Los Angeles, California has experienced a recent bout of earthquakes. It's time to delve into the science of what they are, when they happen, and how to prepare before the Big One shakes the state without freaking out.

Storekeeper cleans up merchandise knocked off shelves by an earthquake in the Los Angeles region on Friday night. Photo credit Jae C. Hong/AP


When the Earth quakes

What, exactly, is an earthquake? Yeah, yeah, yeah, we all know it's when the ground shakes, but surely geoscientists have a more detailed description of the process than that! I recruited seismologist Dr. Julian Lozos, a postdoctoral fellow with the United States Geological Survey and Stanford University, to give a more detailed definition:

An earthquake occurs when some section of a fault breaks and slips, releasing seismic waves and causing the ground to shake. Faults are planes of weakness in the earth's crust. They may appear as lines on maps, but they extend miles deep.


Faults can also spread out away from a neat little line, creating an entire fault zone of weakness. These zones of weakness are typically at the boundaries of tectonic plates, the brittle, rigid chunks of crust and lithosphere that make up the surface of the Earth. Dr. Lozos continues:

Even though faults are weaker than the surrounding rock, they're still strong enough to stay still most of the time. The same friction that heats up your hands when you rub them together, or keeps you from slipping when you take a step, is what allows a fault to resist moving along with the larger motion of the tectonic plates that make up earth's crust.

However, as long as the fault is resisting slipping, the moving plates are putting stress on the fault. Eventually, the amount of built up stress becomes too much for the fault to handle. [...] Part of the fault breaks, and slips until the built up energy is released.

Energy is neither created nor destroyed, so when that potential energy is released, it is converted into kinetic energy. The sides of the fault scrape past each other up to several meters per second, producing seismic waves.


Seismic waves are what you feel, and what causes so much damage. Seismic wave are split into two categories. Body waves travel through the interior body of the Earth, and surface waves travel along the surface of the crust.

Body waves can be either P-waves, a pressure wave that compresses in the direction of propagation, or S-waves, a shear wave that displaces perpendicular to the direction of propagation. Because P-waves are faster than S-waves, the P-wave is the primary wave that always arrives first, while S-waves are the secondary waves that arrive second. After them is the entire array of surface waves: Love waves that shake side to side and Rayleigh waves that roll like the ocean.

The body waves are what seismologists use to determine things like, "Where was that?!" but the surface waves are what break buildings, knock people down, and otherwise make earthquakes a brief period of highly exciting destruction.


Selected seismograms from last night's earthquake. Extracted from USGS data.

The green bars are the P-wave and S-wave arrival times. The P-wave arrives later farther from the earthquake epicentre, and the gap between P-wave and S-wave arrival times increases. For these seismograms, the P-waves amplitude are quite small, with the S-wave barely bigger except at the closest stations. For the most part, the surface waves are the ones producing the iconic dramatic wiggle on the seismogram, with a few even spiking out at the stations nearest the earthquake.


Quantifying Earthquakes: how big is a 5.1?

An earthquake's magnitude is a quantitative measure of the amount of energy released. Last night La Habra, a city within the Los Angeles basin, experienced a magnitude 3.6 earthquake at 8pm, followed by a magnitude 5.1 earthquake an hour later. Switching back to Dr. Lozos:

The amount of the fault that slips, and the distance that it slips, determines the size of the earthquake. A 5.1 like the earthquake in La Habra tonight doesn't require a very large amount of slip or fault area, but in order to produce The Big One - the archetypal M7.8 on the San Andreas, like the 1906 earthquake that destroyed San Francisco - 300 miles of fault need to sustain as much as 30 feet of slip.


Magnitude is a logarithmic scale, so every full number increase represents 10 times more shaking, and 32 times more energy released. The difference between the two earthquakes last night was 1.5, so the second earthquake was more than 30 times larger amplitude on a seismogram, and released over 175 times as much energy.

Qualitative Earthquakes: how did it feel?

It's trite but true: size isn't everything. Sure, it matters, but magnitude alone doesn't determine how much shaking you feel.

Because the bigger earthquakes break more of the fault, the shaking tends to last longer for them. But the further away you are from an earthquake, the gentler the shaking, even if it does last longer. Being right on top of a small earthquake may give you a stronger shake than being a larger distance away from a bigger one.


This shaking is influenced not only by the size and location of the earthquake, but also by the surficial geologic materials. Sediments amplify shaking, so in general valleys and basins will feel an earthquake more intensely than hard bedrock. For Los Angeles, the type and thickness of sediments around the basin were used to create a prediction for how much seismic waves would be amplified by an earthquake.

