Earthquakes

Describing Earthquakes Intensity vs. Magnitude

Intensity Scale Intensity Scale- Modified Mercalli Scale -Uses Roman Numerals example: VI or X -The effects of an earthquake are an indication of an earthquakes intensity. Examples: people awaking, damage to brick and stone structures, and movement of furniture The scale ranges from I, which corresponds to imperceptible events, up to XII which corresponds to total destruction.

Intensity Scale -No mathematical basis for the scale -Ranking based on observed effects -Measure of the actual effects at a certain location -Subjective measure, perceived, based on qualitative descriptions.

Intensity Scale Isoseismal lines identify areas of equal intensity.

Intensity Scale This is an isoseismal map showing the impact of the Alaska Earthquake of July 9, XI on the Modified Mercalli Intensity Scale (Magnitude 7.9)

Intensity Scale Isoseismal map for the earthquake of December 16,1811 New, Madrid Missouri. In the winter of , the central Mississippi Valley was struck by three of the most powerful earthquakes in U.S. history.

Magnitude Scale Richter scale- Arabic Numerals -Decimals to the tenths place Ex: Earthquake Magnitude 9.2 on the Richter Scale Earthquake magnitude is a measure of the amplitude of the seismic waves recorded on a seismogram. -Magnitude scales are logarithmic based on powers of 10. -Seismic wave amplitudes increase by 10 times for each unit of the scale.

Magnitude Scale

Richter scale Ex: a magnitude 6.0 earthquake is 10 times the measured amplitude in a magnitude 5.0 earthquake. -Quakes less than 3.5 on this scale are generally not felt at the surface but can be detected by seismometers. -Quakes from 3.5 up to 5.5 are felt, but there is little structural damage; above 6.0 damage increases dramatically.

Magnitude Scale

Effects of Local Geology The Mercalli Scale is a measure of the effects of an earthquake at a particular place and depends not only on strength (magnitude) of a quake, but also the distance from the place of origin and the local geology at the observation point. A given event will have only one magnitude, but many intensity values, which tend to decrease with distance from the origin, although local conditions can produce anomalies.

Effects of Local Geology Intensities are considerably greater over soft soils than solid rock. Ground shaking- amplitude, duration, and damage increases in poorly consolidated rocks. (all of which increase the intensity)

Effects of Local Geology Liquefaction- a geologic process that affects earthquake intensity -Liquefaction is the temporary change of water saturated soil and sand from solid to liquid state.

Effects of Local Geology The Marina district in San Francisco experienced very high intensities during the Loma Prieta Earthquake in 1989 (Magnitude 7.1). The earthquake was centered 80 km south of the city. Nearby parts of the city, built on hard bedrock, did not experience intensities as high as the Marina District which was built on wet, unconsolidated, landfill.

Effects of Local Geology

Mexico City 1985: 8.0 on the Richter Scale and IX on the Modified Mercalli Intensity Scale. -Buildings were greatly affected although the epicenter was far away (300km). -Acapulco, which was much closer to the epicenter, suffered less damage because it stands firmly on bedrock.

Effects of Local Geology Mexico City 1985

Effects of Local Geology Mexico city is built on a basin filled with weak layers of volcanic ash, gravel, plus sand and clay deposits from an old lake bed.

Effects of Local Geology

Materials like soils and sediments, which are much less rigid than bedrock, respond to the passage of seismic waves with a much greater amplitudes. -In other words, loose, unconsolidated soils will shake much more than solid bedrock.

Earthquake Hazards Direct Hazards (due to ground shaking) Collapse of buildings & structures Broken/fallen power lines (electricity) Broken pipelines (water & gas) Damage to roads and bridges

Earthquake Hazards Indirect Hazards Fire: ground motion breaks fuel lines, fuel tanks and power lines. -Water lines are often broken, reducing the amount of water available to fight fires. Landslides: earthquakes trigger the failure of unstable slopes. Tsunami: the ocean floor rises or falls suddenly due to an earthquake, generating giant waves as much as 30 meters tall.

Earthquake Hazards

Earthquakes and Plate Tectonics Earthquakes are caused by plate interactions along tectonic plate boundaries. Earthquakes occur at each of the three types of plate boundaries: divergent, transform, and convergent. -At divergent boundaries, tensional forces produce shallow focus quakes – At transform boundaries, shear forces produce shallow focus quakes – At convergent boundaries, compressional forces produce shallow- to deep-focus quakes (subduction zones)

Earthquakes and Plate Tectonics

80% of all earthquakes occur in the Circum-Pacific belt, most of these result from convergent boundary activity. 15% occur in the Mediterranean-Himalayan belt remaining 5% occur in the interiors of plates and on spreading ridge centers More than 150,000 quakes strong enough to be felt are recorded each year.

Earthquakes and Plate Tectonics

Areas of Risk in the United States Risk- the impact of natural hazards on people Factors that affect risk: -size of the potential natural hazard -How often they occur -How close they are to people and population density

Areas of Risk in the United States Earthquake risk for the United States is based on earthquake history.

Areas of Risk in the United States The areas at highest risk are near plate boundaries. -California has a very long transform fault called the San Andreas Fault.

Areas of Risk in the United States Southern Alaska is near a subduction zone.

Areas of Risk in the United States Large earthquakes have also happened far from plate boundaries. This is due to zones of weakness in the interior of the North American Plate and tension stress.

Areas of Risk in the United States 1886 Charleston, South Carolina Magnitude 7.3 Intensity X

Areas of Risk in the United States 1812 New Madrid, Missouri- Magnitude