Ch. 11 - Earthquakes and Earthquake Hazards

Class: GEOL-101

Notes:

11.1 What is an Earthquake?

Sketch and describe the mechanism that generates most earthquakes.

Earthquakes

Presentation content:

• An earthquake is ground shaking caused by the sudden and rapid movement of one block of rock sliding past another
  • Rocks slide past one another along fractures in the crust called faults
    • Rock slippage originates in the ground at the focus or hypocenter
    • The epicenter is the point on the ground surface directly above the focus
  • Most faults are locked except for brief, abrupt movements (earthquakes)
  • Stored up energy is released as seismic waves that radiate in all directions from the focus

Notes from the lecture:

• Here we do not have a lot of earthquake activity because we are not on a large plate boundary
• Epicenter is the transpose vertical location of the hypocenter

Discovering the Causes of Earthquakes

Presentation content:

• Energy released from volcanic eruptions, massive landslides, and meteorites can generate earthquake-like waves—but these are usually weak
• Over tens to hundreds of years, stress builds up from plate movement. Eventually, stress along the fault overcomes the frictional resistance, and slip initiates as the rocks break
  • The deformed rocks “snap back” to their original position in a process called elastic rebound

Notes from the lecture:

• A stick breaks pass the elastic limit
• Eventually enough energy is built up, so it breaks, that oscillation is the release of seismic waves

Where do earthquakes occur?

Presentation content:

• Earthquakes occur along both new and preexisting faults in places where differential stresses cause the crust to break
  • Normal
  • Reverse and thrust
  • Megathrust fault
    • In a subduction zone, the boundary forms
    • Produce powerful earthquakes, generate tsunamis
• Strike-slip

Notes from the lecture:

• Example: 8.5 feet displaced fence during the 1906 San Francisco earthquake

Megathrust Faults

Strike-Slip Faults

• Plates moving in opposite direction
• Faults are not linear, they have slips, they have little branches
• Not a single fault, but a fault zone (a series of faults)

Fault Rupture and Propagation

Presentation content:

• Slippage along large faults does not occur instantaneously
  • Initial slip begins at hypocenter and propagates along the fault surface
  • Slippage adds strain to adjacent sections triggering more slippage
  • Slippage mainly travels in one direction
• Fault slip is the amount of displacement on the fault surface

Notes from the lecture:

1. During an earthquake the initial slippage begins at the hypocenter
2. The rupture surface propagates (travels) along the fault surface, at a rate of 2 to 3 kilometers per second.
3. The rupture surface continues to grow until it reaches a section of the fault where the rocks have not been sufficiently strained to rupture.

11.2 Seismology: The Study of Earthquake Waves

Seismology

Presentation content:

• Seismology is the study of earthquake waves
• Earliest studies of earthquake waves date back almost 2000 years to the Chinese

Ancient Chinese Seismograph
Invented by Zhang Heng, was a large hollow jar containing a weight suspended from the top. The suspended weight was connected to the jaws of several large dragon figurines that encircled the container. The jaws of each dragon held a metal ball. When earthquake waves reached the instrument, the relative motion between the suspended mass and the jar would dislodge some of the metal balls into the waiting mouths of frog figurines directly below.

Notes from the lecture:

• When you have any sort of vibration, the device captures it.
• May indicate where the seismic waves are coming from

Instruments That Record Earthquakes

Presentation content:

• Seismographs (or seismometers) record the movement of Earth in relation to a stationary mass on a rotating drum or magnetic tape
• Earthquakes cause vertical and horizontal ground movement

Notes from the lecture:

• Vertical Motion:
  • If there is any sort of vertical ground motion, pen will move up and down

Seismic waves

Presentation content:

• Records obtained are called seismograms
• Types of seismic waves
  • Body waves travel through Earth’s interior
    • Primary (P) waves are compression waves (travel through all materials)
    • Secondary (S) waves are shear waves (Only travel through solid material)

Notes from the lecture:

• See the expansion and contraction of the rope, this is basically what waves are.
• P and S waves are much more subtle, you do not need to feel any floor shaking at all

Surface waves

Presentation content:

• Two general directions of motion
  • One causes the ground to move up and down, similar to the movement of ocean swells
  • The second causes the ground to move side to side
    • Causes the greatest destruction

Body waves versus surface waves

Presentation content:

• P waves:
  • first to arrive at a recording station
  • have the lowest amplitude
• S waves:
  • second to arrive at a recording station
• Surface waves:
  • have the lowest velocity
  • last to arrive at a recording station
  • have the highest amplitude
  • cause the greatest property damage

Notes from the lecture:

• Waves propagate at different speeds in continental crust vs. oceanic crust but overall we do not care about this.

• As you move further away, the P, S and Surface way are further apart from each other
• As you move further away from the epicenter, there is less shaking.
• S waves can travel through liquid (can travel through the outer core)

11.3 Locating the Source of an Earthquake

Explain how seismographs locate the epicenter of an earthquake.

