Chapter 15. Earthquakes and Plate Tectonics. what s the connection? At the boundaries friction causes plates to stick together.

Transcription

1 Chapter 15 Earthquakes and Plate Tectonics what s the connection? As with volcanoes, earthquakes are not randomly distributed over the globe At the boundaries friction causes plates to stick together. Figure showing the distribution of earthquakes around the globe Figure showing the distribution of earthquakes around the globe Earthquakes occur when built up energy causes them to break apart. 1

2 Where Do Earthquakes Occur and How Often? ~80% of all earthquakes occur in the Ring of Fire (most of result from convergent margin activity) ~15% occur in the Mediterranean-Asiatic belt remaining 5% occur in the interiors of plates and on spreading ridge centers more than 150,000 quakes strong enough to be felt are recorded each year S.2 Where do earthquakes form? Figure showing the tectonic setting of earthquakes Where do earthquakes form? ADD ARROWS TO DIAGRAM What is the Elastic Rebound Theory? Explains how energy is stored in rocks Rocks bend until the strength of the rock is exceeded Rupture occurs and the rocks quickly rebound to an undeformed shape Energy is released in waves that radiate outward from the fault S.3 Figure showing the tectonic setting of earthquakes S.4 2

3 What is an Earthquake? Ground movement caused by the sudden release of seismic energy due to tectonic forces. The Focus and Epicenter of an Earthquake The point within Earth where faulting begins is the focus or hypocenter The point directly above the focus on the surface is the epicenter S.5a S.5b Fault Traces Southern Vancouver Island Fault Traces Southern Vancouver Island Wrangellia Pacific Rim Terrane Crescent Terrane 3

4 What are Seismic Waves? Response of earth material to the arrival of energy fronts released by rupture Two types: Body waves P and S Surface waves R and L Body Waves: P and S waves S.6 P or primary waves fastest waves travel through solids, liquids, or gases compressional wave, material movement is in the same direction as wave movement S or secondary waves slower than P waves travel through solids only shear waves - move material perpendicular to wave movement Two Types of Body Waves P Waves P-Waves S Waves Two Types of Body Waves S-Waves 4

5 Surface Waves: R and L waves Two Common Types of Surface Waves Surface Waves Travel just below or along the ground s surface Slower than body waves Rolling and side-to-side movement Especially damaging to buildings S.7 Functioning of Seismograph How is an Earthquake Recorded? Seismographs record earthquake events on a seismogram. S.8 At convergent boundaries, focal depth increases along a dipping zone called a Benioff zone 5

6 How is an Earthquake s Epicenter Located? Using Seismic Wave Behavior P waves arrive first, then S waves, then L and R Average speeds for all these waves is known The difference in arrival times at a seismograph station can be used to calculate the distance from the seismograph to the epicenter. Earthquake Waves: Interpreting a Seismogram WS See Chapter 15 Worksheet Package 1. Identify (i)the first P-waves (ii)the first S-waves (iii)the first L-waves 2. Describe (i)p-waves (ii)s-waves (iii)l-waves 3. Which type of earthquake/seismic waves (i)will not pass through the earth's outer core? (ii)cause the most damage to surface buildings? (iii)pass through all the earth's internal zones? 4. What is the difference in arrival times for the P and S-waves? S.9 How is an Earthquake s Epicenter Located? How is an Earthquake s Epicenter Located? Time-Travel Graphs Show the average travel times for P-waves and S-waves. The farther away a seismograph is from the focus of an earthquake, the longer the interval between the arrivals of the P- and S- waves. S.10 6

7 How is an Earthquake s Epicenter Located? How is an Earthquake s Epicenter Located? Map of Epicenter, using Time-Travel differences of P-waves and S-waves Three seismograph stations are needed to locate the epicenter of an earthquake A circle where the radius equals the distance to the epicenter is drawn The intersection of the circles locates the epicenter S.11a S.11b Magnitude < > 8.0 Earthquake Magnitude Earthquake Effects Usually not felt, but recorded Often felt, only minor damage Slight damage to buildings and other structures May cause a lot of damage in very populated areas Major earthquake. Serious damage. Great earthquake. Can be totally destructive near the epicentre. Approx. number each year 900,000 30, every 5-10 years Can Earthquakes be Predicted? Earthquake Depth and Plate Tectonic Setting S.12a 7

8 Can Earthquakes be Predicted? Earthquake Depth and Plate Tectonic Setting Can Earthquakes be Predicted? Earthquake Prediction Programs include laboratory and field studies of rocks before, during, and after earthquakes monitor activity along major faults produce risk assessments See Chapter 15 Worksheet Package S.12b Can Earthquakes be Predicted? Earthquake Precursors: changes in elevation or tilting of land surface fluctuations in groundwater levels, magnetic field, electrical resistance of the ground seismic dilatancy model seismic gaps S.13 Earthquake Hazards These are important hazards to understand: EQ s commonly strike without warning no time for evacuation not a predictable trend to number of earthquakes, their magnitude or location 1000's of large earthquakes every year ~ 20 are > M7.0 and these account for 90% of the energy released and 80% of all the fatalities EQ Tsunamis and building collapse kills the highest number of people per year (> 1 million during the past century) S.14 8

