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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.

A diagram depicting the tectonic components of a volcanic arc, including the back arc spreading center, back arc convection cell, zone of fractional melting, outer arc trough, outer arc ridge, subducting oceanic lithosphere, and asthenosphere.

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.

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