Page 455 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

AQuantitativeSOLUTION The Sizes of Earthquakes
Two scales for measuring the size of earthquakes were described in this chapter: the amplitude magnitude scale (commonly known as the Richter Scale) and the moment magnitude scale. Although each scale measures a different phenomenon and there is not perfect agreement, the USGS explains that both magnitude scales should yield approximately the same value for an earthquake,1
but the moment magnitude scale is more accurate for measuring the size of the largest earthquakes.
Both the amplitude and moment scales are logarithmic. Each increase of one unit on a logarithmic scale represents an increase of 10 on the corresponding numerical scale, an increase of two logarithmic units represents an increase of 10 × 10 = 102 = 100 on the numerical scale, and so on. We can compare the numerical difference between two earthquakes if we are given magnitude values.
For earthquakes with measured wave amplitudes of 7.0 and 4.0, the difference is 107.0 − 104.0. This is 10(7.0 − 4.0) or 103.0, and 103.0 is 10 × 10 × 10 = 1000. So the amplitude of the
1USGS Earthquake Hazards Program, earthquake.usgs.gov/learn/ glossary/?termID=118.
7.0 earthquake was three logarithmic units larger than the 4.0 earthquake—but this means it was 1000 times larger numerically.
How much larger is magnitude 5.7 than magnitude 3.4? Calcu- late: 10(5.7 − 3.4) = 102.3 = about 200 times larger.
The calculations above compare the size of two earthquakes on the amplitude magnitude scale. But that scale does not directly measure the amount of energy released during an earthquake. The actual amount of energy released is approximately 101.5 times
the amplitude, or approximately 31.6 as much. Let’s now compare the released energy in the same examples as above.
We substitute 101.5 before raising to the power of the ampli- tude magnitude, so the energy released by a 7.0 earthquake is 10(1.5) × (7.0 − 4.0) = 10(1.5 × 3.0) = ~31 600 times as much compared to magnitude 4.0. For the second example, 10(1.5) × (5.7 − 3.4) = 10(1.5 × 2.3) = ~2820 times as much energy released. With these examples, it is easy to see why the infrequent great earthquakes with magni- tudes of 8 and 9 are so destructive: there are incredible amounts of energy released at one time compared to smaller earthquakes that occur with greater frequency.
concepts review
KEy LEARNINg
■ Describe first, second, and third orders of relief, and list Earth’s six major topographic regions.
Tectonic forces generated within the planet dramatically shape Earth’s surface. Relief is the vertical elevation dif- ference in a local landscape. The variations in the physi- cal surface of Earth, including relief, are topography. Orders of relief are convenient descriptive categories for landforms; the coarsest level includes the continental landmasses (portions of crust that reside above or near sea level) and ocean basins (portions of the crust that are entirely below sea level), and the finest level comprises local hills and valleys.
relief (p. 384)
topography (p. 384) continental landmass (p. 385) ocean basin (p. 385)
1. How does the map of the ocean floor in the chapter- opening illustration exhibit the principles of plate tectonics? Briefly analyze.
2. What is meant by an order of relief? Give an example from each order.
3. Explain the difference between relief and topography.
■ Describe the formation of continental crust, and define
displaced terranes.
A continent has a nucleus of ancient crystalline rock called a craton. A region where a craton is exposed is a continental shield. As continental crust forms, it is enlarged through
accretion of dispersed terranes. An example is the Wrangel- lia terrane of the Pacific Northwest and Alaska.
continental shield (p. 387) terrane (p. 389)
4. What is a craton? Describe the relationship of cratons to continental shields and platforms, and describe these regions in North America.
5. What is an accreted terrane, and how does it add to the formation of continental landmasses? Briefly describe the journey and current location of the Wrangellia terrane.
■ Explain the process of folding, and describe the princi- pal types of faults and their characteristic landforms.
Folding, broad warping, and faulting deform the crust and produce characteristic landforms. Compression causes rocks to deform in a process known as folding, during which rock strata bend and may overturn. Along the ridge of a fold, layers slope downward away from the axis, forming an anticline. In the trough of a fold, how- ever, layers slope downward toward the axis; this is a syncline.
When rock strata are stressed beyond their ability to remain a solid unit, they express the strain as a fracture. Rocks on either side of the fracture are displaced relative to the other side in a process known as faulting. Thus, fault zones are areas where fractures in the rock demonstrate crustal movement.
When forces pull rocks apart, the tension causes a normal fault, sometimes visible on the landscape as a scarp, or escarpment. Compressional forces associated with converging plates force rocks to move upward, pro- ducing a reverse fault. A low-angle fault plane is referred
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