A radiocarbon age provides a numerical estimate of when organic matter was formed. As organic matter gets old, its radioactive carbon (₁₄C) disappears by decay. Scientists routinely use radiocarbon ages of wood, charcoal, peat, and shell to estimate the timing of earthquakes in the past 40,000 years.

Radiometric
Radiometric is pertaining to the measurement of geologic time by the analysis of certain radioisotopes in rocks and their known rates of decay.

Stress is the force per unit area acting on a plane within a body. Six values are required to characterize completely the stress at a point: three normal components and three shear components.

Normal stress
The normal stress is that stress component perpendicular to a given plane. If you lean against a door after you close it, you are applying normal stress to the door. Normal stress can either be compressional or tensional.

Compressional Stress
Compressional stress is the stress that squeezes something. It is the stress component perpendicular to a given surface, such as a fault plane, that results from forces applied perpendicular to the surface or from remote forces transmitted through the surrounding rock.

Tensional Stress
Tensional stress is the stress that tends to pull something apart. It is the stress component perpendicular to a given surface, such as a fault plane, that results from forces applied perpendicular to the surface or from remote forces transmitted through the surrounding rock.

Shear Stress
Shear stress is the stress component parallel to a given surface, such as a fault plane, that results from forces applied parallel to the surface or from remote forces transmitted through the surrounding rock.

A body wave is a seismic wave that moves through the interior of the earth, as opposed to surface waves that travel near the earth's surface. P and S waves are body waves. Each type of wave shakes the ground in different ways.

P wave
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.

S wave
An S wave, or shear wave, is a seismic body wave that shakes the ground back and forth perpendicular to the direction the wave is moving.

The innermost part of the earth. The outer core extends from 2500 to 3500 miles below the earth's surface and is liquid metal. The inner core is the central 500 miles and is solid metal.

Creep is slow, more or less continuous movement occurring on faults due to ongoing tectonic deformation. Faults that are creeping do not tend to have large earthquakes.

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.