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