Earthquakes and Tsunamis
Jan 21, 2015
Earthquakes and Tsunamis
Earthquakes
• Late Thursday night (our time), an earthquake occurred of the coast of Japan.
Earthquakes
• Late Thursday night (our time), an earthquake occurred of the coast of Japan.
Earthquakes
• One of the largest earthquakes since we’ve been measuring them (6th strongest).
What is an Earthquake?
• Earthquakes are movements of the earth that release pressure.
• Occur along faults.
- planar breaks in rock where there is movement.
- occur in all sizes.
Faults
Faults
What Causes an Earthquake?
• Build up of strain (gradual or sudden) deforms the earth until the elastic limit is reached – then things snap.
• Elastic Rebound.
• Energy is released in the form of seismic waves.
What Causes an Earthquake?
How do we Measure an Earthquake?
• Most common way is by the Richter Scale.
• Based on the amplitude of the seismic waves.
• Scale is logarithmic! Difference in two whole numbers means in increase in vibrations by a factor of 10!
- ground that moves 1 inch in a 4.0 earthquake moves 10 inches in a 5.0 quake, and 100 inches in a 6.0 quake.
Where Does the Strain Come From?
• Plate tectonics.
• Outermost portion of our planet is broken into different pieces called plates.
• The plates move all the time.
Tectonic Plates
Plate Boundaries
• Most of the action occurs at the boundary between two plates.
• Plates can move away from one another, towards one another, or just slide past one another.
Plate Boundaries
Convergent Plate Boundaries
• When plates run into one another, the more dense plate is forced underneath the less dense plate, a process called subduction.
Convergent Plate Boundaries• When ocean material runs into continental material one gets a trench in the ocean, and a chain of volcanic mountains on the continent.
Convergent Plate Boundaries• When ocean material runs into continental material one gets a trench in the ocean, and a chain of volcanic mountains on the continent.
Convergent Plate Boundaries• When ocean material runs into ocean material one gets a trench in the ocean, and a chain of volcanic islands called an island-arc.
Convergent Plate Boundaries• When ocean material runs into ocean material one gets a trench in the ocean, and a chain of volcanic islands called an island-arc.
Earthquake Damage
• Ground movement
• Fire
• Landslides
• Liquefaction
• Tsunamis
Earthquakes and Tsunamis
• Sudden movement of the sea floor will displace a large volume of water.
Tsunami Speed
• The speed of the tsunami is controlled by the depth of the water.
C (celerity) = √ g x d
where g is gravitational acceleration, and d is the depth of the water in which the wave is travelling.
• Since g is a constant, the speed is mostly a function of water depth (wavelength also plays a role, but we’re neglecting it for now).
Tsunami Speed
• Average depth of the ocean is 4000 m.
C (celerity) = √ g x d
C = √ 9.8 m/s2 x 4000 m
C = √ 39,200 m2/s2
C = 198 m/s
• 1609 meters in a mile, 3600 seconds in an hour.
C = 198 m/s / 1609 m/mile x 3600 s/hr
C = 443 miles/hour!
Tsunami Speed
• As the wave approaches the shore its speed decreases.
C (celerity) = √ g x d
• Aside from slowing, other things happen to the wave as it enters shallower water.
Tsunami Occurrences
Indonesia, December 2004
• 9.1 magnitude earthquake occurred only 8 miles deep.
• Rupture was 1200 km long with 15 m of vertical displacement.
Indonesia, December 2004
Indonesia, December 2004
• Magnitude 8.9.
• Strong enough quake to affect our axis of rotation (a day on earth is now a little shorter).
Japan, March 2011