1. Earthquake Measurement 7-2

2. What You Will Learn
To find an earthquake’s epicenter, you must triangulate by using data from three or more seismometers.
Magnitude is a measure of an earthquake’s strength.
The intensity of an earthquake depends on four major factors.

3. Studying Earthquakes
Scientists use instruments called seismometers, or seismographs, to record seismic waves.
Seismometers record the vibrations of P waves, S waves, and surface waves.
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.
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.
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.
Scientists draw a circle around a seismometer station that has a radius equal to the distance the waves have traveled.
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. This process of locating the epicenter is called triangulation.

8. Earthquake Magnitude
Magnitude is the measure on an earthquake’s strength.
The greater the magnitude, the stronger the earthquake.
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.
Richter-scale values range from 0-9.
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.
Magnitude moment (Mw) represents the:
size of the area of the fault that moves
average distance moved by fault blocks, and
rigidity of rocks in the fault zone.

11. Earthquake Intensity
An earthquake’s intensity is the effect of the earthquake on people.
The Modified Mercalli scale describes earthquake intensity.
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.
Data from past earthquakes are used to create earthquake intensity maps.

14. The Effects of Earthquakes
Effects of earthquakes can vary over a wide area.
Effects depend on the size of the earthquake.
Effects also depend on three other factors: distance from the epicenter, local geology, and type of construction in the area.

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Distance from the Epicenter
The total energy in a seismic wave stays relatively constant as the wave travels.
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.
Therefore, an earthquake is less destructive to areas that are farther away from the epicenter.

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Local Geology
The amount of damage caused by an earthquake depends on the material through which seismic waves travel.
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.
This process is called liquefaction.

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Liquefaction can intensify ground shaking.
Liquefaction can also cause the ground to settle, which can cause structures to tilt or collapse.

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Earthquake-Resistant Construction
Brick and concrete structures are easily damaged by earthquakes.
Wood and steel structures are more flexible and less likely to be damaged.
Shorter buildings, on strong, anchored foundations are also less likely to be damaged.