Locate the epicenter

Presentation content:

• Seismologists first locate the epicenter
  • Developed by using seismograms from earthquakes whose epicenters could easily be pinpointed
    • Travel-time graphs were constructed
  • Using travel-time graphs and triangulation we can locate an epicenter

Notes from the lecture:

• We need at least 3 location in order to triangulate the location of the hypocenter

Using triangulation

Presentation content:

• Using travel-time graphs and triangulation we can locate an epicenter
  • Time interval between first P wave and first S wave
  • Find location on graph where vertical separation between curves is equal to that time interval
  • Read the distance to the epicenter
  • Repeat with two or more seismic stations

Notes from the lecture:

• We can measure the time apart, and use it to triangulate a distance
• Looking for where these different circles intersect based on the arrival times of P and S waves

11.4 Determining the Size of an Earthquake

Distinguish between intensity scales and magnitude scales.

Describe the size of an earthquake

Presentation content:

• Two fundamentally different measurements are used to describe the size of an earthquake
  • Intensity: a measure of the amount of ground shaking at a particular location based on observed property damage
  • Magnitude: quantitative measurement of ground motion based on data from seismic records used to estimate of the amount of energy released at an earthquake’s source

Notes from the lecture:

• Companies measure probabilities of having a location destroyed by an earthquake (measuring property damage)
• Intensity is a very subjective scale
  • Earthquakes do not kill people, buildings do.

Intensity scales

Presentation content:

• The Modified Mercalli Intensity scale was developed using California buildings as its standard
  • Developed in 1902 by Giuseppe Mercalli
  • Based on property destruction in a region
  • Values change based on the distance from the epicenter

Notes from the lecture:

• Totally dependent on distance from the epicenter
• We still use this scale today (though it is very subjective)
• Look at maps and think of how reliable are they.

Magnitude scales: Richter scale

Presentation content:

• Richter magnitude (Charles Richter in 1935)
  • The Richter scale is calculated by measuring the amplitude of the largest seismic wave (usually A wave) recorded on a seismogram
  • Logarithmic scale that accounts for the decrease in wave amplitude with increased distance

Notes from the lecture:

• Purely based on amplitude and time between P and S waves
• Based on tons of dynamite (the amount of energy being released)

Magnitude scales: Moment Magnitude

Presentation content:

• Moment magnitude (MW) measures the total energy released during an earthquake
  • Calculated by the average amount of slip on the fault, the area of the fault surface that slipped, and the strength of the faulted rock
  • Can also be calculated by modeling data from seismograms

Notes from the lecture:

• Total energy released by the earthquake
• Based on the distance from the epicenter but use the maximum distance
• All of these scales have their own ups and downs.
• Measured in kilograms of explosive

11.5 Earthquake Destruction

List and describe the major destructive forces that earthquake vibrations can trigger.

Destruction

Presentation content:

• Amount of destruction attributable to an earthquake varies based on:
  • Magnitude of the earthquake
  • Proximity of a populated area to the epicenter
• Destruction from Seismic Vibrations
  • The amount of damage to structures:
    • The earthquake intensity
    • The duration of the vibrations
    • The nature of the material beneath the structures
    • The nature of building materials and construction practices
• Amplification of seismic waves
  • Soft sediments amplify seismic waves more than solid bedrock

Notes from the lecture:

• Different rock types, different materials affect how vibrations and waves can be intensified
• Seismic vibrations are impacted by the type of materials
• really important for things like bridges and other critical buildings

Liquefaction

Presentation content:

• The phenomenon where loosely packed, waterlogged sediments behave as a fluid during the intense shaking of an earthquake

Notes from the lecture:

• Water displaces, and makes a competent material, incompetent
• Example: Tilted buildings rested on unconsolidated sediment that behaved like quicksand during the 1964 Niigata, Japan, earthquake.

Landslides, subsidence, and fire

Presentation content:

• Landslides and Ground Subsidence
  • Ground shaking causes loose sediments on a slope to slump
  • Often the greatest damage from earthquakes
• Fire
  • Can start when gas and electrical lines are destroyed by an earthquake
  • Broken water lines make fire control nearly impossible

Tsunami

Presentation content:

• A series of large ocean waves (“harbor waves”)
• Most are generated by displacement from a megathrust fault
• Advance across the ocean at 800 km/hr
• In open water, the wave amplitude is less than 1 m and the wavelength can be larger than 700 m
• Close to shore, the water “piles up” and some tsunamis can exceed 30 m in height

Notes from the lecture:

• Large volume of water is displaced
• Water slows down when reaching the shore but it is till relatively fast
• It is not actually until water hits the shore that you actually start to see a wave

Tsunami damage

Presentation content:

• Tsunami damage from the 2004 Indonesian earthquake
  • The tsunami was caused by an undersea earthquake near Sumatra and is one of the deadliest natural disasters
  • 230,000 killed
• Japan tsunami
  • The tsunami generated from the 2011 Tohoku earthquake was 40 m high and a Pacific-wide event, affecting not only Japan but also the west coast of North America

Notes from the lecture:

• Affected nuclear reactors in Japan and released radio-active material to the environment

Tsunami warning system

Presentation content:

• Observations in the Pacific Ocean allow scientists to track tsunamis and issue appropriate warnings to affected areas
  • Seismic observatories report large earthquakes to the Tsunami Warning Center
  • A series of deep-water buoys in the Pacific Ocean detect energy released by earthquakes
  • Tidal gauges measure sea level rise and fall

Notes from the lecture:

• How can we predict tsunamis?
  • We can monitor earthquakes, and triangulate them to determine if there will be potential for a tsunami.
  • We can measure how long it may take to reach a certain island and stuff like that (early warning system)
  • Areas to worry about: Da Fuca Plate
    • Washington, Seattle, Oregon, etc.

