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When it comes to earthquakes, size matters and so does terrain
By Alison Chenshar | CNN Meteorology
Earthquakes can be jelly-like. A simple, but often used analogy, is that if you are sitting in a valley or basin, it acts as a bowl of gelatin and will vibrate more than the rocks around it.
But not all earthquakes are created equal and the ground you walk on can make a big difference.
“Your local geology definitely matters – what are you sitting on,” said Dr. Susan Hogg, a geophysicist with the US Geological Survey. “What is the topography, it is definitely important.”
Earthquakes fall into two basic types of waves: body waves (often called P waves or S waves that travel across the Earth) and surface waves (which travel along the Earth’s surface).
The Earth’s surface is made up of a variety of soil types – from sand to clay to rocks and many other types, so damage from these basic wave types can vary as the earthquake travels through these different types of terrain.
Hugh further explains that while the waves themselves travel the same way, meaning that the P wave is still a P wave, and the S wave is still an S wave, however, their velocities and amplitudes will change depending on the type of rock.
Whether it’s sedimentary rock or small sandy soil, it makes a difference.
Since the particle motion of surface waves is greater than the motion of body waves, surface waves tend to do more damage.
Earthquakes occur on every continent in the world – from the highest peaks of the Himalayas to the lowest valleys like the Dead Sea to biting cold regions of Antarctica. However, the distribution of these earthquakes is not random.
Size matters, and so does the type of terrain
When it comes to earthquakes, size matters very much. The physical size of an earthquake is measured in terms of magnitude. For example, 5.5 is a moderate earthquake, and 6.5 is a strong earthquake. Because the scale is logarithmic, each increase in volume represents a ten-fold increase. Therefore, a magnitude 6.5 earthquake is ten times greater than 5.5 magnitude, and not one time larger as the number suggests.
But just because the magnitude of an earthquake is greater does not always mean that the damage caused is worse.
For example, in January 2010, Haiti was hit by a 7.0 magnitude earthquake. More than 200,000 people lost their lives during this event with losses estimated between $ 7.8 billion and $ 8.5 billion.
In 2019, a magnitude 7.1 earthquake occurred near Ridgecrest, California. For this strongest earthquake, only one person lost his life, with losses estimated at $ 5 billion.
Besides the similar size, the depths were also similar. The Haiti earthquake was 8 miles (13 km) deep, and the California earthquake was 5 miles (8 km) deep. While 8 miles may not seem shallow, it is in terms of earthquakes. Geologically speaking, any earthquake less than 43 miles (70 km) deep is considered shallow. The smaller the earthquake, the greater the possibility of damage because it is closer to the surface.
Why was there such a disparity between deaths and damage from two earthquakes of equal magnitude 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 shakes slowly, the frequency is low. If the ground is shaken quickly, the frequency is high.
Low frequencies mainly affect multi-storey buildings in particular. In fact, the lower the frequency, the more buildings that will be affected. Whereas, high frequencies tend to affect small buildings.
Hesitation was only 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,” Hugh told CNN. “The greater the earthquake, the greater the level of low loud tones. But large earthquakes also release a lot of high-frequency energy. The high-frequency energy is rapidly suppressed as it travels through the ground, so the Haiti earthquake had damaged Port-au-Prince in part because the fault was so close.”
The subsoil is often just as important as size and frequency.
In Haiti and other island countries, you have rocks rising from the surface, on which homes are built, into softer areas that can actually amplify seismic waves.
These factors could locally intensify seismic waves, and thus lead to additional damage.
“In the 1906 California earthquake, some people who lived within 100 miles fell asleep during the earthquake,” said Hugh. “While the New Madrid earthquakes (which occurred in 1811 and 1812 in present-day Missouri), they actually rang church bells in Charleston, South Carolina. It has to do with how waves travel through the crust. There is 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 around it is dispersed and weakened by the different terrain, which means that it does not make it too far into the earth’s crust.
