Images

First motion mechanism. On a seismogram, the first motion is the direction of ground motion as the P wave arrives at the seismometer. Upward ground motion indicates an expansion in the source region; downward motion indicates a contraction.

Small foreshocks on a seismic record. Foreshocks are relatively smaller earthquakes that precede the largest earthquake in a series, which is termed the mainshock. Not all mainshocks have foreshocks.

Photo of Zion Canyon located in Zion National Park in Utah, United States. (Image courtesy of Justin Gould)

The earthquakes (colored dots) in the middle of the continent are intraplate events. Those near the ocean boundary are at the boundary between two plates (interplate).

Diagram of left-lateral motion on fault. If you were to stand on the fault and look along its length, this is a type of strike-slip fault where the left block moves toward you and the right block moves away. See also right-lateral. 

A Love wave is a surface wave having a horizontal motion that is transverse (or perpendicular) to the direction the wave is traveling.

Ripples on a pond are attenuated as they move away from the source. Q is the inverse of attenuation. A high Q means a low attenuation.

Geologic time scale showing eras, periods, and notable events in terms of millions of years ago.

Secular refers to long-term changes that take place slowly and imperceptibly. Commonly used to describe changes in elevation, tilt, and stress or strain rates that are related to long-term tectonic deformation.

The source is the term for the released forces that generate acoustic or seismic waves, also called the earthquake source.

The strike is the trend or bearing, relative to north, of the line defined by the intersection of a planar geologic surface (for example, a fault or a bed) and a horizontal surface such as the ground.

Tsunamigenic is referring to those earthquakes, commonly along major subduction zone plate boundaries such as those bordering the Pacific Ocean, that can generate tsunamis.

YBP is an abbreviation for "years before present".

Formation of Earth: 4.6x10^9 YBP

Dinosaur extinction: 6.5x10^7 YBP

First hominids: 2x10^6 YBP

Last great ice age: 1x10^4 YBP

Declaration of Independence: 2x10^2 YBP

Establishment of UWB: 1x10 YBP

High, medium, and low amplitude. The amplitude is the size of the wiggles on an earthquake recording.

This image of the Los Angeles region combines this information to predict the total amplification expected in future earthquakes from local geologic conditions or site effects.

A diagram showing a slice of the Earth's lithosphere, including continental crust, oceanic crust, asthenosphere, and the upper mantle.

An illustration showing two stick figures doing basement excavations, one with a frown that excavated into the labelled hard, stable bedrock on the left and the other with a smile that excavated above the labelled bedrock surface.  

A diagram of a blind thrust fault resulting in ductile rock layers folding over the fault and leaving the fault under the surface.

A diagram showing three stress types: tensional stress, compressional stress, and shear stress. 

Cross-section of the Earth with lithosphere, mantle, and the core divided into the metallic liquid core and the solid inner core.

En echelon shears crossing street, Hayward Hayward, California. The street shows en echelon shears crossing Foothill Boulevard in San Leandro, just to the north of Hayward. Here the shear zone is quite narrow.