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Earthquake Magnitude
Earthquake Magnitude
When an earthquake happens, scientists want to know how big it is. This “bigness” is called magnitude, and it measures the total amount of energy released during the earthquake. Magnitude is a single number that represents the earthquake’s overall power.
Scientists measure magnitude using data from seismographs, instruments that record the vibrations of the ground. In the past, the Richter scale was commonly used. It gave each earthquake a number based on the height of the largest seismic wave recorded. The Richter scale isn’t used much anymore because it has some important limitations:
It only works well for small to medium earthquakes (generally below magnitude 7). For larger quakes, it doesn’t give accurate results.
It was designed for Southern California. The scale was based on local geology and seismographs in that region, so it doesn’t always work correctly for quakes in other parts of the world.
It only measures the largest seismic wave. This ignores other important factors, like the size of the fault that slipped or how far the rocks moved.
Today, scientists more often use the Moment Magnitude Scale (Mw), which is more accurate for very large earthquakes. The Moment Magnitude Scale (Mw) calculates earthquake magnitude using the actual physics of the fault slip, not just the size of seismic waves. It is based on seismic moment, which depends on three main factors:
The area of the fault that slipped – how large the section of rock that broke and moved was.
The average amount of slip on the fault – how far the rocks moved past each other.
The strength of the rocks (rigidity) – stiffer rocks release more energy when they break.
In formula form, seismic moment (M₀) = fault area × slip × rock strength.
The Moment Magnitude Scale then converts this seismic moment into a single number that represents the earthquake’s size.
This makes it more accurate than the old Richter scale, especially for very large earthquakes anywhere in the world.
Both the Richter Scale and the Moment Magnitude Scale have numerical values that range from 1-10. An earthquake with a magnitude of 1 would release the least amount of energy, while an earthquake with a magnitude of 10 would release the most amount of energy. The magnitude scales are logarithmic, meaning that each whole number increase represents about 32 times more energy released. For example, a magnitude 6 earthquake releases roughly 32 times more energy than a magnitude 5, and about 1,000 times more than a magnitude 4.
Earthquake Intensity
Earthquake Intensity
When an earthquake strikes, people often want to know not just how big it was overall, but also how strongly it was felt in different places. This is what scientists call intensity. Unlike magnitude, which measures the total energy released by an earthquake, intensity describes the effects of the shaking at specific locations.
Intensity can vary widely, even for the same earthquake. Three main factors affect earthquake intensity (how strong the shaking feels in a specific location):
Distance from the Epicenter – The closer you are to where the earthquake starts, the stronger the shaking will feel. Intensity decreases with distance.
Depth of the Earthquake – Shallow earthquakes (closer to the surface) usually cause stronger shaking at the surface than deeper ones, even if they have the same magnitude.
Local Ground Conditions – The type of ground beneath you makes a big difference. Soft soils can amplify shaking, while solid bedrock can reduce it. This is why some neighborhoods in the same city may experience very different levels of damage.
Scientists measure intensity using the Modified Mercalli Intensity (MMI) scale. This scale ranges from I (not felt) to XII (total destruction). It is based on observations, such as whether people felt the quake, whether objects moved, or whether buildings were damaged or collapsed. Today, scientists also collect reports through apps, online surveys, and instruments that measure ground shaking.
Modified Mercalli Intensity Scale
In short, intensity tells us what people experienced and what damage occurred, while magnitude tells us about the earthquake’s energy - how powerful the earthquake was overall. Both are important for understanding the impact of earthquakes and preparing for future ones.
Hazards of Earthquakes
Hazards of Earthquakes
Earthquakes create many different dangers, both from the shaking itself and from the events that the shaking sets off. The most dangerous hazards are listed here:
Ground Shaking
Ground Shaking
This is the most immediate hazard of an earthquake. When rocks along a fault suddenly move, they send out seismic waves that make the ground shake. The intensity of shaking depends on the earthquake’s magnitude, depth, and distance from the epicenter. Ground shaking can collapse buildings, bridges, and roads, and it can injure or kill people who are inside structures that are not built to withstand earthquakes.
Surface Rupture
Surface Rupture
Sometimes the earthquake causes the fault to break the ground surface. This can tear apart roads, railways, pipelines, and buildings that lie directly on the fault. Surface rupture is usually limited to a narrow zone along the fault but can cause severe local damage.
Landslides
Landslides
Earthquakes can make slopes unstable, especially in hilly or mountainous areas. Loose rocks and soil can slide downhill, destroying houses, roads, and farmland in their path. Landslides can bury communities and block rivers, creating additional flooding risks.
Tsunamis
Tsunamis
Underwater earthquakes can displace large volumes of water, creating giant waves called tsunamis. These waves travel quickly across the ocean and can flood coastal areas, destroying buildings, roads, and crops. Tsunamis can reach far from the earthquake’s epicenter, making them especially dangerous for coastal regions.
Liquefaction
Liquefaction
In areas with soft, water-saturated soil, strong shaking can cause the ground to act like a liquid. This is called liquefaction. Buildings can sink, tilt, or even topple over because the soil beneath them temporarily loses its strength. Roads and bridges can also crack or collapse.
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Here’s an interesting connection between liquefaction and zombie legends:
In some earthquake-prone regions with loose, waterlogged soil, strong quakes can cause liquefaction, making the ground behave like a liquid. This can sometimes heave coffins or disturb shallow graves, bringing objects—or rarely even human remains—closer to the surface.
