PLANET EARTH

Fact Sheet 01: Frequency of Occurrence of Earthquakes

Descriptor	Magnitude	Average Annually
Great	8 and higher	1 ¹
Major	7 - 7.9	17 ²
Strong	6 - 6.9	134 ²
Moderate	5 - 5.9	1319 ²
Light	4 - 4.9	13,000 (estimated)
Minor	3 - 3.9	130,000 (estimated)
Very Minor	2 - 2.9	1,300,000 (estimated)
¹ Based on observations since 1900. ² Based on observations since 1990.

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Fact Sheet 02: Year-wise description of Earth Quakes

Number of Earthquakes Worldwide for 2000 - 2005. Located by the US Geological Survey National Earthquake Information Center
Magnitude	2000	2001	2002	2003	2004	2005
8.0 to 9.9	1	1	0	1	2	1
7.0 to 7.9	14	15	13	14	14	9
6.0 to 6.9	158	126	130	140	140	116
5.0 to 5.9	1345	1243	1218	1203	1509	1307
4.0 to 4.9	8045	8084	8584	8462	10894	10264
3.0 to 3.9	4784	6151	7005	7624	7937	5782
2.0 to 2.9	3758	4162	6419	7727	6317	3249
1.0 to 1.9	1026	944	1137	2506	1344	20
0.1 to 0.9	5	1	10	134	103	0
No Magnitude	3120	2938	2937	3608	2939	642
Total	22256	23534	27454	31419	* 31199	* 21390
Estimated Deaths	231	21357	1685	33819	284010	1957

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What is an earthquake and what causes them to happen?

An earthquake is caused by a sudden slip on a fault. Stresses in the earth's outer layer push the sides of the fault together. Stress builds up and the rocks slips suddenly, releasing energy in waves that travel through the earth's crust and cause the shaking that we feel during an earthquake. An EQ occurs when plates grind and scrape against each other. In California there are two plates the Pacific Plate and the North American Plate. The Pacific Plate consists of most of the Pacific Ocean floor and the California Coast line. The North American Plate comprises most the North American Continent and parts of the Atlantic Ocean floor. These primary boundary between these two plates is the San Andreas Fault. The San Andreas Fault is more than 650 miles long and extends to depths of at least 10 miles. Many other smaller faults like the Hayward (Northern California) and the San Jacinto (Southern California) branch from and join the San Andreas Fault Zone. The Pacific Plate grinds northwestward past the North American Plate at a rate of about two inches per year. Parts of the San Andreas Fault system adapt to this movement by constant "creep" resulting in many tiny shocks and a few moderate earth tremors. In other areas where creep is NOT constant, strain can build up for hundreds of years, producing great EQs when it finally releases.

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