- Slides: 22
Warm Up 1. When an earthquake occurs, energy radiates in all directions from its source, which is called the ____. a. fault c. seismic center b. epicenter d. focus 2. The hypothesis that explains the release of energy during an earthquake is called the ____. a. moment magnitude hypothesis b. elastic rebound hypothesis c. Richter hypothesis d. vibration hypothesis 3. Most earthquakes are produced by the rapid release of which kind of energy stored in rock subjected to great forces? a. thermal c. chemical b. elastic d. mechanical Answers: 1) d. 2) b. 3) b.
Measuring Earthquakes Chapter 8, Section 2
Seismographs • Seismographs – instruments that record earthquake waves • When waves from an earthquake come into contact with a seismograph, a weight suspended from a support will remain motionless as Earth moves (providing a reference point) and can draw a diagram showing Earth’s motion • Seismograms – the electronically recorded ground motion from a seismograph
Earthquake Waves • Surface Waves – seismic waves that travel along Earth’s surface, most destructive seismic waves • Surface waves travel along the ground and cause the ground anything resting upon it to move • P waves – push-pull waves; they push (compress) and pull (expand) rocks in the direction the waves travel • S waves – shake the particles at right angles to their direction of travel • Gases and liquids do not transmit s waves, but do transmit p waves • A seismogram shows all three types of waves: the p waves arrive first, then the s waves, followed by the surface waves last
Concept Check • Which seismic wave travels fastest? • P waves
Locating an Earthquake • We can determine the distance to an epicenter by finding the difference between the arrival of p waves and s waves , then looking at a traveltime graph we can determine how far away the epicenter is • Travel-time graphs from three or more seismographs can be used to find the exact location of an earthquake epicenter • About 95% of earthquakes occur in a few narrow zones, and most of these occur around the Pacific Ocean
Finding an Epicenter
Global Distribution of Earthquakes
Concept Check • Where do most earthquakes occur? • Along the edge of the Pacific Ocean
Measuring Earthquakes • Historically, scientists have used two different types of measurements to describe the size of an earthquake – intensity and magnitude • Richter Scale – outdated scale for measuring the magnitude of earthquakes, uses amplitude of the largest seismic wave and uses a logarithmic scale (ten times increase for every increase of 1 on the scale) • Moment Magnitude – derived from the amount of displacement that occurs along a fault zone (surface area of fault) x (avg. displacement along fault) x (rigidity of rock) • Moment magnitude is the most widely used measurement for earthquakes because it is the only magnitude scale that estimates the energy released by earthquakes
Concept Check • Which scale is the most widely used for measuring an earthquake’s magnitude? • Moment Magnitude
Modified Mercalli Scale • Modified Mercalli Scale – rates an earthquake’s intensity in terms of the earthquake’s effects at different locations • It has 12 steps, expressed as roman numerals • An earthquake that can barely be felt is rated as a I • An earthquake that causes near total damage is rated as a XII • The same earthquake can be given different ratings at different locations
Modified Mercalli Scale I. Instrumental Not felt except by a very few under especially favorable conditions. II. Feeble Felt only by a few persons at rest, especially on upper floors of buildings. Delicately suspended objects may swing. III. Slight Felt quite noticeably by persons indoors, especially on the upper floors of buildings. Many do not recognize it as an earthquake. Standing motor cars may rock slightly. Vibration similar to the passing of a truck. Duration estimated. IV. Moderate Felt indoors by many, outdoors by few during the day. At night, some awakened. Dishes, windows, doors disturbed; walls make cracking sound. Sensation like heavy truck striking building. Standing motor cars rocked noticeably. Dishes and windows rattle. V. Rather Strong Felt by nearly everyone; many awakened. Some dishes and windows broken. Unstable objects overturned. Clocks may stop. VI. Strong Felt by all; many frightened and run outdoors, walk unsteadily. Windows, dishes, glassware broken; books off shelves; some heavy furniture moved or overturned; a few instances of fallen plaster. Damage slight. VII. Very Strong Difficult to stand; furniture broken; damage negligible in building of good design and construction; slight to moderate in well-built ordinary structures; considerable damage in poorly built or badly designed structures; some chimneys broken. Noticed by persons driving motor cars. VIII. Destructive Damage slight in specially designed structures; considerable in ordinary substantial buildings with partial collapse. Damage great in poorly built structures. Fall of chimneys, factory stacks, columns, monuments, walls. Heavy furniture moved. IX. Ruinous General panic; damage considerable in specially designed structures, well designed frame structures thrown out of plumb. Damage great in substantial buildings, with partial collapse. Buildings shifted off foundations. X. Disastrous Some well built wooden structures destroyed; most masonry and frame structures destroyed with foundation. Rails bent. XI. Very Disastrous Few, if any masonry structures remain standing. Bridges destroyed. Rails bent greatly. XII. Catastrophic Total damage - Almost everything is destroyed. Lines of sight and level distorted. Objects thrown into the air. The ground moves in waves or ripples. Large amounts of rock may move.
Some Notable Earthquakes
Assignment • Read Chapter 8, Section 2 (pg. 222 -228) • Do Chapter 8 Assessment #1 -29 (pg. 243 -244) • For Section 2: Do #’s 4, 5, 9 -12, 18, 19, 22, & 24 -29