Tsunamis
NOT A TIDAL WAVE!!
Created by the displacement of water
Earthquakes, volcanoes, landslides
How does a wave work?
Start by looking at regular ocean waves
Ocean waves
Wave length, Wave height and motion on open ocean.
Primarily get swells…no breakers
Waves near the shore
Waves break as wave base interacts with the floor. Slows base while top continues at original speedà wave falls over
How are tsunami waves different
Much longer wavelengths
Long period
Normal wave has a period of 5-15 seconds
Tusnami wave has a period of 7 min to 1 hour
Tsunamis travel up to 500 mph depending on water depth
Tsunamis have no expression on open ocean
Size of wave at shore depends on
Seafloor configuration (
Distance from quake
Crescent City 21 ft wave
San Fran 3 ft wave
How are Tsunamis monitored
Tsunami warning system
26 countries participate
Monitors
Sensors measure change in water pressure as waves pass over them
Indonesian Tsunami
Do all earthquakes cause tsunamis?
Why was the tsunami in
What Is required for a warning system to work?
Where else did the tsunami travel?
Northwest Earthquakes
What type of quakes do we get in the northwest due to the configuration of the tectonic plates?
How do we know we get great subduction quakes in the NW if we have never experienced them first hand.
Some Facts and questions
Other subduction zones experience great quakes.
NW subduction zone is very quiet with few quakes…Why?
Questions:
Is subduction still occurring?
Are we monitoring the area properly?
Is the plate moving smoothly so no strain?
Evidence of NW great quakes
Deposits found
Submerged marshes
Drowned forests
Tree rings date to 1699 trees being alive but dead in 1700
tsunami deposits (here and in
Date to the same year
Marine sediments among forest debris
All found from Vancouver island to
Current research on how subduction zone is behaving
Due to subduction zone, coast range is uplifting
S OR,
Central OR and
Coastal areas are being tilted east
Subduction zone is locked and strain is accumulating at ~1.6” per year
In 1987 Cascadia subduction zone declared major threat
Potential to produce quake of mag 9.5 or greater
Turbidite deposits show breaking in past…entire length.
Strain studies show mag 8
Other NW Quakes
Deep quakes on downgoing plate
Nisqually Quake
Normal fault ruptured on subduction plate
1949 7.1 magnitude quake
Magma Quakes
Found offshore at ridges
Within the Cascade volcanic center
Small and shallow
Transform Faults
Offshore at Juan de Fuca and Gorda Ridges
Can be large but low intensity
Inland Crustal Faults
Created by tectonic interactions
Blind fault
Found by seismic reflection and offset
Runs W-E under
Uplifted areas suggest M7 quakes
Considered active fault
3 major faults
Cuts through downtown
Capable of M6-7 quakes
Last ruptures 10,000 yrs ago
East Bank Fault
Oatfield Fault
Other Faults and Quakes
Mount Angel Fault
Scott Mills Quake: March 1993 (M5.6)
5.9 and 6.0
Killed one person
Other Regions with quakes
Basin and Range
Series faults representing a region of crustal extension…Region is very
seismically active
Rock Creek Campground
August 1959: 7.3 quake
Landslide of 40 million cubic yards
Destroyed campground
Fault in lake bed
Major Cities in Basin and Range
All are located near or on faults that are active
Expect mag 7.1
1st recognized ~100 yrs ago. Why more of an issue now?
New
1811-1812
3 earthquakes occurred from Dec-Feb
Magnitudes 8.1, 7.8, 8.0
Quakes occurred on 3 faults
2 are right Lateral strike slip faults
One is a reverse fault
Still seismically active
Why do earthquakes occur here?
Possibly a fail rift zone
Rupture of Reelsfoot thrust fault
Fault crosses river 3 times
Major disruptions of riverbed and flow patterns
Waterfall formed due fault scarp
Flow barriers formed due to fault scarps
Sandblows or blowouts very prevalent
Archeological accounts show this isn’t the first time this has happened in this area
Recent quakes in this region
In 2003 USGS forcast:
7-10% chance of mag 8 in next 50 years
25-40% chance of mag 6 or greater in next 50 yrs
August 1886 mag 7.0
High Mercalli intensity…Why?
Other Regions of seismic activity
Many faults on East coast
Most faults hidden by sediments (blind faults)
Stress builds up over time by plate motion
Isostatic rebound from glaciation
