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Lecture 17 Earthquake Hazards


Lecture 17 Earthquake Hazards Big earthquakes Earthquake damages: aftershocks, amplification, liquefaction, lands, fire Earthquake hazard mitigation – PowerPoint PPT presentation

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Title: Lecture 17 Earthquake Hazards

Lecture 17 Earthquake Hazards
  • Big earthquakes
  • Earthquake damages aftershocks, amplification,
    liquefaction, landslides, fire
  • Earthquake hazard mitigation
  • Tsunami

Big earthquakes
  • appr. annual frequency of earthquakes
  • description mag on the order of
  • great gt8 1
  • major gt7 10
  • strong gt6 100
  • moderate gt5 1000

  • Distribution of earthquakes Mgt5 for 1980-1990.

  • earthquake magnitude and energy equivalence
  • mag energy release appr. equivalence
  • 2 600x1012 ergs 1000 pound of
  • rumbling of trains,
    smallest human can feel
  • 6 600x1018 ergs 1946 Bikini atomic
    bomb test
  • 10 600x1024 ergs annual US energy
  • 1811-1812 New Madrid, Missouri, three major
    earthquakes, M8, largest in contiguous US
  • 1964 Alaska earthquake, M9, 131 death, one of
    the largest ever recorded.

  • examples of most destructive earthquakes
  • 1556 Shanxi Huaxian, China earthquake, 830
    thousands deaths, possibly the greatest natural
    disaster ever.
  • 1976 Tangshang, China M7.8, 240 thousand deaths.
  • 1994 North ridge, CA, M6.7, 61 deaths, damage
    exceeding 15 billion.
  • 1995 Kobe, Japan, M6.9, 5472 death, damage
    exceeding 100 billion.

  • Areas away from plate boundaries are not
    necessarily immune from earthquakes. This is
    damage to Charleston, S. Carolina caused by the
    Aug 31, 1886 earthquake there. This was the
    greatest earthquake in the eastern US. Strong
    vibrations were felt even in Chicago.

Destruction from seismic vibrations
  • The amount of structural damage due to
    vibrations depends on
  • strength of earthquake
  • duration (and after shocks)
  • distance from epicenter
  • the site materials
  • building design, building natural periods

  • Earthquake intensity
  • Modified Mercalli intensity scale (P.408)
    measures the damage from an earthquake at a
    specific location.
  • The intensity ranges from I (not felt) to XII
    (total destruction).
  • Appr. relationship between MM and magnitude and
    ground acceleration (P.409)

  • Damage caused to a five-story JC Penney building,
    Anchorage, Alaska by 1964 Alaskan earthquake.
    Very little structural damage was incurred by the
    adjacent building. (NOAA)

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  • Resonance with building natural periods
  • At close distances, the most powerful vibrations
    are in 0.5-5 Hz, produced by S and short-period
    surface waves (Lg).
  • Typical building of 10 storeys has T1s each
    storey adds 0.1s.

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  • aftershocks
  • After a main earthquake, there are
    aftershocks in the following minutes, hours,
    days, months, or years -- The number of
    aftershocks decreases with time. The chance of
    one or more aftershocks with equal or larger
    magnitude within 7 days can be over 50 in
  • Aftershocks may cause previously damaged, yet
    still standing, structures to collapse. Thus, for
    engineers, damaged public buildings should be
    examined immediately and closed down if necessary
    to minimize the risks of aftershocks.
  • Example 1952 Ken county, CA earthquake (M7.7)
    had a M5.8 aftershock, which caused more damage
    to Bakersfield than the main one.

  • amplification by soft sediments
  • Massive bedrock provides best foundation because
    it passes wave motions on, resulting less
    vibration to building structure.
  • Soft sediments generally amplify the vibrations
    more than solid bedrock.

  • Seismograms from an aftershock of 1989 Loma
    Prieta earthquake show that shaking is greatly
    amplified in soft mud as compared to firmer
    materials. The portion of the Cypress Freeway
    structure in Oakland, CA that stood on soft mud
    (dashed red line) collapsed during the main shock.

  • liquefaction
  • Saturated fine sands and silts are subject to
    liquefaction during earthquake vibrations, in
    which water rises and a stable soil turns into a
    mobile fluid that has weak shear strength.
  • Underground objects such as sewer lines may
    literally float toward the surface and buildings
    settle and collapse.

