Chapter 8 EARTHQUAKES AND EARTHS INTERIOR Earthquake Numbers

Chapter 8 EARTHQUAKES AND EARTH’S INTERIOR

Earthquake Numbers 30, 000 earthquakes per year Most are minor – little damage 100 major earthquakes per year Usually in remote areas

Earthquakes Vibration of Earth caused by rapid release of energy in lithosphere Fault Fracture in Earth where movement has occurred

Earthquakes Focus Point in Earth where an earthquake starts Beneath the surface Seismic Wave Released energy travels as this type of wave Epicenter Location on surface directly above focus

Earthquakes

Faults and Changes to the Earth’s Surface Movement along faults causes surface features of Earth to change Can push up coastlines, mountains and plateaus Vertical Movement Uplifted Produces ridge known as fault scarp Horizontal Movement Offset or Displaced

Faults and Changes to the Earth’s Surface

The San Andreas Fault Extends 1300 km through California and into Pacific Ocean Broken into 100 -200 km segments Segments behave differently � Some creep, some slip, some stay locked for 100 s of years then break 1906 San Francisco Earthquake One side of fault moved 4. 7 meters

The San Andreas Fault

The Cause of Earthquakes Elastic Rebound Hypothesis Most earthquakes produced by rapid release of energy stored in rock that has been subjected to great forces. When strength of the rock is exceeded, it suddenly breaks and releases some of its stored energy as seismic waves

Deformation of Rocks Forces inside Earth’s crust slowly deform rocks This causes the rock to change shape Energy stored Eventually rock breaks and energy is released Rocks slip at weakest point This becomes the focus Movement exerts forces farther along fault and more slippage occurs until energy is depleted Elastic rebound Tendency of rock to spring back after earthquake Like a rubber band

Elastic Rebound

Aftershocks and Foreshocks Aftershock Usually occurs soon after major quake Not all energy gets released during major event Foreshock Small quake preceding major quake Can happen days or possible years before major quake

8. 1 Review Questions What is a fault? Describe the cause of earthquakes. What two things happen when a rock is deformed? How are an earthquake’s fault, focus, and epicenter related? Why do most earthquakes cause little damage and loss of life?

Measuring Earthquakes Seismic Waves Body Waves � travel through Earth’s interior Surface Waves � Travel along surface of Earth

Seismic Waves Push-Pull Compress and Expand Particles move in same direction as wave Travel faster than S waves Can go through liquids and solids

Seismic Waves S Waves Particles move at right angle to waves’ direction Also called transverse waves Travel slower than P waves Travel through solids only

Seismic Waves Surface Waves Happens when body wave reaches surface Travel slower than body waves Surfaces move up-and-down and side-to-side Larger than body waves – most destructive

Seismic Waves

Recording Seismic Waves Seismographs Weight suspended from a support attached to bedrock When wave reaches instrument weight keeps it in one spot while Earth and the support vibrate

Recording Seismic Waves Time record of ground motion during an earthquake Records all types of waves P waves first, then S, then surface

Measuring Earthquakes Two ways to measure Magnitude �Richter �Moment Magnitude Intensity �Modified Mercalli

Richter Scale Outdated Based on height of largest seismic wave Tenfold increase in wave height equals increase of 1 on Richter Scale 5. 0 earthquake is 10 times more powerful than 4. 0 Since seismic waves weaken when they travel this scale is only reliable for small, shallow earthquakes that are close to recording station

Moment Magnitude Estimates energy released by quake Measures Average amount of movement along fault Area of the surface break Strength of the broken rock

Moment Magnitude

Modified Mercalli Scale Rates intensity based on quakes effects at different locations Uses Roman Numerals The same quake can have many different ratings

Locating An Earthquake Measure difference in arrival times of P and S waves P waves always get there first The longer it takes for the S wave to arrive the farther away the epicenter must be Need at least three locations

8. 2 Review List the two categories of seismic waves Describe three ways to measure the size of an earthquake Describe how the epicenter of an earthquake is located

Causes of Earthquake Damage Seismic Shaking Liquefaction Landslides Mudflows Tsunamis

Seismic Shaking The damage to buildings and other structures from earthquake waves depends on several factors the intensity and duration of the vibrations the nature of the material on which the structure is built the design of the structure.

