Lecture 17 Earthquake Hazards

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Lecture 17 Earthquake Hazards

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

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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

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• Distribution of earthquakes Mgt5 for 1980-1990.

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• earthquake magnitude and energy equivalence
• mag energy release appr. equivalence
• 2 600x1012 ergs 1000 pound of
  explosives,
• rumbling of trains,
  smallest human can feel
• 6 600x1018 ergs 1946 Bikini atomic
  bomb test
• 10 600x1024 ergs annual US energy
  consumption
• 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.

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• 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.

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• 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.

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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

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• 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)

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• 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
  California.
• 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.

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• 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.

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• 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.

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• 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.

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

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• 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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Landslides

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

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• 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.
  (USGS)

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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.

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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
  information
• 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)

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• 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

• 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.

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• 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)

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• 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
  up.

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• 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)

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

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• 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).

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

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• 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.