Microearthquake:

An earthquake having a magnitude of 2 or less on the Richter scale.

Microseism:

A more or less continuous motion in the Earth that is unrelated to an earthquake and that has a period of 1.0 to 9.0 seconds. It is caused by a variety of natural and artificial agents.

Microzonation:

The division of a town or county into smaller areas according to their variation in seismic hazard.

Moderate earthquake:

An earthquake having a magnitude of 5 to 6 on the Richter scale.

Modified Mercalli Intensity Scale:

The Mercalli scale rates the intensity of shaking from an earthquake. The ratings vary from I (felt only under especially favourable circumstances) to XII (total destruction).

Mohorovicic discontinuity (the Moho):

The boundary surface or sharp seismic-velocity discontinuity (pronounced Mo-ho-ro-vi-chich) that separates the Earth's crust from the underlying mantle. Named for Andrija Mohorovicic, the Croatian seismologist who first suggested its existence. Under Eastern Canada, it can be found at around 40 km depth.

Moment (of earthquakes):

A measure of earthquake size related to the leverage of the forces (couples) across the area of the fault slip. The rigidity of the rock times the area of faulting times the amount of slip. Dimensions are dyne-cm (or Newton-metres).

Moment magnitude (MW):

Magnitude of an earthquake estimated by using the seismic moment.

Normal fault:

A dip-slip fault in which the rock above the fault plane has moved downward with respect to the rock below.

Oblique faulting:

The slip on the fault has components both along the dip and along the strike of the fault.

Origin time:

The exact time at which an earthquake occurred.

Outer core:

Outer liquid shell of the Earth's core, probably iron with some oxygen; inner radius, 1221 kilometres, outer radius, 3480 kilometres.