Terremotos

EARTHQUAKES

2.1 PLATE TECTONICS, THE CAUSE OF EARTHQUAKES The plates consist of an outer layer of the Earth. This is called the lithosphere. It is cool enough to behave as a more or less rigid shell. Occasionally the hot asthenosphere of the Earth finds a weak place in the lithosphere to rise buoyantly as a plume, or hotspot. The satellite image in Fig. 2.1 below shows the volcanic islands ofthe Galapagos hotspot.

Fig. 2.1 Volcanic islands (Courtesy: NASA)
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The map in (Fig. 2.3) of Earth’s solid surface shows many of the features caused by plate tectonics. The oceanic ridges are the asthenospheric spreading centers, creating new oceanic crust. Subduction zones appear as deep oceanic trenches. Most of the continental mountain belts occur where plates arepressing against one another.

Fig. 2.4 Plate tectonic environments (Courtesy: http://seismo.unr.edu)

There are three main plate tectonic environments (Fig. 2.4): extensional, transform, and compressional. Plate boundaries in different localities are subject to different inter-plate stresses, producing these three types of earthquakes. Each type has its own special hazards.

Fig. 2.5 Juande Fuca spreading ridge (Courtesy: http://seismo.unr.edu)

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Basic Geotechnical Earthquake Engineering

At spreading ridges, or similar extensional boundaries, earthquakes are shallow. They are aligned strictly along the axis of spreading, and show an extensional mechanism. Earthquakes in extensional environments tend to be smaller than magnitude 8 (magnitude of earthquake has beendiscussed in detail later). A close-up topographic picture (Fig. 2.5) of the Juan de Fuca spreading ridge, offshore of the Pacific Northwest, shows the turned-up edges of the spreading center. As crust moves away from the ridge it cools and sinks. The lateral offsets in the ridge are joined by the transform faults. A satellite view (Fig. 2.6) of the Sinai shows two arms of the Red Sea spreading ridge,exposed on land.

Fig. 2.6 Two arms of red sea spreading ridge (Courtesy: NASA)

Extensional ridges exist elsewhere in the solar system, although they never attain the globe-encircling extent the oceanic ridges have on Earth. This synthetic perspective of a large volcano on Venus (Fig. 2.7) is looking up the large rift on its flank. At transforms, earthquakes are shallow, running as deep as 25km. The mechanisms indicate strike-slip motion. Transforms tend to have earthquakes smaller than magnitude 8.5. The San Andreas fault (Fig. 2.8) in California is a nearby example of a transform, separating the Pacific from the North American plate. At transforms the plates mostly slide past each other laterally, producing less sinking or lifing of the ground than extensional or compressionalenvironments. The white dots in Fig. 2.8 locate earthquakes along strands of this fault system in the San Francisco Bay area.

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Fig. 2.7 Large volcano on Venus (Courtesy: NASA/JPL)

Fig. 2.8 The San Andreas fault in California (Courtesy: USGS)

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Basic Geotechnical Earthquake Engineering

Fig. 2.9 (Courtesy: NASA, Topography from NOAA)

At compressional boundaries,earthquakes are found in several settings ranging from the very near surface to several hundred kilometers depth, since the coldness of the subducting plate permits brittle failure down to as much as 700 km. Compressional boundaries host Earth’s largest quakes, with some events on subduction zones in Alaska and Chile having exceeded magnitude 9. This oblique orbital view of Fig. 2.9 looking eastover Indonesia shows the clouded tops of the chain of large volcanoes. The topography of Fig. 2.9 shows the Indian plate, streaked by hotspot traces and healed transforms, subducting at the Javan Trench. Sometimes continental sections of plates collide; both are too light for subduction to occur. The satellite image (Fig. 2.10) below shows the bent and rippled rock layers of the Zagros Mountains in...