Intraplate Earthquakes
• New Madrid, Missouri: December 1811, January 1812, February 1812
• Eastern U.S. earthquakes less frequent, but more widely felt
• Crust is older and transmits stress
Intraplate Earthquakes
Earthquake Size and Characteristics
• How big is it?• Answer in terms of perceived
effects: intensity• Answer in terms of amount of
energy released: magnitude
Earthquake Intensity
• Mercalli Intensity Scale developed in 1902
• Based on effects• Local small quake
is similar to distant large quake
http://www.scec.org/instanet/01news/images/NorthridgeSMap.gif
Felt effects using Mercalli Scale
Hypothetical M7.8 earthquake
Felt effects using Mercalli Scale
Landers 1992 earthquake:
Felt intensity by zip code
Earthquake Magnitude• Richter Magnitude
Scale (ML) derived by Charles Richter in 1935– Uses maximum
amplitude of earthquake waves on seismograph
– Logarithmic scale• Richter magnitude less
accurate above M6.5
Problems with scales• Moment magnitude is measure of total energy
expended during earthquake– Determined from long-period waves– Moment = (shear strength of rocks) x (surface area
of rupture) x (slip distance on fault)– Each number is 32X energy– Each 2 numbers =1000X
– This is the most common scale for quakes >3.5
Another example of M=1/f
1995Kobe earthquake,Japan
1989Loma Prieta earthquake,SF Bay Area
Liquifaction
Water in sediment causes solid rock to behave like a liquid.
Liquifaction
Secondary Ground Effects• Surface rupture-scarp• Earthquakes often trigger landslides• Can also cause liquefaction
– Soils become almost liquid when shaken, solidify when shaking stops
– Significant damage to structures atop liquefied sediments
• Fires
Surface rupture-scarp
1906 earthquake surface rupture.8’ fence offset above
http://mnw.eas.slu.edu/Earthquake_Center/1906EQ/1906thumb.htmlAnd http://quake.wr.usgs.gov/info/1906/images/fenceoffset_big.html
Australia, 1968, M6.8
Armenia, 1988, M6.9
Borah Peak, ID: 1983 M7.3
California, 1979, M6.9
Landslides caused by 2002 Denali Fault earthquake
1965 Seattle quake M6.5
Liquifaction
Water in sediment causes solid rock to behave like a liquid.
This residential and commercial building sank more than three feet into the partially liquefied soil.
:
Liquifaction: Niigata, Japan, 1964
San Francisco 1906 M8.3
Secondary effects: Fire
Long term probability• Probability of where and when an earthquake will
strike used to construct risk map
Earthquake Hazards2% chance in next 50 years
The San Francisco Bay Area
Las Cruces:
15-18%
chance
of M5
in
50 yrs.
Sample Test Questions
2. What does the elastic rebound theory describe? A) the build-up and release of stress during an earthquake B) the fluctuations in groundwater prior to an earthquake C) the formation of mountain ranges by successive earthquakes D) the uplift of the crust in response to erosion
Use the following to answer questions 3-4:
3. Point A, where slip initiated during the earthquake, is called the __________. A) dip B) epicenter C) focus D) strike
4. Point B is called the earthquake __________. A) dip B) epicenter C) focus D) strike Ans: B
Use the following to answer questions 9-12:
9. What causes the up-and-down wiggles on the seismogram? A) electromagnetic
pulses C) ground vibrations
B) tsunami waves D) variations in air pressure Ans: C
10. Which set(s) of waves travels through the Earth's interior? A) set A B) set B C) sets A and B D) sets A, B, and C Ans: C
11. Which set(s) of waves is/are most likely surface waves? A) set A B) set B C) set C D) Sets A, B, and C are all surface waves. Ans: C
14. Which of the following correctly lists the order in which seismic waves arrive at a seismograph station?
A) P waves surface waves S waves B) P waves S waves surface waves C) S waves P waves surface waves D) surface waves P waves S waves