What is an earthquake?
An episode of ground shaking
Causes of Earthquake
Slip on a FAULT (fracture on which sliding occurs) or formation of a new fault
Movement of magma in a volcano
Explosion of a volcano
Giant Landslide
Meteorite Impact
Underground nuclear-bomb tests
How Is Rock Deformed?
Earth's internal heat mechanical energy
Tectonics forces squeeze, stretch, bend, and break rock in the lithosphere
Differential Stress
Tensional stress: stretches rock
Compressional stress:
squeezes rocks
Shear stress: slippage and translation
Tensional stress
stretches rock
Compressional stress:
squeezes rocks
Shear stress:
slippage and translation
Deformation
Stress=pressure
Strain=deformation of a rock
What happens?The Elastic Rebound Theory
A change in shape of a material that disappears instantly when stress is removed
Features of Faults
Features of Faults
Fault Scarp
Displacement
the amount of slip on a fault
Vertical OR horizontal
Fault Scarp
displacement of the ground surface
Types of Faults
Dip Slip
Strike Slip
When an earthquake occurs...
Movement on a fault releases energy that travels outward as waves
When an earthquake occurs
SEISMIC WAVES are produced
P-waves
Some animals can hear the P waves of an earthquake. Usually we only feel the bump and rattle of these waves
S-waves
This wave moves rock up and down
Surface Waves
L-waves- Love wave
R-waves- Rayleigh wave
How are earthquakes recorded?
Seismograph, seismometer
Seismic Waves
Magnitude of quakes
Measuring Earthquakes
Two ways:
The amount of damage
The amount of energy released
1) Amount of Damage
Measure of the intensity of an earthquake by the amount of damage caused
Mercalli Intensity Scale
Ranges from I (only detected by seismic instruments to XII (complete destruction
2) Energy Released
Determined by the maximum amplitude of ground motion (= MAGNITUDE)
The Richter scale is one (and the original) of many magnitude scales
Moment Magnitude (M)
How large was the largest earthquake ever recorded?
Reliable measurements since about 1900
1960 in Chile: 9.5
2004 Tsunami-causing Quake - Sumatra, Indonesia: 9.1
Most recent: 2005 Northern Sumatra
What seismic waves tell us...
Earth's material & structure
Ground behaviors during an Earthquake
1 Ground Shaking
2 Ground Rupture/Displacement
3 Landslides
4 Liquefaction
Tsunamis
1 Ground Shaking
Surface waves are most damaging
Severity of shaking depends on:
Distance to epicenter
Depth of focus
Direction of rupture
Geology
4 Liquefaction
The abrupt loss of strength of a wet sediment due to ground shaking
Earthquakes and Tsunamis
Wave caused by the vertical displacement of ocean water
Requires a Trigger
What triggers tsunamis?
Earthquakes
Rapid uplift or subsidence of the sea floor: earthquake
Precursor vs. Prediction
Geologic History
What is Seismic Gap?
Can humans effect earthquakes?
Excess weight on the crust
Deep liquid waste injections
Underground nuclear explosions
So then... Earthquakes in Minnesota?
The first record of an earthquake in Minnesota was in 1860. Although the exact date is unknown, the shock was fairly strong in the central part of the State. Very little information is available about this earthquake.
The largest earthquake recorded in Minnesota was in 1975 near Morris
Magnitude 5.0, Intensity VI
Caused minor damage to walls and foundations of basements
The most recent record of an earthquake in Minnesota was in Granite Falls in 1994. It had a magnitude of 3.1 on the Richter scale.
Since 1860, Minnesota has recorded 18 earthquakes, more than half of them during the last 35 years.
Most occur along a line that runs from the southwest to the northeast through Ortonville, Morris, Alexandria, Staples and Nisswa