What is an Earthquake?

Transcription

1 Earthquakes

2 What is an Earthquake? Earthquake - sometimes violent shaking of ground caused by movement of Earth s tectonic plates; creates seismic waves Often followed by smaller earthquakes (aftershocks); can occur for days/weeks Most take place in Pacific Ring of Fire Most caused by movement along faults Some caused by human activity

3 What is an Earthquake? Human activities that trigger quakes: Storing large amounts of water behind a dam Drilling and injecting liquid into wells Coal Mining Oil Drilling Nuclear Tests Fracking

4 What is Stress? Earth s plates are in constant motion Friction between plates can lock a fault Continued relative motion between plates leads to increasing stress build up Stress - total force acting on crustal rocks per unit of area Fault ruptures - releases stored energy

5 Types of Faults Fault - any fracture or system of fractures along which Earth moves 3 types Reverse (thrust) - compression (crust shortened) causes horizontal and vertical movement - common at convergent boundaries Most powerful earthquakes (megathrust)

6 Types of Faults Normal - tension (crust extended) causes horizontal and vertical movement - common at divergent boundaries Major earthquakes (up to magnitude 8) Strike-slip - shear causes horizontal movement - common at transform boundaries Earthquakes with magnitude less than 7

7 Earthquake Depth Shallow Focus - depth < 70km Mid Focus - depth 70km - 300km Deep Focus - depth 300km - 700km Benioff Zone - seismically active subduction zone where older/colder oceanic crust descends beneath another plate Shallower = more damage

8 Earthquake Waves Seismic Waves - vibrations of ground during an earthquake 3 types

9 P-Wave Primary Waves - squeeze and push rocks in direction along which waves are traveling Volume of rock changes length as a P-wave passes through it Compressional movement

10 S-Wave Secondary Waves - second set of waves felt Motion causes rocks to move at right angles in relation to direction of waves Both P-waves and S-waves pass through Earth s interior - body waves

11 Surface Wave Surface Waves - travel along Earth s surface Cause ground to move sideways and up & down Cause most destruction because they cause most movement of ground, and take longest time to pass

12 Locating Earthquake Focus - point where first body waves generated by earthquake originate Usually several km below Earth s surface Epicenter - point on Earth s surface directly above focus Surface waves originate from epicenter and spread out

13 Locating Earthquakes Travel times for P and S waves used to pinpoint location 3+ seismic stations needed P and S waves change speed and direction when they encounter different materials

14 Measuring Earthquakes Seismometers - sense and record movements in Earth s surface Show intensity and duration of earthquakes Richter Scale - rates earthquakes on amount of energy released No accurate for large magnitude earthquakes Based on amplitude of wave and distance to epicenter

15 Earthquake Magnitude Moment Magnitude Scale - measure of energy released during an earthquake Based on rigidity of Earth times average amount of slip on fault times size of area that slipped Each increase in magnitude corresponds to a 32-fold increase in seismic energy

16 Earthquake Intensity Mercalli Scale - uses observations of people who experienced the earthquake to estimate its intensity

17 Effects of Earthquakes Damage to structures and property Falling debris, fire, injury, loss of life Trigger natural disasters: Avalanches and Landslides Surface Faulting Ground Rupture Tsunamis Liquefaction

18 Tsunami Tsunami - long-wavelength, long-period sea waves produced by sudden movement of large volumes of water - underwater earthquakes km/hr for thousands of km s Caused by earthquakes of magnitude 7.5+ Massive flooding

19 Liquefaction Liquefaction - water saturated material temporarily loses its strength and transforms from a solid to a liquid Causes rigid structures (buildings/bridges) to tilt or sink Flash floods

20 Predicting Earthquakes Can t predict Can estimate probability of a quake in next few decades along well- understood faults Earthquake warning systems - provide notification before ground surface has begun to move

21 Preparing for Earthquakes Know the risks Make a plan Get a kit