When it comes to earthquakes, size matters but so does the terrain

By Allison Chinchar, CNN Meteorologist

Earthquakes can be like Jell-O. A simple, yet often used analogy is that if you’re sitting in a valley or basin, it acts like a bowl of gelatin and it will shake more than surrounding rock.

But not all earthquakes are created equal and the ground you walk on can make all the difference.

“The local geology definitely matters — what you’re sitting on,” said Dr. Susan Hough, a geophysicist with the US Geological Survey. “What the topography is, it definitely matters.”

Earthquakes are broken down into two basic wave types: body waves (often called P-waves or S-waves which travel through the Earth) and surface waves (which travel along the Earth’s surface).

The surface of the Earth is made up of a variety of soil types – from sand to clay to rock and many others, so the damage resulting from those basic wave types can vary as an earthquake travels through these varying types of terrain.

Hough explains further that while the waves themselves travel the same way, in the sense that a P wave is still a P wave, and a S wave is still a S wave, however, their speeds and amplitudes will change depending on the rock type.

Whether it is sedimental rock or a young sandy soil, it makes a difference.

Because the particle motion of surface waves is larger than that of body waves, surface waves tend to cause more damage.

Earthquakes occur on every continent in the world — from the highest peaks in the Himalayan Mountains to the lowest valleys like the Dead Sea to the bitter cold regions in Antarctica. However, the distribution of these quakes is not random.

Size matters, and so does the type of terrain

When it comes to earthquakes, the size is very important. The physical size of an earthquake is measured in magnitude. For example, a 5.5 is a moderate earthquake, and a 6.5 is a strong earthquake. Because the scale is logarithmically based, each whole number increase in magnitude represents a tenfold increase. So, a 6.5 magnitude quake is 10 times bigger than a 5.5 magnitude, not one times bigger like the number implies.

But just because the magnitude of an earthquake is bigger does not always mean the resulting damage is worse.

For example, in January 2010, a magnitude 7.0 quake struck Haiti. More than 200,000 people lost their lives during that event with estimated damages between $7.8 and $8.5 billion.

In 2019 a 7.1 magnitude quake struck near Ridgecrest, California. For this stronger quake only one person lost their life, with an estimated $5 billion in damages.

Besides the magnitude being similar, the depths were also similar. The Haiti quake was 8 miles (13 km) deep, and the California quake was 5 miles (8 km) deep. While 8 miles may not sound shallow, it is in terms of earthquakes. Geologically speaking, any earthquake that is less than 43 miles (70 km) deep is considered shallow. The shallower an earthquake is, the more likely damage will occur since it is closer to the surface.

So why was there such a disparity between the fatalities and damages from two quakes with such similar magnitudes and depths? The answer has a lot to do with plate tectonics and how buildings are constructed.

Earthquakes emit low and high frequencies. If the ground vibrates slowly, it is low frequency. If the ground vibrates quickly, it’s more of a high frequency.

Low frequencies mainly affect multistory buildings in particular. In fact, the lower the frequency, the bigger the buildings that will be affected. Whereas high frequencies tend to affect small buildings.

Frequency was just one factor in why the Haiti earthquake was so devastating.

“The earthquake itself, like most large earthquakes, released energy with a wide range of frequencies,” Hough tells CNN. “The bigger the earthquake, the greater the level of booming low tones. But big earthquakes also release a lot of high-frequency energy. The high-frequency energy gets damped out quickly as it travels through the earth, so the Haiti earthquake was damaging to Port-au-Prince in part because the fault rupture was so close.”

Subsoil is often just as important as magnitude and frequency.

In Haiti and other island nations, you have rocks that rise from the surface, on which houses are built, to much softer zones which can actually amplify the seismic waves.

These factors can locally intensify the seismic waves, therefore leading to additional damage.

“In the 1906 California earthquake, some people living 100 miles away slept through the quake,” Hough said. “Whereas the New Madrid earthquakes (which happened in 1811 and 1812 in present-day Missouri), it actually rang church bells in Charleston, South Carolina. That has to do with how the waves travel through the crust. There’s a difference.”

California’s terrain varies widely. There are active faults, mountain ranges, valleys, basins and beaches. When an earthquake occurs in California, the energy is scattered around and gets attenuated by the varying terrain, which means it just doesn’t make it very far out into the crust.

