Earth Science 8.3 Earthquake Hazards Earthquake Hazards
Causes of Earthquake Damage • The Prince William Sound earthquake struck Alaska in 1964 and was the most violent earthquake to hit North America in the 20th century. • It had a moment magnitude of 9.2 and lasted 3 to 4 minutes. It left 131 people dead and thousands homeless damaging the major ports and towns of Alaska. • Had this earthquake happened in more populated areas, it’s affect would have been devastating.
Causes of Earthquake Damage • An earthquake as powerful as the 1964 earthquake in Alaska can cause catastrophic damage. • But even less powerful earthquakes still pose a severe danger. • Earthquake related hazards include seismic shaking, liquefaction, landslides, mudflows and tsunamis.
Causes of Earthquake Damage • Seismic Shaking: • The ground vibrations caused by an earthquake, called seismic shaking, are the most obvious hazard of an earthquake. • Seismic waves interact to jolt and twist structures. Buildings made of unreinforced brick may collapse. • Wood frame buildings may remain intact but can be knocked clear off their foundations.
Causes of Earthquake Damage • Seismic Shaking: • Seismic shaking is generally strongest closest to the epicenter. • Strong seismic shaking can occur in areas of loose soil or filled in land far from an epicenter. • The filled soil magnifies the effects of the seismic waves. • Structures in such an area can experience severe damage though far from the epicenter.
Causes of Earthquake Damage • Liquefication: • Where sediment layers and rock are saturated with water, earthquakes can cause a process called liquefaction. • When liquefaction occurs, what had been stable soil suddenly turns into liquid.
Causes of Earthquake Damage • Liquefication: • The liquid cannot support buildings or other structures. Buildings and bridges may settle and collapse. • Underground storage tanks or sewer lines may float to the surface.
Causes of Earthquake Damage • Landslides and Mudflows: • Earthquakes can trigger different types of mass movements. These mass movements can often do more destruction and loss of life than the initial quake. • Earthquakes often cause loose rock and soil on slopes to move. These movements are called landslides.
Causes of Earthquake Damage • Landslides and Mudflows: • In areas where the water content of the soil is high, an earthquake can start a mudflow. During a mudflow, a mixture of soil and water slides downhill rapidly burying everything beneath.
Causes of Earthquake Damage • Tsunamis: • A tsunamisis a wave formed when the ocean floor shifts suddenly during an earthquake. • In 2004, a magnitude 9.4 earthquake west of the island of Sumatra in the Indian ocean produced devastating tsunamis.
Causes of Earthquake Damage • Tsunamis: • Without warning, the huge waves struck coastal areas of Indonesia, Sri Lanka, Thailand and several other countries killing nearly 300,000 people.
Causes of Earthquake Damage • Tsunami’s form when an earthquake pushes up a slab of ocean floor along a fault. • An underwater landslide or volcanic eruption can also trigger a tsunami. • Once formed, a tsunami resembles the ripples created when you drop a stone in the water.
Causes of Earthquake Damage • A tsunami wave in open ocean is only a meter high but moves very fast; at hundreds of miles per hour. • As the wave enters shallower water near a shore, the water slows down and the waves begin to pile up. • When the waves do hit shore, they are much bigger due to the compression and hit with devastating force sweeping inland for miles.
Reducing Earthquake Damage • Earthquake damage depends on severalfactors. • Two very important factors are the strength and duration of the seismic shaking and the materials and design of structures. • Earthquake damage and loss of life can be reduced by determining the earthquake risk for an area, building earthquake resistant structures, and following earthquake safety procedures.
Reducing Earthquake Damage • The first step to reduce earthquake risk is to asses the danger or probability of an earthquake in an area. Earthquakes are most common along the boundaries of Earth’s tectonic plates. • Scientists use several methods to determine earthquake risk. • One way is to study historical records of earthquakes and look at where they have occurred in the past.
Reducing Earthquake Damage • Scientists also use devices to measure the uplift, subsidence, and strain on rocks in the fault areas. • They also study seismic gaps. A seismic gap is an area along a fault where there has not been any earthquake activity for a long period of time. • Scientists hypothesize that the buildup of strain along a seismic gap will eventually lead to an earthquake.
Reducing Earthquake Damage • Scientists also look for small warning signs that an earthquake may be about to strike. They may measure water levels and pressure in wells, radon gas emissions, small changes in the electromagnetic properties of rocks even. • Efforts at short term prediction of earthquakes are at present far from adequate to allow timely warning.
Reducing Earthquake Damage • Seismic Design safety: • Many modern cities in earthquake prone areas, such as San Francisco and Los Angeles, have modern building codes that take earthquakes into account.
Reducing Earthquake Damage • Seismic Design safety: several examples • Steel building frames are often reinforced with cross-braces. • Buildings are sometimes mounted on large rubber and steel pads called base-isolators that absorb shock. • Wooden frame houses are often reinforced and bolted to their foundations more securely. • Flexible pipes and automatic shutoff valves prevent rupture of gas and water lines and leakage of gas or fluids.
Computer Lab: • After completing worksheet 8.3; use the internet to research and find information on the following topic. Write a short mini-report of three paragraphs minimum on the following: DO NOT COPY, CUT OR PASTE: USE YOUR OWN WORDS • In recent years, earthquake proofing has become an important legal mandate in many cities around the world. Research earthquake proofing and find at least 5 different design techniques that are used to help earthquake proof a building.