Over the past month, a spate of earthquakes has captured people’s attention. A magnitude 6.9 off of Eureka, north of San Francisco. A pair of magnitude 4-5 earthquakes in the Los Angeles basin. A magnitude 4.8 near the Yellowstone caldera in Wyoming. And now, a massive magnitude 8.2 off the coast of Chile that even generated a tsunami. Is Earth spiraling out of control? Are end times are around the corner? Far from it.
In fact, these earthquakes illustrate a couple of things to bear in mind when it comes to seismicity. First, there are no strong patterns in the distribution of earthquakes over time. Second, earthquakes happen all the time. When we hear about a quake in the news, it’s likely because it affected a notable area (like a major U.S. city, for example).
Each year, there are thousands upon thousands of earthquakes felt around the world. According to the U.S. Geological Survey using data going back to 1900, there are over 14,000 earthquakes of magnitude 4 or greater every year. That works out to be approximately 40 per day (if they were equally distributed—more on that later). Even what we consider large earthquakes (magnitude 6 or greater) occur, on average, over 150 times per year. Earthquakes are not rare occurrences on the planet, but where they occur along the plate boundaries strongly influences whether anyone notices.
But let’s back up for a moment for a little Earthquake 101. Why are we having quakes in the first place? Earthquakes are generated by releases of stress caused by interactions between and within the earth’s tectonic plates. These Californian and Chilean earthquakes are caused by interactions between plates, either with one plate sliding underneath the other (subduction) or sliding side-by-side (transform). This motion isn’t smooth, so plates can get caught on one other, building stress until it is released catastrophically in the form of an earthquake. Earthquakes all share this same characteristic: stress building in the earth’s interior that is released suddenly, sending seismic waves rushing outwards. Some of these waves are what we feel when the ground beneath our feet moves during a quake.
Earthquakes are mainly distributed along the boundaries of plates, with a smattering of earthquakes in places within the plates (usually marking older tectonic features or active volcanic areas). So, people who live near these plate boundaries should expect to feel earthquakes from time to time, which is exactly what happened in San Francisco, Los Angeles, and Chile. These are earthquake-prone areas where geologists can safely say that earthquakes are likely to occur (unlike the middle of Ohio, where the North American plate is stable).
Before we get to why location is important, let’s talk about time. Earthquakes are distributed randomly through time—that is, when large earthquakes occur does not follow any statistically significant pattern. When you look at a random distribution of events across time, there will be times when earthquakes will cluster and times when earthquake activity will be noticeably lower. However, no matter how hard people have looked for patterns in this signal, there isn’t any. Sure, there are plenty of people who claim (falsely) to “predict” earthquakes based on patterns of seismicity, the moon, sunspots, planetary alignments, and more. But for all these cases, there is no scientifically-valid evidence to support their predictions.
The best we can offer is a probability that an earthquake might occur along a fault in a given period of time. This is why it is better to be prepared for an earthquake if you live in LA or San Francisco because in places along plate boundaries, we know the likelihood of future earthquakes is high.
Even when looking at massive earthquakes like Chile’s, there is no statistically-robust correlation between such large events and other large earthquakes that might follow—that is to say, one earthquake does not cause others far away. They can trigger nearby earthquakes (called aftershocks) because as stress is released on one part of the fault, it might increase on another. However, there is no evidence that this works on a global scale, so triggering of distant earthquakes isn’t strongly supported.
So, why does it seem like we’re having a lot of earthquakes right now?
We could be in one of those clusters of randomly distributed earthquakes. Looking back at the past month of earthquake data, it turns out that there were 483 earthquakes of magnitude 4.5 or greater around the planet. Over the month before that, there were 574 earthquakes globally of magnitude 4.5 or greater. This means that there were actually fewer significant earthquakes over the past month than the month prior.
That being said, the singular Chilean earthquake is a rare event. We have to go back to a magnitude 8.3 earthquake in the Sea of Oshotsk off eastern Russia in May 2013 for the last earthquake as large as the Chilean earthquake. On average, there is an earthquake of magnitude 8 or greater about once a year. However, clustering really doesn’t explain why it seems like we’re having more earthquakes than usual.
This is where location becomes important. During February, there were many earthquakes as large or larger than the California earthquakes, but they happened in places like remote China, the middle of the Atlantic Ocean, and distance island chains in the south Pacific—not exactly headline-grabbing locations where the earthquake would be felt by lots of people. However, the California earthquakes occurred near highly populated areas and, for the Los Angeles earthquakes, during TV broadcasts that were instantly shared across the world.
That is the biggest reason why we’re tricked into thinking that disasters like earthquakes and volcanic eruptions might be happening more frequently: location, location, location (and the rapid dissemination of news on the Internet). Have a few earthquakes near places where people live, and it suddenly seems like earthquakes are in the news a lot more often. We’re suddenly thinking that more earthquakes are occurring. This is the same as the supposed “rule of three” for celebrity deaths or airplane crashes.
So, even though it might seem that we’re having a lot of earthquakes, it is actually just a product of:
1. Location2. Constant news coverage
Earth is still working like usual, relieving the stress that is built up during the interactions of tectonic plates. It’s just that we humans want to find patterns in the chaos to explain why disasters occur when they do. But sometimes, those patterns just don’t exist.