planet earth earthquake_powerpoint_presentaion
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- 1.Earthquakes and Earths Interior This 4-floor apartment building pancaked during the 1999 Izmit earthquake in Turkey, killing all occupants. The thousands of deaths and billions of dollars in damage from this event underscore the fact that earthquakes are one of the deadliest natural disasters faced by mankind.
2. Earthquakes are vibrations of the earth caused by the rupture and sudden movement of rocks that have been strained (deformed) beyond theirelasticlimit. Earthquakes occur along faults .Faultsare breaks in the lithosphere where regions of rock move past each other. Most major faults occur alongtectonic plate boundaries . Thefocusis the point on the fault where the rupture begins.Theepicenteris the point on the earths surface directly above the focus.When the fault ruptures,wavesof energy spread out in all directions. 3. Elastic rebound theorystates that the waves of energy from an earthquake result from the sudden release of stored up strain energy in rock as it deforms. Much like a rubber band stretched past its breaking point, the rock on either side of a fault snaps suddenly to a new position, releasing energy in the process. 4. Types of Faults The majority of earthquakes (90%) are caused by rocks rupturing in response to tectonic stresses at active plate margins. 5. Faults can be divided depending on the direction of relative displacement.There are 2 main categories.
- Dip-slip faults- where the displacement is vertical
Relative displacement is largely a function of the type of tectonic stress the rock is under.
- Strike-slip faults- where the displacement is horizontal.
Types of Faults 6. Types of Tectonic Stress 7. Dip-Slip Faults - Normal Faults
- Normal faultsresult fromtensional stressesalongdivergent boundaries .
- The hanging wall block moves down relative to the footwall block.
- Low Richter magnitudes due to the tendency of rocks to break easily under tensional stress.
- Shallow focus (less than 20 km) because the lithosphere is relatively thin along diverging plate boundaries.
Examples - all mid-ocean ridges; Continental Rift Valleys such as the basin and range province of the Western U.S. and the East African Rift Valley 8. Dip-Slip Faults - Reverse Faults
- Reverse faultsare faults that result from horizontal compressional stresses where the hanging wall block has moved up relative to the footwall block.
- Reverse faulting occurs along convergent boundaries.
There are two types of converging plate boundaries. 1. Subduction boundaries where oceanic lithosphere is pushed beneath either oceanic or continental lithosphere. 2.Collision boundaries where two plates with continental lithosphere collide. 9. Subduction Boundaries
- At subduction boundaries there is a continuum of stress along the subducting plate.Shallow focus earthquakes can be generated near the trench, but focal depths can reach down to 700 km as earthquakes are generated along the subducting plate.
- Rocks are strong under compression and can store large amounts of strain energy before they rupture.Therefore, these earthquakes can be very powerful.
- 1960 Southern Chili = 9.5
- 1964 Alaska = 9.2
10. Collision Boundaries
- At collision boundaries two plates of continental lithosphere collide resulting in fold-thrust mountain belts.
- Earthquakes occur due to the thrust faulting and range in depth from shallow to about 200 km.
Example:The Himalayas from the collision of India with Asia 11. Strike-Slip Faults - Transform Faults
- Strike-slip faults where the relative motion on the fault has taken place along a horizontal direction due to shear stresses acting on the lithosphere.
- Can be right lateral or left lateral.
- Earthquakes along these boundaries tend to be shallow focus with depths usually less than about 100 km.Richter magnitudes can be large.
