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L11: EARTHQUAKE HAZARD

EARTHQUAKE INTENSITY

Intensity: Qualitative description at a location, as evidenced by observed damage and human reactions

Magnitude: Quantitative measure of the size and strength of an earthquake Qualitative Measurements

-Use descriptions from those affected. Can infer intensity from past events with no recordings

INTENSITY SCALESIntensity is measured using a scale: often ranging from not felt to total damage

Several scales used worldwide:

Most english speaking countries: modified Mercalli intensity (MMI)Japan: Japanese Meterological Agency (JMA)

Central and Eastern Europe: Medvedev-Spoonheuer-Karnik (MSK)

MODIFIED MERCALLI INTENSITY SCALE

Originally developed by Mercalli (Italian Sesimologist)Modified by others, including C. Ricter (American Seismologist), to correspond to California conditions

Ranges from I XII

ISOSEISMAL MAPS

Contour map of earthquake intensityBased on damage observed and reports

OTHER INTENSITY MAPS

Isoseismal maps are good to use but:They take a while to create Only as good as the records and reports allow

Would like something almost real time, i.e. ShakeMaps:

Created moments after an earthquake, and updatedUse recorded values to determine intensity

EARTHQUAKE DAMAGE

Factors include:

Earthquake parameters: magnitude, duration of shaking, epicenter location, depth, etc.

AftershocksSite conditions, ex. rock vs soil

Building style and materials** Hypocenter Depth

- Shallower earthquakes produce stronger shaking

Aftershocks

- Occur often gradually decreasing over time after a main shock- Can be almost as strong as the main shock

- Example: 2002, November 3rd M7.9 Denali Earthquake (Alaska)

Distance from Epicenter

- Waves attenuate more with distance travelled

- Can depend on the region: i.e. western N. A. attenuates faster than eastern N. A. Example: Mexico City, 1985

- Magnitude 8.1- Subduction zone earthquake

- 8,000 lives lost

- Severity of damage related to near-surface conditions*Mexico City built over drained Lake Texcoco

- Ground motion of soft lake sediments amplified by surface waves

- The damage in Mexico City was in a large part due to COINCIDENCE between the dominant periodof the ground shaking and the natural period of vibration of these high-rise structures.

- Most severe damage to almost 400 buildings between 7 and 18 storeys in height.

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LOCAL SITE AMPLIFICATION

Rocks and Soils have a different responsefrequency content

amplitude of ground motions

0-30m is a good indicator of site response (measure Vs30)

Local site amplification effects include:

Broadband amplification

Resonance amplification

Focusing (defocusing) of seismic wavesSurface wave generation at basin edges

BROADBAND AMPLIFICATIONSeismic waves increase amplitude travelling through softer materials

RESONANCE AMPLIFICATION

Seismic waves get trapped in an acoustic mediumCertain frequencies have stronger amplifications

FOCUSING (DEFOCUSING)

Waves combine or disperse depending on bedrock topography

SURFACE WAVE GENERATIONBasin edges are known to generate surface waves

Seismic waves are amplified and periods are longer

LIQUEFACTION

Liquefaction: phenomenon in which the strength of soil is reduced by rapidand violent shaking

Occurs in saturated soils in which the space between particles is filled with water

Liquefied soil behaves like a liquid

Does not have the strength to support a loadLANDSLIDES

Soils or clays along a slope fail and shift downwards

i.e. Force of gravity overcomes the cohesive strength of the soilLemieux, Ontario

- Settled around 1850 as a milling and farming community

- Purchased by South Nation River Conservation Authority and Ministry of Natural Resources for

Ontario in 1989- All homes were moved or destroyed by 1991

Lemieux Landslide, 1993- Caused by heavy rainfall

- Covered 17 hectares- Crater dimensions = 680m long by 320m wide by 18m deep

- 2.5-3.5 million cubic meters of debris

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LANDSLIDES

Other landslides in the region are thought to be caused from earthquakes15 landslides dated around 4550 years ago

Large deformation of soil and sand areas dated at 7060 years ago

BUILDING STYLE AND MATERIALS

Earthquakes dont kill, buildings do.Very few people have died as a direct result of an earthquake. Most deaths occur from secondary disasters; i.e.

tsunami, fire, building collapse, etc.BUILDING RESPONSEDifferent building materials behave differently when subjected to external deformation forces

The same material can behave differently depending on the type of external deformation forces

TensionCompression

Shear

Resonance amplification also affects buildings

ELASTIC LIMITMaterials behave elastically below this limit (i.e. Return to original shape)

Above this limit, two possiblities:

Abrupt failure; stone, brickPlastic deformation; wood, steel

Messina: structures were hard; failed at elastic limit

San Francisco: structures were soft; failed after plastic behavior

EARTHQUAKE DAMAGE

Which type of seismic wave is most damaging to structures?P-waves (compression, tension)

S-waves (shear)Surface waves (tension, compression and shear)

S-WAVES!

Shearing motion is often most damaging

S-waves tend to be the strongest waves close to the epicenterTension, compression already accounted for in building design (gravity)

Surface waves take some distance to fully develop (not as strong near source)

However these can be stronger away from the source

MITIGATION AGAINST SEISMIC HAZARDS

Steps1.Classify Seismic Hazard vs. Risk

2.Identify high/low risk areas3.Determine likelihood of a certain level/type of seismic loading

4.Mitigate the hazard as best as possible!

HAZARD VS. RISKSeismic Hazard: shaking irrespective of consequence

Risk = Hazard * Vulnerability

Vulnerability: likelihood that a community will suffer injuries, deaths or property damage from a hazardousevent

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SEISMIC HAZARD

Can be found either:Deterministically: maximum level of shaking possible

Probabilistically: likelihood of above a certain level of shaking over a specified time frame

What do we need to know?

Seismic sources in the area (historical seismicity, paleoseismic studies)Distance to these sources

Types (or sizes) of earthquakes

SEISMICITYHistorical Seismicity:

Earthquakes often occur where they have in the past

Plate tectonics:Earthquakes often occur along plate boundaries

Paleoseismic studies:

has a young fault (

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NATIONAL BUILDING CODE OF CANADA

Includes a uniform hazard spectrum which buildings must be designed to withstandDifferent for each city

Amplification factors for the different site classes

CURRENT RESEARCH

Modelling basin effects3D vs 1D amplification

Soil strength and amplification

Large scale velocity mappingBASIN EFFECTS

Can see several subsurface basins from microzonation and fundamental period maps

Currently have 3 soil/rock seismometer pairs in these basins

Objective: separate 1-D amplification from 3-D amplification