Earthquakes Causes - tectonics and faults Magnitude - energy and intensity Earthquake geography...

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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
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Transcript of Earthquakes Causes - tectonics and faults Magnitude - energy and intensity Earthquake geography...

Page 1: Earthquakes Causes - tectonics and faults Magnitude - energy and intensity Earthquake geography Seismic hazards - shaking, etc. Recurrence - frequency.

Earthquakes

• Causes - tectonics and faults

• Magnitude - energy and intensity

• Earthquake geography

• Seismic hazards - shaking, etc.

• Recurrence - frequency and regularity

• Prediction?• Mitigation and preparedness

Page 2: Earthquakes Causes - tectonics and faults Magnitude - energy and intensity Earthquake geography Seismic hazards - shaking, etc. Recurrence - frequency.

Causes: accumulated strain leads to fault rupture

- the elastic rebound model

Page 3: Earthquakes Causes - tectonics and faults Magnitude - energy and intensity Earthquake geography Seismic hazards - shaking, etc. Recurrence - frequency.

North American tectonic regimes(much simplified)

Page 4: Earthquakes Causes - tectonics and faults Magnitude - energy and intensity Earthquake geography Seismic hazards - shaking, etc. Recurrence - frequency.

Styles of faulting

Page 5: Earthquakes Causes - tectonics and faults Magnitude - energy and intensity Earthquake geography Seismic hazards - shaking, etc. Recurrence - frequency.

Causes: fault movement releases energy as seismic waves radiating from

rupture

Seismic waves

Page 6: Earthquakes Causes - tectonics and faults Magnitude - energy and intensity Earthquake geography Seismic hazards - shaking, etc. Recurrence - frequency.

Seismic wave forms

P wave

S wave

L wave(Rayleigh wave)

L wave(Love wave)

Page 7: Earthquakes Causes - tectonics and faults Magnitude - energy and intensity Earthquake geography Seismic hazards - shaking, etc. Recurrence - frequency.

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

Page 8: Earthquakes Causes - tectonics and faults Magnitude - energy and intensity Earthquake geography Seismic hazards - shaking, etc. Recurrence - frequency.

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.

Page 9: Earthquakes Causes - tectonics and faults Magnitude - energy and intensity Earthquake geography Seismic hazards - shaking, etc. Recurrence - frequency.

The Richter scale nomogram

Nomogram

1

2

3

Steps

Page 10: Earthquakes Causes - tectonics and faults Magnitude - energy and intensity Earthquake geography Seismic hazards - shaking, etc. Recurrence - frequency.

Z

Locating the epicentre:X, Y and Z are seismograph stations

YX

220 km

epicentre

280 km

150 km

Page 11: Earthquakes Causes - tectonics and faults Magnitude - energy and intensity Earthquake geography Seismic hazards - shaking, etc. Recurrence - frequency.

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

Page 12: Earthquakes Causes - tectonics and faults Magnitude - energy and intensity Earthquake geography Seismic hazards - shaking, etc. Recurrence - frequency.

Saturation of non-

moment scales

Page 13: Earthquakes Causes - tectonics and faults Magnitude - energy and intensity Earthquake geography Seismic hazards - shaking, etc. Recurrence - frequency.

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

Page 14: Earthquakes Causes - tectonics and faults Magnitude - energy and intensity Earthquake geography Seismic hazards - shaking, etc. Recurrence - frequency.

Source: GSHAP, Switzerland

Earthquake geography

Page 15: Earthquakes Causes - tectonics and faults Magnitude - energy and intensity Earthquake geography Seismic hazards - shaking, etc. Recurrence - frequency.

Seismic hazard: North & Central America

Page 16: Earthquakes Causes - tectonics and faults Magnitude - energy and intensity Earthquake geography Seismic hazards - shaking, etc. Recurrence - frequency.

Seismic hazards

• Locating faults• Estimating recurrence: history and

geology• Measuring relative motions and

crustal deformation• Learning from analogies• Assessing probabilities

Page 17: Earthquakes Causes - tectonics and faults Magnitude - energy and intensity Earthquake geography Seismic hazards - shaking, etc. Recurrence - frequency.

Locating faults: Seattle Fault (LIDAR image)

Page 18: Earthquakes Causes - tectonics and faults Magnitude - energy and intensity Earthquake geography Seismic hazards - shaking, etc. Recurrence - frequency.

Prediction:

where will the next

earthquake in the Bay

Area occur?

San Francisco

San Jose

Santa Cruz

Berkeley

Oakland

San

Andreas

Hayw

ard

Page 19: Earthquakes Causes - tectonics and faults Magnitude - energy and intensity Earthquake geography Seismic hazards - shaking, etc. Recurrence - frequency.

The Hayward fault runs

through UC Berkeley campus

(US $1 billion seismic upgrade

program)

Lawrence Livermore

UC Berkeley

Page 20: Earthquakes Causes - tectonics and faults Magnitude - energy and intensity Earthquake geography Seismic hazards - shaking, etc. Recurrence - frequency.

San FranciscoCity Hall, 1906

Recurrence - historical records

Page 21: Earthquakes Causes - tectonics and faults Magnitude - energy and intensity Earthquake geography Seismic hazards - shaking, etc. Recurrence - frequency.

Recurrence:geological evidence

e.g. Pallett Creek, CA

(after Sieh, et al. 1989)

Page 22: Earthquakes Causes - tectonics and faults Magnitude - energy and intensity Earthquake geography Seismic hazards - shaking, etc. Recurrence - frequency.

Prediction:current crustal deformation

Page 23: Earthquakes Causes - tectonics and faults Magnitude - energy and intensity Earthquake geography Seismic hazards - shaking, etc. Recurrence - frequency.

