Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Chapter 16 Earthquakes What is an earthquake? • Shaking of the ground caused by the sudden release of energy stored in the rocks. • When rocks are stressed, potential energy is stored in them. • Rocks can behave plastically upto a certain limit (elastic limit), after which they fail and release the energy. • Energy waves produced by an earthquake are called Seismic Waves. • The generation of an earthquake is explained by Elastic Rebound Theory. • According to this theory, the sudden release of progressively stored strain in rocks causes movement along a fault and an earthquake is generated. Causes of earthquakes • • • • Movement along a fault Volcanic eruptions Plate movement Subsidence Earthquake terminology • Focus: the point inside the earth where seismic waves first generate. • Epicenter: the point on ground surface directly above the focus. It may be any geographic locality. Seismic waves • 2 types –Body waves: traveling inside the earth –Surface waves: travel on the earth’s surface. • Body waves: –P-waves (primary) –S-waves (secondary) • P-wave: is the fastest (4-7 km/sec) and is the first to arrive at a recording station. • P-wave: compressional or longitudinal wave in which rocks vibrate back and forth parallel to the direction of wave propagation. • S-wave: slower (2-5 km/sec) and arrives after the P-wave at a recording station. • It’s a transverse wave in which the rock vibrates perpendicular to the direction of wave propagation. • Surface waves: –slowest waves generated by an earthquake. –They take longer to pass and cause more property damage. EQ measurement • The instrument used to measure an EQ is called a seismometer. • Seismograph: paper or electronic record of an EQ. World seismograph stations Locating EQ Epicenter • Travel time of the seismic waves from the focus to the seismograph station is used to determine the EQ epicenter. • P and S waves gradually separate because they travel at different velocities. • The P-S interval is compared with a standard travel-time curve. • Data from one station can be used to determine only distance to the station and not the direction. • At least three stations are required to determine the location of the epicenter. Measuring the size of an EQ • Two ways of determining the EQ size; –Intensity –Magnitude Measuring the size of an EQ • EQ Intensity: it is a measure of how an area has been damaged by the EQ. • Modified Mercalli Scale is used to measure intensity. • EQ magnitude: it’s a measure of the amount of energy released by an EQ. • EQ magnitude is reported on the Richter scale. • It is done by measuring the height (amplitude) of a specific wave. • ML = log10A(mm) + (Distance correction factor) Magnitude (log Scale) 1-2 3 4 5 6 7 8 9 Possible Effects Normally only detected by instruments Only faintly felt Faint tremor causing little damage Structural damage Distinct shaking, less well-constructed buildings collapse Most buildings destroyed Major structures destroyed Ground seems to shake-catastrophic Effects of EQ • Primary effects: occur immediately from ground shaking • Secondary effects: damage caused by; Effects of EQ –Floods –Fire –Landslides –Liquefaction: wet, saturated soil changes from solid to liquid as a result of shaking. –Tsunamis Global distribution of EQ • 3 belts; –Circum-Pacific belt –Mediterranean-Himalayan belt –Continental interior Mitigating EQ effects • • • • Avoiding EQ prone zones Foundation design Structural modifications Building codes Predicting EQ • Rock monitoring: rock properties begin to change before its failure. • Water levels: increase or decrease. • Radon emission: increases before an EQ. Predicting EQ • Seismic gap: regular pattern of EQ occurrence. • Animal behavior: snakes, dogs. Assignment 2 Rivers, groundwater, lakes Ohio EPA Great lakes, lake erie Relative proportions Maximum Contaminant Level Primary contaminants Secondary