Images

The residual is the difference between the measured and predicted values of some quantity.

A sand boil is sand and water that come out onto the ground surface during an earthquake as a result of liquefaction at shallow depth.

Slickensides are polished striated rock surfaces caused by one rock mass moving across another on a fault.

Subduction zone of the Juan de Fuca Plate and North America Plate, creating the Mt. St. Helens and Mt. Hood volcanoes.

The time history is the sequence of values of any time-varying quantity (such as a ground motion measurement) measured at a set of fixed times. Also termed time series.

Ripples on a pond are attenuated as they move away from the source. When you throw a pebble in a pond, it makes waves on the surface that move out from the place where the pebble entered the water. The waves are largest where they are formed and gradually get smaller as they move away. This decrease in size, or amplitude, of the waves is called attenuation.

Photo of a Kinemetrics FBA-23 accelerograph. The device is black and square with rounded edges on a green background.

Cross-section of the Benioff zone resulting from a by the interaction of a downgoing oceanic crustal plate with a continental plate.

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

Diagram of dipping slab with different focal depth events. The focal depth refers to the depth of an earthquake hypocenter.

The geologic time scale showing each eon, era, period, epoch, and their associated ages.

Photo of bridge.  Geotechnical means referring to the use of scientific methods and engineering principles to acquire, interpret, and apply knowledge of earth materials for solving engineering problems.  

Horst and graben diagram. A graben is a down-dropped block of the earth's crust resulting from extension, or pulling, of the crust. 

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

Plot showing least squares fit. When plotting data points on a graph, the least-squares-fit is the line or curve that comes closest to going through all the points.

The Landers fault (red lines at left) and the Hector Mine fault and its aftershocks (red lines and circles at right). The Hector Mine earthquake was a magnitude 7.1, and it produced thousands of aftershocks.

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 P wave, or compressional wave, is a seismic body wave that shakes the ground back and forth in the same direction and the opposite direction as the direction the wave is moving.

Diagram of period.  The period is the time interval required for one full cycle of a wave.

Moment = ? A D

? = shear modulus = 32 GPa in crust, 75 GPa in mantle

A = LW = area

D = average displacement during rupture

The New Madrid seismic zone in Central U.S.

A seismic zone is an area of seismicity probably sharing a common cause.