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