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Earthquakes
• 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
P wave
S 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 scaleSteps:
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
Nomogram
1
2
3
Steps
Z
Locating the epicentre:X, Y and Z are seismograph stations
YX
220 km
epicentre
280 km
150 km
Earthquake magnitude:scales based on rupture dimensions
(equivalent to energy released )
• Mo= seismic moment.
= * A * d, where 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 non-
moment scales
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
Source: GSHAP, Switzerland
Earthquake geography
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?
San Francisco
San Jose
Santa Cruz
Berkeley
Oakland
San
Andreas
Hayw
ard
The Hayward fault runs
through UC Berkeley campus
(US $1 billion seismic upgrade
program)
Lawrence Livermore
UC Berkeley
San FranciscoCity Hall, 1906
Recurrence - historical records
Recurrence:geological evidence
e.g. Pallett Creek, CA
(after Sieh, et al. 1989)
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)
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
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 onSan Andreas fault (long segment) in the Bay Area
Rupture
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
periodic periodicrandom
body\surface surface/body
DamageDamage
heavy light heavyheavy light heavy
ridgebasin basin
Bedrock topography underlying
Fraser delta
Earthquakes don’t kill;
buildings do!
Building harmonicsBuildings 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 aTIFF (Uncompressed) decompressor
are needed to see this picture.
The response of mud-brick buildings to
ground shaking
QuickTime™ and aTIFF (LZW) decompressor
are needed to see this picture.
QuickTime™ and aTIFF (LZW) decompressor
are needed to see this picture.
The 2 000-yr old citadel in Bam, Iran
Pre-earthquake
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
QuickTime™ and aTIFF (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
>>>>
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 1988 1996
UBC = Uniform Building Code
Public education?
Post-earthquake adjustments
Compare: abandonment of Antigua
Guatemala (mid-C18th) vs. reconstruction of Lisbon (post-
1755), San Francisco (post-1906), Kobe (post-1995).
Cascadia: megaearthquakes at the
plate boundary
Mw = 9.2?
9.2 (1964)
9.3(2005)
Earthquake sequences, Nankai Trough and Cascadia
or
here
?
S U W W? Y
The scientistsKenji Satake Alan Nelson Brian Atwater
Buried marsh soils as evidence for interplate earthquakes at Cascadia
Y
UW
S