Chapter 7 Section 2
Studying Earthquakes
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Scientists use instruments called seismometers, or seismographs, to record seismic waves.
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Seismometers record the vibrations of P waves, S waves, and surface waves.
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Seismometers also record the time it takes for waves to arrive at a seismometer station.
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Seismometers create a tracing of earthquake motion called a seismogram.
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Seismograms help to locate the earthquakes epicenter, the point on Earth’s surface directly above the
earthquake’s starting point.
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The earthquake’s starting point inside Earth is called the focus.
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The earthquake’s starting point inside Earth is called the focus.
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The epicenter is directly above the focus.
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The lag time between the arrival of P waves and S waves tells scientists how far the waves have
traveled.
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Scientists draw a circle around a seismometer station that has a radius equal to the distance the waves
have traveled.
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Scientists draw circles around three seismometer stations and find the point of intersection.
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The point at which all circles intersect is the epicenter.
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This process of locating the epicenter is called triangulation.
Earthquake Magnitude
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Magnitude is the measure on an earthquake’s strength.
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The greater the magnitude, the stronger the earthquake.
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In the past, the Richter scale was used to describe earthquake strength. Now, scientists use the
magnitude moment scale.
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The Richter Scale measures ground motion from an earthquake and adjusts for distance to find an
earthquake’s magnitude.
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Richter-scale values range from 0-9.
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Each increase of one number represents a tenfold increase in strength.
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The magnitude moment scale is a more accurate measure of earthquake strength.
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Magnitude moment (Mw) represents the:
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size of the area of the fault that moves
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average distance moved by fault blocks, and
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rigidity of rocks in the fault zone.
Earthquake Intensity
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An earthquake’s intensity is the effect of the earthquake on people.
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The Modified Mercalli scale describes earthquake intensity.
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Intensity ranges from barely noticeable to total destruction of an area.
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Earthquake intensity maps show the level of intensity expected in different areas that experience the
same earthquake.
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Data from past earthquakes are used to create earthquake intensity maps.
The Effects of Earthquakes
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Effects of earthquakes can vary over a wide area.
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Effects depend on the size of the earthquake.
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Effects also depend on three other factors: distance from the epicenter, local geology, and type of
construction in the area.
Distance from the Epicenter
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The total energy in a seismic wave stays relatively constant as the wave travels.
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Seismic waves grow increasingly larger as they move away from the epicenter.
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As seismic waves grow larger, the amount of energy at any one point decreases.
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Therefore, an earthquake is less destructive to areas that are farther away from the epicenter.
Local Geology
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The amount of damage caused by an earthquake depends on the material through which seismic waves
travel.
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Seismic waves are particularly dangerous when they travel through water-saturated soil or sediment.
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When seismic waves shake water-saturated sediment or soils, sediment grains lose contact with each
other and are surrounded by water.
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This process is called liquefaction.
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Liquefaction can intensify ground shaking.
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Liquefaction can also cause the ground to settle, which can cause structures to tilt or collapse.
Earthquake-Resistant Construction
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Brick and concrete structures are easily damaged by earthquakes.
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Wood and steel structures are more flexible and less likely to be damaged.
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Shorter buildings, on strong, anchored foundations are also less likely to be damaged.