Geology Earthquakes

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