball of iron and other metals that can reach temperatures up to 10,800°F. The extreme heat from that inner core emanates through its surrounding layers—first through the outer core, mostly made of liquid iron and nickel, and then on to the mostly solid rock layer called the mantle. This heating process causes constant movement in the mantle, which makes the Earth’s crust above it move, too.
The crust comprises a patchwork of giant, individual rock slabs called tectonic plates. Sometimes when two plates are sliding against each other, the friction between their jagged edges causes them to temporarily get stuck. The pressure builds until it can finally overcome the friction, and the plates finally go their separate ways. At that point, all the pent-up energy is released in ripples—or seismic waves—that literally shake the land sitting on the Earth’s crust.
2. Scientists can’t predict earthquakes, but they can occasionally forecast them.
Unfortunately, there’s no fancy device that warns us whenever an earthquake is coming. But while scientists can’t predict exactly when or where an earthquake will occur, they can occasionally forecast the probability that one will hit a certain area sometime soon (and if that sounds a little vague, it’s because it is). For one, we know where the tectonic plates border each other, and that’s where the high-magnitude earthquakes occur. The Ring of Fire, for example, is an area along the rim of the Pacific Ocean where approximately 81 percent of the world’s biggest earthquakes happen. We also know that especially large earthquakes are sometimes preceded by tiny quakes called foreshocks (though they can’t be identified as foreshocks unless a larger earthquake actually hits—if that doesn’t happen, they’re just regular, small earthquakes). When small quakes near a plate boundary coincide with other geological changes, it can indicate that a big earthquake is coming.
In February 1975, for instance, the Chinese city of Haicheng experienced possible foreshocks after months of shifts in land elevation and water levels, so officials ordered its million residents to evacuate immediately. The next day, a 7.0-magnitude earthquake rocked the region. Though there were 2000 casualties, it’s estimated that 150,000 could have been killed or injured if nobody had fled.
3. There’s a very small chance that “The Big One” will occur in the next year.
That said, successful forecasts like Haicheng’s are rare, and scientists spend a lot of time monitoring known fault lines—the borders between plates—to try to determine how much pressure is building up and when it might cause a problem. It’s not an exact science.
One fluctuating forecast is for “The Big One,” a huge earthquake that’s expected to hit the San Andreas Fault Zone, an 800-mile network of fault lines that runs from Northern to Southern California, sometime in the future. Right now, the USGS forecasts a 31 percent chance that a 7.5-magnitude quake will hit Los Angeles in the next 30 years and a 20 percent chance that such a quake will occur in San Francisco’s Bay Area.
The likelihood of “The Big One” is partially dependent on other earthquakes in that fault zone. After two back-to-back quakes hit Ridgecrest, California, in 2019, seismologists observed pressure changes in the surrounding fault lines, and a study published in July 2020 suggested that the chances of “The Big One” happening in the next year may have increased to 1.15 percent—three to five times likelier than previously thought.
4. Underwater earthquakes can cause tsunamis.
Because so much of Earth’s surface is covered in water, many earthquakes don’t touch land at all, but that doesn’t mean they don’t affect people. When plates shift on the ocean floor, the energy displaces the water above them, causing it to rise dramatically. Then, gravity pulls that water back down, which makes the surrounding water form a massive wave, or tsunami.
Earthquakes can also indirectly cause tsunamis by altering the landscape. On July 9, 1958, a 7.8-magnitude earthquake hit Lituya Bay in northeastern Alaska, causing a rockslide on a bordering cliff. As an estimated 40 million cubic yards of rock rushed into the bay, the force created an estimated 1720-foot wave—the largest tsunami of all time.
5. Alaska also holds the record for the largest earthquake in the U.S.
The boundary between the North American and Pacific plates runs through and around Alaska, which means that Alaskans are no strangers to earthquakes; according to the Alaska Earthquake Center, one is detected in the state about every 15 minutes.
