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Chapter 8 EARTHQUAKES

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Your textbook estimates there are approximately 30,000 earthquakes occur every year. According the USGS, several million earthquakes occur each year on average. On average only about 75 per year are very large. Most of these earthquakes occur in remote locations where very little harm is done. Occasionally, these large earthquakes occur in areas with a large population of people and they are one of the most destructive forces on earth.

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EARTHQUAKE DAMAGE Turkey JapanTaiwan Chile Haiti California

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WHAT CAUSES AN EARTHQUAKE? The surface of the earths crust is made of plates. Below these plates is hot molten rock called magma. The heat from this magma causes the plates to move. There are boundaries between the different plates. When the plates move it creates pressure and friction between these plate boundaries. When the pressure that builds up is released, an earthquake occurs.

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WHERE AN EARTHQUAKE OCCURS When an earthquake occurs, there is a specific place usually along a plate boundary where the pressure is released and a large seismic wave is produced. The actual specific place where the pressure is released is called the focus. Most earthquake foci occur at great depths. The epicenter is a location on the surface of the earth located directly above the focus.

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EPICENTER AND FOCUS

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EPICENTERS AND FAULT LINES When geologists are trying to describe to the general public the location of an earthquake, it is not necessary to explain how deep the earthquake was. Most people generally would like to know the location of the quake in relation to a familiar place. That is why epicenters are most often used. A fault line is the fracture or boundary between two plates. It is most commonly where earthquakes occur.

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WHAT IS THE ELASTIC REBOUND HYPOTHESIS? The elastic rebound hypothesis is an explanation for how seismic waves are produced and generated. Basically, if you think of the earths crust as a stick, when the stick is bent, it stores elastic potential energy. Eventually the stick (earths crust) will build up enough energy to overcome the bonds that hold the stick together. When this occurs the stick will break. When the stick breaks, it releases all its stored energy, producing a wave (seismic wave).

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ELASTIC REBOUND HYPOTHESIS

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WHAT ARE AFTERSHOCKS AND FORESHOCKS If you think of the analogy of the stick from the elastic rebound hypothesis, it is easy to understand where aftershocks and foreshocks come from. If you were to bend the stick, you might hear cracks occurring before the stick snapped in half. These cracks are the equivalent to a foreshock in the earths crust. Geologists can sometimes detect foreshocks occurring before a large quake.

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FORESHOCKS AND AFTERSHOCKS Foreshocks are small earthquakes that often occur before a large earthquake. Aftershocks are small earthquakes that occur after a large earthquake occurs. These were recorded from the Japan earthquake that occurred on March 11, 2011.

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SEISMOLOGY AND THEIR INSTRUMENTS Seismology is the study of earthquakes. Waves produced by earthquakes are called seismic waves. Instruments that record seismic waves are called seismographs. Seismographs produce a written or digital record of seismic waves produced from earthquakes. The recorded seismic waves are called seismograms.

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SEISMOGRAPHS AND SEISMOGRAMS

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SEISMIC WAVES Seismologists can tell a lot about earthquakes and the earths interior from seismic waves. There are basically three main types of seismic waves. Surface Waves these waves only travel through the earths outer layer. These waves move similarly to the way an ocean wave moves. These waves move both up and down, as well as side to side. Surface waves are the most destructive seismic waves.

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SEISMIC WAVES The other two types of waves are called body waves because they can travel through the earth. The first body waves are called P waves or primary waves. P waves are the fastest seismic waves. Sometimes they are also called push-pull waves or compression waves because they compress and expand the rocks as they travel through the earth. They move similarly to a spring being compressed.

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SEISMIC WAVES The second body wave is called an S wave or secondary wave. S waves shake the ground at right angles to the direction the wave is traveling.

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SEISMIC WAVES

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SEISMOGRAMS AND SEISMIC WAVES Seismic waves are recorded on seismograms. P waves are the fastest waves and are about 1.7 times faster than S waves. Surface waves are the slowest and are about 90 percent of the speed of the S waves. Knowing the speed of the waves can tell seismologists the distance to an earthquakes epicenter and if at least three distances are known, the exact location can be determined by triangulation.

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READING A SEISMOGRAM

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READING A DISTANCE TRAVEL TIME GRAPH

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FINDING THE LOCATION OF AN EPICENTER

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WHERE DO EARTHQUAKES OCCUR? About 95% of most earthquakes occur along active plate boundaries. Most of these occur along the Circum-Pacific belt, also known as the Pacific Ring of Fire. Another belt where many earthquakes occur is called the Mediterranean-Asian belt.

