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  • Elastic rebound theory states that the waves of energy from an earthquake result from the sudden release of stored up strain energy in rock as it deforms. When the rock ruptures the rock on either side of a fault snaps suddenly to a new position, releasing the stored up strain energy in the process.

Earthquake Presentation Earthquake Presentation Presentation Transcript

  • Earthquakes Chapter 3 Earthquakes and Their Damages: Shaking Ground, Collapsing Buildings Chapter 4 Earthquake Prediction and Tectonic Environment
  • Earthquakes Defined
    • Earthquakes are vibrations of the earth caused by the rupture and sudden movement of rocks that have been strained (deformed) beyond their elastic limit. The forces that cause deformation and the build-up of strain energy in the rock are referred to as stresses .
    • Earthquakes occur along faults . Faults are fractures in the lithosphere where regions of rock move past each other ( displaced ).
    • Refer to page 33-35.
    • The focus is the point on the fault where rupture occurs and the location from which seismic waves are released.
    • The epicenter is the point on the earth’s surface directly above the focus.
    • When the fault ruptures, waves of energy called seismic waves spread out in all directions.
    • Refer to page 44.
    Earthquake Terminology
  • The Elastic Rebound Theory Refer to page 33-35
  • Types of Faults
    • The majority of earthquakes (90%) are caused by rocks rupturing in response to tectonic stresses at active plate margins.
    • Refer to pages 36-37.
  • Types of Tectonic Stress
    • Tensional Stress (extensional stress)
    • Compressional Stress
    • Shear Stress
    • Refer to page 36-37.
  • Types of Faults
    • Faults can be divided depending on the direction of relative displacement. Relative displacement is largely a function of the type of tectonic stress the rock is under.
    • There are 2 main categories.
      • Dip-slip faults - where the displacement is vertical
    • Strike-slip faults - where the displacement is horizontal.
  • Dip-Slip Faults - Normal Faults
    • Normal faults result from tensional stresses along divergent boundaries .
    • The hanging wall block moves down relative to the footwall block.
    • Low Richter magnitudes due to the tendency of rocks to break easily under tensional stress.
    • Shallow focus (less than 20 km) because the lithosphere is relatively thin along diverging plate boundaries.
    • Refer to pages 36-37, 41.
    Examples - all mid-ocean ridges; Continental Rift Valleys such as the basin and range province of the Western U.S. and the East African Rift Valley
  • Normal Fault Examples Left: Fault scarp near Hebgen Lake, Montana, after the magnitude 7.1 earthquake of August 18, 1959, shows a displacement of 5.5 to 6.0 m. Right: This section of the normal fault scarp was produced by the earthquake of October 28, 1983, at Borah Peak, Idaho. Left: Dixie Valley-Fairview Peaks, Nevada earthquake December 16, 1954
  • Normal Fault Example – The Basin and Range of Nevada, Utah, and Adjacent Areas
    • The Faults of the Basin and Range occupy a spreading zone accompanying the Northwest drag of the Pacific Plate against the North American Plate, which moves slightly south of west.
    • Refer to page 41.
    Lower Right: The Wasatch Front is a high fault scarp east of the Salt Lake basin. There has not been an earthquake of any consequence since the founding of Salt Lake city in 1847. However, the Salt Lake City area should still consider themselves at high risk for major earthquakes.
  • Dip-Slip Faults - Reverse Faults
    • Reverse faults result from compressional stresses along convergent boundaries .
    • The hanging wall block has moved up relative to the footwall block.
    • A Thrust Fault is a special case of a reverse fault where the dip of the fault is less than 15 o
    • Refer to pages 39-41.
    • There are two types of converging plate boundaries.
      • Subduction boundaries
      • Collision boundaries
    Left: Thrust Fault
  • Subduction Boundaries
    • At subduction boundaries there is a continuum of stress along the subducting plate. Shallow focus earthquakes can be generated near the trench, but focal depths can reach down to 700 km as earthquakes are generated along the subducting plate.