The US Geological Survey automatically-generates shakemaps as an indication of how much ground shaking was experienced at different locations. Here's the shakemap for the magnitude 5.1:


The red star marks the epicentre of the earthquake; red lines mark known faults in the area. The places closest to it felt strong shaking (yellow), dropping down to moderate and light shaking farther from the epicentre. However, the squishy sediments in the valley bottoms amplified shaking so people as far away as Victorville may have felt the earthquake, while the closer Thousand Oaks and Santa Clarita had weak to no shaking thanks to the solid rocks of the coastal mountains under their communities.

But, shakemaps are built from single-point measurements at seismographs around the valley. Despite the extensive efforts of the USGS, the Los Angeles basin has far more individual humans in it than seismographs, so why not ask them what they felt? Enter the Did You Feel It? program, an effort to build a better picture of how people at different distances, on different geologies, and in different buildings feel an earthquake.


Last night's 3.1 was small enough that people nearest the epicentre possibly mistook it for a truck passing, and people farther away didn't notice it at all. But the 5.1? That caught people's attention.

Dr. Chris McKenney, a postdoc at CalTech in Pasadena was in the submillimeter astronomy lab in Pasadena during the earthquake. He described it to me as,

I was in my lab, and it just felt like I couldn't keep my balance for several seconds. Then I looked up and noticed all the lights that hang from the ceiling were swaying. It was long enough that my co-worker and I had time to have a conversation: "Is this an eathquake" "Yes, I do believe it is." lonnnng pause. "Is it still going?" "Yep, maybe we should get under the desks or something?"


Afterwards, he teased that all lab equipment is now tested as being earthquake safe. Observation Deck regular Ghost in the Machine was also rocked by the quake, and is spending the weekend updating an emergency preparedness kit.


The newborn son of one of my friends proved himself a true Californian by sleeping through the quake entirely, echoing the experience of documentary-maker Louis Theroux:

If you're in the Los Angeles area, please help seismological research by filling out a a Did You Feel It? report. Even report it if you didn't feel it — negative reports are important to keep the USGS from over-estimating felt intensity from scattered positive reports in areas with weak shaking.


What's going on in LA?

This is just the latest earthquake that has hit Los Angeles in recently, so what's going on? Are last night's earthquakes in La Habra connected to the Westwood earthquakes? And does this mean the Big One is coming and Los Angeles is about to fall into the ocean? Here's Dr. Lozos again for the seismological context:

Long story short again, some of the faults haven't been able to hold themselves together, and they've been slipping.

Short story longer, Los Angeles has tons of faults. Make whatever jokes about the city you will, but geologically speaking, the greater Los Angeles area sits atop a complex network of faults, of different types and orientations and behaviors. There are some famous ones - the San Andreas, which does not actually run through Los Angeles proper, or the Hollywood Fault, which has had a lot of press these days, or the Puente Hills, which had a deadly 5.9 in 1987 - but there are literally hundreds of smaller faults crisscrossing the region. With so many of them, it's really not unusual for L.A. to be having earthquakes, and in fact, the past 20 years have been unusually quiet.


Any time we get more than one earthquake in a region in a short period of time, the question always comes up of, "Are these related?" Although it'd be really nice to be able to give a clear-cut answer, real life complexity makes a simple yes or no answer impossible.

Any time we have an earthquake, no matter how big or small, the distribution of stresses around the fault that had the quake changes. Some areas get a decrease in stress, and faults in those areas are less likely to have an earthquake after the first one than they were beforehand. But some areas get an increase, and faults in those spots are brought closer to failure. The larger the initial earthquake, the bigger the stress change, and the larger of an area it affects.


After a 2010 earthquake in Baja, the stress along the slipped fault decreased, while the stress at the locked ends increased. Image credit Ross Stein (USGS)/Shinji Toda (Kyoto University)

Within the context of the recent earthquakes, Dr. Lozos is reluctant to connect them too firmly:

So, it's possible that the quake in Westwood last week put the fault in La Habra closer to failure. But maybe not - the Westwood quake was a 4.4, which isn't very big, and it's not very close to La Habra. It's very possible that the fault in La Habra was just about ready to go anyway, without help from any other faults.


What is certain is that the magnitude 5.1 in La Habra put stress on the neighbouring faults, triggering a series of aftershocks last night and into this morning. Aftershocks happen on faults very close to the one that just failed, corresponding with places that have an increase in stress. The physics is the same as any other earthquake, so the only reason we call them aftershocks is because they happened after a larger earthquake.

Recent earthquakes in the Los Angeles basin, colour-coded by time and scaled by magnitude.


Update 8:30pm PDT: Seismologist Dr. Jascha Polet, Associate Professor of Geophysics, Cal Poly Pomona, has a map of everything that's been shaking the basin today, and for the past several years. She's now updated it with La Habra's earthquake and aftershock sequence:

Those beach-ball symbols are how geophysicists indicate the focal mechanism of an earthquake. A focal mechanism is a description very first motions of the earthquake as indicated by the characteristics of the seismic waves. The stress for the main 5.1 earthquake and the large 4.1 aftershock in Rowland Heights are similar — a mix of compression and shear — but not identical. It's close enough that they might both be motion on the same fault plane shifting with a mix of strike-slip and reverse thrust motion.


