P wave:

Also called primary, longitudinal, irrotational, push, pressure, dilatational, compressional, or push-pull wave. P waves are the fastest body waves and arrive at stations before the S waves, or secondary waves. Their velocity in the crust varies between 5.0 and 7.0 km/s. The waves carry energy through the Earth as longitudinal waves, moving particles in the same line as the direction of the wave. P waves can travel through all layers of the Earth. P waves are generally felt by humans as a bang or thump.

Paleoseismology:

The study of ancient (prehistoric) earthquakes from their geological evidences.

Passive margin:

Continental margin formed during initial rifting apart of continents to form an ocean; frequently has thick sedimentary deposits.

Period (wave):

The time interval between successive crests in a sinusoidal wave train; the period is the inverse of the frequency of a cyclic event.

Phase: (seismic)

The onset of a displacement or oscillation on a seismogram, indicating the arrival of a different type of seismic wave.

Plates and plate tectonics:

The crust and upper mantle of the earth are made up of about a dozen large plates and several smaller ones that are constantly moving. The movements are very slow - only a few centimetres per year. Where the plates rub against one another, strain builds up, especially at the edges. When the strength of the rock is exceeded, the earth's crust may break and suddenly shift by several metres, causing an earthquake.

Precursor:

A change in the geological or geophysical conditions that is a forerunner to earthquake generation on a fault. Precursors cannot reliably be recognized as such beforehand.

Prediction (of earthquakes):

The forecasting in time, place, and magnitude of an earthquake; the forecasting of strong ground motions. Currently, there is no reliable method of predicting earthquakes.

Probability:

The number of cases that meet a given description divided by the total number of (equally likely) cases possible.

Probability of exceedence of a given earthquake size:

The odds that the size of a future earthquake will exceed some specified value.

Accelerograph:

A type of seismograph used to measure ground acceleration as a function of time.

Active fault:

A fault along which slip has occurred in recent geological time, or where earthquake foci are located.