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Plate Tectonics And Earthquakes 4

A bad earthquake at once destroys our oldest associations: the earth, the very emblem of solidity, has moved beneath our feet like a thin crust over a fluid;—one second of time has created in the mind a strange idea of insecurity, which hours of reflection would not have produced.

—Charles Darwin , 1835, notes for The Voyage of the Beagle

CHAPTER

Photo by Peter W. Weigand.

LEARNING OUTCOMES

Movements along tectonic-plate edges are responsible for many large earthquakes. After studying this chapter, you should:

• be able to describe the types of movements along tectonic-plate edges and the resultant earthquake magnitudes.

• be able to explain why subduction-zone earthquakes have the greatest magnitudes.

• understand the seismic-gap method of forecasting earthquakes.

• recognize the relationship between buildings and earthquake fatalities.

OUTLINE

• Tectonic-Plate Edges and Earthquakes

• Spreading-Center Earthquakes

• Convergent Zones and Earthquakes

• Subduction-Zone Earthquakes

• Continent-Continent Collision Earthquakes

• The Arabian Plate

• Transform-Fault Earthquakes

During the Northridge earthquake, the ground moved rapidly to the north and pulled out from under this elevated apartment building, causing it to fall back onto its parking lot in Canoga Park, California, 17 January 1994.

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80 Chapter 4 Plate Tectonics and Earthquakes

together—such as India slamming into Asia to uplift the Himalayas—involve incredible amounts of energy. This results in Earth’s greatest earthquakes.

Moving from an idealized plate, let’s examine an actual plate—the Pacific plate. Figure 4.2 shows the same type of plate-edge processes and expected earthquakes. The Pacific plate is created at the spreading centers along its eastern and southern edges. The action there produces smaller earthquakes that also happen to be located away from major human populations.

The slide-past motions of long transform faults occur: (1) in the northeastern Pacific as the Queen Charlotte fault, located near a sparsely populated region of Canada; (2) along the San Andreas fault in California with its famous earthquakes; and

The past decade brought a staggering number of mega-killer earthquakes ( table 4.1 ). The causes of these earthquakes are best understood using their plate- tectonic settings.

Tectonic-Plate Edges And Earthquakes Most earthquakes are explainable based on plate-tectonics theory. The lithosphere is broken into rigid plates that move away from, past, and into other rigid plates. These global- scale processes are seen on the ground as individual faults where Earth ruptures and the two sides move past each other in earthquake-generating events.

Figure 4.1 shows an idealized tectonic plate and assesses the varying earthquake hazards that are concentrated at plate edges:

1. The divergent or pull-apart motion at spreading centers causes rocks to fail in tension. Rocks rupture relatively easily when subjected to tension. Also, much of the rock here is at a high temperature, causing early failures. Thus, the spreading process yields mainly smaller earthquakes that do not pose an especially great threat to humans.

2. The slide-past motion occurs as the rigid plates fracture and move around the curved Earth. The plates shear and slide past each other in the dominantly horizontal movements of transform faults. This process creates large earthquakes as the irregular plate boundaries retard movement because of irregularities along the faults. It takes a lot of stored energy to overcome the rough surfaces, nonslippery rocks, and bends in faults. When these impediments are finally over- come, a large amount of seismic energy is released .

3. The convergent or push-together motions at subduction zones and in continent-continent collisions cause rocks to fail in compression. These settings store immense amounts of energy that are released in Earth’s largest tectonic earthquakes. The very processes of pulling a 70 to 100 km (45 to 60 mi) thick oceanic plate back into the mantle via a subduction zone or of pushing continents

TRANSFORM FAULT

TRANSFORM FAULT

Plate movement

Larger earthquakes

Slide-past motion

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Figure 4.1 Map view of an idealized plate and the earthquake potential along its edges.

Mega-Killer Earthquakes, 2003–2011

Year Place Magnitude Deaths Tectonic Setting 2011 Japan 9.0 ̃22,000 subduction

2010 Haiti 7.0 ̃230,000 transform fault

2008 China 7.9 87,500 continent collision

2005 Pakistan 7.6 88,000 continent collision

2004 Indonesia 9.1 ̃245,000 subduction

2003 Iran 6.6 31,000 continent collision

TABLE 4.1

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Spreading-Center Earthquakes 81

(3) at the southwestern edge of the Pacific Ocean where the Alpine fault cuts across the South Island of New Zealand (see figures 3.5 and 3.6).

The Pacific plate subducts along its northern and western edges and creates enormous earthquakes, such as the 2011 Japan seism, the 1964 Alaska event, and the 1931 Napier quake on the North Island of New Zealand.

Our main emphasis here is to understand plate-edge effects as a means of forecasting where earthquakes are likely to occur and what their relative sizes may be.

Spreading-Center Earthquakes A look at earthquake epicenter locations around the world (see figure 2.20) reveals that earthquakes are not as common in the vicinity of spreading centers or divergence zones as they are at transform faults and at subduction/collision zones. The expanded volumes of warm rock in the oceanic ridge systems have a higher heat content and a resultant decrease in rigidity. These heat-weakened rocks do not build up and store the huge stresses necessary to create great earthquakes.

