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EARTHQUAKES

What causes earthquakes?

The tectonic plates of the Earth are always in motion. Plates that lie side by side may not move very easily with respect to one another; they "stick" together and occasionally they slip. These slips (from a few inches to many feet) create earthquakes and can often be very destructive to human lives and structures.

What is an epicenter?

An epicenter is the point on the Earth's surface that is directly above the hypocenter, or point where earthquakes actually occur. Earthquakes do not usually occur at the surface of the Earth but at some depth below the surface.

What is a fault?

A fault is a fracture or a collection of fractures in the Earth's surface where movement has occurred. Most faults are inactive, but some, like California's San Andreas fault, are quite active. Geologists have not discovered all of the Earth's faults, and sometimes earthquakes occur that take the world by surprise, like the one in Northridge, California, in 1994. When earthquakes occur on faults that were previously unknown, they are called blind faults.

What does an earthquake feel like?

Smaller earthquakes or tremors feel disorienting at first. You feel a sense that the room is spinning, as if you are becoming dizzy. Usually preceding an earthquake, when the initial tremors hit, you can hear the sounds of things rattling that you have never heard before, like glasses rubbing against each other and windows vibrating. With larger earthquakes, as the earth nearby tears or opens, you can hear a very loud rumbling sound that is similar to a train driving by.

What is the significance of the San Andreas fault?

The infamous San Andreas fault is the border between the North American and the Pacific tectonic plates. This fault lies in California and is responsible for some of the major earthquakes that occur there. Los Angeles is on the Pacific Plate but San Francisco is on the North American Plate. The Pacific Plate is sliding northward with respect to the North American Plate and, as a result, Los Angeles gets about half an inch closer to San Francisco every year. In a few million years, the two cities will be neighbors.

Will California eventually fall into the ocean?

No, it will not. The famous San Andreas Fault, which runs along the western edge of California from the San Francisco Bay Area to Southern California, is known as a transverse fault. This means that the western side of the fault, which includes places like Monterey, Santa Barbara, and Los Angeles, is sliding northward with respect to the rest of the state. In a few million years, the state's two largest urban areas, San Francisco and Los Angeles, will be right next to each other. The fault is moving at about two centimeters (just under an inch) a year.

Was San Francisco destroyed by earthquake or by fire in 1906?

In 1906 a very powerful earthquake struck San Francisco, California, which sparked a fire that destroyed much of the city. In an effort to preserve San Francisco's image with residents and would-be visitors, official policy regarding the disaster stated that it was not the earthquake but mostly the fire that destroyed the city. Official books and publications produced after the earthquake referred to both the fire and the earthquake as having caused the damage. In fact, the earthquake did considerable damage to the city and killed hundreds.

Which states are earthquake-free?

While a 20-year period isn't an excellent indicator, there were four states that had no earthquakes between 1975 and 1995: Florida, Iowa, North Dakota, and Wisconsin.

Is there a high risk of earthquakes in the Midwestern United States?

Great earthquakes struck the New Madrid, Missouri, area in 1811 and 1812. They caused considerable damage (some areas experienced shaking at the level of XI on the Mercalli scale) and were felt as far away as the East Coast. The potential exists for future earthquakes in the region, since earthquakes have occurred there before. Planning and preparedness continues throughout the region, centered at the junction of Missouri, Arkansas, Illinois, Kentucky, Tennessee, and Mississippi.

What should I do in the event of an earthquake?

Duck, cover, and hold! Duck under a table, counter, or any area that can provide protection from falling objects. Cover the back of your head with your hands to help protect against flying debris. Hold on to the leg of the table or anything solid to ride out the shaking.

Is it safe to stand in a doorway during an earthquake?

While a doorway is a nice, structurally sound place to be during an earthquake, officials have found that many people are injured when a door swings open and closed during an earthquake, so you may want to avoid standing in a place where your fingers can become crushed.

What is the Richter scale?

The Richter scale measures the energy released by an earthquake. It was developed in 1935 by California seismologist Charles F. Richter. With each increase in Richter magnitude, there is an increase of 30 times the energy released by an earthquake. For example, a 7.0 earthquake has 30 times the power of a 6.0, and an 8.0 is 900 times as powerful as a 6.0. Each earthquake only has one Richter magnitude. The strongest earthquakes are in the 8.0 range—8.6 for Alaska's 1964 earthquake, and 8.0 for China's 1976 earthquake in Tangshan.

What is the Mercalli scale?

The Mercalli scale measures the power of an earthquake as felt by humans and structures. It was developed in 1902 by Italian geologist Giuseppe Mercalli. The Mercalli scale is written in roman numerals and it ranges from I (barely felt) to XII (catastrophic). The Mercalli scale can be mapped surrounding an epicenter and will vary based on the geology of an area.

The Mercalli Scale of Earthquake Intensity

I Barely felt

II Felt by a few people, some suspended objects may swing

III Slightly felt indoors as though a large truck were passing

IV Felt indoors by many people, most suspended objects swing, windows and dishes rattle, standing autos rock

V Felt by almost everyone, sleeping people are awakened, dishes and windows break

VI Felt by everyone, some are frightened and run outside, some chimneys break, some furniture moves, causes slight damage

VII Considerable damage in poorly built structures, felt by people driving, most are frightened and run outside

VIII Slight damage to well-built structures, poorly built structures are heavily damaged, walls, chimneys, monuments fall

IX Underground pipes break, foundations of buildings are damaged and buildings shift off foundations, considerable damage to well-built structures

X Few structures survive, most foundations destroyed, water moved out of banks of rivers and lakes, avalanches and rockslides, railroads are bent

XI Few structures remain standing, total panic, large cracks in the ground

XII Total destruction, objects thrown into the air, the land appears to be liquid and is visibly rolling like waves

How many really big earthquakes occur each year?

On average, there are about 100 earthquakes of magnitude 6.0-6.9, about 20 of magnitude 7.0-7.9, and two huge 8.0-8.9 earthquakes each year. Many of these really big earthquakes occur in the ocean, so we don't hear much about them.

Is a magnitude 10 the top of the Richter scale?

While the media often refers to the Richter scale as being on a scale of 1 to 10, there is no upper limit, even though the strongest quakes are not as high as 10. It is incorrect to assume that a 7 is on a scale of 1 to 10 because the magnitudes are based on the energy released and it is a logarithmic scale.

 
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