1. JapanTsunami_SendaiAirport_3-11-11

2. OnagawaTown,MiyagiPrefecture_3-26-11

3. What is an Earthquake?
We inhabit a fragile built environment of houses, buildings & transportation systems that is anchored in Earth’s crust
This environment is vulnerable to seismic vibration, ground rupture, landslides and tsunamis

4. What is an Earthquake?
Plate movements generate forces at the boundaries that can be described in terms of stress, strain and strength
Stress – local forces per unit area that cause rocks to deform
Strain – relative amount of deformation
Rocks fail – break – when they are stressed beyond a critical value called their strength

5. What is an Earthquake?
Earthquakes are the result of stress that builds up over time, as tectonic forces deform rocks on either side of a fault
They occur when the stress exceeds the strength of the rocks, which suddenly break along a new or preexisting fault
The two blocks of rock on each side of the fault slip, releasing the stress suddenly, causing an earthquake, which generates seismic waves

6. Elastic Rebound Theory
Faults remain locked while strain energy accumulates in the rocks on either side, causing them to deform until a sudden slip along the fault releases the energy
Elastic means the rocks spring back to their undeformed shape when the fault unlocks
The distance of displacement is called the fault slip

7. Photo: http://www.winona.edu/geology/MRW/mrwimages/elasticrebound.jpg

8. Focus and Epicenter
Focus – point at which the slip begins – somewhere below the surface
Most earthquakes in continental crust have focal depths from 2 – 20 km (rocks behave in a ductile manner below 20 km)
Subduction zone earthquakes can have foci as great as 690 km deep
Epicenter – the geographic point on Earth’s surface directly above the focus

9. Photo: http://www.yorku.ca/esse/veo/earth/image/1-10-15.JPG

10. Fault Rupture Does not happen all at once
Starts at focus and expands outward on fault plane at ~2 – 3 km/s
Rupture stops when stress can no longer break the rocks
Size of earthquake is related to total area of fault rupture

11. Fault Rupture
Most earthquakes are very small and the rupture never breaks the surface
However, in large, destructive earthquakes, surface breaks are common
Ex: 1906 San Francisco EQ caused surface displacements averaging 4 m (13 ft.) along a 400 km section of the San Andreas

12. Tree displaced 15 ft (from where person is standing) Photo: http://www
.ac.uk/Resources/EarthSci/Tectonics/images/ranch.jpg

13. Fault Rupture
Faulting in largest Earthquakes can extend more than 1000 km and the slip can be as large as 20 m (~60 ft)
Stored strain energy is released in the form of frictional heating and seismic waves

14. Foreshocks and Aftershocks
Aftershocks occur as a consequence of a previous EQ of larger magnitude
Their foci are distributed in and around the rupture plane of the main shock
They can last from weeks to years
They can compound damage from the main shock

15. Foreshocks and Aftershocks
Foreshocks are small earthquakes that occur near, but before, a main shock
Many large earthquakes have been preceded by foreshocks
Scientists have tried to use them to predict large earthquakes
Hard to distinguish foreshocks from other small earthquakes

16. Seismic Waves Ground vibrations produced by an earthquake
Enable us to locate earthquakes and determine type of faulting that produced them
4 types:
Body Waves
a. P waves
b. S waves
Surface Waves
a. Rayleigh waves
b. Love waves

17. Primary or P Waves Travel through Earth and are
first to arrive at seismic station
Compressional waves
Can be thought of as
push-pull waves: they push
or pull particles of matter in
the direction of their travel

18. Secondary or S Waves
Follow the P waves through Earth, and arrive second at the seismic station
Shear waves
Displace material at right angles to their path of travel

19. Surface Waves Arrive last after traveling around Earth’s surface
Speed slightly less than S waves
Rayleigh waves – travel in rolling motion over surface
Love waves – shake the ground in sideways motion

20. Locating the Epicenter
Time interval between P and S wave arrival depends on distance waves have traveled from focus
If three or more seismic stations know the distance, then the epicenter can be located using triangulation

21. Measuring the Size of an Earthquake
Magnitude of an earthquake is the main factor that determines the intensity and potential destructiveness of an earthquake
Two scales:
Richter magnitude
Moment magnitude

22. Richter Magnitude Developed by Charles Richter in 1935
Each earthquake is assigned a number on a logarithmic scale
Two earthquakes differ by one magnitude if the size of their ground motions differs by a factor of 10
This means the ground motion of a magnitude 6 earthquake is 10 times greater than a magnitude 5 and 100 times greater than a magnitude 4
The energy released as seismic waves increases by a factor of 33 for each Richter unit

23. Moment Magnitude
Seismologists now prefer a measure of EQ size more directly related to the physical properties of faulting that causes the EQ
Moment magnitude is the product of the area and the average slip across the fault break
It increases by about 1 unit for every 10-fold increase in the area of faulting
It produces roughly the same numerical values as Richter’s method, but can be measured from seismograms and determined by field measurements of the fault

24. Earthquake Size and Frequency
Large earthquakes occur much less often than small ones
Worldwide figures of earthquake size per year:
1,000,000 with magnitudes greater than 2.0
100,000 greater than 3.0
1000 greater than 5.0
10 greater than 7.0
Earthquakes with magnitude above 8.0 occur about once every 3 years
Very large ones like the 2004 Sumatra quake (magnitude 9.2), 1964 Alaska (9.2) and 1960 Chile (9.5) are rare

25. Shaking Intensity
Amplitude of shaking depends on distance from fault rupture
Damage from shaking depends on distance from populated areas
Estimated shaking determined with modified Mercalli intensity scale – values from I (not felt) to XII (damage total) (for full scale see page 307 of textbook)

26. Shaking Intensity maps of 1906 & 1989 San Francisco Earthquakes
Photo:
Boatwright_BA_intensity.jpg

27. Earthquakes and Faulting
Most earthquakes occur at plate boundaries
Largest earthquakes occur at convergent boundaries on megathrust faults that form where one plate subducts beneath another
Exs: Sumatra (2004), Alaska (1964) & Chile (1960): largest EQ ever recorded, magnitude 9.5

28. Intraplate Earthquakes
A small percentage of earthquakes occur in plate interiors
Foci are shallow and occur mostly on continents
Many occur on old faults that use to be part of plate boundaries and are now areas of crustal weakness
Examples include some of most famous in American history: New Madrid, Missouri ( ), Charleston, South Carolina (1886), and Cape Ann, near Boston Massachusetts (1755)

29. Regional Fault Systems
Zones of deformation between plate boundaries usually have a network of interacting faults – a fault system – rather than a single fault
Ex: in California, the “master fault” is the San Andreas, however, there are many subsidiary faults on either side that generate large earthquakes.
Most of the damaging earthquakes in California during the last century have occurred on these subsidiary faults

30. San Andreas Fault System
Photo:
earthq3/map1a.gif

31. Earthquake Destructiveness
Over the last century, earthquakes worldwide have caused an average of 13,000 deaths per year and hundreds of billions of dollars of damage
Two California earthquakes – 1989 Loma Prieta (mag 7.1 & $10 billion in damage) and 1994 Northridge (mag 6.8 & $40 billion in damage) – were among the costliest disasters in U.S. history because of nearby urban areas

32. Loma Prieta Earthquake Damage

33. Nimitz Freeway after the Loma Prieta Earthquake, 1989
Photo:

34. Column collapse along Cypress Viaduct, Loma Prieta EQ, 1989
Photo: GSA, Explore Earthquakes CD-Rom