Chapter 7 Section 3
Earthquake Hazard
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Earthquake hazard is a measure of how likely an area is to have damaging earthquakes in the future.
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Earthquake-hazard level is determined by past seismic activity.
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California has a very high earthquake-hazard level.
Earthquake Forecasting
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Forecasting earthquakes is difficult.
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Scientists study earthquakes to discover patterns in earthquake strength and frequency.
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The relationship between earthquake strength and frequency is based on the amount of energy released
during earthquakes.
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Stronger earthquakes are much rarer than weaker earthquakes.
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Millions of small earthquakes release the same amount of energy as one large earthquake does.
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The gap hypothesis is a way to forecast earthquake location, strength, and frequency.
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The gap hypothesis states that sections of active faults that have had relatively few recent earthquakes are
likely to be the sites of strong earthquakes in the future.
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The areas along an active fault where relatively few earthquakes have happened are called seismic gaps.
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Stress has a long time to build at seismic gaps.
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When a fault breaks at a seismic gap, the sudden release of stress can cause a large-magnitude earthquake.
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In 1988, scientists predicted a > 6.5 magnitude earthquake would happen within 30 years in a seismic gap
near Santa Cruz.
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The 1989 Loma Prieta earthquake of magnitude 6.9 happened in the gap.
Reducing Earthquake Damage
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Much of the loss of human life during earthquakes is caused by buildings that collapse.
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Retrofitting can make older buildings more earthquake-resistant.
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Common ways to retrofit include strengthening structures with steel and fastening the building to its
foundation.
Are You Prepared for an Earthquake?
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You can plan ahead to protect yourself and your property from earthquake damage.
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Safeguard your home by putting heavy objects on low shelves.
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Ask your parents about having your home strengthened.
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Find places that are safe within each room of your home and outside your home.
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Make a plan to meet with others in a safe place after the earthquake.
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Store water, nonperishable food, a fire extinguisher, flashlight with batteries, radio, medicines, and a first
aid kit in a safe place.
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If an earthquake happens when you are indoors, stay indoors until the earthquake stops.
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Crouch or lie face down under a table or desk in the center of the room.
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If you are outside, stay outside.
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Lie down away from buildings, power lines, or trees and cover your head with your hands.
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If you are in a car, stop the car and remain inside.
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After the earthquake, stay calm and get your bearings as quickly as possible.
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Identify immediate hazards, such as downed power lines, broken glass, or fire hazards.
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Stay out of damaged buildings.
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Return home only when someone in authority says it is safe.
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Remember that there may be aftershocks, which may cause more damage.
Tsunamis
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Earthquakes on the ocean floor can generate tsunamis.
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A tsunami is an extremely long wave that can travel across the ocean at speeds of up to 800 km/h.
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Tsunamis most often form when an earthquake causes a vertical movement of the sea floor, which displaces
an enormous volume of water.
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In the open ocean, tsunami waves can seem very small.
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As tsunami waves enter shallow water along a coastline, the energy of the waves is compressed.
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The waves get rapidly taller. By the time they reach shore, waves can be taller than 30 m.
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Tsunamis can cause damage and loss of life by smashing into and washing away anything in their path.
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Almost 150 tsunamis happened worldwide during the 20th century.
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In 2004, an undersea earthquake of magnitude 9.3 caused a tsunami that killed more than 280,000 people
and left 1.25 million people homeless.
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Tsunamis are monitored by most of the nations that border the Pacific Ocean.
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These nations provide seismic and tide data to the Pacific Tsunami Warning Center (PTWC) in Hawaii.
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If a tsunami has been generated by an undersea earthquake, the center sends a bulletin to warn officials in
threatened areas.