    • Rocks are strong under compression and can store large amounts of strain energy before they rupture. Therefore, these earthquakes can be very powerful.
      • 1960 Southern Chili = 9.5
      • 1964 Alaska = 9.2
      • Refer to pages 39-41.
  • Reverse Fault Example –Subduction Zones Chile 1960
    • On May 22, 1960 the largest earthquake on record struck the coast of Chile with a Mw of 9.5. The earthquake ruptured along a 1,000 km length of the subduction zone. In Chile, the earthquake and the tsunami that followed took more than 2,000 lives. From Chile the tsunami radiated outward, killing 61 people in Hawaii and 122 in Japan.
    • Refer to pages 39-41
    • Case in Point page 55.
    Left: Stuck to the subducting plate, the overriding plate gets squeezed. Right: An earthquake along a subduction zone happens when the leading edge of the overriding plate breaks free and springs seaward, raising the sea floor and the water above it. This uplift starts a tsunami.
  • Collision Boundaries
    • At collision boundaries two plates of continental lithosphere collide resulting in fold-thrust mountain belts.
    • Earthquakes occur due to the thrust faulting and range in depth from shallow to about 200 km.
    • Refer to pages 39-41.
    Example: The Himalayas from the collision of India with Asia
  • Reverse Fault Example – Collision Zones Bam, Iran 2003
    • Earthquakes in Iran and neighboring regions (e.g., Turkey and Afghanistan) are closely connected to their position within the active Alpine-Himalayan belt near the convergence of the Arabian and Eurasian plate.
    Map showing the Alpine-Himalayan Belt. Below: Before and after pictures of the 2,000 yr. old citadel in the city of Bam, the largest mud-brick structure in the world. The Mw 6.5 Bam Earthquake destroyed nearly 80% of the adobe buildings killing more than 26,000 people.
  • Strike-Slip Faults - Transform Faults
    • Strike-slip faults result from shear stresses acting on the lithosphere along transform boundaries .
    • Horizontal motion can be right lateral or left lateral.
    • Earthquakes along these boundaries tend to be shallow focus with depths usually less than about 100 km. Richter magnitudes can be large.
    • Refer to pages 37-38.
  • Transform Fault Example The San Andreas Fault System
    • The San Andreas Fault is the main strand of a zone of parallel faults resulting from interaction between the relative northwest movement of the Pacific Plate compared with the North American Plate.
  • Intraplate Earthquakes
    • These are earthquakes that occur in the stable portions of continents. Many of them occur as a result of re-activation of ancient faults, although the causes of some intraplate earthquakes are not well understood. Refer to pages 42-44.
    • Charleston, South Carolina, 1886
    • Mw = 7.3, Mercalli Intensity of IX
    Right: The Charleston Earthquake resulted from movements along a segment of the East Coast Fault system, a series of faults trending Northeast near the boundary between continental crust and Atlantic Oceanic crust. The ancient faults are believed to be associated with the early stages of opening of the Atlantic Ocean 200 mya.
  • Earthquake Seismic Waves
    • Body waves travel through the interior (body) of the earth as they leave the focus. They include P-waves and S-waves .
    • Surface waves travel parallel to the earth’s surface. They are the slowest and most damaging. They include Love and Rayleigh Waves .
    • Refer to pages 44-45.
  • P-waves
    • “ Primary ” waves – fastest. Arrive at distant locations first.
    • P ush- p ull waves - Compress and extend in the direction of wave travel.
    • Refer to pages 39-41.
  • S-waves
    • “ Secondary ” waves – second fastest. Arrive at distant locations second.
    • Shear waves – Travel at right angles to the direction of wave travel.
    • Refer to pages 39-41.
  • Surface Waves
    • Surface waves travel parallel to the earth’s surface. They are the slowest, involve the greatest ground motion, and are therefore most damaging.
