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Chapter 10: Problem 5

Explain the differences among the scales used to describe the size of an earthquake.

Short Answer

Expert verified
The three primary scales used to describe the size of earthquakes are the Richter Magnitude Scale, Modified Mercalli Intensity Scale, and Moment Magnitude Scale. The Richter Scale measures energy released based on seismic wave amplitude, but has limitations in measuring very large earthquakes. The Modified Mercalli Intensity Scale measures observed effects and provides a subjective assessment but does not quantify energy released. The Moment Magnitude Scale measures energy released based on seismic moment and is more accurate, especially for larger earthquakes. Each scale offers unique insights into earthquake size, considering factors like energy released, effects, and geological aspects.

Step by step solution

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1. Introduction to Earthquake Scales

There are three primary scales used to describe the size of earthquakes: the Richter Magnitude Scale, Modified Mercalli Intensity Scale, and the Moment Magnitude Scale. Each scale measures different aspects of an earthquake, either the energy released or felt intensity, and provides a way to compare earthquakes of varying sizes. Let's discuss each scale in detail.
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2. Richter Magnitude Scale

The Richter Magnitude Scale, developed by Charles F. Richter in 1935, is a logarithmic scale that measures the energy released by an earthquake. It is calculated using the amplitude of seismic waves recorded on a seismograph. The Richter scale ranges from 0 to 10, with each increase of 1 representing a ten-fold increase in the amplitude of seismic waves and approximately 31.6 times more energy released. Limitations of the Richter scale include its inability to accurately measure very large earthquakes and its sensitivity to the distance between the earthquake epicenter and the recording seismograph.
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3. Modified Mercalli Intensity Scale

The Modified Mercalli Intensity Scale, originally developed by Giuseppe Mercalli in 1902 and later modified by Harry O. Wood and Frank Neumann in 1931, measures the observed effects, or intensity, of an earthquake. This scale uses Roman numerals from I (barely felt) to XII (total destruction) to describe the degree of damage, ground motion, and human perception of an earthquake. Unlike the Richter and Moment Magnitude scales, the Modified Mercalli Intensity scale does not measure the energy released by an earthquake; it only provides a subjective assessment of the earthquake's impact on people and structures.
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4. Moment Magnitude Scale

The Moment Magnitude Scale (Mw), introduced by Hiroo Kanamori and Thomas C. Hanks in 1979, also measures the energy released by an earthquake, but uses more accurate seismological data. It is based on the seismic moment, calculated using the area of the rupture, slip along the fault, and rigidity of the rocks affected by the earthquake. The Moment Magnitude Scale is logarithmic, similar to the Richter scale, but provides a more accurate assessment of the earthquake's energy release, particularly for very large earthquakes (magnitude greater than ~7). Nowadays, the Moment Magnitude Scale is the most commonly used scale for earthquake magnitudes.
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5. Comparing the Scales

In summary: - Richter Magnitude Scale: Measures energy released, based on seismic wave amplitude, limited in measuring very large earthquakes. - Modified Mercalli Intensity Scale: Measures observed effects, does not quantify released energy, and provides subjective assessments. - Moment Magnitude Scale: Measures energy released, based on seismic moment, more accurate for very large earthquakes, most commonly used. Each scale offers unique insights into the size of an earthquake, taking into account the energy released, effects, and the geological factors involved. By combining the information provided by these scales, scientists can analyze and compare earthquakes of various sizes more accurately.

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Most popular questions from this chapter

What are the four types of seismic waves? Which are body waves, and which are surface waves?

The northeast-trending Ramapo fault crops out north of New York City near the east coast of the United States. Precambrian gneiss forms the hills to the northwest of the fault, and Mesozoic sedimentary rock underlies the lowlands to the southeast. (You can see the fault on Google Earth \(^{\mathrm{TM}}\) by going to Lat \(41^{\circ} 10^{\prime} 21.12^{\prime \prime}\) N Long \(74^{\circ} 5^{\prime} 12.36^{\prime \prime} \mathrm{W}\). Once you're there, tilt the image and fly northeast along the fault.) Where the fault crosses the Hudson River, there is an abrupt bend in the river. A nuclear power plant was built near this bend. Geologic studies suggest that the Ramapo fault first formed during the Precambrian, was reactivated during the Paleozoic, and was the site of major displacement during the Mesozoic rifting that separated North America from Africa. Imagine that you are a geologist with the task of determining the seismic risk of the fault. What evidence of present-day or past seismic activity could you look for?

Explain how the vertical and horizontal components of an earthquake motion are detected on a seismometer.

On the seismogram of an earthquake recorded at a seismic station in Paris, France, the S-wave arrives six minutes after the P-wave. On the seismogram obtained by a station in Mumbai, India, for the same earthquake, the difference between the P-wave and S-wave arrival times is 4 minutes. Which station is closer to the epicenter? From the information provided, can you pinpoint the location of the epicenter? Explain.

How are long-term and short-term earthquake predictions made? What is the basis for determining a recurrence interval, and what does a recurrence interval mean?
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