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THE CIRCUM-PACIFIC BELT (PACIFIC RING OF FIRE)

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HOW TO DETERMINE THE MAGNITUDE OF AN EARTHQUAKE The old familiar scale that was used for determining an earthquakes magnitude is called the Richter Scale. The Richter Scale is based on the amplitude of the largest seismic wave recorded on a seismogram. It is a logarithmic scale which means that if the amplitude of a 5.0 seismic wave 10 times greater in amplitude than a 4.0 seismic wave. So the amount of shaking or intensity (energy released) of a 5.0 earthquake is 33 times greater than a 4.0.

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UNDERSTANDING THE RICHTER SCALE

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LIMITATIONS OF THE RICHTER SCALE AND THE USE OF THE MOMENT MAGNITUDE SCALE The problem with the Richter Scale is that the intensity of waves decrease with the distance from the epicenter. Because of this, the Richter Scale is only accurate out to about 500 km (300 miles). Seismologists use another method called the Moment Magnitude Scale, which is believed to be more accurate. The Moment Magnitude Scale is based on three things; the average displacement along the fault, the surface area of the fault, and the rigidity (strength) of the rock.

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DESTRUCTION FROM EARTHQUAKES At 5:36 PM, March 27 (Good Friday), 1964, an enormous earthquake occurred about 75 miles east of Anchorage. It was the largest recorded earthquake ever recorded in North America. It was the second largest earthquake ever recorded. (1960 Chile earthquake was the largest) It had a moment magnitude of 9.2 and lasted about 3 to 4 minutes. (1960 Chile earthquake was a 9.5) Luckily, very few people lived in Alaska at the time, and it occurred on Good Friday (a holiday), so only 131 people died. Which is a relatively small number considering the enormous magnitude of the earthquake.

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1964 ALASKAN EARTHQUAKE

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WHAT LESSONS CAN WE LEARN FROM THE 1964 ALASKA EARTHQUAKE? Scientists and engineers have learned a great deal from the 1964 Alaska earthquake. Approximately $300 million dollars worth of damage. Many building and homes were destroyed by the intensity and duration of the shaking. Steel framed and wood framed buildings and home withstood better than unreinforced stone or brick homes and buildings.

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WHAT TYPES OF BUILDING DESIGNS WORK?

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EARTHQUAKE PROOF BUILDINGS

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EARTHQUAKE PROOF FOUNDATIONS

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SHEAR STRENGTH ON WALLS

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BUILDING DAMAGE FROM THE EARTHQUAKE IN HAITI (JANUARY 12, 2010)

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EARTHQUAKE IN HAITI (2010)

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WHAT WAS DIFFERENT BETWEEN THE EARTHQUAKES IN ALASKA (1964) AND HAITI (2010)? The earthquake in Haiti was only a magnitude 7.0 compared with Alaskas 9.2 (1964). That is nearly 1000 times less intensity. Alaska only had 131 fatalities. Haiti had over 300,000 fatalities The earthquake in Haiti made 1,000,000 people homeless, and 250,000 homes were destroyed. The primary difference was the building designs. In addition to building design, population size was also a factor.

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WHAT ARE SOME OTHER FACTORS THAT CAUSE DAMAGE DURING AND AFTER AN EARTHQUAKE? Normally, under ordinary conditions soil that has a small amount of moisture in it is very stable. The cohesive properties of water actually make the soil more stable and help hold the soil together. But, seismic waves can cause the bonds of the soil and water to break and cause the stable damp soils to become like an unstable soup of mud. This is known as liquefaction of soils.

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LIQUEFACTION OF SOILS

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THE RESULTS OF LIQUEFACTION OF SOILS

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LIQUEFACTION OF SOILS DURING THE 1964 ALASKAN EARTHQUAKE

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WHAT ARE SOME OTHER FACTORS THAT CAUSE DAMAGE FROM AN EARTHQUAKE? The largest loss of life during the 1964 Alaskan earthquake as well as many other earthquakes like the one that occurred in Japan on March 11, 2011 are seismic sea waves called tsunamis. Tsunamis are usually caused when an earthquake occurs on the floor of the ocean. The underwater quake quickly displaces a large volume of water. Tsunamis are hard to detect in the ocean, and most are only about a foot tall, and travel at speeds up to 500 MPH.

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TSUNAMIS As a tsunami approaches shore, it slows down, builds up height, sometimes approaching 100 feet or more. Before the tsunami reaches shore, witnesses have often reported a vacuum effect occurs and all the water rushes out to sea, exposing the sea floor. When this happens, usually within five minutes or less the main wave comes rushing in at enormous speed. After the main wave, other waves may follow.

