Earthquakes

Where do EQ's occur?

• Think specifically and globally

• Foci

Where do earthquakes usually occur?

• At plate boundaries

• Because of movement or slip along a fault or between plate boundaries

How do eq occur?

• Elastic rebound theory

  • Rocks bend and store elastic energy

  • Frictional resistance holding rocks together at fault is overcome at weakest point (focus)

  • Eqs occur as deformed rock "springs back" to original shape which is called elastic rebound

Generalized rock mechanics curve:

• depends on response of rocks to stress

1. Elastic strain

   • Strain/stress recovery

2. Plastic strain

   • Ductile deformation

3. Fracture

   • Brittle deformation and stored elastic strain energy is released

Earthquakes

• Rocks on both sides of existing fault deformed by tectonic forces

• Rocks bend and store elastic strain energy

• Frictional resistance holding rocks together at fault is overcome at weakest point (focus)

• Eqs occur as deformed rock "springs back to original shape"

• When force is applied to rocks until their internal strength is exceeded, they break and snap back to their original shape

• Eq is vibration of earth due to rapid release of stored elastic strain energy during faulting

  • Energy wave radiate in all directions from source, the focus (epicenter directly above focus on surface)

  • Instruments, called, seismographs, around world record earthquakes

• Most earthquakes result when movement occurs along fractures (faults)

• The point at which fracturing begins, where energy is released, is an earthquake's focus

• The epicenter is the point at the surface directly above the surface

  • Hypocenter = focus

  • Shallow focus < 70 km

  • Intermediate 70 - 300 km

  • Deep focus > 300 km

  • Shallow focus earthquakes are most destructive

• Occur when a fault moves

• Produces offset (displacement as strain released

• Causes

  • Formation of new fault

  • Sudden slip on an existing fault

• Magma earthquakes

  • Movement of magma

  • Explosion of volcano

  • Giant landslides

  • Meteorite impact

  • Underground nuclear bomb tests

Seismology

• Study of EQ waves

• Seismographs

  • Record seismic waves

  • Records movement of earth in relation to a stationary mass on a rotating drum

  • Records obtained in seismograph

We need two of these seismograms, one oriented N-S another E-W

How to record/measure seismic waves?

• Study of EQ waves = seismology

• Seismographs/seismometers

  • Record seismic waves

  • Records movement of earth in relation to a stationary mass on a rotating drum

Types of Seismic waves

1. Surface waves

   • Travel along outer part of earth

   • Cause great destruction

   • Exhibit greatest amplitude and slowest velocity

   • 2 motions for surface waves

     • Back and forth

     • Rolling

   • Most are:

     • R waves = rayleigh waves

       • Slower than love waves

       • Move forward while the individual particles of material move in an elliptic path oriented in the direction of wave movement

     • L-waves = love waves

       • Individual particles of the material only move back and forth in a horizontal plane perpendicular to the direction of wave travel

       • Damaging buildings

2. Body waves

   • Travel through earth's interior

   • 2 types of body waves:

     • Primary waves

       • Push-pull (compress and expand) motion

       • Travel through solids, liquids, and gases

       • Travel fastest, lowest amplitude

     • Secondary waves

       • Shake motion at right angles to direction of travel

       • Travel only though solids

       • Slower velocity than P waves

       • Slightly greater amplitude than P waves

• P and s waves velocities are determined by the density and elasticity of the materials through which they travel

  • Move slowly through dense rock

  • Move quickly through elastic rocks

Locating source of earthquakes

• Focus

  • Place where EQ waves originate

• Epicenter

  • Location on surface directly above focus

• Epicenter located using differences in velocities of P and S waves

• Locating  epicenter of an EQ

  • 3 station recordings needed

  1. Each station determines time interval between arrival of P and S waves

  2. Travel time graph

     1. Determine each station's distance to epicenter

  3. Circle with radius equal to distance to epicenter drawn around each station

  4. Where circles intersect is epicenter

How is an EQ epicenter located?

• Seismic wave behavior

  • P waves arrive first, then s waves, then L and R

• AVG speeds for all these waves is known

• After an EQ the difference in arrival times at a seismograph station can be used to calculate the distance from the seismograph  to the epicenter

Locating the epicenter

• Time-distance graphs for p-waves and s waves illustrate that the difference between the arrival times of the two waves is a function of distance between a seismograph and an EQ's focus

• The father the waves travel, the greater the P-S time interval

• If P-S time intervals are known from at least three seismograph stations, then the epicenter of any EQ can be determined

  • Use a compass and set it to the distance from the focus to each seismic station

• A circle where the radius equals the distance to the epicenter is drawn

• The intersection of the circles locates the epicenter

Distance equal to radius of circle where 3 circles intersect = epicenter

Install several seismometers in relatively close to each other

• Memory the arrival of time of the shockwaves

EQ's

• Range from 5-700 km

• Foci classified as

  • shallow (< 70 km)

  • intermediate (70 km and 100 km)

  • Deep (>300 km)

• Patterns exist

  • Shallow focus = along oceanic ridge system almost all deep focus EQs occur in circumstances

  • Pacific belt

  • HWY: (subduction zones)

Where are earthquakes

1. 80% circum-pacific belt

2. 15% Mediterranean-asiatic belt

3. 5% of interior plates, movement, mid - ocean ridges

   • Every year: 150,000 EQs but 900,000 records one

Subduction (Benoiff) Zones

• Dipping seismic zones common to convergent plate boundaries where one plate is subducted beneath another

Size of earthquakes

• Described by:

  • Intensity

    • Degree of shaking at locality

  • Magnitude

    • Estimates amount of energy released

  • Intensity scales

    • Modified mercalli intensity scale:

    • Destruction may not be true measure of EQ severity

  • Magnitude scales

    • Richter scale

      • Based on amplitude of largest seismic wave recorded

      • Each unit of richter magnitude increase corresponds to tenfold increase in wave amplitude & 32-fold energy increase

Finding the magnitude of an EQ

• Need difference in arrival time of P & S wave and amplitude of largest wave

• P-S (time sec)

• Largest amplitude wave (MM)

Earthquake destruction

• Types:

  • Liquefaction of ground

    • unconsolidated with water turn into fluid

  • Tsunamis

  • Landslides and ground failures

  • Fire

• Destruction depends on magnitude, distance from epicenter, types of rock/material

• Would you build your home on bedrock or mud

• Tsunamis

  • inappropriately called "tidal waves" (nothing to do with sun or moon)

  • Result from motion along fault or ocean floor or undersea landslide

  • In open ocean height < 1 m

  • In shallower waters, water can be > 30 m

• Ground failure, 1964 alaska

Can EQ be predicted?

• Short range-predictions

  • No reliable methods exists

• Long range forecasts

  • Probability of certain magnitude EQ occurring on time scale of 30 to 100 years (gap theory)

  • Assumes EQ cyclical

    • Use paleo seismology

  • Used to

    • develop building code

    • Assist in land use planning

What will be on exam 4

Shorelines

EQ

Geologic time

Plate tectonics