The Big Picture

Major earthquakes are caused by the movement of huge blocks of the Earth's crust. These blocks are pulled apart and wrenched by larger tectonic forces that are caused ultimately by heat escaping from the deep interior of the Earth.

The outer portion of the Earth is made up of plates that move in different directions over the hotter interior. These plates can slide sideways, pull apart, or collide with one another. Huge systems of faults develop to accommodate the motions between the plates. When these faults move abruptly in the cooler, brittle, outermost part of the Earth's crust, called the seismogenic zone, earthquakes are produced. Most earthquakes occur along plate boundaries.

The western United States is on top of a broad boundary between the North American and Pacific Plates. The North American Plate extends east to Iceland; the Pacific Plate extends west to Japan. The Pacific Plate is moving northwest, scraping horizontally past North America at a rate of about 2 inches per year (5 cm/yr). The most famous fault within this mostly strike-slip plate boundary is the San Andreas Fault in California, which moves on average 1 1/3 inches every year (3 1/3 cm). More than a fifth of this overall plate boundary motion (greater than 1 cm) occurs east of the Sierra Nevada. Between the Sierra Nevada and Colorado Plateau is the Basin and Range province, a large area of the North American Plate that is extending or spreading out. This extension, which began over 20 million years ago, has produced a mountain and valley topography, and many faults. Some of these faults are still active today.

Located entirely in the Basin and Range province, Nevada is a region of high average elevation, relatively thin continental crust, high levels of heat flowing out of the Earth, and a distinct mountain and valley topography. The crust in Nevada is both extending and shearing, largely in response to the motion between the Pacific and North American Plates. The extension is resulting in normal-slip faults that bound down-dropped blocks (basins), uplifted blocks (mountains), and tilted blocks (combination mountain and basin). Strike-slip faults also occur in this extending region but are fewer in number than the normal-slip faults. Why this region is pulling apart or extending is debated. Some suggest that hot upwelling from below causes the spreading. Others conclude that the elevation is so high that this area may be simply collapsing under its own weight. The white arrows show the relative motion with respect to the eastern part of the figure (the "spike" pictured in the Colorado Plateau), which corresponds to the stable part of North America. In eastern Nevada, the relative motion is about 1/6 of an inch (4 mm) per year westward. The Sierra Nevada block is moving about 0.5 inch (12 mm) per year to the northwest. The Walker Lane belt is a northwest-trending region in western Nevada and eastern California of diverse orientations of mountain ranges and faults, many strike-slip faults, and high relative levels of background earthquake activity. The Walker Lane belt, is a transition zone between the strike-slip plate boundary system and extensional tectonics of the Basin and Range province. The belt contains many strike-slip faults and has had major strike-slip earthquakes, such as the 1932 Cedar Mountain earthquake (magnitude 7.1). Western Nevada appears to have a higher earthquake hazard than the rest of the state because of the strike-slip activity in addition to the normal-slip faults.

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