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crust

Earth's outermost layer, which varies in thickness from about 5km to 60km and is separated from the mantle by the Moho Discontinuity
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epicenter

point of Earth's surface directely above an earthquake's focus
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focus

in an earthquake, the point beneath Earth's surface where energy release occurs
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inner core

very dense, solid center od the Earth that is made of mostly iron with smaller amounts of oxygen, silicon, sulfur, or nickel
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magnitude

measure of the energy released by an earthquake
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mantle

largest layer inside Earth, lying directly above the outer core and that is made mostly of silicon, oxygen, magnesium, and iron
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normal fault

break in rock due to tension forces, where rock above the fault surface moves downward in relation to rock below the fault surface
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outer core

liquid core that surrounds Earth's solid inner core, and that is made mostly of iron
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primary waves

waves that travel outward from an Earthquake's focus and cause particles in rocks to move back and forth in the same direction the wave is moving
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reverse fault

break in rock due to compression forces, where rocks above the fault surface move upward and over the rocks below the fault surface

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Why does the earth shake when there is an earthquake?

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Why does the earth shake when there is an earthquake?

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While the edges of faults are stuck together, and the rest of the block is moving, the energy that would normally cause the blocks to slide past one another is being stored up. When the force of the moving blocks finally overcomes the friction of the jagged edges of the fault and it unsticks, all that stored up energy is released. The energy radiates outward from the fault in all directions in the form of seismic waves like ripples on a pond. The seismic waves shake the earth as they move through it, and when the waves reach the earth’s surface, they shake the ground and anything on it, like our houses and us! (see P&S Wave inset)

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What causes earthquakes and where do they happen?

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What causes earthquakes and where do they happen?

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The earth has four major layers: the inner core, outer core, mantle and crust. (figure 2) The crust and the top of the mantle make up a thin skin on the surface of our planet. But this skin is not all in one piece – it is made up of many pieces like a puzzle covering the surface of the earth. (figure 3) Not only that, but these puzzle pieces keep slowly moving around, sliding past one another and bumping into each other. We call these puzzle piecestectonic plates, and the edges of the plates are called the plate boundaries. The plate boundaries are made up of many faults, and most of the earthquakes around the world occur on these faults. Since the edges of the plates are rough, they get stuck while the rest of the plate keeps moving. Finally, when the plate has moved far enough, the edges unstick on one of the faults and there is an earthquake.

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An earthquake is what happens when two blocks of the earth suddenly slip past one another. The surface where they slip is called the fault or fault plane. The location below the earth’s surface where the earthquake starts is called thehypocenter, and the location directly above it on the surface of the earth is called the epicenter.

Sometimes an earthquake has foreshocks. These are smaller earthquakes that happen in the same place as the larger earthquake that follows. Scientists can’t tell that an earthquake is a foreshock until the larger earthquake happens. The largest, main earthquake is called the mainshock. Mainshocks always haveaftershocks that follow. These are smaller earthquakes that occur afterwards in the same place as the mainshock. Depending on the size of the mainshock, aftershocks can continue for weeks, months, and even years after the mainshock!

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Places where plates slide past each other are called transform boundaries. Since the plates on either side of a transform boundary are merely sliding past each other and not tearing or crunching each other, transform boundaries lack the spectacular features found at convergent and divergent boundaries. Instead, transform boundaries are marked in some places by linear valleys along the boundary where rock has been ground up by the sliding. In other places, transform boundaries are marked by features like stream beds that have been split in half and the two halves have moved in opposite directions.

Perhaps the most famous transform boundary in the world is the San Andreas fault, shown in the drawing above. The slice of California to the west of the fault is slowly moving north relative to the rest of California. Since motion along the fault is sideways and not vertical, Los Angeles will not crack off and fall into the ocean as popularly thought, but it will simply creep towards San Francisco at about 6 centimeters per year. In about ten million years, the two cities will be side by side!

Although transform boundaries are not marked by spectacular surface features, their sliding motion causes lots of earthquakes. The strongest and most famous earthquake along the San Andreas fault hit San Francisco in 1906. Many buildings were shaken to pieces by the quake, and much of the rest of the city was destroyed by the fires that followed. More than 600 people died as a result of the quake and fires. Recent large quakes along the San Andreas include the Imperial Valley quake in 1940 and the Loma Prieta quake in 1989.