By:• Derek Stoen• Carlos Valdez• Emily Dong

Interplate

Crustal

Earthquakes that occur in the fault zones at plate boundaries

One plate tries to move, stress builds up, and causes plates to slip

Slipping creates an earthquake

7.1 largest known, 7.5 largest expectedDepths of 45-60 km in Juan De Fuca and

Gorda PlatesTwo Magnitude 7 events every 130 yearsFive events greater that 6.0 since 1909

From offshore deformation front to Western Coastal Ranges and Olympic Mountains

Occurs above fault zoneRare compared to interplate earthquakes4 known in last 1000 years

Occurs on Vancouver Island, North Cascades, Seattle Fault

Portland Fault Possible

Ground ShakingStructural HazardsLiquefactionTsunamis

Most seismic damage is due to ground shaking

Influenced by distance from the epicenter, height, stiffness, and density of the soil

Soils amplify at certain frequencies and attenuate (loose energy) at other frequencies

Common types of waves:• Body waves - P and S waves• Surface waves - Love and Rayleigh waves

P-wave S-wave

Rayleigh Wave Love Wave

As waves travel from stiff materials to soft materials they typically slow down and their stress amplitude decrease, while their displacement amplitudes increase

Attenuation of waves: loss of energy (decreases wave amplitude) over time and distance from the seismic source• Material attenuation: heat generation and slipping of grains

(hysteretic)• Geometric attenuation: radiation damping (energy per unit

volume decreases) High frequency amplitudes attenuate over the distance from

the epicenter to the site. Farther distances receive lower frequency waves

Ground motion frequencies determine the severity of the damage to the structure

Structures handle vertical loads very well, but the same cannot be said for horizontal loads

Structural hazards depends on magnitude, frequency and duration of ground motion

Structures have natural frequencies that depend on mass and stiffness of each floor

The most damaging affects occur when ground shaking frequencies are close to the natural frequencies of the structure

Example: 1985 Mexico City Earthquake• 8.1 magnitude, epicenter was over 200 miles away in the Pacific Coast• High plasticity clay layers 30 meters thick amplified the low frequency motions

(period ~2 seconds)• This matched the natural frequency of many buildings in that area• Because of the long duration, brittle structures weakened and they too matched

the frequency of the earthquake• About 90% of all 10 to 15 story buildings were destroyed• 9,500 died

Local example:• 1965 Seattle Tacoma

Earthquake Damage to unreinforced

buildings and buildings along the waterfront

$50 million dollars in damage• 1949 Olympia Earthquake Severe damage to older

masonry buildings 10 schools were condemned

and abandoned $150 million dollars in damage

Damage at the Washington State Training School for Boys in Chehalis, 1949

Loss of soil strength causes soil to appear to flow like a liquid

Occurs in saturated cohesionless soils under undrained conditions when disturbed repeatedly

Cohesionless soils densify when loaded because of the rearrangement of grains. When saturated, this tendency for densification causes excess pour water pressures to increase and effective stresses to decrease

Two main types of liquefaction:• Flow liquefaction: cyclically triggered, but

statically driven• Cyclic Mobility: developed incrementally

during earthquake shaking Liquefaction causes lateral spreading

landslides, loss of bearing pressure for foundations and roadways

Example: Niigata Earthquake (magnitude of 7.5)

Caused by rapid seafloor movements

In the open sea, heights are usually less than 1 meter tall

As they approach the shore, they slow down, but the height increases, sometimes up to 20 meters Caused by Tsunami from the Good

Friday Earthquake, Alaska 1964

LiquefactionTsunamisLandslidesSeismic Hazards

ExplanationLiquefaction susceptibility: Moderate to HighLiquefaction susceptibility: Low to Moderate

Liquefaction susceptibility: Very Low

Bedrock

Peat deposits

ftp://198.187.3.44/geology/pubs/ofr04-20/ofr04-20_king_liq.pdf

Liquefaction

www.pmel.noaa.gov/pubs/PDF/wals2794/wals2794 pdf

Tsunamis

High Risk

Medium Risk

Low Risk

http://pubs.usgs.gov/of/2006/1139/pdf/of06-1139sh2.pdf

Landslides

http://earthquake.usgs.gov/regional/pacnw/hazmap/seattle/index php

Seismic Hazards

www.usgs.org

Plate Orientation

Figure 13. Epicenters and dates of the largest Pacific Northwest earthquakes that occurred between 1872 and 1987. The large symbols are epicenters of earthquakes whose maximum intensities were reported as VIII or greater on the Modified Mercalli Intensity Scale (MM) (Table 1); smaller symbols are MM intensities of VII. The locations of principal volcanoes in the region are also shown.

http://www.pnsn.org/INFO_GENERAL/NQT/f13.html

Historic Earthquakes

http://www.crew.org/papers/CREWCascadiaFinal.pdf

• Scenario M9.0 Earthquake•Bridge Functionality (pic)•Peak Ground Accelerations (pic)•Secondary Hazards

•Fire•Hazardous Materials•Building Vulnerabilities•Transportation

Prediction