Images

Liquefaction potential map of the San Francisco Bay Area, California. (Image courtesy of the Association of Bay Area Governments-ABAG)

The natural frequency is the frequency at which a particular object or system vibrates when pushed by a single force or impulse, and not influenced by other external forces or by damping. If you hold a slinky by one end and let it hang down and then give it one push up from the bottom, the rate of up-and-down motion is its natural frequency.

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.

A diagram showing the Earth's crust with continental and oceanic crust.

The crust is the outermost major layer of the earth, ranging from about 10 to 65 km in thickness worldwide. The uppermost 15-35 km of crust is brittle enough to produce earthquakes.

Diagram of dip. Dip is the angle that a planar geologic surface (for example, a fault) is inclined from the horizontal.

Acceleration, velocity, and displacement records. 

Displacement is the difference between the initial position of a reference point and any later position. The amount any point affected by an earthquake has moved from where it was before the earthquake.

When seismologists think of an earthquake, this image of a seismogram is what comes to their minds.

Diagram showing the epicenter and hypocenter. 

The epicenter is the point on the earth's surface vertically above the hypocenter (or focus), point in the crust where a seismic rupture begins.

Strike-slip, normal, and reverse faults. A reverse fault with a small dip angle is called a thrust fault.

Photo of fault gouge. Fault gouge is crushed and ground-up rock produced by friction between the two sides when a fault moves.

Fault plane diagram. (Image courtesy of Kian H. Chong, Univ. of California, Davis)

Fault scarp produced by the M7.1 Hector Mine, Calfornia earthquake. (Photo by Ken Hudnut, U.S. Geological Survey)

Focal mechanism examples. A fault plane solution is a way of showing the fault and the direction of slip on it from an earthquake, using circles with two intersecting curves that look like beach balls. Also called a focal-mechanism solution.

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.

Liquefaction and lateral spreading occurred along the west side of Sunset Lake trailer park in Tumwater.

Plot showing 440 Hertz and 880 Hertz. Hertz (Hz) is a unit of measurement named in honor of Heinrich Hertz, the German physicist who in 1887 first produced electromagnetic waves. Expresses the frequency in cycles per second; 1 Hz = 1 cycle of rise and fall of a wave per second .

Map showing intensity for the New Madrid earthquake. (Image courtesy of the Central U.S. Earthquake Consortium)

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).

Iceland and plate motions. Kinematic refers to the general movement patterns and directions of the earth's rocks that produce rock deformation.