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Earthquake Hazards. Key ideas: The amount of damage an earthquake causes depends on its magnitude, and where it occurs. Safe building practices can limit loss of life and damage to property. Damage from Earthquakes.
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Earthquake Hazards Key ideas: The amount of damage an earthquake causes depends on its magnitude, and where it occurs. Safe building practices can limit loss of life and damage to property.
Damage from Earthquakes • Earthquakes have a tremendous destructive power; they strike quickly, and many, many times unexpectedly. Within seconds or minutes thousands of people can die, and buildings turn into piles of debris. • Most earthquakes occur at the boundaries of plate tectonics, or faults. (Faults are planes along which masses of rocks are displaced)
Damage from Earthquakes An earthquake can destroy any man made structure (buildings, bridges, roads, etc) in several ways: • Ground shaking • Aftershocks and fire • Tsunamis
Ground Shaking Ground shaking is caused by the P waves (the ground is shook side to side) and the S waves (the ground is shook up and down). Most buildings can withstand up and down shaking, but few will resist to side to side vibration.
Ground Shaking • Buildings that are built on solid rock have better chances to remain intact than buildings that are built on soft rocks. Soft rocks or loose soil can temporarily take some of the properties of a liquid during an earthquake. This is called liquefaction. • For example, San Francisco’s Marina district sits on a landfill which was used to extend the city into San Francisco Bay. In the 1989 Loma Prieta earthquake this landfill liquefied, shaking buildings off their foundations. Liquefaction caused great loses in Turkey (1999), Alaska (1964) Mexico City (1985).
Aftershocks and Fire • Most earthquakes are followed by a series of smaller ones, originating close to the focus of the large earthquake. These smaller earthquakes are called aftershocks. Although the magnitude of these aftershocks is considerably smaller than the initial earthquake, many still standing but weakened buildings might collapse during the aftershocks..
Fires • Imagine that an earthquake affects a heavily populated area, with miles and miles of gas pipes running under the ground. The ground movement breaks apart the gas lines, and a spark is enough to start a devastating fire. • After the 1906 San Francisco earthquake the fires destroyed about 3000 buildings and burned about 11 square Km of the city.
Tsunamis • Tsunamis are huge waves created by a displacement of water - a landslide, volcanic eruption, or slippage of the boundary between two of the earth's tectonic plates – thick slabs of rock that carry the Earth's continents and seas on an underground ocean of much hotter, semi-solid material.
Tsunamis • Tsunamis can travel up to 600 mph (965 km/hour, or 521 knots) at the deepest point of the water, but slow down as they get near the shore, eventually hitting the shore at 30 to 40 mph (48 to 64 km/h or 26 to 35 knots). The energy of the wave's speed is transferred to height and sheer force as it approaches the shore.
Earthquakes Facts • On of the most destructive earthquakes recorded since 1900 occurred on July 27, 1976, in Tangshan, China, when the official death count reach 255,000 for a 7.5 magnitude quake. Estimated death counts, however, reached as high as 655,000.
Earthquake Facts • The highest toll for an earthquake-tsunami combination since 1900 took place on December 28, 1908, when a 7.2 magnitude quake struck Messina, Italy, killing an estimated 70,000 to 100,000 people. • The deadliest earthquake ever recorded is believed to have occurred on January 23, 1556, in Shansi, China, killing 830,000 people.
Earthquake facts • The worst tsunami in recent history followed the August 27, 1883, the eruption of the volcano Krakatau. The resulting wave swept over the Indonesian islands of Java and Sumatra, ultimately killing 36,000 people.
Sources • U.S. Geological Survey - www.usgs.gov • Pacific Tsunami Warning Center - www.prh.noaa.gov/ptwc • University of Oregon Seismology Dept - www.geophys.washington.edu • http://earthquake.usgs.gov/eqcenter/recenteqsus/
Earthquake Risk • Where do major earthquakes occur? What areas in the world are at risk? To answer this question, let’s remember that earthquakes are closely related to movement of plate tectonics or movement of packs of rocks along faults. Hence, earthquakes happen everywhere in the world where these conditions are met.
Earthquake risk in the US • The west coast of the United States is the most tectonically active. • Notice that California has the highest risk for major earthquakes to happen. Los Angeles and San Francisco, two of the largest cities in the US are close to one of the most active faults in the world: the San Andreas Fault.