On March 28, 1964, a 9.2-magnitude earthquake—the largest ever recorded in the U.S.—hit Prince William Sound, a body of water that borders the Gulf of Alaska. Not only did the initial force level buildings and homes, but it also generated a series of landslides, tsunamis, and other earthquakes (called aftershocks) that affected communities as far as Oregon and California.
Scientists discovered that the earthquake had happened because the Pacific plate wasn’t just rubbing up against the North American plate—it was actually slipping under it. The area where these plates converge is known as a “subduction zone.” Occasionally, the pressure builds up and causes a major movement, or megathrust, when it finally releases. Though experts still couldn't predict these movements, studying the damage did help Alaskans shore up their defenses for future earthquakes. Officials passed better building codes, and the town of Valdez, which sat on unstable land, was actually moved four miles east.
6. The world's largest recorded earthquake happened in Chile.
The 1960 earthquake near Valdivia, Chile, was larger than Alaska’s earthquake four years later, but the conditions that caused it were similar. The Nazca plate, which runs beneath the Pacific Ocean along South America’s west coast, is slipping under the South American plate (which is beneath the continent itself). On May 22, 1960, there was a huge shift along a 560- to 620-mile length of the Nazca plate, causing a catastrophic, record-breaking earthquake with a magnitude of 9.5. Just like in Alaska, this quake set off a series of tsunamis and aftershocks that decimated whole towns. It’s difficult to quantify the damage, but it’s estimated that at least 1655 people died and another 2 million people ended up homeless.
7. An earthquake can leave genetic scars on a species.
Approximately 800 years ago, an earthquake near Dunedin, New Zealand, thrust a section of its coast upward and wiped out the bull kelp that had lived there. New bull kelp soon started settling in the area, and their descendants today look indistinguishable from the neighboring kelp that never got displaced. In July 2020, scientists published a study in the journal Proceedings of the Royal Society B showing that the two kelp populations actually have different genetic makeup. Their findings suggest that earthquakes—and similar geological catastrophes—can have an extremely long-lasting impact on the biodiversity of the affected area.
8. The Richter scale for measuring earthquakes isn’t always accurate.
In 1935, Charles Richter devised a scale for determining an earthquake’s magnitude by measuring the size of its seismic waves with a seismograph. Basically, a seismograph is an instrument with a mass attached to a fixed base; the base moves during an earthquake, while the mass does not. The movement is converted into an electrical voltage, which is recorded by a moving needle onto paper in a wave pattern. The varying height of the waves is called amplitude. The higher the amplitude, the higher an earthquake scores on the Richter scale (which goes from one to 10). Since the scale is logarithmic, each point is 10 times greater than the one below it.
But seismic wave amplitude in one specific area is a limited metric, especially for larger earthquakes that affect pretty vast regions. So, in the 1970s, seismologists Hiroo Kanamori and Thomas C. Hanks came up with a measurement called a “moment,” found by multiplying three variables: distance the plates moved; length of the fault line between them; and rigidity of the rock itself. That moment is essentially how much energy is released in an earthquake, which is a more comprehensive metric than just how much the ground shakes.
To put it in terms the general public could grasp, they created the moment magnitude scale, where the moment is converted to a number value between one and 10. The values increase logarithmically, just like they do on the Richter scale, so it’s not uncommon for newscasters or journalists to mistakenly mention the Richter scale when they’re actually talking about the moment magnitude scale.
9. The moon has earthquakes, too.
Aptly called moonquakes, these seismic shifts can happen for a few reasons (that we know of so far). Deep moonquakes are usually because Earth’s gravitational pull is manipulating the moon’s interior structures. A surface-level quake, on the other hand, is sometimes the result of a meteoroid impact or the stark temperature change between night and day. But in May 2019, scientists suggested a possible fourth reason for shallower shakes: The moon is shrinking as its core cools, and this process is causing shifts in its crust. As the crust shifts, the scarps—or ridges—that we see on the moon’s surface may shift, too.