In contrast, the East Coast has an older crust. When an earthquake occurs, its echo resonates like waves produced by a ripple in water. Waves can travel for hundreds of miles, and are usually much farther east than in California.
“There are three important factors with earthquakes,” Hugh said. “There is energy coming out at the source, and there is amplified by local geology when it reaches a site, and then there is something that happens between them.” “In between that really matters the East Coast versus the West Coast.”
Haiti also has a topographical aspect to it. Port-au-Prince is mostly located at sea level, with sandy deposits in those low-lying areas. But at only 10-15 miles, the elevation increases several thousand feet in more mountainous terrain with more solid boulders at the surface.
The vibration is amplified by the low sandy sediments in Port-au-Prince, but also over some hills and hills in Haiti due to the topographical effect.
But we must also build structures according to the soil and / or rocks we are building on.
Building on solid ground provides more stability to buildings because the rocks mainly absorb waves. Hugh referred to the magnitude 7.8 earthquake that struck Nepal in 2015 and flattened multi-storey buildings in the capital, Kathmandu.
“In Kathmandu in 2015, there was booming amplification because it is a lakebed area, but the valley was rolling back and forth over a period of five seconds, and you can see that on a closed TV. You had things veering in one side … a thousand, two thousand, Then three thousand and four thousand. It’s a fairly slow movement, and it was strong because of the bottom of the lake. But the effect on the buildings depends on the size of the buildings. “
Hugh uses an analogy for a massive swell in the ocean, stating that the waves would be damaging if the boat crawled violently. For a large ship in a large bulge, its bow rises as the stern goes down, which creates pressure inside the boat. If the ship is smaller than the bulge, the entire ship goes up and down – essentially it goes along the voyage.
When the Earth becomes liquid
Another important contributor to earthquake damage comes from earthquake-induced landslides and their liquefaction, known collectively as Earth Failure.
The U.S. Geological Survey has a ground failure product that provides near-real-time regional estimates of landslide and liquefaction hazards induced by earthquakes.
“It takes time for responders and first-time experts to survey actual damage in the area, so our product provides early estimates of where attention is focused and response planning,” according to the USGS.
Although the models provide initial awareness, general extent, and indicate areas in which they are most likely to occur, they do not predict very specific events.
Using satellite imagery, the US Geological Survey was able to map more than 23,000 landslides caused by strong shaking across the island of Hispaniola from the 2010 Haiti earthquake.
But landslides are only one component of a ground failure.
Liquefaction is a process in which a water saturated sediment is shaken with sufficient force to begin acting more like a liquid than a solid.
“There is something called nonlinearity, and it turns out that if you try to shake the soft sediment vigorously, this isn’t a bowl of Jell-O so much as a sandbox,” says Hugh.
For example, Hough explains that if you shake a sandbox vigorously, it will stop acting like rock. Things will change on the level of grain size and this process absorbs energy.
A tweet appeared during a magnitude 6.0 earthquake that struck India in 2021 showing how the liquefaction occurred.
“If sand is saturated with water, as I imagine it in many places in India, it could start to act like a liquid. Leaching has several consequences for vibration: Some potentially harmful vibrations are absorbed, which could be a good thing, but if the ground under the structure starts to act Like a liquid, the structure will not have a solid foundation. It’s almost as if it is sitting on shifting sand. Even a well-built building can tip over, ”Hugh told CNN.
Any aftershocks will add to the damage because the buildings may have already suffered structural damage from the initial earthquake. Building on a slope, or particularly on soft ground, can sink the foundations and allow the waves to double the devastating impact of an earthquake.
It is also important to note that what works in one disaster does not work in another.
It is often reported that buildings in Haiti are not built to the same standards as buildings 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 experience a major hurricane in any given year than it is from a major earthquake.
Hough explains that they have a building style where they put roofs that are very heavy for hurricanes, so the roof does not blow out. But when an earthquake does happen, the extremely heavy concrete roof is dislodged and threatens the underlying structure, which likely already has some element of building weakness to begin with.
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