Types of Stress

(3)

Types of Deformation

(3)

Deformation is the change that occurs 

1. Elastic- temporary and reversible, like a balloon
2. Plastic- permanent, the folding and flowing of a rock
3. Brittle - permanent, fracturing of the rock, characteristic of cold temperatures; earthquakes produce this deformation

Where is the brittle deformation zone?

Where is the plastic deformation zone?

Fault types

(2 main kinds)

1. Dip-slip faults (vertical motion)
2. Strike-slip faults (horizontal motion) 

Fault types:

Dip-Slip Faults 

(3)

Fault Types:

Stride-Slip Faults

(2)

Stride-slip faults are a product of shear stress, occur at transform boundaries 

1. Right-lateral 
2. Left-lateral 

The vibrations produced by the rapid release of energy

* Most EQ's are produced by the breaking along a fault 

Fault types 

(picture)

San Francisco Earthquake (1906)

The April 18, 1906 earthquake was one of the largest to occur in North America in historic times (M8.3). The earthquake was centered near SF. Despite the severity of the ground vibrations, the greatest amount of damage (80%) was caused by the ensuing fires. The city was not built to withstand earthquakes. Not only did buildings collapse, but both water and gas pipes were ruptured. The gas leaks caught fire and there was no water to stop the flames. One of the most devastating fires (known as the "ham and eggs fire") was started by a housewife making ham and eggs without realizing that her chimney had fallen away. The fire ravaged the city for three days and burned an area of 520 city blocks. About 700 people died directly from the earthquake, and up to 5000 deaths occurred in the ensuing months due to epidemics of hideous diseases. An outbreak of bubonic plague was reported in over 150 cases.

elastic rebound moves structure deformation of rock into "s" shape

6 cm/year

Determining rate of motion

--Fault creep

200 yrs. since last EQ

rate of motion = 6cm/yr.

200 x 6 cm/yr = 1200 cm or 12 m or 36 ft is the rate of motion that would be produced by an EQ 

Fault creep- an area that has not experienced an EQ

An area where most EQ's occur in the rim of the pacific ocean; corresponds with the "ring of fire" aka where most volcanic eruptions occur; considered most dangerous place on earth 

Earthquakes occur when....

Stress builds up over time 

Earthquake Depth 

(occur beneath earths surface)

Less than 70 km = shallow EQ

70-300 km = Intermediate EQ

greater than 300 km = Deep EQ

Mid-oceanic Ridge --> shallow Earthquakes

Subduction Zones --> shallow and intermediate EQ's

*Deep EQ's occur most often on land 

Benioff Zone

Earthquake activity associated with subduction zones 

Hypocenter

Epicenter

Hypocenter-- the focus or source of the EQ, beneath the surface 

Seismic Waves

Vibrations that travel in all directions away from the hypocenter and measured by a seismograph. 

Seismic waves will slow down as they travel through looser and looser ground materials. Notice that the amplitude (height) of the waves also increases in the loosest materials, thus generating greater degrees of ground shaking.

Types of Seismic Waves

Seismic waves are produced when an EQ occurs 

1. Body Waves

• Primary  waves (P-waves) 
• Secondary waves (S-waves)

2. Surface Waves (L-Waves) 

• Rayleigh waves
• Love Waves 

Body waves 

Body waves can travel through the earths interior 

Seismic Waves

Picture 

Primary Waves

• Also known as p-waves
• change the volume of the rock as a wave passes through 
• Compressional 
• analogous to S-waves
• Slinky-like 
• DOES NOT change shape

Secondary Waves

• Also known as S-waves
• Changes the Shape of the rock as the wave passes through
• Rope-like
• change in shape or temperature deformation

Surface Waves

• Only on the surface
• restricted to traveling on the exterior of earth
• Also known as L-waves
• "L" stands for "long waves"

Rayleigh Waves 

• Change in shape as wave passes through
• rotate backwards 

Love Waves

• Change in shape as wave passes through 
• Side-to-side motion 

Order of waves 

• begins with p-waves
• Then s-waves
• last is L-waves 

More about P, S and L waves

P-S Interval 

• P-waves move 6 km/second
• S-waves move 3.5 km/second 
• L-waves are the most dangerous even though they are the slowest
• P-S Interval-- the greater the interval, the farther away you are from the EQ.  It is based on the speed of the two waves traveling through the same kind of material 

Triangulation 

A method of finding the epicenter of an EQ by determining 3 locations that felt the EQ 

Factors used to Measure Earthquakes

1. Intensity
2. Magnitude

Measuring Earthquakes:

INTENSITY

• Intensity= the effects of an EQ at any given location
• Modified Merrcalli Intensity Scale--> 1= EQ not felt; to 12= Total damage 
  • this is determined by polling people who experienced the EQ
  • An intensity map shows different areas affected by the EQ and the different intensity levels. EX: In Northridge EQ, Santa Monica felt an intensity of 9 due to soft ground

Measuring Earthquakes:

MAGNITUDE

• Magnitude is the directed measure of the strength of an EQ, traditionally measured by a Richter Scale
  • Richter scale is not an accurate measure for very strong EQ
• Today we use the "magnitude scale" 

Magnitude Scale 

A logarithmic scale (10x's)

• Must know the P-S interval and amplitude of S-wave (height of wave) 

EX: P-S interval = 24 seconds

      Amplitude = 23

~ therefore the EQ was a M5