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TSUNAMI IN JAPAN

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TSUNAMI IN THAILAND DURING THE 2004 INDIAN OCEAN EARTHQUAKE

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HOW A TSUNAMI OCCURS In 2004, 230,000 people died from a tsunami that occurred in the Indian Ocean. In 2011, nearly 16,000 people died from a tsunami in Japan. In 1964, 119 out of the 131 people who died from the Alaskan earthquake died from tsunamis.

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WHAT LESSONS CAN WE LEARN? If at all possible dont live too close to sea level. If a large earthquake occurs, get to high ground quickly. The plate itself can sometimes lower in elevation. The government has created the Pacific Tsunami Warning Center, located in Hawaii and in coastal communities located around the Pacific Ocean. If a large earthquake occurs, buoys in the ocean can usually detect if a tsunami will occur and if it is not already too late, a warning will be issued.

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WHAT ARE SOME OTHER FACTORS THAT CAUSE DAMAGE FROM AN EARTHQUAKE? Another major damaging affect of many earthquakes is large landslides. During the 1964 Alaskan earthquake, many people lost their homes and land due to the land collapsing beneath their feet and broken foundations. In 1958, a landslide triggered by an earthquake caused a massive megatsunami in Lituya Bay, Alaska that reached a record height of over 1,700 feet.

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LITUYA BAY, ALASKA

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1964 ALASKAN EARTHQUAKE 200 acres of land were swept out to sea in the Turnagain Heights area of Anchorage, AK

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WHAT ARE SOME OTHER FACTORS THAT CAUSE DAMAGE FROM AN EARTHQUAKE? Believe it or not, fires can become a major hazard as a result of an earthquake. During an earthquake, gas and electrical lines often rupture. When this happens fires can occur. Often times, broken water lines occur as well, making it very difficult to put out the fires. In 1906, in San Francisco, 80% of the city burned down from a fire that was caused by an earthquake.

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THE SAN FRANCISCO FIRE OF 1906

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EARTHQUAKE PREDICTIONS So far, scientists have been unable to successfully predict earthquakes with any consistent accuracy. By measuring movements in the ground, pressure and water levels in aquifers, radon gas emissions from fractures, scientists can gather clues that can help them assess the risk of an earthquake. Scientist can also use patterns of when earthquakes have occurred in the past to locate a seismic gap or an area along a fault which has not had an earthquake for a long time and therefore is likely to have one in the future.

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CAN INFORMATION GAINED FROM EARTHQUAKES BE USEFUL? Seismic waves that travel through the earth give clues to geologists about the composition of the earth. If the inside of the earth was all the same material, the seismic waves would travel straight through. But that is not how they travel. In fact, seismic waves bend or refract as they travel through the earth. Based on seismic waves, scientists believe the earth is made up of three zones based on their composition.

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THE THREE ZONES OF THE EARTH BASED ON COMPOSITION These three zones based on composition are the crust, the mantle and the core.

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THREE ZONES OF THE EARTH BASED ON COMPOSITION The Crust A thin, rocky outer layer made of ocean crust and continental crust. Made up of mostly rocks like basalt, gabbro (ocean), and granite (continental). The Mantle A solid rocky material but behaves like a plastic material that can bend and flow. It is made up of mostly ultramafic rocks like peridotite. The Core Believed to be an iron-nickel alloy material.

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FOUR ZONES OF THE EARTH BASED ON PHYSICAL PROPERTIES The earth can also be divided into four different zones based on physical properties. The Lithosphere (brittle) The crust and the upper mantle make up the solid lithosphere. The Asthenosphere (ductile) The rocks in the asthenosphere are softer than the upper mantle and lithosphere. These rocks are close to their melting points and can deform very easily.

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FOUR ZONES OF THE EARTH BASED ON PHYSICAL PROPERTIES The Outer Core This layer is hot enough to melt rock completely. Therefore this layer is a liquid layer. The flow of metallic iron in this region is believed to create the earths magnetic field. The Inner Core Even though this layer is hot enough to melt the rock, the immense pressures created by the extreme depths force this material into a solid state.

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FOUR ZONES OF THE EARTH BASED ON PHYSICAL PROPERTIES

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EARTHS LAYERS AND COMPOSITION Another discovery about the earths composition is something called the Mohorovicic discontinuity. A Croatian scientist discovered it and most scientists just refer to it as the Moho. The Moho is a boundary layer at about 50 km deep, where the crust is separated from the mantle. Scientists also determined the core was liquid because S waves cant travel through a liquid so the core creates a shadow zone.

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THE MOHO AND SHADOW ZONES