Chapter 8 Earthquakes

About 3 million earthquakes w/1+ magnitude occur every year—about 1 every 10 seconds! Highest recorded: 9.5 on the Richter scale in Chile in 1960! Seismology: study of earthquakes The majority of quakes occur: in earthquake zones where a large # of faults are located along tectonic plates

What causes earthquakes? deformation: change in shape of rock in response to stress -elastic deformation: rocks are stretched & break releasing energy which leads to an earthquake -elastic rebound: sudden return of elastically deformed rock to its original shape (This is the CAUSE OF AN EARTHQUAKE)

  Plate Motion at Major Fault Type Tectonic Boundaries Transform boundary Strike-slip Fault Convergent boundary Reverse Fault Divergent boundary Normal Fault

BODY WAVE SURFACE WAVE S Waves (shear waves) Surface Waves P Waves (pressure waves) S Waves (shear waves) Surface Waves -AKA primary waves secondary waves -travel thru solids, liquid, & gas -don’t travel thru liquids -travel: affect upper few kilometers of Earth’s surface -fastest & 1st detected -2nd fastest -speed: travel slowly -most destructive -moves: back & forth (squeezed & stretched) side to side -1st type moves: up & down, around -2nd type moves:

Locating Earthquakes epicenter: point on surface above quake’s starting point focus: point inside Earth where a quake begins

Determining Time & Location of Quakes S-P Time Method—used to find epicenter Step 1: collect several seismograms from same quake from different locations Step 2: place seismograms on a time-distance graph Step 3: line up the first P-wave seismogram tracing w/the P-wave time-distance curve Step 4: line up the first S-wave tracing w/S-wave time-distance curve Step 5: find distance of each station from quake by reading horizontal axis on graph Step 6: after finding distances, can locate the quake’s epicenter.

Locating Earthquakes seismographs: instrument at or near Earth’s surface that records seismic waves on a seismogram seismogram: tracing of quake motion

Measuring Quake Strength & Intensity magnitude: measure of strength of quake The Richter Scale: scale that measures ground motion from quake & adjusts for distance to find its strength intensity: measure of degree a quake is felt & amount of damage caused—use the Modified Mercalli scale

Magnitude Estimated Effects 2.0 3.0 4.0 5.0 6.0 7.0 Only detected by seismograph 3.0 Felt at epicenter 4.0 Felt by people in area 5.0 Damage at epicenter 6.0 Widespread damage 7.0 Great, widespread damage

Modified Mercalli Intensity Scale numerical scale from I-XII--describes increasing quake intensity levels Level I-----not felt by most people to Level XII-----total damage of an area

Earthquakes and Society Predicting when, where, & at what strength earthquakes will occur is difficult. Earthquake Hazard: measurement of how likely an area is to have damaging future quakes

To decrease earthquake damage, buildings, homes, and bridges can be strengthened to decrease damage. technology sensors in roof that control building shift to counteract movement steel cross braces flexible pipes base isolators to absorb wave energy Safeguard homes-in addition to strengthening house, heavier objects on floor

Earthquakes and Buildings Retrofitting: making older buildings quake resistant

New technology for earthquake resistant buildings -mass damper: motion sensors in roof that detects movement & send messages to a computer which signals controls in roof to shift the mass damper to counteract the building movement -steel cross braces: braces between floors to counteract pressure that pushes & pulls at side of building during quake -active tendon system: sensors in roof that indicate the building is moving -flexible pipes: pipes w/flexible joints that twist & bend w/out breaking -base isolators: layers of rubber & steel wrapped around a lead core that absorb waves & prevent them from traveling thru buildings

Preparing for an Earthquake Before an earthquake: 1. safeguard your home (heavier objects on bottom shelves 2. if possible, strengthen your home 3. find safe places w/in each room 4. make a plan to meet in a safe place 5. store water, food, fire extinguisher, flashlight, batteries, radio, medicine, & first-aid kits

When the shaking starts: 1. if indoors, lie face down under a table 2 When the shaking starts: 1. if indoors, lie face down under a table 2. if outdoors, lie face down away from danger 3. if in a car, stop, remain inside After the shaking stops: 1. stay calm & get your bearings 2. remove yourself from danger 3. go home if told it is safe 4. beware of aftershocks & more damage