In places like Haiti, where zombie folklore is strong, stories of the dead “returning” may have been influenced by such natural phenomena. People might see disturbed graves or partially exposed coffins after an earthquake, and interpret it through cultural and spiritual beliefs. Combine this with the local traditions of Voodoo and bokors (who were believed to raise zombies), and you get a rich mix of real events and supernatural interpretation.
So while zombies are primarily a product of folklore, unusual natural events like liquefaction could have reinforced or inspired certain stories, making it seem as if the dead were coming back to life. It’s a fascinating example of how natural disasters can influence myths and legends.
Fires
Fires
Earthquakes often damage gas lines, power lines, or electrical equipment, which can start fires. In crowded cities, fires can spread quickly and cause more damage than the shaking itself. Combined with broken water lines or limited firefighting access, fires are a serious post-earthquake hazard.
San Franciso 1906
San Franciso 1906
The most famous earthquake-related fire in U.S. history happened during the 1906 San Francisco earthquake. Here’s what happened:
On April 18, 1906, a massive earthquake with a magnitude of around 7.9 struck San Francisco, California. The shaking lasted about 45–60 seconds and caused widespread ground rupture and building damage.
While the earthquake itself destroyed many structures, it was the fires that followed that caused the greatest devastation. Broken gas lines, overturned stoves, and damaged electrical lines ignited flames throughout the city.
The fires raged for three days, consuming about 500 city blocks and leaving over 250,000 people homeless. Water mains had been broken by the earthquake, making it extremely difficult for firefighters to put out the flames.
In total, the earthquake and resulting fires killed an estimated 3,000 people and destroyed much of San Francisco, making it one of the deadliest and most destructive natural disasters in U.S. history.
In short, earthquakes are dangerous not just because of the shaking but also because of the chain reaction of hazards they can set off. That’s why scientists and emergency planners study both primary and secondary hazards to reduce risks in earthquake-prone areas.
Earthquake Occurrence
Earthquake Occurrence
Earthquakes are not spread evenly around the Earth—they tend to happen in specific areas where the Earth’s tectonic plates interact. Three major zones stand out: the Circum-Pacific Belt, the Alpine-Himalayan Belt, and the Mid-Ocean Ridges.
Circum-Pacific Belt
Also called the “Ring of Fire,” this zone circles the Pacific Ocean. It is full of subduction zones, where one tectonic plate slides under another. The pressure and friction along these boundaries cause frequent strong earthquakes and volcanic activity. Because this region circles the Pacific Ocean, many tsunamis occur in this region as well. About 81% of our planet’s largest earthquakes occur here. Countries like Japan, Chile, and the west coast of the U.S. are part of this belt.Alpine-Himalayan Belt
This belt stretches from the Mediterranean region through the Middle East to the Himalayas. It forms where continental plates collide, such as the African Plate pushing into the Eurasian Plate. These collisions create mountain ranges and powerful earthquakes, like those in Turkey, Iran, and Nepal. About 17% of the world’s largest earthquakes occur here.Mid-Ocean Ridges
Mid-ocean ridges, like the Mid-Atlantic Ridge, are underwater mountain chains where tectonic plates are moving apart. As the plates separate, magma rises to form new crust. This movement creates smaller but frequent earthquakes along the ridges.
The New Madrid Fault
The New Madrid Fault
Most people think of earthquakes as something that only happens in places like California, but the central United States has its own powerful earthquake zone called the New Madrid Fault (or New Madrid Seismic Zone). This fault system runs through parts of southeastern Missouri, northeastern Arkansas, western Tennessee, and western Kentucky—hundreds of miles south of Illinois, but still close enough to matter.
The New Madrid Fault formed millions of years ago when the North American continent began to pull apart but never fully split. Today, that ancient rift remains a weak spot in the Earth’s crust, where stress sometimes builds up and releases as earthquakes.
Between December 1811 and February 1812, a series of massive earthquakes struck the region near New Madrid, Missouri. These were some of the strongest quakes ever recorded in the continental U.S.—estimated around magnitude 7.5 to 8.0. The shaking was so powerful that it changed the course of the Mississippi River, created temporary waterfalls, and caused land to sink and rise in large areas.
Even though Lockport, Illinois, is about 300 miles away, people there would have felt those earthquakes. Historical accounts say the shaking was noticeable as far away as Chicago, Detroit, and even the East Coast.
Today, smaller earthquakes still happen along the New Madrid Fault every year, though most are too weak to feel. Scientists continue to monitor the area closely because another large quake could one day affect much of the central United States—including Illinois.
The Sandwich Fault
The Sandwich Fault
The Sandwich Fault Zone is a strike-slip fault that runs northwest from Oswego Illinois to Ogle County, passing through Lee County. While it has been largely inactive and under-researched, there was a 3.8 magnitude earthquake caused by the Sandwich fault in 2010 that could be felt by residents of nearby Lockport. This earthquake struck at 3:59 AM CST, approximately 5 miles east of Sycamore, Illinois, in DeKalb County. It was felt by an estimated 18,000 people across several states, including Illinois, Wisconsin, and Iowa. The tremor caused minor shaking but no significant damage. This earthquake woke a sleeping Mr. Wason that was living in Lockport at the time!
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