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Geology - Earthquakes

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    Study Guide Geology – Earthquakes 1. Monitoring Earthquakes Earthquakes may happen suddenly, but scientists have powerful tools to study them. By carefully measuring seismic waves, geologists can figure out where an earthquake happened, how deep it was, and how strong it was. Let’s break this down step by step. 1.1 Seismometers and Seismograms Earthquakes are detected using an instrument called a seismometer . A seismometer contains a suspended, pendulum - like mass that stays as still as possible while the ground moves around it. Here’s how it works: • When the ground shakes, the frame of the seismometer moves. • The suspended mass moves much less. • This difference in motion is recorded. The recording device is called a seismograph , and the record it produces is known as a seismogram . A seismogram looks like a series of wavy or “squiggly” lines that show how the ground moved during the earthquake. Seismograph stations all over the world can record waves from the same earthquake. By comparing data from different stations, scientists can calculate: • the location of the earthquake, • its depth , and • its strength . These comparisons can also give clues about the types of rocks the seismic waves traveled through.

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    Study Guide 1.2 Depth of Focus The depth of focus is the vertical distance between the focus (where the earthquake starts underground) and the epicenter (the point directly above it on Earth’s surface). Key facts about earthquake depth: • The deepest earthquakes occur at about 670 km (400 miles) below the surface. • 85% of earthquakes have a shallow focus , up to 70 km (40 miles) deep. • 12% have an intermediate focus , between 70 and 350 km (40 – 210 miles) . • Only 3% are deep - focus earthquakes, occurring between 350 and 670 km (210 – 400 miles) . Most earthquakes are shallow because rocks near the surface are brittle and break easily. Deeper in the Earth, rocks are hotter and behave more like soft, flowing material, so they are less likely to fracture suddenly. 1.3 Travel - Time Curves and Loca Ɵ ng Earthquakes P waves and S waves begin at the focus at the same time, but they travel at different speeds. P waves move faster, so they arrive at seismograph stations first. Important idea: • The farther a station is from the earthquake, the greater the time gap between the arrival of P waves and S waves. Scientists use this time difference to create a travel - time curve , which shows how far the station is from 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 where three or more circles intersect .

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    Study Guide 1.4 First - Mo Ɵ on Studies Seismic data can also reveal how rocks moved along a fault during an earthquake. These are called first - motion studies . Scientists look at the first movement recorded at each station: • A push means the rock moved toward the station. • A pull means the rock moved away from the station. By analyzing push - and - pull patterns from many stations, scientists can determine: • the possible orientation of the fault , and • the direction of movement along that fault. If the fault’s orientation is already known, the direction of movement can be identified very accurately. 1.5 Measuring Earthquake Strength: Intensity One way to describe an earthquake’s strength is by its intensity , which focuses on damage and human experience . The Modified Mercalli Scale ranks intensity from 1 to 12 : • Low numbers mean little or no damage.

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    Study Guide • High numbers indicate severe 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: Magnitude A more precise way to measure earthquakes is by magnitude , which tells us how much energy was released. The most familiar system is the Richter scale : • It ranges roughly from 2 to 8.6 . • It is logarithmic , not linear. This means: • Each whole - number increase represents 10 times greater wave amplitude . • Each increase releases about 30 times more energy . For example: • A magnitude 3 earthquake shakes the ground 10 times more than a magnitude 2. • It releases about 30 times more energy . The Richter scale works well up to about magnitude 7 . For larger earthquakes, scientists now use methods based on fault movement and surface rupture . Using this approach, the largest earthquake measured so far was magnitude 9.5 during the 1960 event in Chile. 1.7 How O Ō en Do Earthquakes Happen? Every year: • Over 100,000 magnitude 2 earthquakes occur (these are “just felt”). • Several thousand magnitude 4 – 5 earthquakes cause noticeable damage.

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    Study Guide • About 15 – 20 earthquakes of magnitude 7 or greater cause serious destruction. In the United States, most earthquakes happen in tectonically active areas , such as the Pacific coast and western fault zones. Fewer earthquakes occur in the central and eastern states, but they can still be very damaging because the crust there is older and more brittle , allowing seismic waves to travel farther. 2. E ff ects of Earthquakes Earthquakes do more than shake the ground. They can damage buildings, reshape the land, and even affect the oceans. Let’s look at the main effects in a clear and easy way. 2.1 Structural Damage and Fires The most immediate effect of an earthquake is ground shaking caused by seismic waves. This shaking can: • crack walls, • collapse buildings, and • damage bridges and roads. In strong earthquakes, gas mains may break , which can start many fires at once . These fires can be just as dangerous as the shaking itself. Sometimes, small earthquakes called foreshocks happen before a major earthquake. Scientists study them because they can act as warning signs that a larger earthquake may be coming. After the main earthquake, aftershocks often occur. There can be thousands of aftershocks , and although many are smaller, they can still cause serious damage — especially to buildings that were already weakened. 2.2 Mass - Was Ɵ ng Events and Liquefac Ɵ on Earthquake shaking can trigger mass - wasting events , such as landslides. These fast - moving events can: • destroy homes, • block roads, and

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    Study Guide • cause loss of life. One especially dangerous type is liquefaction . This happens when: • loose sediment is saturated with water , and • shaking causes the sediment to behave like a thick liquid. During liquefaction: • the ground loses strength, • buildings may sink 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 Movement Earthquakes can cause the ground to permanently shift along faults. This movement can be: • vertical , creating a steep step in the land called a fault 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 about 7 meters (25 feet) . 2.4 Seismic Sea Waves (Tsunamis) When an earthquake causes the sea floor to suddenly rise or fall , it can push a huge amount of water upward. This creates seismic sea waves , commonly called tsunamis . Tsunamis are very different from normal ocean waves:

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