Ch. 5 Earthquakes
I. Over millions of years, 3 types of stress change the shape & volume of rock. A. Tension – pulls or stretches crust 1. occurs where plates are moving apart. B. Compression – squeezes crust together 1. plates push against each other until rock folds or breaks. C. Shearing – rock slips in opposite directions
II. Most faults occur along plate boundaries. There are 3 kinds of faults: A. Normal fault 1. is at an angle 2. occurs where plates diverge (pull apart), resulting in tension 3. California’s Owens Valley
B. Reverse fault 1. Same structure as normal fault 2. movement is opposite 3. occurs where crust is compressed 4. Rocky Mountains & Klamath Mtns.
C. Strike-Slip fault 1. Rock on either side slips past each other 2. Little up or down motion 3. Occurs where crust is being sheared - at sliding boundaries 4. San Andreas fault
III. Over millions of years forces of plate movement change Earth’s surface by folding, stretching, and uplifting the crust. A. Folding - rock is compressed, causing it to bend, but not break. It folds over on itself 1. anticline – an upward fold (arch) 2. syncline – folds down; creates a valley 3. Himalayas (Asia); Alps (Europe)
B. Stretching – Tension pulls two plates away from each other creating normal faults 1. creates fault-block mountains 2. two faults form parallel to each other with a block in-between 3. plates spread; block rises 4. Basin and Range region of U.S.
C. Uplifting – areas of crust that are pushed upwards (mountains and plateaus) 1. Plateaus are large, flat blocks of rock that have been lifted 2. Colorado Plateau in Four Corners region (AZ, UT, CO, & NM)
An earthquake is shaking caused by sudden movement of crust along a fault. A. Earthquakes begin in lithosphere within about 60 miles of surface. B. Focus – area beneath surface where released stress creates an earthquake. C. Epicenter – spot on surface directly above focus.
V. Seismic Waves carry earthquake energy away from focus, through Earth’s interior, and across the surface. There are 3 types of Seismic Waves.
A. P Waves – Primary waves compress and expand the earth like an accordion. 1. fastest B. S Waves – Secondary waves vibrate up and down and side to side 1. slower than P waves C. Surface Waves – When P and S waves hit the surface, some become surface waves. 1. move more slowly (than P or S waves) 2. can produce severe ground movements 3. roll like ocean waves, or vibrate side to side
VI. Earthquakes are measured using 3 different methods A. Mercalli Scale 1. measures destructive strength (effect) at a certain place 2. gives lower ratings at different locations because intensity usually decreases farther away from epicenter
B. Richter Scale 1. rates magnitude based on seismic wave 2. best for measuring nearby earthquakes C. Moment Magnitude Scale 1. measures total energy released 2. useful for near and far away earthquakes
VII. Comparing earthquakes A. An earthquake’s magnitude tells how much energy was released B. For each number increase, the amount of ground shaking increases by a factor of 10 (10 • x) C. For each number increase, the amount of energy released increases by a factor of 30! (30 • x)
VIII. Geologists use seismic waves to locate an earthquake’s epicenter A. P waves travel faster and arrive at seismograph sooner than S waves B. seismologists measure the difference in arrival times to calculate distance to epicenter C. Geologists use several, distant seismographs to determine epicenter
IX. Geologists use various instruments to monitor earthquakes A. Seismograph 1. measures vibrations of the earth 2. a very heavy weight that hardly moves 3. drum vibrates underneath 4. pen records movement on drum’s paper
B. Tiltmeter 1. measures tilting/rising of ground 2. two bulbs connected by hollow tube 3. liquid flows into one bulb and is measured on a scale C. Creep meter 1. measures horizontal movement 2. wire stretched across fault line 3. movement of weight measured
D. Laser-ranging 1. measures horizontal movement 2. laser bounces off reflector 3. increases in distances measured very precisely E. GPS 1. measures horizontal & vertical movement 2. uses signals bounced off satellites to very precisely measure changes
X. Geologists use monitoring data to map faults and try to predict earthquakes A. Seismic waves bounce off faults allowing scientists to calculate its length & depth B. Along some faults, rocks lock together and build up great energy C. Geologists have monitored fault movements trying to predict earthquakes, but have been unsuccessful
XI. Geologists evaluate risk based on where faults are active, location of past earthquakes, & where most damage has occurred A. The most active faults are near plate boundaries B. Scientists use Mercalli scale maps to see which areas around faults suffer most damage C. Studying intensity maps and historic earthquakes helps estimate future damage
XII. Damage is caused by shaking, liquefaction, aftershocks, and tsunami A. Shaking caused by seismic waves damages structures, gas & water lines, utility poles, and triggers landslides 1. Loose soil shakes more violently than solid rock B. Liquefaction – soil turns into liquid mud
C. Aftershock – a smaller earthquake that strikes after a larger, main earthquake 1. occur hours, days, or months later D. Tsunami – water displaced by an earthquake that forms a large swell or wave 1. wave is low in open ocean 2. may rise into large wave in shallow water 3. can be extremely destructive
XIII. Enhancing earthquake safety A. If inside, cover 1. get under a heavy table or desk 2. crouch against an inner wall. Avoid windows & things that could fall on you B. If outside, move to open space 1. stay clear of buildings, power lines & trees C. Be prepared; have supplies on hand 1. water, canned/dried food, first-aid kit and knowledge to use it
D. Safer buildings & utilities 1. reinforce buildings & objects inside 2. base-isolated buildings 3. flexible joints on utility lines 4. automatic shut-off valves