    • Refer to pages 39-41.
    • They include:
    • Love Waves - complex, horizontal motion
    • Rayleigh Waves - Rolling or elliptical motion.
  • Seismographs
    • Seismographs are instruments that detect and record ground shaking produced by earthquake waves.
    • Due to their different speeds, the different waves arrive at the seismograph at different times: first P-waves arrive, then S-waves, then surface waves.
    • Refer to pages 45-46.
  • Seismogram
    • Seismogram - the record of an earthquake as recorded by a seismograph. It is a plot of vibrations versus time.
    • Refer to pages 45-46.
  • Locating the Epicenter Via Seismograms
    • P-waves are faster than S-waves, and the time gap between their arrival at a seismograph increases precisely with distance from the focus.
    • Basically, the lag time between the arrival of your first recorded P-wave and first recorded S-wave is proportional to distance traveled.
    • Refer to pages 46-47.
  • Locating the Epicenter Via Seismograms
    • We can use the lag time between the P-waves and S- waves to calculate the distance to an earthquake !
    • If we do this for a minimum of three different seismic stations , we can precisely locate the epicenter. In the figure, each circle has a radius equal to the distance to the earthquake from three separate seismic stations. All three circles intersect at only one point -- the epicenter!
    • Refer to pages 46-47.
  • Earthquake Measurement
    • Richter Magnitude Scale
    • - M L ; based on the highest amplitude wave measured on a seismogram, corrected for distance from the seismograph to the epicenter
    • - ranges from 1.0 (smallest) to infinity, but 9.0 is typically the highest possible value for an earthquake.
    • logarithmic scale: each whole unit on the Richter scale represents a ten-fold increase in wave amplitude (ground shaking) and an ~ thirty fold increase in the energy released.
    • Refer to page 48-50.
    Above: The magnitude of the earthquake can be estimated using an earthquake nomograph, on which a straight line is plotted between the P-S time (distance) and the maximum wave amplitude. This line intersects the central line at the approximate magnitude of the earthquake.
  • Earthquake Measurement Richter Magnitude Scale
  • Earthquake Measurement
    • The Local Magnitude Scale developed by Richter was strictly valid only for certain frequency and distance ranges. Therefore new magnitude scales were developed all calibrated to Richter's original method . These include body-wave magnitude, M B , and surface-wave magnitude, M S , each valid for a particular frequency range.
    • Because of the limitations of all three magnitude scales, a new scale, known as moment magnitude, or M W , was developed. M W is a measure of the seismic moment , or total energy expended during an earthquake. M W depends on the rock strength, area of rock broken, and amount of offset across the fault.
    • Refer to page 49.
  • Earthquake Measurement
    • Modified Mercalli scale
    • based on people’s reported perceptions of shaking (subjective), and the type and extent of damage produced (objective).
    • ranges from I (not felt by people) to XII (catastrophic destruction)
    • Refer to page 47.
    • An example of the Modified Mercalli scale follows in the next slide.
  • Modified Mercalli Scale Example: The 1994 Northridge, California Earthquake Right: A map of Modified Mercalli intensities for the 1994 Northridge, California Earthquake. Left: TriNet ShakeMap showing the distribution of shaking during the 1994 Northridge Earthquake.
  • Modified Mercalli Scale vs. The Richter Scale
  • Fault Creep
    • Not all fault movements result in violent earthquakes. Some faults move slowly and fairly continuously, a movement called fault creep .
    • Fault creep never killed anyone, but, as shown in these pictures, it can cause damage to roads or other structures.
    • Refer to pages 36.
  • Earthquake Hazards
    • “ earthquakes don’t kill people, buildings do”.
    • Earthquake Damage Susceptibility Depends on
    • Magnitude of the earthquake - the higher the magnitude, the more intense the shaking, the longer the duration of shaking, and the greater the displacement. Refer to slides 35-37.