11.6 Where Do Most Destructive Earthquakes Occur?

Locate Earth’s major earthquake belts on a world map.

Where do destructive earthquakes occur?

Presentation content:

• About 95% of earthquakes along fault surfaces where tectonic plates interact
• The zone of greatest seismic activity is called the circum-Pacific belt
• The largest earthquakes occur along megathrust faults of convergent plate boundaries
• The Alpine-Himalayan belt is another region of strong earthquakes
  • Collision of the African and Indian Plates with the Eurasian Plate
• Divergent plate boundaries are associated with frequent but weak seismic activity
• Transform faults tend to generate large earthquakes on a cyclical basis

Notes from the lecture:

• A lot of earthquakes and tsunamis here
• circum-Pacific plate + African + Arabian + Indian plates
• Multiple plates converging.

Previous larger earthquakes:

• 1811 was a very big earthquake in magnitude

11.7 Earthquakes: Predictions, Forecasts, and Mitigation

Compare and contrast the goals of short-range earthquake predictions and long-range forecasts.

Short-Range Predictions

Presentation content:

• The goal is to provide a warning of the location and magnitude of a large earthquake within a narrow time frame

• Must have a narrow range of uncertainty regarding location and timing. Most efforts have failed or produced false alarms

• Currently, no reliable methods exist for making short-range earthquake predictions

• Research has concentrated on monitoring possible precursors of major earthquakes:

  • Monitor changes in ground elevation
  • Measure strain in the rocks
  • Measure changes in groundwater level
  • Frequency of foreshocks

Notes from the lecture:

• Still an area of active research (not a deterministic method)
• Goal: Provide early warning
• Satellite telemetry:
  • Monitor movement of plates
  • I a plate is not moving for a long time it generates stress, which may be a trigger
• We do not know what the trigger is
• Probability-based predictions.

Long-Range Forecasts

Presentation content:

• Based on the probability of earthquakes of a certain magnitude occurring at a time

• Useful guide for building codes, dams, roadways

• Based on evidence that many large faults break in a cyclical manner, producing earthquakes of roughly the same magnitude at roughly similar intervals

• Seismic gaps are tectonically quiet zones along a fault where strain is currently building up

  • The stored strain will be released in a future earthquake
  • Strain can be estimated using known rate of plate movement

• Paleoseismology is the study of prehistoric earthquakes
  • By digging a trench across a fault zone, scientists look for evidence of ancient faulting (mud volcanoes and offset sedimentary strata)

Notes from the lecture:

• "Every 1000 years there is a large earthquake"
• Minimizing Earthquake Hazards:
  • Retrofit with Cross Braces
  • 20' thick concrete pad w/ 947 pilings 200' deep

End of Chapter 11 - Concept Checks

11.1 What Is an Earthquake?

1. What is an earthquake?
2. 3. How are hypocenters and epicenters related?
      Explain what is meant by elastic rebound.
      11.2 Seismology: The Study of Earthquake Waves
3. 2. 3. Briefly describe how a seismograph works.
         List the major differences between P, S, and surface waves.
         Which of the three basic types of seismic wave is likely to cause the greatest destruction to buildings?
         11.3 Locating the Source of an Earthquake
4. 2. What information does a travel–time graph provide?
      Briefly describe the triangulation method used to locate the epicenter of an earthquake.
      11.4 Determining the Size of an Earthquake
5. 2. 3. What does the Modified Mercalli Intensity scale tell us about an earthquake?
         How much more energy does a magnitude 7.0 earthquake release than a magnitude 6.0 earthquake?
         Why is the moment magnitude scale favored over the Richter-like magnitude scales for large earthquakes?
         11.5 Earthquake Destruction
         List three factors that influence the amount of destruction that seismic vibrations cause to human-made structures.
         In addition to the destruction created directly by seismic vibrations, list three other types of destruction associated with earthquakes.
         What is a tsunami? How are tsunamis generated?
         11.6 Where Do Most Destructive Earthquakes Occur?
         What zone on Earth has the greatest amount of seismic activity?
         Which type of plate boundary is associated with Earth’s most destructive earthquakes?
         11.7 Earthquakes: Predictions, Forecasts, and Mitigation
         Are accurate, short-range earthquake predictions currently possible using modern seismic instruments?
         What is the value of long-range earthquake forecasts?