Aftershocks decrease exponentially over time. After the magnitude 6 earthquake that shook Christchurch, New Zealand in 2011, the aftershocks continued for a few weeks after the main earthquake. The catastrophic megaquake in Tōhoku, Japan later that year produced substantial aftershocks for months after the main event. For the magnitude 5.1 in LA, it's normal to have over a hundred aftershocks, but the city won't keep shaking for too much longer.

A Forecast for Impeding Doom!

Alright, fine, what about foreshocks? Could these be foreshocks for the Big One? Eh, hard to say. In the words of Dr. Lozos,

Because the physics of a foreshock is the same as the physics of any other earthquake, there's no way to tell something is a foreshock until the bigger earthquake happens.


We only know if an earthquake is a foreshock later after a bigger earthquake happens. So, anyone claiming that these earthquakes are foreshocks of the Big One are either speculating, or time-travellers from a post-apocalyptic future desperately trying to get us to prepare.

You might be getting the impression that foreshocks and aftershocks don't help all that much in trying to predict anything. You're right:

Some earthquakes have tons of aftershocks, some have very few. Some big earthquakes have tons of foreshocks, and some have none at all. It's also possible for there to be a large swarm of small earthquakes without a single one that could be considered even remotely big. There are a lot of different fault behaviors, and science doesn't understand them all, but we do know there's no specific pattern that suggests the Big One is coming.


The Big One

But, we all know California is a seismically active area, and that The Big One, a major earthquake, is coming. Is it time to run in circles, scream and shout? Nah, not unless you have fun doing it.

Every time there's an earthquake in California, people talk about whether or not it's going to set off The Big One, or even a Pretty Big One (like Northridge in 1994 or Loma Prieta in 1989). The Big One is definitely the seismic boogeyman, and because science just plain isn't at a place where we can even begin to predict earthquakes, we cannot say for sure that it's not going to happen soon. But that's also where people's uncertainty - and the media - love to jump in with phrases like, "Larger event possible in the near future!"

A larger event is possible in the near future, but it always is. Any earthquake changes the stresses in the crust around it, and may influence other faults to break.


How much did the La Habra earthquake change the local stress system? It's causing aftershocks, so could it trigger The Big One? Going back to magnitudes, last night was a 5.1. A Pretty Big One would be a magnitude 7.1, 900 times bigger. The only way the exciting-but-wee 5.1 is going to trigger something that big is if a fault was so close to breaking loose that all it needed was a tiny tap. Metaphorically speaking, that's housecat rubbing against the ankles of an elephant, triggering a stampede. Sure, it can happen, but most of the time something that big is going to ignore the tiny disruption.

We're all going diiieeee! ...eventually

The 5% chance of a larger earthquake sometime in the 24 hours following the La Habra is true enough, but that's actually a fairly standard probability for any earthquake. The only reason it's circling now is because an earthquake just happened, so people are primed to care about it.


One day, the Big One will happen. It might even be this week, although the odds of that happening are really no greater than they ever are. When it happens, yes, it will be a tragedy and people will die, but the entire city is not going to be destroyed.

In 1906, we had no idea how to predict where earthquakes would happen, or how to prepare for them. We've come a long way since then, and California takes its risk of earthquakes seriously. We have good building codes and emergency plans, and are even testing a prototype earthquake early-warning system. Of course, they could be better, so feel free to pressure politicians into prioritizing disaster research and preparedness!

Like a near-miss solar storm or asteroid flyby, seismologists will use this window of awareness as an opportunity to update their emergency plans, stockpile some water, and maybe even improve the seismic safety of their homes. Dr. Lozos is no exception, reminding us:


I'd suggest using these increased odds as an excuse to check your earthquake kit, because it's always good to make sure the water and food in are still fresh, but it's definitely not something to lose sleep over.


Disasters happen everywhere, so that's good advice for everyone outside of California, too. Along with food and water, consider packing a flashlight, warm clothes, spare medication, toilet paper, a bit of cash, and anything else you would be seriously inconvenienced to live without for 72 hours of post-disaster chaos. Designate a meet-up location and an out-of-area emergency contact to find your family if the phone-lines get swamped, and consider volunteering for your local emergency response organization.

Every year, California runs a state-wide Shakeout earthquake drill. This year, Los Angeles is getting a real-life reminder that they live where the ground rolls, and it pays to prepare.


This article involved a scary amount of people with higher degrees in seismology: Thank you to Dr. Lozos for obliging with a late-night interview, and to Dr. Jascha Polet, Dr. Eric Fielding and doctoral candidate @jeffersonite for corrections in support of greater scientific accuracy. All figures courtesy of theUSGS. For a flashback to historical doom in Pasadena, check out this history of rockets at JPL.