    • Distance from the epicenter – Seismic waves attenuate (amplitude diminishes) with distance. Refer to slide 38.
    • Surface Geology – Surface Faulting and ground rupture, and soil amplification. Refer to slides 39-42.
    • Integrity of Structures and Type of Construction - Building codes; Building material: concrete/masonry vs. wood/steel. Structural Integrity. Refer to slides 43-53.
    • Integrity of Utilities.
    • Population Density, building density, time of day, etc . - the more people and buildings, the greater the potential for structural damage and death.
  • Earthquake Magnitude and Ground Acceleration
    • Ground acceleration is the rate of increase in velocity, or the strength of the shaking. During an earthquake, the ground accelerates from being stationary to a maximum velocity before slowing and reversing its movement.
    • Acceleration is normally designated as some proportion of the acceleration due to gravity (g). 1.0 g is the acceleration felt by a freely falling body.
    • Refer to pages 50-51.
  • Earthquake Magnitude and Shaking Time
    • An increase in magnitude significantly increases the time of shaking, and the potential damage. Refer to pages 51-52.
    Richter Magnitude Duration of Strong Ground Shaking in Seconds 8 – 8.9 30 – 90 7 – 7.9 20 – 50 6 – 6.9 10 – 30 5 – 5.9 2 – 15 4 – 4.9 0 - 5
  • Magnitude and Fault Displacement
    • The amount of displacement during fault movement and the length of surface rupture are generally proportional to the magnitude of the accompanying earthquake.
    • Refer to page 50.
    Above: Graph of the relationship between maximum displacement and earthquake magnitude on all types of faults. Left: Graph of the relationship between surface rupture length and earthquake magnitude on all types of faults
  • Distance from the Epicenter Seismic waves attenuate with distance. Refer to page 52.
    • Ground rupture and surface faulting occur due to horizontal or vertical displacement of faults that break the surface. Damage can result from the ground shifting upward (called uplift ) or downward (called subsidence ).
    Surface Faulting and Ground Rupture Land Uplift and Subsidence Above: The photo shows a fault scarp -- a cliff created by movement along a fault. This scarp formed during the 1992 Landers, CA, quake. Left: The 1999 Taiwan earthquake caused ground movement over a fault rupture of 50 miles. The photo shows a fresh scarp cutting across the running track at a local high school.
  • Soil Amplification
    • Shaking is amplified as waves travel from solid bedrock to unconsolidated sediment to water-saturated sediment.
    • Refer to page 52.
  • Liquefaction
    • Liquefaction is a quicksand like condition that occurs in water-saturated soil and rock. The shaking of earthquake waves causes the soil or rock to turn into a weak, fluid-like mass . Structures built on areas that liquefy may fall over or sink. Refer to pages 52-53.
    Above: Buildings in Niigata, Japan, fell over when the sediments below them liquefied during the 1964 earthquake The figure below shows how liquefaction can occur. Shaking of water-saturated soils causes the particles to settle, driving the water out from between the particles and forcing it upward, thus liquefying the areas above.
  • Landslides
    • Vibration of water saturated sediment can force water into pore spaces between sediment grains reducing friction and permitting the mass to slide down slope. Refer to pages 52-53.
    Turnnagin Heights,Alaska,1964
  • Integrity of Structures and Type of Construction
    • The Uniform Building Code provides a seismic zonation map that indicates the level of construction standards for buildings
    • Concrete and masonry structures are brittle and thus more susceptible to damage.
    • Wood and steel structures are more flexible and thus less susceptible to damage.
    • Refer to page 82-83.
  • Base Shear
    • Most damage and collapse of structures occurs due to sideways movement of the ground from earthquake waves. This process is called horizontal ground acceleration , or base shear . Refer to pages 75-79.
    • Base shear causes the building to deform from a rectangle into a parallelogram.
  • Base Shear
    • Base shear causes buildings constructed on so-called “cripples” -- short walls that raise the building up from its foundation -- to fall sideways.
