Transcript EARTHQUAKES
What are they? EARTHQUAKES How are they measured? 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. • 6.1-6.9 – 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? • In a day? 80 80 800 8000 (choose one) 800 8000 • That’s about one every ten seconds. • But we don’t feel all of them, do we? No, most are smaller earthquakes. Frequency of Earthquakes Worldwide Descriptor Magnitude Great 8 or higher Major 7–7.9 Annual average 1 17 1 2 2 Strong 6–6.9 134 Moderate 5–5.9 1,319 Light 4–4.9 c. 13,000 Minor 3–3.9 c. 130,000 Very minor 2–2.9 c. 1,300,000 National Earthquake Information Center, U.S. Geological Survey. 2