earthquakes

earthquake

• a series of vibrations induced in the earth's crust by the abrupt rupture and rebound of rocks in which elastic strain has been slowly accumulating.
• epicenter- the point on the earth's surface vertically above the focus of an earthquake.
• focus- the location where the earthquake begins

Seismic Waves

• P Waves- a longitudinal earthquake wave that travels through the interior of the earth and is usually the first conspicuous wave to be recorded by a seismograph.
• S Waves- a transverse earthquake wave that travels through the interior of the earth and is usually the second conspicuous wave to reach a seismograph.
• L Waves- an earthquake wave that travels around the earth's surface and is usually the third conspicuous wave to reach a seismograph.

Instruments and Measurements

• Intensity: The severity of earthquake shaking is assessed using a descriptive scale – the Modified Mercalli Intensity Scale.
• Magnitude: Earthquake size is a quantitative measure of the size of the earthquake at its source. The Richter Magnitude Scale measures the amount of seismic energy released by an earthquake.

San Fransico Earthquake

Magnitude and Intensity

• The 1906 San Francisco earthquake struck the coast of Northern California at 5:12 a.m. on April 18 with an estimated "moment magnitude" of 7.8 and a maximum "Mercalli intensity" of "XI" ("Extreme"). Severe shaking was felt from Eureka on the North Coast to the Salinas Valley, an agricultural region to the south of the San Francisco Bay Area of the long vast Central Valley.

Damage and Casualties

• Despite a quick response from San Francisco's large military population, the city was devastated. The earthquake and fires killed an estimated 3,000 people and left half of the city's 400,000 residents homeless.
• Damage: see next photo

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