Earthquakes
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Causes - tectonics and faults
Magnitude - energy and intensity
Earthquake geography
Seismic hazards - shaking, etc.
Recurrence - frequency and regularity
Prediction?
Mitigation and preparedness
Causes: accumulated strain
leads to fault rupture
- the elastic rebound model
North American tectonic regimes
(much simplified)
Styles of faulting
Causes: fault movement releases energy as
seismic waves radiating from rupture
Seismic waves
Seismic wave forms
S wave
P wave
L wave
(Rayleigh wave)
L wave
(Love wave)
Earthquake magnitude:
scales based on seismograms
• ML=local (e.g. Richter scale) - based on amplitude
of waves with 1s period within 600 km of
epicentre.
• Mb=body-wave (similar to above)
• Ms=surface wave (wave periods of 20s measured
anywhere on globe
• Mo=seismic moment
• Mw= moment magnitude
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.
The Richter scale
nomogram
2
1
Steps
3
Nomogram
Locating the epicentre:
X, Y and Z are seismograph stations
280 km
220 km
Y
X
epicentre
Z
150 km
Earthquake magnitude:
scales based on rupture dimensions
(equivalent to energy released )
• Mo= seismic moment.
= m * A * d, where m is the shear modulus
of rock; A is the rupture area, and d is
displacement
• Mw= moment magnitude.
= 2/3 * log Mo - 10.7
N.B. moment scales do not saturate
Saturation
of nonmoment
scales
Earthquake magnitude:
scales based on shaking intensity
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.
Sichuan earthquake, May 12, 2008
Earthquake geography
Source: GSHAP, Switzerland
Seismic hazard: North & Central America
Seismic hazards
• Locating faults
• Estimating recurrence: history and
geology
• Measuring relative motions and
crustal deformation
• Learning from analogies
• Assessing probabilities
Locating faults:
Seattle Fault (LIDAR image)
Prediction:
where will
the next
earthquake
in the Bay
Area occur?
Berkeley
Oakland
San Francisco
San Jose
Santa Cruz
The Hayward
fault runs
through UC
Berkeley
campus
(US $1 billion
seismic upgrade
program)
Lawrence
Livermore
UC Berkeley
Recurrence - historical records
San Francisco
City Hall, 1906
Prediction:
current crustal
deformation
Prediction: crustal velocity (mm/yr)
from repeated GPS measurements at permanent
stations
Why are all stations
moving to NW?
Learning from analogues
(Turkey - California)
N.B. A probability of
70% over 30 years is
equivalent to a daily
probability of
1 : 15 000
The Bay Area:
earthquake
probabilities
(AD20002030)
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
Individual seismic hazards
• Shaking = accelerated ground motion
• Liquefaction = failure of waterlogged sandy
substrates
• Landslides, dam failures, etc.
• Tsunamis = seismic sea waves
• Fire, etc.
Predictions of shaking intensity on
San Andreas fault (long segment) in the Bay Area
Shaking and liquefaction: the importance of
surficial geology
Building collapse as a result of soil
liquefaction, Niigata, Japan, 1964
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
Ground motion, structural damage and basin
morphology: Mexico City, 1985
Damage
heavy
light
heavy
body\surface
surface/body
basin
periodic
ridge
random
basin
periodic
Bedrock
topography
underlying
Fraser delta
Earthquakes
don’t kill;
buildings do!
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
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
Pre-earthquake
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
The response of
mud-brick
buildings to
ground shaking
The 2 000-yr old
citadel in Bam, Iran
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
Post-earthquake
(Dec. 2003)
“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)
<< << wall collapse, Pakistan, 2005
Complete collapse of multi-storey
apartment, Pakistan, 2005 >>>>
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
<<<< pancaking of ‘soft-storey’
buildings near Algiers (May, 2003);
Bridge collapse
Loma Prieta earthquake, CA (1989)
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
Preparedness: Modifying the building code in
the western US
1969
1976
UBC = Uniform Building Code
1988
1996
Public education?
Post-earthquake
adjustments
Compare:
 abandonment of Antigua Guatemala
(mid-C18th) vs.
 reconstruction of Lisbon (post-1755),
San Francisco (post-1906), Kobe (post1995).
Cascadia: megaearthquakes
at the plate boundary
Mw = 9.2?
9.2 (1964)
9.3
(2005)
or here?
Earthquake sequences,
Nankai Trough and Cascadia
S
U
W
W?
Y
Kenji Satake
The scientists
Alan Nelson
Brian Atwater
Y
U
W
S
Buried marsh soils as evidence for
interplate earthquakes at Cascadia