  • Effects of liquefaction. The tilted building
    rests on unconsolidated sediment that behaved
    like quicksand during the 1985 Mexican
    earthquake. (J.L. Beck)

  • Mud volcanoes produced by 1989 Loma Prieta
    earthquake. They formed when geysers of sand and
    water shot from the ground, an indication of
    liquefaction. (R.Hilton)

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  • A photo of Turnagain Heights landslides caused by
    the 1964 Alaskan earthquake.

  • Landslides of Turnagain Heights, Alaska caused by
    the 1964 Alaskan earthquake. In less than 5 min,
    as much as 200 m of the bluff area was destroyed.

Fire caused by earthquakes
  • San Francisco in flames after the 1906
    earthquake. The greatest destruction was caused
    by the fires, which started when gas and
    electrical lines were severed.

Fire hazards after earthquakes
  • Ignited by broken gas and electrical lines,
    toppled stoves
  • Added fuel from chemicals, rubbers, gas stations
  • Fire services not alerted because of lack of
  • Roads blocked by earthquake damages
  • Water in emergency tanks underground run out

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  • Shaking hazard map based on past earthquake
    activities and how far shaking extends from quake
    sources. Colors show the levels of horizontal
    shaking that have a 10 chance of being exceeded
    in a 50-year period. (USGS)

  • Aerial view of the collapsed freeway interchange
    between I-5 and the Antelope Valley Freeway
    (State 14) after Northridge Earthquake, Jan. 17,
    1994 (Mw 6.7). (photo Kerry Sieh)

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  • Tsunami means "great wave in harbor" in Japanese.
    The name is fitting as these giant waves have
    frequently brought death and destruction to
    harbors and coastal villages.
  • In physics, tsunami is just like ordinary wind
    driven ocean waves. They are gravity waves
    Gravity is the primary restoring force of the
    motions. But tsunamis are distinguished by
    particularly long periods (200-2000s) and
    wavelengths of tens of kilometers.

  • A tsunami hit Hilo, Hawaii on April 1, 1946, 4h
    55m after it originated from a large earthquake
    in Aleutian trench. Tsunami runup reached 16m.
    Total 159 killed in the five main islands,
    including 96 deaths in Hilo. No warning was
    issued. (UC Berkeley)

  • Tsunami (continued)
  • These long period ocean waves can travel
    thousands of kilometers across the ocean with the
    speed (500-9500 km/hr) equivalent to that of
    jetliners. The wave speed decreases with the
    decrease of ocean depth.
  • The height of a tsunami in the open ocean is
    usually less than 1 meter (so it can pass
    undetected), but the waves can sometimes exceed
    30 m as they slow down in shallow water and pile

  • Illustration of a tsunami generated by
    displacement of ocean floor. The wave speed
    decreases with the decrease of ocean depth. The
    height of a tsunami in the open ocean is usually
    less than 1 meter (so it can pass undetected),
    but the waves can sometimes exceed 30 m as they
    slow down in shallow water and pile up. (Tarbuck
    and Lutgents)

  • Tsunami are excited by large-scale submarine
    displacements of water due to submarine
    landslides, submarine volcanic eruptions, sea
    bottom faulting (earthquakes), etc.

  • Tsunami at Hawaii
  • Being in the middle of the Pacific Ocean,
    surrounded by a ring of earthquakes'', Hawaii
    is exposed to real damages and threats of
    tsunamis. On 1946, April Fools Day, a large
    earthquake occurred in Aleutian trench, 4h 55m
    later, large tsunami hit Hawaii. Tsunami sunup
    reached 16m. Hilo was the most affected. Total
    159 killed in the five main islands, including 96
    deaths in Hilo. No warning was issued.
  • Two years later, the now Tsunami Warning System
    in Hawaii was set up. The system took full
    advantage of the fundamental relationship between
    tsunamis and earthquakes (1) large earthquakes
    can generate tsunamis (2) seismic waves travel
    at 30 to 60 times the speed of a tsunami (i.e.
    minutes vs hours).

  • Tsunami travel times to Honolulu, Hawaii from
    various locations of Pacific rim. (Tarbuck and

  • Examples of tsunami warning
  • 1957, March, 9. A magnitude 8 quake occurred in
    the Aleutian Trench. A Tsunami Watch was issued.
    4h55m later, 10 ft waves hit Hilo. However Kauai
    had larger damages. The runup reached 32 ft. No
    loss of life.
  • 1960, May 22. A magnitude 8.5 quake shook Chile.
    A large tsunami was immediately excited. It
    traveled 6600 miles in 15h and hit the Hawaii
    islands. Hilo again had the most extensive damage
    with 61 deaths. The tsunami warning was very
    accurate but the public response was a failure.
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