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

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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

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<<<< 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