lecture on Earthquake epicenter, hypocenter-4.ppt

lecture on Earthquake epicenter, hypocenter-4

1. Earthquakes
•Causes - tectonics and faults
•Magnitude - energy and intensity
•Earthquake geography
•Seismic hazards - shaking, etc.
•Recurrence - frequency and regularity
•Prediction?
•Mitigation and preparedness

2. Causes: accumulated strain
leads to fault rupture
- the elastic rebound model

3. North American tectonic regimes
(much simplified)

4. Styles of faulting

5. Causes: fault movement releases energy as
seismic waves radiating from rupture
Seismic waves

6. Seismic wave forms
P wave
S wave
L wave
(Rayleigh wave)
L wave
(Love wave)

7. Earthquake magnitude:
scales based on seismograms
•M
L=local (e.g. Richter scale) - based on amplitude
of waves with 1s period within 600 km of
epicentre.
•M
b=body-wave (similar to above)
•M
s=surface wave (wave periods of 20s measured
anywhere on globe
•M
o
=seismic moment
•M
w
= moment magnitude

8. The Richter scale
Steps:
1. Measure the interval (in seconds) between
the arrival of the first P and S waves.
2. Measure the amplitude of the largest S
waves.
3. Use nomogram to estimate distance from
earthquake (S-P interval) and magnitude
(join points on S-P interval scale and S
amplitude scale).
4. Use seismograms from at least three
geographic locations to locate epicentre
by triangulation.

9. The Richter scale
nomogram
Nomogram
1
2
3
Steps

10. Z
Locating the epicentre:
X, Y and Z are seismograph stations
Y
X
220 km
epicentre
280 km
150 km

11. Earthquake magnitude:
scales based on rupture dimensions
(equivalent to energy released )
•M
o
= seismic moment.
=  * A * d, where  is the shear modulus
of rock; A is the rupture area, and d is
displacement
•M
w
= moment magnitude.
= 2/3 * log M
o
- 10.7
N.B. moment scales do not saturate

12. Saturation
of non-
moment
scales

13. e.g. Mercalli, Rossi-Forel, San
Francisco scales
MMI (=Modified Mercalli Index)
I Not felt
…..
VI Felt by all. Many frightened and
run outdoors. Persons walk unsteadily.
Pictures fall off walls. Furniture
moved, trees shaken visibly.
….
XII Damage nearly total. Objects
thrown into air.
Earthquake magnitude:
scales based on shaking intensity
Sichuan earthquake, May 12, 2008

14. Source: GSHAP, Switzerland
Earthquake geography

15. Seismic hazard: North & Central America

16. Seismic hazards
•Locating faults
•Estimating recurrence: history and
geology
•Measuring relative motions and
crustal deformation
•Learning from analogies
•Assessing probabilities

17. Locating faults:
Seattle Fault (LIDAR image)

18. Prediction:
where will
the next
earthquake
in the Bay
Area occur?
San Francisco
San Jose
Santa Cruz
Berkeley
Oakland
S
a
n

A
n
d
r
e
a
s
H
a
y
w
a
r
d

19. The Hayward
fault runs
through UC
Berkeley
campus
(US $1 billion
seismic upgrade
program)
Lawrence
Livermore
UC Berkeley

20. San Francisco
City Hall, 1906
Recurrence - historical records

21. Recurrence:
geological evidence
e.g. Pallett Creek,
CA
(after Sieh, et al. 1989)

22. Prediction:
current crustal
deformation

23. Prediction: crustal velocity (mm/yr)
from repeated GPS measurements at permanent
stations
Why are all stations
moving to NW?

24. Learning from analogues
(Turkey - California)

25. The Bay Area:
earthquake
probabilities
(AD2000-
2030)
N.B. A probability of
70% over 30 years is
equivalent to a daily
probability of
1 : 15 000

26. Probabilities, yes!
but prediction, no!
•1996 - Earthquake prediction group of Japanese
Seismological Survey voluntarily disbands (after
Kobe)
•2000 - British researcher argues that prediction
of main shock impossible at present; immediate
goal should be prediction of aftershock location
and magnitude

27. Individual seismic hazards
•Shaking = accelerated ground motion
•Liquefaction = failure of waterlogged sandy
substrates
•Landslides, dam failures, etc.
•Tsunamis = seismic sea waves
•Fire, etc.

28. Predictions of shaking intensity on
San Andreas fault (long segment) in the Bay Area
R
u
p
t
u
r
e

29. Shaking and liquefaction: the importance of
surficial geology

30. Building collapse as a result of soil
liquefaction, Niigata, Japan, 1964

31. Liquefaction and the urban fire hazard:
San Francisco, 1906
2-6 m of lateral
displacement in old
marsh soils -> 300
breaks in water lines
City lost 90% of water
supply; fires raged out
of control
Photos: Archives, Museum of San Francisco

32. Ground motion, structural damage and basin
morphology: Mexico City, 1985
periodic periodicrandom
body\surface surface/body
DamageDamage
heavy light heavyheavy light heavy
ridge
basin basin

33. Bedrock
topography
underlying
Fraser delta

34. Earthquakes
don’t kill;
buildings do!
Building harmonics
Buildings at high risk
•URM = unreinforced
masonry;
•open lower storeys;
•poor ties to
foundations
and between storeys;
•lack of cross-bracing;
•poor quality materials.
Collapsed school building, Ying Xiu,
Sichuan, China (May 12, 2008);
>10,000 children died in this earthquake
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.

36. The response of
mud-brick
buildings to
ground shaking
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
The 2 000-yr old
citadel in Bam, Iran
Pre-earthquake
Post-earthquake
(Dec. 2003)

37. “Much of the building is
done by people putting up
their own houses. But they
cannot afford proper
materials and do not use
skilled labour. There are
many small kilns producing
bricks but because of
demand these are not fired
for the 28 days needed to
make them strong.”
Mohsen Aboutorabi,
Professor of Architecture,
(BBC News, 2003/12/30,
discussing the Bam earthquake in
which ~40,000 died)
Muzaffarabad,
Pakistan
(October 8, 2005
M 7.7; depth 10km)

38. << << wall collapse, Pakistan, 2005
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
<<<< pancaking of ‘soft-storey’
buildings near Algiers (May, 2003);
Complete collapse of multi-storey
apartment, Pakistan, 2005 >>>>

39. Bridge collapse
Loma Prieta earthquake, CA (1989)

40. Preparedness (examples)
Buildings - site selection, design to code,
retrofit, upgrade codes;
Strengthen bridges, dams, pipelines;
Earthquake drills - houses, schools, search
& rescue;
Emergency planning - survival kits,
evacuation routes, fire prevention, utility
failures, communication alternatives,
education

41. Preparedness: Modifying the building code in
the western US
1969 1976 1988 1996
UBC = Uniform Building Code

42. Public education?

43. Post-earthquake
adjustments
Compare:
abandonment of Antigua Guatemala
(mid-C18th) vs.
reconstruction of Lisbon (post-1755),
San Francisco (post-1906), Kobe (post-
1995).

44. Cascadia: megaearthquakes
at the plate boundary
M
w = 9.2?

45. 9.2 (1964)
9.3
(2005)

46. Earthquake sequences,
Nankai Trough and Cascadia
o
r

h
e
r
e
?
S U W W? Y

47. The scientists
Kenji Satake Alan Nelson Brian Atwater

48. Buried marsh soils as evidence for
interplate earthquakes at Cascadia
Y
U
W
S