Seismic Shaking Waves jolt and twist structures Unreinforced brick may collapse Wood-frames may move off foundation Shaking strongest closest to epicenter If building is on loose soil or filled land can be affected relatively far away

Liquefaction When soil and rock are saturated with water Stable soil suddenly turns into liquid Buildings and bridges collapse Underground storage tanks float to surface

Landslides Landslide Loose rock and soil on slopes move

Mudflow Water content of soil is high Soil and water slide downhill

Tsunami Wave formed when ocean floor shifts Creates ripple effect Moves at hundreds of kilometers per hour In deep open waters not really noticeable Water piles up in shallow waters

Reducing Damage depends on Strength and duration of shaking Materials and design of structures Ways to reduce damage Determine risk for area Build earthquake resistant structures Follow safety precautions

Assessing Earthquake Risk Most frequent along tectonic plate boundaries Study historical records Measure uplift, subsidence and strain in rocks near active faults Study seismic gaps Area along fault that hasn’t had earthquake for long time Look for warning signs Measure water level and pressure in wells Radon gas emissions Small changes in electromagnetic properties of rocks Short term predication not really successful

Earthquake Distribution

Seismic Safe Designs Steel frames reinforced with cross-braces Building mounted on base isolators Large rubber and steel pads Wood framed homes bolted to foundation Gas lines made of flexible pipes and equipped with automatic shut-off valves Flexible joints in water mains

Earthquake Safety Drop, Cover and Hold Indoors Get under sturdy table or desk Stay close to inner walls Cover head Avoid windows, mirrors and top heavy furniture Outdoors Move to open area Sit down Avoid vehicles, power lines, trees and buildings

8. 3 Review Describe five hazards caused by earthquakes. Explain how earthquake-related damage can be reduced. What is a tsunami? What is a seismic gap? �Next Page

8. 3 Review A builder in Alaska has a choice of two sites for a building: one is on filled land next to the ocean, and the other is inland on solid ground. Both sites are the same distance from an active fault. Which site should the builder choose? Explain In an earthquake-prone area, it has been many years since the last earthquake along a fault. Should residents be concerned about a future earthquake? Explain

Layers Defined by Composition We know Earth has different layers because seismic waves don’t travel in straight line at same speed Increased pressure causes waves to move faster Waves get refracted, or bent, as they travel Three layers crust mantle core

Crust Thin, rocky outer layer Oceanic 7 km thick Made of basalt and gabbro Density of 3. 0 g/cm 3 Fairly young Continental 8 -75 km thick Mostly a granite rock known as granodiorite Density of 2. 7 g/cm 3 Very old

Mantle 82% of volume here Solid, rocky shell Depth of 2890 km Common rock is peridotite Density of 3. 4 g/cm 3

Core Sphere Made of iron-nickel alloy Density of 13 g/cm 3 Due to extreme pressure

Layers by Physical Properties Based on composition Crust, mantle and core Based on Physical properties Lithosphere, asthenosphere, lower mantle, outer core and inner core

Lithosphere Outermost layer Crust and upper mantle Cool, rigid shell 100 km thick

Asthenosphere Below lithosphere Soft, weak layer Rocks close to melting point Easily deformed Lower lithosphere and asthenosphere are part of upper mantle

Lower Mantle Reaches from depth of 600 km to base of mantle More rigid Due to pressure Still very hot and can gradually flow

Inner and Outer Core Outer Liquid 2260 km thick Flow of metallic iron causes Earth’s magnetic field Inner Sphere 1220 km thick High temperature but is solid because of extremely high pressure

Discovering Earth’s Layers 1909 – Andrija Mohorovicic Studied seismic records and found velocity of waves increased abruptly at 50 km below Europe Boundary separates crust from mantle Mohorovicic discontinuity – Moho for short 1906 Found boundary between mantle and outer core Found P waves bent around liquid outer core and P waves that did travel through core arrived much later than expected Region called shadow zone

Discovering Earth’s Layers Bent wave paths only explained by core being composed of material different from mantle P waves bend around core Like being able to hear people talking around corner without seeing them Found S waves can’t do this and could not travel through core Now they knew core was liquid

Discovering Earth’s Layers

Discovering Earth’s Composition Studied Seismic data Rock samples from crust, mantle and meteorites Performed high pressure experiments on Earth materials Took small samples of rocks and metals, heated and squeezed them, then measured speed of S and P waves through them Seismic data and rock samples from drilling tell us continental crust is made of lowdensity granite and oceanic crust is basalt

Discovering Earth’s Composition Since lava comes from partially melted asthenosphere scientists have inferred composition by doing lab experiments Showed partially melting peridotite produces substance very similar to lava Meteorites Assumed to be composed of original material from which Earth was formed Found iron, nickel and peridotite Crust has smaller percentage of iron compared to meteorites so scientists think the dense iron sank during formation of Earth

8. 4 Review Questions Describe Earth’s layers based on composition. Describe Earth’s layers based on physical characteristics. What evidence led scientists to conclude that Earth’s outer core is liquid? Explain Why are meteorites considered important clues to the composition of Earth’s interior?