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  • A magnitude 7.8 earthquake struck San Francisco early on the morning of April 18, 1906.

  • The event marked the first time that the consequences of a major US natural disaster were recorded on film.

  • A subsequent report demonstrated that the earthquake occurred on the northern half of a single continuous structure that we now know as the San Andreas Fault.

  • The 1906 earthquake and subsequent fire killed more than 3000 San Francisco residents

  • and left over half the city's population homeless.

  • Today, nearly 40 million people live in California with many of them concentrated in major cities located close to the San Andreas and other fault systems.

  • What can we do to mitigate the potential damages and loss of life from a future dangerous earthquake?

  • To answer that question we need to understand the nature of earthquake hazards.

  • When earthquakes occur the earth shakes for seconds or maybe even minutes.

  • However much of the damage with earthquakes is not a result of the shaking itself but may be driven by other factors such as the design of buildings and the underlying geology.

  • Our objective for this lesson is to familiarize you with the principle hazards associated with earthquake activity and the factors that can magnify their impact.

  • One obvious consequence of shaking associated with an earthquake is the collapse of buildings and other structures.

  • Rapid vertical and horizontal movements can shift homes off their foundations,

  • collapse multi-story office blocks and destroy elevated roadways.

  • The colors on this map indicate the severity of the shaking associated with the San Francisco earthquake.

  • The red color indicates that shaking was most intense along the fault,

  • and the transition from red to yellow to green indicates that shaking decreased as we get farther to the east.

  • This graph illustrates that instruments at some stations near the source of the 1994 Northridge earthquake recorded ground shaking that approached 1g.

  • A vertical acceleration of 1g is sufficient to overcome gravity, throwing objects into the air.

  • Cities in areas of seismic risk typically have strict building regulations that require that new structures withstand accelerations of 0.5g or more with little damage.

  • Even 60 miles or 95 kilometers from the epicenter, there is sufficient shaking to cause damage to older buildings.

  • But shaking isn't just about the distance from the fault.

  • Look again at this map of the 1906 earthquake.

  • Notice this red area here around Santa Rosa.

  • Even though Santa Rosa was more than 30 kilometers from the fault, is suffered some of the worst damage as a result of the earthquake.

  • The underlying geology and building design can combine to exaggerate or dampen the effects of earthquake shaking.

  • or example, the 49-story Transamerica Pyramid shook for more than a minute during a major earthquake

  • but no one was seriously injured and the tower was undamaged.

  • Buildings or structures have a natural resonance or frequency of vibration related to the motion of an earthquake.

  • The greatest damage occurs when ground motion matches the resonance of a building.

  • For example, taller structures will exhibit greater oscillations back and forth with slower ground motions.

  • In contrast, short structures, like two-story homes, will vibrate more violently with faster ground motions.

  • The same earthquake will produce different shaking effects in different earth materials.

  • Bedrock typically produces more frequent, smaller vibrationsthe type of shaking that is more likely to damage shorter structures such as two-story homes.

  • Weaker materials such as muddy soils can amplify ground shaking and produce larger, low resonance vibrations that are more dangerous for multi-story structures.

  • Elsewhere the shaking may cause ground failure. Landslides can occur in areas with steep slopes, blocking roads and burying homes under debris.

  • Liquefaction occurs when shaking causes the compaction of sediment increasing water pressure and causing water and water-saturated material to be ejected at the surface.

  • In this example, a block of granite sinks into the underlying saturated sands when the material is shaken vigorously.

  • Local features like these sand boils or sand volcanoes are often produced.

  • Elsewhere, liquefaction results in subsidence of the land surface and causes objects to collapse into the slurry of water and sediment.

  • In the most extreme events this can cause whole apartment buildings collapse as the ground beneath them gives way.

  • Fault movement during earthquakes can result in adjacent pieces of the land surface being displaced by up to several meters.

  • Some blocks may move up or down, while others shift from side to side.

  • This is one of the easier hazards to avoid, the general rule isDon't build anything on or near an active fault.”

  • We see examples of the effects of ground shaking and ground failure following the magnitude 9.2 Great Alaska earthquake of 1964.

  • This remains the largest US earthquake ever recorded.

  • Liquefaction, subsidence, and landslides swallowed buildings and left behind a chaotic urban landscape.

  • The town of Valdez was moved to a more stable location following the earthquake.

  • We was one major learning objective for this lesson. How confident are you that you could complete this task?

A magnitude 7.8 earthquake struck San Francisco early on the morning of April 18, 1906.

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B1 中級

地震の危険性 I.地盤の故障 (Earthquake Hazards I: Ground Failure)

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    maggieolulu に公開 2021 年 01 月 14 日
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