Transcript Intensity Scale

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, 1958 .
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 1811-12,
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
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
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
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.
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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)
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Earthquakes and Plate Tectonics
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
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 7.4 - 8.0