We don’t really have a way to compare earthquakes in one part of the world with another.

I mean, of course, we have a  way of measuring these events and giving them a number  that translates to strength. But our methods are kind of….shaky.

Earthquakes like the one that hit New  Jersey in early 2024 can feel like a bigger deal than ones that are  technically the same strength in California. And it’s not just because of their novelty. It turns out that earthquakes in  the eastern US really do go farther and hit harder than their western counterparts, and it’s all to do with the rocks  they encounter along the way.

So here is how earthquake waves are  fundamentally different in different places, and what makes our measurements way  less meaningful than you might think. [♪ INTRO] Let’s start out with the fact that we are  not all working with the same scale. Earthquakes happen when rocks break  and slip along geological faults, releasing energy that travels  through the ground as seismic waves. These waves are recorded by  appropriately named seismographs, which show how much the Earth  moves at a particular location.

Then, that measurement can be used to  calculate the strength of the quake. One of the most famous measures of  earthquake strength is the Richter Scale, which was developed back in 1935 by the  American seismologist Charles Richter. It is an objective measure of  the earthquake’s magnitude based on the height of the  tallest peak on a seismogram.

It's also logarithmic, meaning that a  quake measuring 6 on the Richter scale is 10 times stronger than one measuring 5, and a hundred times stronger than magnitude 4. But the Richter scale isn’t as objective  as you might imagine, because Dr. Richter devised his scale using only data from a single type of seismograph in California.

Most of the earthquakes in California  happen at a similar depth and behave in a similar way, so the calculations  worked locally but couldn’t fully capture the behavior of earthquakes  elsewhere around the world, which is where most earthquakes happen. So if you read about a  magnitude 5 earthquake today, they probably don’t mean  magnitude 5 on the Richter scale. Instead, it is an earthquake  with a moment magnitude of 5.

Helpfully though, it is actually  pretty similar to the Richter scale. The moment magnitude is a more  broadly applicable measure, calculated from several  different seismograph readings. And it is related to the physical properties  of the fault that caused the earthquake, including the rigidity of the rock, the size of the fault that slipped,  and the amount it slipped by.

The numbers are then converted into a scale  that closely mirrors the Richter scale, to be consistent with that famous  but flawed magnitude measurement. It's just a little trick  that we played on everybody. Everybody thought that we're still using  the Richter scale, but we're not anymore.

And yet, despite the improvements made  over the years, even the most objective measures cannot paint a complete  picture of earthquakes in every region. Quakes of similar magnitude  can have very different effects depending on where in the world they occur. For a start, the rocks that seismic  waves travel through have a big effect on how far they’ll go and how far  from the epicenter you’ll feel them.

It’s a bit like sound waves  traveling through solid materials. Compare the ringing sound that goes  for ages when you strike solid metal, to the bonk that you hear  when you hit a piece of wood. And if you whack a rotten piece of wood, the thunk tells you that those sound  waves aren’t getting very far at all!

And this is a similar situation between  the east and west coasts of the US. On the East coast, the geology  is old, solid, and continuous, and the seismic waves travel  great distances with ease. As a result, shaking can be  felt from much farther away.

In comparison, the west coast sits  near a tectonic plate boundary and the bedrock is all convoluted  with lots of cross-cutting faults. So the seismic waves are slowed and  attenuated, and shaking isn’t felt as far. So in 2011, shaking from a magnitude  5.8 quake near Mineral, Virginia was felt over 950 kilometers away.

While in 2014, a magnitude 6  earthquake in Napa, California was only felt 402 kilometers  away, despite releasing about twice the energy of the Virginia quake. So even when different earthquakes  are measured on the same scale, an almost magnitude 6 earthquake in  Virginia is going to spread out to a totally different maximum distance than  a magnitude 6 earthquake in California. And it’s not just the US that experiences  this quirk of seismic transmission.

The Gujarat earthquake in India in  2001 had a moment magnitude of 7.7, and resulted in a huge  death toll of 20,000 people, with over 150,000 people injured. It was what’s known as an intraplate earthquake, originating from a previously unknown  fault within rocks that are compressed. As a result, shaking was felt over 1000  kilometers away in the capital, New Delhi.

In comparison, the Sumatra Andaman  Earthquake, which caused the devastating Boxing Day tsunami in 2004, had an  estimated magnitude of at least 9.1. But while the tsunami caused  widespread devastation,   the shaking from the quake was felt mostly  in line with the fault that slipped. Despite being much stronger  than the Gujarat quake, shaking from this event was barely  felt in Malaysia, 600 kilometers away.

But local geology controls more than just  the distance that seismic waves travel. It also affects the intensity of  shaking wherever they happen to hit. So some earthquakes feel bigger than  their magnitude rating might suggest.

In contrast to the objective  magnitude at its epicenter, intensity is a subjective measure  of an earthquake’s strength. In the US it’s classified on the  12-point modified Mercalli scale ranging from 1, not felt at  all, to 12, extreme shaking. In general, being closer to the epicenter means you’ll feel more intense  shaking for any given earthquake size.

But it’s not always a  straightforward relationship, because areas that are underlain by  softer sediments often experience much more intense shaking than those  with a solid bedrock underground. That’s because seismic waves travel  slower through soft sediments, but at the same time they increase in amplitude. The waves can also kind of bounce back  and forth between sedimentary layers, so the shaking goes on for longer.

You can think of it almost like  a bowl of jello on a table. If you hit the table leg, the  tabletop will move a little, but the jello will jiggle for  much longer and much more. This is another reason that earthquakes  on the North American east coast often seem more intense.

There are many areas of Washington  DC that are underlain by sediment, and so experience this jello  effect, resulting in more damage than you would expect from a  relatively small earthquake. Local sedimentary effects are not  unique to the US east coast, though. On the west coast, the whole of the Los  Angeles basin has around 10 kilometers of sediment underneath it, and can  experience shaking five times more intensely than in sites in the surrounding mountains.

And a similar process leads to higher  intensities of shaking all around the world. Notably, the 1985 earthquake near Mexico  City shook the capital to its core, because the city is built on an ancient lakebed. And earthquakes in 2010 and 2011  in Christchurch, New Zealand were especially devastating, because the  city is built on the Canterbury plain, underlain by gravel washed down  from the nearby Southern Alps.

In fact, our tendency to build towns on the flat, soft grounds of plains and basins,  rather than in the mountains, hasn’t done us any favors when it  comes to amplified earthquakes. Geological amplification causes a  kind of intense high-frequency shaking that’s particularly damaging to buildings,  right where we like to build them! So, whenever an earthquake strikes,  spare a thought for the ground underfoot.

Magnitude can give us an  idea of the energy released, but it’s the geological  context that really matters. And while there may be way more  quakes in the US’s mountainous west, the ones in the ancient, sedimentary  east are deceptively intense. [♪ OUTRO]