9 What are the Destructive Effects of Earthquakes? Ground Shaking - amplitude, duration, and damage increases in poorly consolidated rocks Building collapse Fire Tsunami Ground failure Economic and Societal Impacts S.15a S.15b Seismic Risk Map North America Seismic Risk Map Greater Victoria Belmont 9

10 How are the Size and Strength of an Earthquake Measured? Modified Mercalli Intensity Map 1994 Northridge, CA earthquake Magnitude 6.7 Modified Mercalli Scale (Intensity) subjective measure of damage done and people s reactions isoseismal lines identify areas of equal intensity How are the Size and Strength of an Earthquake Measured? Modified Mercalli Scale (Intensity) S.16 Earth s Layered Structure How are the Size and Strength of an Earthquake Measured? Richter Scale (Magnitude) measures total amount of energy released by an earthquake; independent of intensity Nomogram S-waves P-waves Amplitude of the largest wave produced by an event is corrected for distance and assigned a value on an open-ended logarithmic scale S.17 Seismic Wave Paths Through the Earth S.18a 10

11 Earth s Layered Structure Earth s Layered Structure S-waves P-waves Seismic Wave Paths Through the Earth S.18 Layers Defined by Composition Crust Continental crust - Upper crust = granitic rocks - Average density is about 2.7 g/cm 3 - Up to 4 billion years old Oceanic crust - Basaltic composition - Density about 3.0 g/cm 3 - Young -180 million years or less S.19a Earth s Layered Structure Earth s Layered Structure Layers Defined by Composition Mantle Below crust to a depth of 2900 kilometers Composition of uppermost mantle is the igneous rock peridotite (changes at greater depths). Core Below mantle Sphere with a radius of 3486 kilometers Composed of an iron-nickel alloy Average density of nearly 11 g/cm 3 S.19b Layers Defined by Physical Properties Lithosphere Crust and uppermost mantle (about 100 km thick) Cool, rigid, solid Asthenosphere Beneath the lithosphere Upper mantle To a depth of about 660 kilometers Soft, weak layer that is easily deformed S.20a 11

12 Earth s Layered Structure Earth s Layered Structure Layers Defined by Physical Properties Lower Mantle km, more rigid layer Rocks are very hot and capable of gradual flow. Inner Core Sphere with a radius of 1216 km Behaves like a solid Outer Core Liquid layer, 2270 km thick Convective flow of metallic iron within generates Earth s magnetic field S.20b Earth s Layered Structure The Shadow Zone Discovering Earth s Layers Moho Velocity of seismic waves increases abruptly below 50 km of depth Separates crust from underlying mantle Shadow Zone Absence of P waves from about 105 degrees to 140 degrees around the globe from an earthquake Can be explained if Earth contains a core composed of materials unlike the overlying mantle S.21 S.22a 12

13 The Shadow Zone The Shadow Zone ONLY P-waves ONLY P-waves NO S-waves Shadow Zone NO P-waves or S-waves NO S-waves Shadow Zone NO P-waves or S-waves S.22b S.22c Earthquake Waves: The Shadow Zone WS Earthquake Waves: The Shadow Zone WS See Chapter 15 Worksheet Package 13

14 Earthquake Waves: The Shadow Zone WS Earthquake Waves: The Shadow Zone WS See Chapter 15 Worksheet Package Earthquakes and Tsunamis How does an earthquake form a tsunami? 14

15 Earthquakes and Tsunami s Generation of a Tsunami An earthquake under the ocean has the potential to form a tsunami. The earthquake must vertically displace overlying water (extensional or compressional faults - not shearing) Extension Compression Shearing S South Asian Boxing Day Event Biggest earthquake in 40 years! Magnitude km off the west of Northern Sumatra Generated a disastrous tsunami in 12 countries S.24 Above: Countries most effected by the 2004 tsunami second largest recorded earthquake occurred at a subduction zone 1,600 km of faultline slipped about 15 m released energy equivalent to 23,000 Hiroshima atomic bombs 15

16 Before and After photographs from Phuket, Thailand 2004 Boxing Day tsunami A village near the coast of Sumatra lays in ruin after the tsunami. 16

17 How does an earthquake form a tsunami? We CANNOT stop the geologic processes. We CANNOT stop the population growth/expansion. Therefore, we must try to reduce (mitigate) the hazards How do we mitigate the hazards from earthquakes? Scientific study Reinforce buildings Education Disaster plans How do we mitigate the hazard from tsunamis? Monitoring technology-intensive high costs for many poorer countries often no technology available reliant on the Pacific Tsunami Warning Center tsunami in 1998 was not detected Education S.25a S.25b 17

18 UNIT TEST - Chapters Exercise 6 Seismic Waves! Complete Worksheets Assignments: Chapter 13 Plate Tectonics Notes & Vocab Chapter 14 Plate Tectonics & Volcanoes Notes & Vocab Chapter 15 Plate Tectonics & Earthquakes Notes & Vocab Chapter 16 Plate Tectonics & Mountain Building Notes & Vocab For Review and Test Prep: Go to esc11.weebly.com Flashcard Library (home page) volcano photo review link to practice quizzes and review PowerPoints Quizlab Chapters Practice Test S.26 S.27 18