Chapter 8. Earthquakes. The New Madrid Earthquakes.
Eyewitnesses to the 1811-1812 earthquakes in New Madrid, Missouri, reported seeing bright flashes of light and a dull glow in the sky over a wide area. Reeking sulfurous odors also accompanied the quakes. Many survivors were convinced that the quakes were a heavenly sign meant to frighten the local citizens back to church. As a result, church attendance in the area skyrocketed between 1811 and 1812.
The strongest earthquake recorded to date occurred in Chile in 1960. It measured 9.5 on the Richter scale. This is equivalent to detonating more than 1 billion tons of TNT!
There is a branch of Earth Science devoted to earthquakes called seismology. Seismology is the study of earthquakes. Earthquakes are complex, and they present many questions for seismologists, the scientists that study earthquakes.
The largest and most active earthquake zone lies along the plate boundaries surrounding the Pacific Ocean.
As tectonic plates push, pull, or scrape against each other, stress builds up along the faults near the plates’ edges. In response to this stress, rock in the plates deforms.
Deformation is the change in the shape of the rock in response to stress. Rock along a fault deforms in mainly two ways – in a plastic manner, like a piece of molded clay, or in an elastic manner, like a rubber band.
Plasticdeformation does not lead to earthquakes. Elastic deformation, however, does lead to earthquakes. While rock can stretch farther than steel without breaking, it will break at some point. Think of elastically deformed rock as a stretched rubber band. You can stretch a rubber band only so far before it breaks. When the rubber band breaks, it releases energy, and the broken pieces return to their un-stretched shape.
Like the return of the broken rubber band pieces to their un-stretched shape, elasticrebound is the sudden return of elastically deformed rock to its original shape. Elastic rebound occurs when more stress is applied to rock than the rock can withstand. During elastic rebound, rock releases energy that causes an earthquake.
Earthquakes are not a rare phenomenon. In fact, more than 3 million earthquakes happen each year, about one every 10 seconds! Most earthquakes are too weak to be felt by humans. The Ring of Fire, a volcanic zone which lies along the plate boundaries surrounding the Pacific Ocean, is also the world’s largest and most active earthquake zone.
Earthquakes differ in strength and in the depth at which they begin. These differences depend on the type of tectonic plate motion that produces the earthquake. Examine the chart and the diagram below to learn how earthquakes differ.
Many people assume that major earthquakes in the United States occur only on the West Coast. However, major quakes have occurred in South Carolina and Missouri---far from any active plate boundaries. The four major tremors of the 1811-1812 earthquakes in New Madrid, Missouri, were so intense that, according to reports, they altered the flow of the Mississippi River and rang church bells in Boston!
Remember rock releases energy when it springs back after being deformed. This energy travels in the form of seismic waves. Seismic waves are waves of energy that travel through the Earth. Seismic waves that travel through the Earth’s interior is called body waves. There are two types of body waves: P waves and S waves. Seismic waves that travel along the Earth’s surface is called surface waves. Different types of seismic waves travel at different speeds and move the materials that they travel through differently.
Seismographs reveal that two main groups of seismic waves are generated by the slippage of a rock mass.
Surface waves consist of two types of wave motion. One motion produces a complex up-and-down motion similar to ocean swells, while the other surface wave whips the ground from side-to-side without any vertical motion. Surface waves have a longer time interval between crests and are referred to as L-Waves. These waves cause the greatest destruction.
“S” Waves- (secondary) shake the particles at right angles to the direction of wave travel. This can be illustrated by tying one end of a rope to a post and shaking the other end. S waves do not travel through liquids and gasses, only through solids.
“P” Waves- (primary) are compressional waves and travel through solids, liquids and gasses. This wave action is like that generated by human vocal chords as they move air to create sound.
The epicenter is the location on the surface of the Earth directly above the focus (source) and can be measured by a seismograph.
Seismograph- A device used to measure the low frequency waves generated by an earthquake. They are placed at over 100 locations throughout the world and provided geologists with data about the Earth’s composition, by understanding the characteristics of the low frequency waves generated by an earthquake.
Seismograph of an Earthquake
Seismographs are instruments located at or near the surface of the Earth that record seismic waves. When the waves reach a seismograph, the seismograph creates a seismogram. A seismogramis a tracing of earthquake motion created by a seismograph.
Seismologists use seismograms to calculate when an earthquake started. An earthquake starts when rock slips suddenly enough along a fault to create seismic waves. Seismologists find an earthquake’s start time by comparing seismograms and noting the difference in arrival times of P waves and S waves.
Seismologists also use seismograms to find an earthquake’s epicenter. An epicenter is the point on the Earth’s surface directly above the earthquake’s starting point. A focusis the point inside the Earth where an earthquake begins.
The Richter scaleis commonly used to measure earthquake strength. It is named after Charles Richter, an American seismologist who developed the scale in the 1930’s.
The Richter Scale is used worldwide to describe earthquake magnitude. Using Richter’s scale, the magnitude is determined by measuring the amplitude of the largest wave recorded on the seismograph. Large-magnitude earthquakes will cause the seismograph pen to be displaced farther than small-magnitude earthquakes.
There is a pattern in the Richter scale relating an earthquakes magnitude and the amount of energy release by the earthquake. Each time the magnitude increases by 1 unit, the amount of energy released becomes 31.7 times larger. For example, an earthquake with a magnitude of 5.0 on the Richter scale will release 31.7 times as much energy as an earthquake’s with a magnitude of 4.0 on the Richter scale.
Earthquake hazardmeasures how prone an area is to experiencing earthquakes in the future. An area’s earthquake-hazard level is determined by past and present seismic activity. The West Coast, for example, has a very high earthquake-hazard level because it has a lot if seismic activity.
Predicting when and where earthquakes will occur and how strong will be a difficult task. However, by closely monitoring active faults and other areas of seismic activity, seismologists have discovered some patterns in earthquakes that allow them to make some broad predictions.
Engineers have devised giant shock absorbers for buildings. The shock absorbers contain a ferrofluid solution that becomes rigid in a magnetic field. When an earthquake occurs, a computer controls the electromagnets in the shock absorbers to dampen the vibrations!
Being in an earthquake is a startling experience. Afterward, you should not be surprised to find yourself and others puzzled about what happened. You should try to calm down, get your bearings, and remove yourself from immediate danger, such as downed power lines, broken glass, and fire hazards. Be aware that there may be aftershocks. Recall your earthquake plan, and follow it through.
The study of earthquakes has led to many important discoveries about the Earth’s interior. Seismologists learn about the Earth’s interior by observing how seismic waves travel through the Earth. Likewise, seismic waves on other cosmic bodies allow seismologists to study the interiors of those bodies.
In July 1969, humans set foot on the moon for the first time. They brought with them a seismograph. Not knowing if the moon was seismically active, they left nothing to chance – they purposely crashed their landing vehicle back to the moon’s surface after they left to create artificial seismic waves. What happened after that left seismologists astonished.
“Moonquakes” fall into three categories: deep quakes, which result from the gravitational pull of the Earth; shallow quakes, which may be caused by the heating and cooling of the moon’s surface; and quakes caused by collision of objects with the moon’s surface.