Earthquakes
Source: NPS
Source: USGS
Seismic Hazard
Source: USGS
California
Tectonics:
Present
Source: USGS
California Tectonics: Past
p://www.nps.gov/prsf/naturescience/images/Subduction-animation_1.gif
What Is An Earthquake?
An earthquake is the vibration of Earth caused by a rapid
release of energy in the form of waves
http://geography.sierra.cc.ca.us/booth/California/1_lithosphere/earthquakes.htm
Types of Earthquake Waves
 Surface waves
• Complex motion
• Slowest velocity of all waves
 Body waves
1. Primary (P) waves
• Push-pull (compressional) motion
• Travel through solids, liquids, and gases
• Greatest velocity of all earthquake waves
2. Secondary (S) waves
• "Shake" motion
• Travel only through solids
• Slower velocity than P waves
Seismic Waves
P-waves
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decompressor
are needed to see this picture.
S-waves
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decompressor
are needed to see this picture.
A seismogram records wave
amplitude vs. time
http://ww2.lafayette.edu/~malincol/Geol120/seismogram.GIF
Locating an Epicenter
1. Measure the difference in arrival of p and s waves.
Step 2 of locating the epicenter
•
2. Use a graph to
determine the
distance from the
monitoring station
to the epicenter.
Steps 3, 4 & 5 of Locating the Epicenter
3. Draw a circle, centered around the monitoring
station with a radius equal to the distance from the
epicenter.
4. Repeat two more
times.
5. The point where all
three circles
intersect is the
location of the
Epicenter.
Measuring The Size of An
Earthquake
• Intensity – determined by effects on people,
structures, and the environment
• Magnitude – measures energy released at the
source of the earthquake.
Magnitude From Amplitude
http://www.earthquakes.bgs.ac.uk/earthquakes/education/eq_booklet/dia_richter_scale.jpg
Abbreviated Modified Mercalli Intensity Scale
Intensity
Witness observations
I
Felt by very few people; barely noticeable
II
Felt by a few people, especially on upper floors.
III
Noticeable indoors, especially on upper floors, but may not be recognized as an
earthquake. Hanging objects swing.
IV
Felt by many indoors, by few outdoors. May give the impression of a heavy truck
passing by.
V
Felt by almost everyone, some people awakened. Small objects move. Trees and
poles may shake.
VI
Felt by everyone. Difficult to stand. Some heavy items of furniture move, plaster
falls. Slight damage to chimneys possible.
VII
Slight to moderate damage in well-built, ordinary structures. Considerable damage
to poorly built structures. Some walls may fall.
VIII
Little damage in specially built structures. Considerable damage to ordinary
buildings, severe damage to poorly built structures. Some walls collapse.
IX
Considerable damage to specially built structures, buildings shifted off foundations.
Noticeable cracks in ground. Wholesale destruction. Landslides.
X
Most masonry and frame structures and their foundations destroyed. Ground badly
cracked. Landslides. Wholesale destruction.
XI
Total damage. Few, if any, structures standing. Bridges destroyed. Wide cracks in
ground. Waves seen on ground.
XII
Total damage. Waves seen on ground. Objects thrown up into air.
Did You Feel It?
http://earthquake.usgs.gov/eqcenter/dyfi.php
Earthquake Hazards
•
•
•
•
•
•
Ground shaking
Liquefaction
Ground Rupture
Landslides
Tsunamis
Fire
http://www.abag.ca.gov/bayarea/eqmaps/gif99/bayas06m.gif
Shaking
Liquefaction
http://www.tulane.edu/~sanelson/images/liquefaction.gif
Niigata Earthquake, Japan, 1964:
Tilting of apartment buildings
http://cee.uiuc.edu/sstl/education/liquefaction/Pictures/APTS.jpg
Asian Earthquake and Tsunami of 2004
What Happened and Why?
Magnitude 9 earthquake
 Largest earthquake since the
9.2 magnitude Alaskan
earthquake (1964)
 Energy release equivalent of
23,000 atom bombs such as
the one that destroyed
Hiroshima
 Why did it occur here?
Wind-generated Waves vs. Tsunamis
Earthquakes and Tsunamis
What, besides earthquakes, can cause a tsunami?
Tsunami Risk in California
• California is at risk of tsunamis that are
generated locally and from a distance.
• 1964: 10 killed and 35 injured when
tsunami from the 1964 Alaska earthquake
reached Crescent City (Oregon/CA border).
• A Cascadia earthquake could bring a
tsunami to Northern CA in ~ 15 minutes.
Short-term Prediction
Precursors: May or may not occur
• Uplift
• Foreshocks
• Anomalous animal behavior
• Changes in water levels in wells
• Release of radon gas
• Changes in velocity of P waves
Long-Range Forcasting
• Based on knowledge of when and where past
earthquakes have occurred.
Paleoseismology – record of past offsets and
recurrence interval
 Seismic gaps: a segment of an active geologic
fault or subduction zone that has not slipped in a
very long time; they are often considered
susceptible to future strong earthquakes.
Bay Area EQ
Probabilities
Hayward – Rodgers Creek
Faults have the highest
probabilities
Forecasting
(probability) vs.
prediction
Source: USGS
Paleoseismology - the study of prehistoric
earthquakes.
http://www.tulane.edu/~sanelson/images/paleosseis.gif
http://pubs.usgs.gov/fs/1999/fs152-99/images/faults.jpg
M7 1868 Hayward earthquake
• 30 fatalities, 5 in San Francisco (12th
most lethal US earthquake)
• $350,000 (>$5-100M in 2007 dollars) in
damage in San Francisco alone
• Extensive damage in San Leandro,
Hayward, and Fremont (total population
less than 2000)
• Bay Area population was 260,000 (it is
now 27 times larger) Source: USGS
1868 Hayward
Earthquake
(“The Great
SF
Earthquake”)
38°
Source: USGS
37.5°
-121°
Earthquake of M > 6.8 on the Hayward Fault?
A major earthquake today on the Hayward fault
• would impact more than 5 million people and
• Cause estimated total economic losses to
residential and commercial properties would
likely exceed $165 billion.
• Other factors, such as fire, damage to
infrastructure and related disruption would
substantially increase the loss
http://www.usgs.gov/newsroom/article.asp?ID=1899&from=rss