Measurement/Classification:

The Richter Magnitude Scale:
3.0-3.5   Minor
4.0-4.9   Light
5.0-5.9   Moderate
6.0-6.9   Strong - Damage in an area 100 mi. across
7.0-7.9   Major  - Serious damage over a large area
8.0-8.9   Great  - Major damage for several hundred miles
9.0-9.9   Great  - Major damage for thousands of miles

The Richter magnitude scale, also known as the local magnitude (M_L) scale, assigns a number computed on a base-10 logarithmic scale of the horizontal amplitude of the largest displacement, so the shaking amplitude of a magintude 8 quake is 10 times larger than a magnitude 7 quake.
Each whole number step in the magnitude scale corresponds to the release of about 31 times more energy than the amount associated with the preceding whole number value.

Though still widely used, the Richter scale has been superseded by the moment magnitude scale, which gives generally similar values.

What are the earthquake magnitude classes? at USGS
The Richter Magnitude Scale at USGS
Richter magnitude scale at Wikipedia

Intensity

Richter Magnitude	Expected Modified Mercalli Maximum Intensity (MMI) (at epicenter)
2	I-II	Usually detected only by instruments
3	III	Felt indoors
4	IV-V	Felt by most people; slight damage
5	VI-VII	Felt by all; many frightened and run outdoors; damage minor to moderate
6	VII-VIII	Everybody runs outdoors; damage moderate to major
7	IX-X	Major damage
8+	X-XI	Total and major damage

Source: California Geological Survey     See Intensity Map for Loma Prieta earthquake

Terms:

dip-slip fault - A fault where the relative movement is approximately vertical. They can be normal where the crust is extended so one side drops relative to the other or reverse (thrust) where one side is pushed into the other causing it to rise.

FEMA - Federal Emergency Management Agency
IGPP - Institute of Geophysics and Planetary Physics

M_W (moment magnitude)

The magnitude calculated from an earthquake's total energy (seismic moment). The seismic moment is a function of the amount of slip on a fault, the area of the fault that slips, and the average strength of the rocks that are faulted. Because MW is directly related to the energy released by an earthquake, it is a uniform means of measuring earthquake magnitude and has become the standard measure of earthquake magnitude in modern seismology.

M_S (surface-wave magnitude)
The magnitude of an earthquake determined from surface waves on a seismogram from a teleseismic earthquake (one located more than 20¡ away). Surface waves are seismic waves that travel over the surface of the Earth versus than those that travel through the Earth, such as P-waves and S-waves. MS magnitudes are measured from surface waves that have a period of about 20 seconds.

M_L (local magnitude)
A numerical calculation that defines the strength (magnitude) of an earthquake based on seismograms from stations within 600 km; also commonly known as the Richter magnitude. As initially defined by Charles Richter, ML represented the largest deflection of the needle on a standard seismograph at a distance of 100 km from the epicenter of a shallow earthquake that was recorded in southern California.

NEHRP - National Earthquake Hazards Reduction Program
NSMN - USGS National Strong-Motion Network
NSMP - National Strong-Motion Project
SCEC - Southern California Earthquake Center

strike-slip fault - A fault where the relative movement is approximately horizontal. Most of the San Andreas fault is strike-slip.

thrust fault or reverse fault - A type of dip-slip fault where one area is pushed into another causing it to rise. The 1983 M6.4 Coalinga earthquake was caused a thrust fault.

UCERF - Uniform California Earthquake Rupture Forecast
USGS - U.S. Geological Survey

What do do during an Earthquake :

• If you're indoors, stay there. Get under -- and hold onto --a desk or table, or stand against an interior wall. Stay clear of exterior walls, glass, heavy furniture, fireplaces and appliances. The kitchen is a particularly dangerous spot. If you're in an office building, stay away from windows and outside walls and do not use the elevator.
• If you're outside, get into the open. Stay clear of buildings, power lines or anything else that could fall on you.
• If you're driving, move the car out of traffic and stop. Avoid parking under or on bridges or overpasses. Try to get clear of trees, light posts, signs and power lines. When you resume driving, watch out for road hazards.
• If you're in a mountainous area, beware of the potential for landslides.
  Likewise, if you're near the ocean, be aware that tsunamis are associated with large earthquakes. Get to high ground.
• If you're in a crowded public place, avoid panicking and do not rush for the exit. Stay low and cover your head and neck with your hands and arms.

What to do in an Earthquake (preparation, during, after)

Earthquakes at PhysicalGeography.net

The Severity of an Earthquake at USGS

USGS Earthquake Glossary
 
    Visual GLossary
    Glossary of Earthquake and Related Terminology
     

Earthquakes
last updated 6 July 2009