6. 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
7. 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.
10. 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
11. 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
24. 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
25. Individual seismic hazards
• Shaking = accelerated ground motion
• Liquefaction = failure of waterlogged sandy
substrates
• Landslides, dam failures, etc.
• Tsunamis = seismic sea waves
• Fire, etc.
29. 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
30. Ground motion, structural damage and basin
morphology: Mexico City, 1985
periodic periodic
random
bodysurface surface/body
Damage
heavy light heavy
ridge
basin basin
32. 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.
33.
34. 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)
35. “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)
36. << << 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 >>>>