  • Base Shear
    • The most deadly type of failure from base shear is “story-shift”, in which the sideways acceleration causes floors to shift resulting in the collapse of either individual floors or the whole building -- a situation called pancaking . Few or no occupants survive such collapses.
  • Mitigation of Base Shear Refer to pages 79-81.
    • Shear walls : diagonal braces or plywood sheeting built into the walls, to keep the building from deforming during base shear.
    • Bolting to the foundation , so the building does not slide off .
  • Mitigation of Base Shear
    • Base isolation : putting the building on large rubber pads, rolling wheels, or slippery Teflon plates. This allows the ground to move under the building, thus isolating the building somewhat from ground motion.
    Basement of Veterans Administration Medical Center, Long Beach, CA
  • Mitigation of Base Shear
    • Seismic joints : areas of flexible material, like rubber, form connections between different parts of a building. These allow the separate areas of the building to shake independently.
    The photograph shows a seismic joint (the dark vertical stripe of rubbery material) between two halves of a building.
    • The building practices described on the last few slides make a huge difference in quake survival! The bar graph shows us the connection between year of construction and amount of damage to buildings during the 1995 Kobe, Japan, earthquake. More recent buildings were built to stricter codes, and thus fared much better during the quake.
    • Case in Point page 90.
    Mitigation of Base Shear
  • Base Shear
    • In addition to buildings, highway overpasses, bridges, and multi-decked freeways also suffer major damage from base shear, most commonly due to the failure of the concrete supporting columns. Refer to pages 75-79.
    Left and Right: The photo shows the collapse of freeways from the 1994 Northridge, CA, quake. Left and Right: Collapse of expressways from the 1995 Kobe, Japan, quake.
  • Mitigation of Base Shear
    • New construction practices to reduce failure of highway overpasses include:
    • 1. Horizontal rebar wrapping added when casting concrete columns (left below).
    • 2. Retro fitting existing columns with steel jackets to make them stronger and more flexible (right below).
    • Refer to pages 75-79.
  • Building Vibration and Oscillation
    • Buildings have natural vibration frequencies in the same range as earthquake waves. Shaking of the building will be amplified when the frequency of the building is close to the frequency of the waves produced by the earthquake. Refer to pages 80-81, 83.
    Adjacent buildings of different heights will sway at different frequencies, and therefore can collide during an earthquake.
  • Indoor Hazards
  • Aftershocks
    • Aftershocks are earthquakes that follow the largest shock of an earthquake sequence. The main earthquake changes the stress pattern in areas around the epicenter, and the crust must adjust to these changes.
    • Aftershocks can continue over a period of weeks, months, or years. In general, the larger the main shock, the larger and more numerous the aftershocks, and the longer they will continue.
  • Fires
    • Fires commonly break out during quakes due to ruptured gas lines or downed electrical lines. Impassible roads and ruptured water mains compound the problem. In some urban quakes, fires have caused more damage than the ground shaking itself . Case in Point page 88.
    Photos show an uncontrolled fire in San Francisco after the 1989 Loma Prieta quake.
  • Some of the Most Catastrophic Earthquakes in Terms of Casualties
    • The largest earthquakes do not necessarily kill the most people. Most of the high death counts come from countries notable for poor building construction or unsuitable building sites. Refer to pages 82-83.
    • Based on knowledge of earthquakes and the tectonic environments that control them scientist can make reasonable forecasts about where and how large an earthquake will occur at some future time.
    • However they have been less successful in finding ways to make short term predictions. Earthquakes appear to be inherently unpredictable.
    • Nevertheless, great research effort has gone toward finding a reliable system for short term earthquake prediction. One main avenue of research has been the study of earthquake precursors.
    Earthquake Prediction / Forecasting
    • Ground deformation: Measurements taken in the vicinity of active faults sometimes show that prior to an earthquake the ground is uplifted or tilts due to the strain building on the fault.
