1. Earthquake Science
(Seismology)

2. Announcements for next week’s lab
Magnetometer survey, working in groups
Please bring:
small notebook
pen
graph paper
umbrella (if raining)
Be prepared to remove all metal (watches, belt buckles, jewelry, etc.)

3. March 28, Alaska
M = 9.1
Up to 12 m vertical displacement

6. October 17, 1989 - Loma Prieta, CA
$6 B in property damage

7. January 17, 1994 - Northridge, CA
Mw = 6.7
> $15 B in property damage

8. January 17, Kobe, Japan
Mw = 6.7
5000 dead, > $200 B in property damage

9. August 17, Izmit, Turkey
M = 7.4
15,700 casualties

10. December 26, Sumatra
M = 9.15
250,000 deaths

11. What are earthquakes? (… and what earthquakes are not!)
Earthquake phenomenology (… fault, epicentre, seismic waves, magnitude)

12. Earthquake: A sudden, violent dislocation in the subsurface caused by stress buildup on a fault.
Fence offset during the Great Earthquake (1906)
4 m
San Andreas
Fault

13. An earthquake is not:
… a giant crack that opens up and swallows buildings whole
…like an underground explosion
… defined by a point in space or time
… easily predictable

14. Fault: A surface across which two blocks can move relative to each other.

15. San Andreas Fault Trace, California

16. Focus: The location on a fault where earthquake rupture initiates.
Focal depth

18. Hypocentre: The calculated position of an earthquake focus.
(almost synonymous)

19. Epicentre: That point on the Earth's surface directly above the hypocentre.
Epicentral
Distance

20. Seismic Waves 101 Body waves Fast Slow P waves
Primary (or compressional)
S waves
Secondary (or shear)
Surface waves
Love waves
Rayleigh waves

22. Northern Peru, Magnitude 7.5SS P
Rayleigh S
ELFO
6.6 min
2005/09/26 01:55: km depth

23. Estimating Epicentral Distance (D)
For a distant earthquake:
D ~ 1000 Dt
Dt in minutes, D in km
For a local earthquake:
D ~ 8 Dt
Dt in seconds, D in km Dt
e.g., D ~ 1000* = 5100 km
Note: Just a crude estimate!

24. Earthquake epicentres are located by triangulation
Requires Dt from at least 3 stations

25. Magnitude: A measure of the strength of an earthquake, as determined by seismographic observations.
Different magnitude formulas are used. Examples:
Richter magnitude
Surface-wave magnitude

26. Small earthquakes happen often, but large earthquakes are infrequent...

28. Surface-wave magnitude formula
(most common)
MS = log10(A) log10(D)
A = amplitude in microns (mm)
D = epicentral distance in degrees
(divide by 111 to go from km to degrees)

29. Magnitude Calculation
672 mm
Magnitude Calculation
Example
D ~ (5100/111) ~ 46 degrees
MS = log10(A) log10(D)
= 7.4