The static size of the Earth implies that crust must be destroyed at about the same rate it is being created. Plate Tectonics provides the mechanism used to recycle the Earth’s crust. Three boundary types are shown here. Video lecture discusses four basic plate boundaries.

Do subducting plates slide smoothly past one another?

Frictional stress builds up along a locked subduction-zone boundary. When that stress exceeds a critical value, a sudden failure occurs along the fault plane that can result in a "mega-thrust" earthquake releasing strain energy and radiating seismic waves. [See Divergent and Convergent Plate Boundaries for more-detailed depiction.

How is stress stored between tectonic plates?

Rock is deformed as it builds up strain in the plates at locked plate boundaries. Stress and strain increase along the contact until the friction is overcome and rock breaks. Video lecture showing demonstration of elastic rebound and brittle material using a yardstick.

Do faults break all at once, or in many short segments?

An asperity is an area on a fault that is stuck or locked. Scientists study areas along long fault zones that have not had earthquakes in a long time in order to determine where the next earthquake may occur; as long faults move, all areas of it will, at some point, become "unstuck" causing an earthquake relative the the size of the asperity that finally breaks.

What are the 4 basic classes of faults?

These animations of four faults are simplified examples of fault motion intended to show basic movement. Video lecture has classroom demonstration of faults and folds.

What happens when the crust is stretched?

Over most of the last 30 million years, movement of hot mantle beneath the region caused the surface to dome up and then partially collapse under its own weight, as it pulled apart. Currently, there is very little actual stretching going on, and the small amount is concentrated on the Western and Eastern edges of the Basin and Range.

GPS - Measuring Plate Motion

Highly accurate measurements made by the GPS system allow scientists to record millimeter-scale slip on faults that cannot ordinarily be measured. This record of land movement provides a critical key to understanding plate tectonics, plate-boundary interaction, volcano deformation, and more. Scientists have placed

hundreds of GPS stations across the Western U.S., in an attempt to learn more about the events building up to earthquakes along the San Andreas Fault system and the Cascadia Subduction Zone. On a narrower scale, they are also used to monitor deformation of active volcanoes.

Earth Structure

Same earthquake, different stations; why do the seismograms look different?

One seismic station can give information about how far away the earthquake occurred, but yields little other information. The cartoonish amplified ground motions show the compressive P wave, the shearing S wave, and the rolling surface wave motions recorded by many stations with their characteristic seismograms. See also Travel-time curves.

How do we capture the motion of an earthquake?

Modern seismometers include 3 elements to determine the simultaneous movement in 3 directions: up-down, north-south,and east-west. Following an earthquake, the ground responds to P, S, and surface waves by moving in all directions. Each direction of movement gives information about the earthquake.

How do seismographs work?

Animations of a drum-style vertical seismograph stations that record vertical and horizontal motion. Although the drum-roll seismographs are used only for museum-type venues, they illustrate the basic principles of operation.

How do earthquakes reveal secrets of Earth's interior?

Seismic tomography is an imaging technique that uses seismic waves generated by earthquakes and explosions to create computer-generated, three-dimensional images of Earth's interior. Human CAT scans are often used as an analogy. Here we simplify things and make an Earth of uniform density with a slow zone that we image as a magma chamber.

Why do seismic waves travel a curving path through the Earth?

$Refraction$ $Refraction$

Seismic waves through the Earth follow the same laws of refraction and reflection as any other wave at interfaces. When they encounter boundaries between different media, the wave will react according to Snell’s law, and the angle of refraction across the boundary will depend on the velocity of the second media relative to the first. The

$Refraction$ $Refraction$

angle of reflection will be equal to the angle of incidence. Various material properties (i.e., elastic moduli) control the speed and attenuation of seismic waves. Before we answer the question posed in the title, we will step through animations increasing in complexity to introduce the concept of refraction.

$Refraction$ $Refraction$

$Refraction$

Earthquakes

How many different ways can an earthquake shake us?

An earthquake generates seismic waves that 1) penetrate the Earth as body waves (P & S) or 2) travel as surface waves (Love and Rayleigh). Each wave has a characteristic speed and style of motion. Here we exaggerate the motion by bouncing a building to show what sensitive instruments record as seismic waves arrive at the station.

Where do travel-time graphs come from?

A travel time curve is a graph of the time that it takes for seismic waves to travel from the epicenter of an earthquake seismograph stations varying distances away. The velocity of seismic waves through different materials yield information about Earth’s deep interior.

How can you model earthquakes in the classroom?

This block-and-sandpaper model can be used to teach the concept of elastic rebound and how energy is stored and released. Earthquakes can provide a useful context for teaching or reviewing many basic physics concepts, such as sliding and static friction, forms of energy and conversion from one form to another, and the elastic properties of materials.

Seismic Signatures

Seismograph stations don't just record earthquakes; they record anything that shakes the instrument. These animations show the growth of seismograms from a variety of ground-shaking events. When the ground is jarred energy is released in the form of seismic waves that radiate from the source source in all directions. The

different types of energy waves shake the ground in different ways and travel through the earth at different velocities. Seismologists are trained to distinguish between events. The animations in this set were done in collaboration with the US Geological Survey and the Mount St. Helens Institute in recognition of the 30th Anniversary of the

1980 Mount St. Helens eruption.

Volcanoes

Volcano Monitoring

Precursory seismicity, deformation of the crater floor and the lava dome, and, to a lesser extent, gas emissions provided telltale evidence of forthcoming eruptions, which is why we selected these three methods for our first volcano monitoring animations. The animations in this set were done in collaboration with the

US Geological Survey & Mount St. Helens Institute in recognition of the 30th Anniversary of the 1980 Mount St. Helens eruption.