    • Foreshocks: Small earthquakes that precede a large quake by a few seconds to a few weeks. The pattern and intensity of foreshocks usually increase in magnitude and may cluster or migrate down a fault to the place where the main shock will eventually occur.
    • Water Level in Wells: As rocks become strained in the vicinity of a fault, new fractures may form causing a change in the path of groundwater subsequently causing changes in the water levels in wells.
    • Emission of Radon Gas - Radon is an inert gas that is produced in the radioactive decay of uranium. Radon is inert and remains in rock until some event forces it out. Deformation resulting from strain may form fracture in the rock which could serve as pathways for the Radon to escape into groundwater resulting in background radioactivity in wells.
    • Abnormal Animal Behavior.
    Precursor Events Refer to page 63-65.
  • Precursor Events
    • The only successful prediction of a major earthquake was the 1975 magnitude 7.3 Haicheng Earthquake in northeastern China. The prediction was based on
    • foreshock activity,
    • bulging of the ground surface near the fault,
    • changes in groundwater levels,
    • and strange animal behavior.
    • Based on these observations, over 1 million people were ordered to evacuate and remain outside in the winter cold. The earthquake collapsed more than 90% of the houses and, were it not for the prediction, would have killed hundreds of thousands of people.
  • Earthquake Prediction / Forecasting
    • No reliable short-term precursors have been found. Therefore research today focuses on longer-term warnings or forecasts . In this approach, geologists attempt to identify regions where large earthquakes are likely to occur within the next several years or decades. While this does not provide short-term warnings, it is useful for long-range planning for building codes and emergency response services.
  • Migrating Earthquakes
    • A few faults show a sequential migration of earthquakes along the fault with time believed to be the result of progressive earthquake failure . For faults that follow this pattern it is believed that one earthquake triggers the next. Refer to pages 67-68, Case in Point page 87.
    Since 1939, there have been seven earthquakes measuring over magnitude 7.0 along the North Anatolian Fault in Turkey, each occurring at a point progressively further west. By analyzing the stresses caused along the fault by each earthquake, they were able to forecast the 1999 Izmut earthquake. It is thought that an earthquake will soon strike further west along the fault, possibly in the heavily populated city of Istanbul.
  • Siesmic Gaps
    • A seismic gaps is a section of an active fault that has not had a recent earthquake. The theory here is that if a portion of a fault has been “locked” for some time (i.e. has not had an earthquake in a long time), then strain may have built up to especially high levels there, and a large quake may occur in the near future.
    • Refer to page 66, Case in point page 85.
    • Recurrence interval is a statistical estimate of the expected time interval between an event of a given magnitude. It is a statistical probability!
    • The theory is that faults should behave in the future like they have behaved in the past, producing a characteristic number of quakes of particular sizes over a given time interval . Refer to pages 63-64, 68-70, Case in Point page 88.
    Earthquake Regularity For nearly 150 years, the San Andreas Fault moved at regular intervals near Parkfield generating earthquakes of ~ magnitude 6 at intervals averaging about 22 years. This series prompted seismologist to predict that there was approximately a 95% chance that the next M6 earthquake would occur between 1985 and 1993. So the USGS installed a wide variety of expensive instruments around the Parkfield area. There, however was no M6 earthquake in this region until September 28, 2004.
  • Earthquake Regularity This map, based on the history of earthquakes on particular faults in the San Francisco region, shows the predicted probabilities of one or more magnitude 6.7 or greater quakes occurring on these faults by 2032. Refer to pages 70-73.
    • Paleoseismology is the study of the long-term geologic history of faults to determine their earthquake history and possible future activity. The methodology assumes that the earthquakes produced by the fault have a characteristic recurrence interval . Refer to page 65-66.
    Paleoseismology Older sedimentary layers are offset more because they have experienced a series of fault movements. Radiocarbon dating of organic layers broken along the fault can reveal the maximum dates of fault movements.