12. Body wavestravel through the interior (body) of the earth as they leave the focus.They includeP-wavesandS-waves .P - waves Primary waves P ush- p ull waves S waves Secondary wavesS hearwavesEarthquake Seismic Waves 13. Earthquake Seismic Waves Surface wavestravel parallel to the earths surface.They are the slowest and most damaging.They includeLoveandRayleigh Waves . Love Waves- complex, horizontal motion Rayleigh Waves- Rolling or elliptical motion. 14. Seismographsare instruments that detect and record ground shaking produced by earthquake waves.Due to their different speeds, the different waves arrive at the seismograph at different times: first P-waves arrive, then S-waves, then surface waves. 15. Seismogram - the record of an earthquake as recorded by a seismograph.It is a plot of vibrations versus time. 16. Remember: P-waves are faster than S-waves.Therefore, as this graph shows,the time gap between their arrival at a seismograph increases precisely with distance from the quake. Therefore, lag time is proportional to distance traveled. For example, in the graph here we see that a time gap of 30 seconds between P- and S- corresponds to a distance of about 340 kilometers (210 miles) from the quake. 17. We can use thelag timebetween the P-waves and S- waves to calculate thedistance to an earthquake ! If we do this for a minimum ofthree different seismic stations , we can precisely locate the epicenter. In the figure, each circle has a radius equal to the distance to the earthquake from three separate seismic stations. The three circles intersect at only one point -- the epicenter! 18. Earthquake Measurement Richter Magnitude scale - M L ; based on thehighest amplitude wavemeasured on a seismogram, corrected for distance from the seismograph to the epicenter - ranges from 1.0 (smallest) to infinity, but 9.0 is typically the highest possible value for an earthquake. - logarithmic scale: each whole unit on the Richter scale represents a ten-fold increase in wave amplitude (ground shaking) and an ~ thirty fold increase in the energy released. Below:The magnitude of the earthquake can be estimated using an earthquake nomograph, on which a straight line is plotted between the P-S time (distance) and the maximum wave amplitude.This line intersects the central line at the approximate magnitude of the earthquake. 19. Earthquake Measurement Modified Mercalliscale - based on peoples reportedperceptions of shaking , and thetype and extent of damage produced - ranges from I (not felt by people) to XII (catastrophic destruction) An example of the Modified Mercalli scale follows in the next slide. 20. A map of Modified Mercalli intensity for the 1994 Northridge, California, earthquake. 21. Not all fault movements result in violent earthquakes. Some faults move slowly and fairlycontinuously, a movement calledfault creep .Fault creep never killed anyone, but, as shown in these pictures, it can cause damage to roads or other structures. 22. Earthquake Hazards and Mitigation
- Now that you are familiar with some important concepts related to earthquakes and their measurement, we shall now consider the specific types of hazards generated by earthquakes, and the specific steps people can take to mitigate (reduce) those hazards.
- The hazards we will review are:
- ground shaking
- uplift or subsidence of land
23. Ground Shaking An old saying among geologists is thatearthquakes dont kill people, buildings do.The vast majority of deaths in earthquakes occur when ground shaking from earthquake waves (particularly S-waves and surface waves) causes buildings or other structures collapse, killing the people inside.Most damage and collapse of structures like buildings, bridges, and roads occurs due tosideways movementof the ground from earthquake waves. This process is calledhorizontal ground acceleration , orbase shear . 24. Base shear causes the building to deform from a rectangle into a parallelogram, causing damage such as that shown in the photos right. Base shear causes buildings constructed on so-called cripples to fall sideways, causing damage such as that shown in the photos left. 25. The most deadly type of failure from base shear is story-shift, in which the sideways acceleration causes floors to shift and collapse onto one another -- a situation calledpancaking . Few or no occupants survive such collapses. 26. In addition to buildings,highway overpasses, bridges, and multi-decked freewaysalso suffer major damage from base shear. The photo shows the collapse of a double-deck freeway from the 1994 Northridge, CA, quake. 27. Collapse of the Hanshin expressway from the 1995 Kobe, Japan, quake. Collapse of freeways is most commonly caused byfailure of the concrete supporting columns , as the photo shows. 28. The Worst Earthquake of the Twentieth Century
- On July 28, 1976, at 3:45 a.m., while 1 million inhabitants of Tang Shan, China, slept, a 7.8 magnitude quake leveled the city.Hardly a building was left standing, and the few that did withstand the first quake were destroyed by a second, magnitude 7.1, which struck at 6:45 p.m. the same day.When the wreckage was cleared, 240,000 people were dead.Losses were large because most of the buildings had not been constructed to withstand an earthquake.They had unreinforced brick walls.When the ground started to shake, the walls collapsed, the roofs caved in, and the sleeping inhabitants were crushed.
29. Building practices make a huge difference in quake survival! The bar graph shows us the connect