Geology - Earthquakes - Quick Guides | Geology

Preview (6 of 17 Pages)

100%

Geology - Earthquakes - Page 1 preview image

Loading page ...

Study GuideGeology–Earthquakes1. Monitoring EarthquakesEarthquakes may happen suddenly, but scientists have powerful tools to study them. By carefullymeasuring seismic waves, geologists can figure outwherean earthquake happened,how deepitwas, andhow strongit was. Let’s break this down step by step.1.1 Seismometers and SeismogramsEarthquakes are detected using an instrument called aseismometer.A seismometer contains a suspended, pendulum-like mass that stays as still as possible while theground moves around it.Here’s how it works:•When theground shakes, the frame of the seismometer moves.•The suspended mass moves much less.•This difference in motion is recorded.The recording device is called aseismograph, and the record it produces is known as aseismogram. A seismogram looks like aseries of wavy or “squiggly” lines that show how the groundmoved during the earthquake.Seismograph stations all over the world can record waves from the same earthquake. By comparingdata from different stations, scientists can calculate:•thelocationof the earthquake,•itsdepth, and•itsstrength.These comparisons can also give clues about the types of rocks the seismic waves traveled through.

Geology - Earthquakes - Page 2 preview image

Loading page ...

Study Guide1.2 Depth of FocusThedepth of focusis the vertical distance between thefocus(where the earthquake startsunderground) and theepicenter(the point directly above it on Earth’s surface).Key facts about earthquake depth:•The deepest earthquakes occur at about670 km (400 miles)below the surface.•85%of earthquakes have ashallow focus, up to70 km (40 miles)deep.•12%have anintermediate focus, between70 and 350 km (40–210 miles).•Only3%aredeep-focusearthquakes, occurring between350 and 670 km (210–400 miles).Most earthquakes are shallow because rocks near the surface arebrittleand break easily. Deeper inthe Earth, rocks are hotter and behave more like soft, flowing material, so they are less likely tofracture suddenly.1.3 Travel-Time Curves and LocaƟng EarthquakesP wavesandS wavesbegin at the focus at the same time, but they travel at different speeds. Pwaves move faster, so they arrive at seismograph stations first.Important idea:•The farther a station is from the earthquake, thegreater the time gapbetween the arrival ofP waves and S waves.Scientists use this time difference to create atravel-time curve, which shows how far the station isfrom the earthquake’s origin.To find the exact location:1.Each station draws a circle on a map.2.The radius of the circle equals the calculated distance to the earthquake.3.The earthquake’s location is wherethree or more circles intersect.

Geology - Earthquakes - Page 3 preview image

Loading page ...

Study Guide1.4 First-MoƟon StudiesSeismic data can also revealhow rocks movedalong a fault during an earthquake. These are calledfirst-motion studies.Scientists look at the first movement recorded at each station:•Apushmeans the rock moved toward the station.•Apullmeans the rock moved away from the station.By analyzing push-and-pull patterns from many stations, scientists can determine:•the possibleorientation of the fault, and•thedirection of movementalong that fault.If the fault’s orientation is already known, the direction of movement can be identified very accurately.1.5 Measuring Earthquake Strength: IntensityOne way to describe an earthquake’s strength is by itsintensity, which focuses ondamage andhuman experience.TheModified Mercalli Scaleranks intensity from1 to 12:•Low numbers mean little or no damage.

Geology - Earthquakes - Page 4 preview image

Loading page ...

Study Guide•High numbers indicatesevere destruction.However, this scale has limits:•Damage depends on population density.•Building design and construction matter.•Ground type (solid bedrock or soft sediment) affects shaking.•Earthquakes in uninhabited areas are hard to rate because there is little visible damage.1.6 Measuring Earthquake Strength: MagnitudeA more precise way to measure earthquakes is bymagnitude, which tells us how muchenergywasreleased.The most familiar system is theRichter scale:•It ranges roughly from2 to 8.6.•It islogarithmic, not linear.This means:•Each whole-number increase represents10 times greater wave amplitude.•Each increase releases about30 times more energy.For example:•A magnitude 3 earthquake shakes the ground10 times morethan a magnitude 2.•It releasesabout 30 times more energy.The Richter scale works well up to about magnitude7. For larger earthquakes, scientists now usemethods based onfault movement and surface rupture. Using this approach, the largestearthquake measured so far wasmagnitude 9.5during the 1960 event in Chile.1.7 How OŌen Do Earthquakes Happen?Every year:•Over 100,000magnitude 2 earthquakes occur (these are “just felt”).•Several thousandmagnitude 4–5 earthquakes cause noticeable damage.

Geology - Earthquakes - Page 5 preview image

Loading page ...

Study Guide•About15–20earthquakes of magnitude7 or greatercause serious destruction.In the United States, most earthquakes happen intectonically active areas, such as the Pacificcoast and western fault zones. Fewer earthquakes occur in the central and eastern states, but theycan still be very damaging because the crust there isolder and more brittle, allowing seismic wavesto travel farther.2. Effects of EarthquakesEarthquakes do more than shake the ground. They can damage buildings, reshape the land, andeven affect the oceans. Let’s look at the main effects in a clear and easy way.2.1 Structural Damage and FiresThe most immediate effect of an earthquake isground shakingcaused by seismic waves. Thisshaking can:•crack walls,•collapse buildings, and•damage bridges and roads.In strong earthquakes,gas mains may break, which can startmany fires at once. These fires canbe just as dangerous as the shaking itself.Sometimes, small earthquakes calledforeshockshappen before a major earthquake. Scientistsstudy them because they can act aswarning signsthat a larger earthquake may be coming.After the main earthquake,aftershocksoften occur. There can bethousands of aftershocks, andalthough many are smaller, they can still cause serious damage—especially to buildings that werealready weakened.2.2 Mass-WasƟng Events and LiquefacƟonEarthquake shaking can triggermass-wasting events, such as landslides. These fast-moving eventscan:•destroy homes,•block roads, and

Geology - Earthquakes - Page 6 preview image

Loading page ...

Study Guide•cause loss of life.One especially dangerous type isliquefaction. This happens when:•loose sediment issaturated with water, and•shakingcauses the sediment to behave like a thick liquid.During liquefaction:•the ground loses strength,•buildings maysink or tilt, and•areas that seemed stable can suddenly fail.Even buildings built on what seemed like solid ground can be badly damaged if liquefaction occurs.2.3 Permanent Ground MovementEarthquakes can cause the ground topermanently shiftalong faults.This movement can be:•vertical, creating a steep step in the land called afault scarp, or•horizontal, where the ground slides sideways.Horizontal movement is especially destructive because it can:•rip apart roads,•break pipelines, and•tear through buildings that cross the fault line.Although this movement is powerful, the amount of displacement usually does not exceed about7meters (25 feet).2.4 Seismic Sea Waves (Tsunamis)When an earthquake causes thesea floor to suddenly rise or fall, it can push a huge amount ofwater upward. This createsseismic sea waves, commonly calledtsunamis.Tsunamis are very different from normal ocean waves:

Preview Mode

This document has 17 pages. Sign in to access the full document!

Report

Geology - Earthquakes

Study Now!

Document Details

Related Documents

Geology - Weathering and Erosion

Geology - The Shorelines

Geology - The Ocean Floor

Geology - The Earth's Components

Geology - Sedimentary Rocks

Geology - Running Water

Geology - Plate Tectonics

Geology - Our Solar System

Geology - Mountain Building

Geology - Metamorphic Rock Types

Company

Explore

Study Tools