In contrast, the East Coast has an older crust. When an earthquake happens, it reverberates like the waves produced by a ripple in water. The waves can travel for hundreds of miles, usually much farther in the East than in California.

“There’s three important factors with earthquakes, there’s energy that leaves the source, there’s amplification by the local geology when it gets to a site, and then there’s what happens in between,” Hough said. “It’s the in between that really matters for East Coast versus West Coast.”

Haiti also has a topographical aspect to it. Port-au-Prince sits mostly at sea level, with sandy sediments in those low-lying areas. But just 10-15 miles away, the elevation increases several thousand feet into a more mountainous terrain with harder rock at the surface.

Shaking is amplified by low-lying sandy sediments in Port-au-Prince, but also on some of Haiti’s hills and ridges due to a topographic effect.

But we must also build structures according to the soil and/or rock that we are building on.

Constructing on harder ground provides more stability for the buildings because essentially the rock absorbs the waves. Hough cited the 2015 magnitude 7.8 earthquake that struck Nepal and leveled multistory buildings in the capital of Kathmandu.

“In Kathmandu in 2015, there was a booming amplification because it’s a lake bed zone, but the valley was sloshing back and forth with a five-second period, and you can see that on closed captioned TV. You had things that went to one side … one one thousand, two one thousand, and then back three one thousand, four one thousand. It’s a fairly slow motion, and it was strong due to the lake bed. But the effect on buildings depends on the size of the buildings.”

Hough uses an analogy of a big swell in the ocean explaining that waves will be damaging if they jostle the boat violently. For a large ship on a big swell its bow would go up while the stern goes down, generating stress within the boat. If the ship is smaller than the swell, the entire ship just goes up and down — essentially going along for the ride.

When the ground becomes a liquid

Another significant contributor to earthquake damage comes from earthquake-triggered landslides and liquefaction, collectively known as ground failure.

The USGS has a ground failure product that provides near-real time regional estimates of landslide and liquefaction hazards triggered by earthquakes.

“It takes time for first responders and experts to survey the actual damage in the area, so our product provides early estimates of where to focus attention and response planning,” according to the USGS.

Though the models provide initial awareness, overall extent, and indicate areas in which they are most likely to have occurred, they do not predict very specific occurrences.

Using satellite imagery, the USGS was able to map more than 23,000 landslides that were triggered by the strong shaking across the island of Hispaniola from the 2010 Haiti quake.

But landslides are just one component of ground failure.

Liquefaction is a process where water-saturated sediments are shaken hard enough to start behaving more like a liquid rather than a solid.

“There is something called non-linearity, and it turns out that if you try to shake soft sediments really hard, it’s not a bowl of Jell-O as much as it is a sandbox,” Hough says.

For example, Hough explains that if you shake a sandbox really hard, it’s going to stop acting like rock. Things are going to shift around at grain-size level and that process absorbs energy.

A tweet surfaced during a 6.0 magnitude quake that struck India in 2021 showing how liquefaction occurred.

“If the sand is water-saturated, as I imagine it is in many places in India, it can start to behave like a liquid. Liquefaction has a couple of consequences for shaking: some of the potentially damaging shaking gets absorbed, which can be a good thing, but if the ground beneath a structure starts behaving like a liquid, the structure no longer has a solid foundation. It’s like it’s sitting on quicksand. Even a well-built building can just tip over,” Hough told CNN.

Any aftershocks will further the damage since buildings could be already structurally compromised from the initial quake. Building on a slope, or especially soft ground, can lead to the sinking of the foundations and allow the waves to multiply the devastating impact of the earthquake.

It’s also important to note that what works in one disaster does not work in another.

It is often mentioned that buildings in Haiti are not built to the same standards that buildings are in California, New Zealand or Chile where earthquakes are also common. While this is true, it only tells part of the story.

Haiti is more likely to be hit by a major hurricane in any given year than they are by a major earthquake.

Hough explains that they have a building style where they put very heavy roofs on for hurricanes, so the roof doesn’t blow off. But when an earthquake happens, the very heavy concrete roof gets displaced and compromises the underlying structure, which likely already had some element of building vulnerability to begin with.

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CNN’s Judson Jones and Taylor Ward contributed to this report.