EARTHQUAKES How are they measured? What are they?.

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Presentation transcript:

EARTHQUAKES How are they measured? What are they?

Recent World Earthquake Activity

What is an earthquake? EQ's are the release of energy built up in Earth’s crust. Most occur along fault lines (remember yesterday?) Friction prevents the crust from moving as stress builds up. When the stress overcomes the friction, movement occurs…

Measuring Earthquakes (1) Mercalli Index Measure of intensity based on descriptions of structural damage and what is felt Scale of I to XII Often dependent on the proximity to EQ focus –One EQ will have many Mercalli scores Examples II. A few people might notice movement if they are at rest and/or on the upper floors of tall buildings. VI. Everyone feels movement. People have trouble walking. Objects fall from shelves. X. Most buildings and their foundations are destroyed. XII. Almost everything is destroyed. Objects are thrown into the air.

Measuring Earthquakes (2) Richter Magnitude Measures the energy released from an earthquake 1 – 10 scale Every EQ has a unique magnitude, but effects will vary due to distance, ground conditions, etc. Examples Less than 3.5 – Generally not felt, but recorded – Can be destructive in areas up to about 100 kilometers across where people live. 8 or greater – Great earthquake. Can cause serious damage in areas several hundred kilometers across.

How many… In a day? (choose one) In a day? That’s about one every ten seconds. But we don’t feel all of them, do we? No, most are smaller earthquakes.

DescriptorMagnitudeAnnual average Great8 or higher1 Major7– Strong6– Moderate5–5.91,319 2 Light4–4.9c. 13,000 Minor3–3.9c. 130,000 Very minor2–2.9c. 1,300,000 Frequency of Earthquakes Worldwide National Earthquake Information Center, U.S. Geological Survey.