EARTHQUAKES

Features of Earthquakes

Seismic Waves Seismic waves are waves of energy generated by the sudden breaking or motion of Earth’s crust. Seismic wave motion

Animations of P and S waves and Raleigh and Love Surface Waves

Primary Waves—”P” Waves Primary waves move from the focus outward in all directions with a back-and-forth motion. It is the first type of wave to reach a seismic station because it takes the most direct route.

Secondary Waves—”S” Waves Secondary waves move from the focus outward in all directions in an “S” shape. It is the second wave to reach a seismic station because it does not take as direct a route as a P wave.

Surface Waves Surface waves travel only through the crust (not through mantle). They do the most damage to structures. They arrive at a seismic station after P and S. The deeper the earthquake’s focus, the less damaging the surface waves are (they weaken as they travel through crust). There are two main types of surface wave: –Love waves (named for a British mathematician who developed a model for the wave) –Raleigh waves (named for a British scientist who predicted their existence).

Surface Waves: Love Waves Love waves are the fastest surface waves and move the ground from side-to-side. They move only through the surface of the crust.

Raleigh Wave

Surface Waves: Raleigh Waves A Raleigh wave rolls along the ground just like a wave rolls across a lake or an ocean. Because it rolls, it moves the ground up and down, and side-to-side in the same direction that the wave is moving. Most of the shaking felt from an earthquake is due to the Raleigh wave, which can be much larger than the other waves.

Earthquake Stress How stress is stored in Earth’s crust: Stretched crust: Seismic Waves traveling through Earth:

Focus vs. Epicenter The focus is the point inside Earth where the crust actually breaks or moves. The epicenter is the point on Earth’s surface directly above the focus.

Locating an Epicenter A seismograph is an instrument that registers the different waves and records the time that each arrives. Locating an epicenter

Seismographs in Hawaii

How a Seismograph Works How Stuff Works

Seismograms

Using the Information Seismologists usually describe earthquakes based on their distances from the seismograph. –Local events occur less than 62 miles away. –Regional events occur 62 miles to 870 miles away. –Teleseismic events are those that occur at distances greater than 870 miles.

Earth’s Layers At the very center of Earth is a solid, dense inner core made mostly of iron with smaller amounts of nickel, oxygen, silicon, and sulfur. –There is some new evidence that the core is made up of radioactive material. Pressure from the layers above causes the inner core to be solid. –The inner core spins at a different speed than the rest of the planet. Above the solid inner core lies the liquid outer core, which is made mainly of iron.

More About Earth’s Layers Earth’s mantle is the largest layer, lying directly above the outer core. –It is made mostly of silicon, oxygen, magnesium, and iron. –The mantle often is divided into an upper part and a lower part based on changing seismic wave speeds. –A portion of the upper mantle, called the asthenosphere (as THE nuh sfihr), consists of weak rock that can flow slowly.

Earth’s Crust The outermost layer of Earth is the crust. Together, the crust and a part of the mantle just beneath it make up Earth’s lithosphere. The lithosphere is broken into a number of plates that move over the asthenosphere beneath it. The thickness of Earth’s crust varies. –It is more than 37 miles thick in some mountainous regions and less than 3 miles thick under some parts of the oceans. The crust is generally less dense than the mantle (that’s why it floats on top of it).

Inside Earth

How We Theorize What’s In There

More on How We Know (or Think We Know) What’s Inside The speeds and paths of seismic waves change as they travel through materials with different densities. By studying seismic waves that have traveled through Earth, scientists have identified different layers with different densities. In general, the densities increase with depth as pressures increase. Studying seismic waves has allowed scientists to map Earth’s internal structure without being there.

The Shadow Zone Early in the twentieth century, scientists discovered that large areas of Earth don’t receive seismic waves from an earthquake. In the area on Earth between 105° and 140° from the earthquake focus, no waves are detected. This area is called the shadow zone.

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More on the Shadow Zone Secondary waves are not transmitted through a liquid, so they stop when they hit the liquid outer core. Primary waves are slowed and bent but not stopped by the liquid outer core. Because of this, scientists concluded that the outer core and mantle are made of different materials. Primary waves speed up again as they travel through the solid inner core. The bending of primary waves and the stopping of secondary waves create the shadow zone.

Layer Boundaries Seismic waves change speed as they pass through layers of Earth.

The Moho Layer Seismic waves speed up when they pass through the bottom of the crust and enter the upper mantle. This boundary between the lithosphere and the asthenosphere is called the Mohorovicic discontinuity--or the Moho layer.

More on Moho

Watch Earthquake Footage

Earthquake Resistant Structures

Liquefaction