Prediction: crustal velocity (mm/yr)from repeated GPS measurements at permanent

stations

Why are all stations moving to NW?

Page 24: Earthquakes Causes - tectonics and faults Magnitude - energy and intensity Earthquake geography Seismic hazards - shaking, etc. Recurrence - frequency.

Learning from analogues

(Turkey - California)

Page 25: Earthquakes Causes - tectonics and faults Magnitude - energy and intensity Earthquake geography Seismic hazards - shaking, etc. Recurrence - frequency.

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

Page 26: Earthquakes Causes - tectonics and faults Magnitude - energy and intensity Earthquake geography Seismic hazards - shaking, etc. Recurrence - frequency.

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

Page 27: Earthquakes Causes - tectonics and faults Magnitude - energy and intensity Earthquake geography Seismic hazards - shaking, etc. Recurrence - frequency.

Individual seismic hazards

• Shaking = accelerated ground motion

• Liquefaction = failure of waterlogged sandy substrates

• Landslides, dam failures, etc.• Tsunamis = seismic sea waves• Fire, etc.

Page 28: Earthquakes Causes - tectonics and faults Magnitude - energy and intensity Earthquake geography Seismic hazards - shaking, etc. Recurrence - frequency.

Predictions of shaking intensity onSan Andreas fault (long segment) in the Bay Area

Rupture

Page 29: Earthquakes Causes - tectonics and faults Magnitude - energy and intensity Earthquake geography Seismic hazards - shaking, etc. Recurrence - frequency.

Shaking and liquefaction: the importance of surficial geology

Page 30: Earthquakes Causes - tectonics and faults Magnitude - energy and intensity Earthquake geography Seismic hazards - shaking, etc. Recurrence - frequency.

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

Page 31: Earthquakes Causes - tectonics and faults Magnitude - energy and intensity Earthquake geography Seismic hazards - shaking, etc. Recurrence - frequency.

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

Page 32: Earthquakes Causes - tectonics and faults Magnitude - energy and intensity Earthquake geography Seismic hazards - shaking, etc. Recurrence - frequency.

Ground motion, structural damage and basin morphology: Mexico City, 1985

periodic periodicrandom

body\surface surface/body

DamageDamage

heavy light heavyheavy light heavy

ridgebasin basin

Page 33: Earthquakes Causes - tectonics and faults Magnitude - energy and intensity Earthquake geography Seismic hazards - shaking, etc. Recurrence - frequency.

Bedrock topography underlying

Fraser delta

Page 34: Earthquakes Causes - tectonics and faults Magnitude - energy and intensity Earthquake geography Seismic hazards - shaking, etc. Recurrence - frequency.

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.

Page 35: Earthquakes Causes - tectonics and faults Magnitude - energy and intensity Earthquake geography Seismic hazards - shaking, etc. Recurrence - frequency.
Page 36: Earthquakes Causes - tectonics and faults Magnitude - energy and intensity Earthquake geography Seismic hazards - shaking, etc. Recurrence - frequency.

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)

Page 37: Earthquakes Causes - tectonics and faults Magnitude - energy and intensity Earthquake geography Seismic hazards - shaking, etc. Recurrence - frequency.

“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)

Page 38: Earthquakes Causes - tectonics and faults Magnitude - energy and intensity Earthquake geography Seismic hazards - shaking, etc. Recurrence - frequency.

<< << 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

>>>>

Page 39: Earthquakes Causes - tectonics and faults Magnitude - energy and intensity Earthquake geography Seismic hazards - shaking, etc. Recurrence - frequency.

Bridge collapse Loma Prieta earthquake, CA (1989)

Page 40: Earthquakes Causes - tectonics and faults Magnitude - energy and intensity Earthquake geography Seismic hazards - shaking, etc. Recurrence - frequency.

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

Page 41: Earthquakes Causes - tectonics and faults Magnitude - energy and intensity Earthquake geography Seismic hazards - shaking, etc. Recurrence - frequency.

Preparedness: Modifying the building code in the western US

1969 1976 1988 1996

UBC = Uniform Building Code

Page 42: Earthquakes Causes - tectonics and faults Magnitude - energy and intensity Earthquake geography Seismic hazards - shaking, etc. Recurrence - frequency.

Public education?

Page 43: Earthquakes Causes - tectonics and faults Magnitude - energy and intensity Earthquake geography Seismic hazards - shaking, etc. Recurrence - frequency.

Post-earthquake adjustments

Compare: abandonment of Antigua

Guatemala (mid-C18th) vs. reconstruction of Lisbon (post-

1755), San Francisco (post-1906), Kobe (post-1995).

Page 44: Earthquakes Causes - tectonics and faults Magnitude - energy and intensity Earthquake geography Seismic hazards - shaking, etc. Recurrence - frequency.

Cascadia: megaearthquakes at the

plate boundary

Mw = 9.2?

Page 45: Earthquakes Causes - tectonics and faults Magnitude - energy and intensity Earthquake geography Seismic hazards - shaking, etc. Recurrence - frequency.

9.2 (1964)

9.3(2005)

Page 46: Earthquakes Causes - tectonics and faults Magnitude - energy and intensity Earthquake geography Seismic hazards - shaking, etc. Recurrence - frequency.

Earthquake sequences, Nankai Trough and Cascadia

or

here

?

S U W W? Y

Page 47: Earthquakes Causes - tectonics and faults Magnitude - energy and intensity Earthquake geography Seismic hazards - shaking, etc. Recurrence - frequency.

The scientistsKenji Satake Alan Nelson Brian Atwater

Page 48: Earthquakes Causes - tectonics and faults Magnitude - energy and intensity Earthquake geography Seismic hazards - shaking, etc. Recurrence - frequency.

Buried marsh soils as evidence for interplate earthquakes at Cascadia

Y

UW

S