ICELAND The style of spreading-center earthquakes can be appreciated by looking at the earthquake history of Iceland, a nation that exists solely on a hot-spot–fed volcanic island portion of the mid-Atlantic ridge spreading center ( figures 4.3 and 4.4 ). The Icelandic geologist R. Stefansson reported on catastrophic earthquakes in Iceland and stated that in the

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Pacific plate

San Andreas fault

Queen Charlotte fault

Alpine fault

Figure 4.2 The Pacific plate is the largest in the world; it underlies part of the Pacific Ocean. Its eastern and southern edges are mostly spreading centers characterized by small- to intermediate-size earthquakes. Three long transform faults exist along its sides in Canada (Queen Charlotte), California (San Andreas), and New Zealand (Alpine); all are characterized by large earthquakes. Subduction zones (shown by black triangles) lie along the northern and western edges, from Alaska to Russia to Japan to the Philippines to Indonesia to New Zealand; all are characterized by gigantic earthquakes. From P. J. Wyllie, The Way the Earth Works. Copyright © 1976 John Wiley & Sons, Inc., New York. Reprinted with permission of John Wiley & Sons, Inc.

portions of the country underlain by north-south-oriented spreading centers, stresses build up to cause earthquakes too small to destroy buildings or kill people. These moderate- size earthquakes tend to occur in swarms, as is typical of volcanic areas where magma is on the move. Iceland does have large earthquakes, but they are associated with

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Figure 4.3 Iceland sits on top of a hot spot and is being pulled apart by the spreading center in the Atlantic Ocean. Triangles mark sites of some active volcanoes.

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82 Chapter 4 Plate Tectonics and Earthquakes

east-west-oriented transform faults between the spreading- center segments.

RED SEA AND GULF OF ADEN Iceland has been built on a mature spreading center that has been opening the North Atlantic Ocean basin for about 180 million years. What would a young spreading center and new ocean basin look like? Long and narrow. In today’s world, long and narrow ocean basins exist in northeast Africa as the Red Sea and the Gulf of Aden ( figure 4.5 ). Following is a model explaining how spread- ing began: the northeastern portion of Africa sits above an extra-hot area in the upper mantle. The heat contained within this mantle hot zone is partially trapped by the blanketing effect of the overlying African plate and its embedded continent ( figure 4.6 a). The hot rock expands in volume, and some liquefies to magma. This volume expan- sion causes doming of the overlying rocks, with resultant uplift of the surface to form topography ( figure 4.6 b). The doming uplift sets the stage for gravity to pull the raised landmasses downward and apart, thus creating pull-apart faults with centrally located, down-dropped rift valleys, also described as pull-apart basins ( figure 4.6 c). As the fracturing/faulting progresses, magma rises up through the cracks to build volcanoes. As rifting and volcanism continue, seafloor spreading processes take over, the down-dropped linear rift valley becomes filled by the ocean, and a new sea is born ( figure 4.6 d).

Figure 4.5 reveals another interesting geometric feature. Three linear pull-apart basins meet at the south end of the Red Sea; this point where three plate edges touch is called a triple junction. Three rifts joining at a point may concen- trate mantle heat, or a concentration of heat in the upper mantle may begin the process of creating this triple junction. Earth’s surface may bulge upward into a dome, causing the elevated rocks to fracture into a radial pattern ( figure 4.7 ). Gravity can then pull the dome apart, allowing magma to well up and fill three major fracture zones, and the spreading process is initiated.

The triple junction in northeast Africa is geologically young, having begun about 25 million years ago. To date, spreading in the Red Sea and Gulf of Aden has been enough to split off northeast Africa and create an Arabian plate and to allow seawater to flood between them. But the East African Rift Valley has not yet been pulled far enough apart for the sea to fill it (see figure 4.5 ). The East African Rift Valley is a truly impressive physiographic feature. It is 5,600 km (3,500 mi) long and has steep escarpments and dramatic val- leys. Beginning at the Afar triangle at its northern end and moving southwest are the domed and stretched highlands of Ethiopia, beyond which the Rift Valley divides into two major branches. The western rift is markedly curved and has many deep lakes, including the world’s second deepest lake, Lake Tanganyika. The eastern rift is straighter and holds shallow, alkaline lakes and volcanic peaks, such as Mount

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Figure 4.5 Topography in northeastern Africa and Arabia. Northeastern Africa is being torn apart by three spreading centers: Red Sea, Gulf of Aden, and East African Rift Valley. The spreading centers meet at the triple junction in the Afar Triangle.

Figure 4.4 Looking south along the fissure at Thingvellir, Iceland. This is the rift valley being pulled apart in an east-west direction by the continuing spreading of the Atlantic Ocean basin. Photo by John S. Shelton.

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Asthenosphere

Melting

Oceanic crust melting

Magma

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Hot rocks

(a) Stage 1, Centering

(b) Stage 2, Doming

(c) Stage 3, Rifting

(d) Stage 4, Spreading

Lithosphere

Extension due to heating

Hot region in mantle

Figure 4.6 A model of the stages in the formation of an ocean basin. (a) Stage 1, Centering: Moving lithosphere centers over an especially hot region of the mantle. (b) Stage 2, Doming: Mantle heat causes melting, and the overlying lithosphere/continent extends. The increase in heat causes surface doming through uplifting, stretching, and fracturing. (c) Stage 3, Rifting: Volume expansion causes gravity to pull the uplifted area apart; fractures fail and form faults. Fractures/ faults provide escape for magma; volcanism is common. Then, the dome’s central area sags downward, forming a valley such as the present East African Rift Valley. (d) Stage 4, Spreading: Pulling apart has advanced, forming a new seafloor. Most magmatic activity is seafloor spreading, as in the Red Sea and the Gulf of Aden.