Engineering Geology and Geohazards Lecture 14: Earthquake hazards and Mitigation

Question 1

In the last decade, how many people have died from earthquakes?

Answer 1

In the last decade some three-quarters of a million
people have died due to earthquakes, mostly in the
developing world, where the risk is increasing due
to rapid population growth and urbanisation.

Question 2

Describe the setting for particularly damaging/ disastrous earthquakes

Answer 2

A geological setting favouring
seismic strain release by large
earthquakes.

• Large numbers of people
  vulnerable to earthquake hazards.
• Underdeveloped economic
  conditions: poor quality housing,
  lack of infrastructure and
  emergency services, lack of
  scientific and technical capability,
  corruption.
• The type and location of the
  earthquake, and potential for
  triggering secondary hazards, eg. tsunami, fire, etc.
• Unfavourable ground response.
  Time of day when earthquake
  occurs.

Question 3

Describe the primary/direct hazards of an earthquake

Answer 3

Ground shaking caused by release of
elastic strain energy.

Manifested as seismic waves.

Sumatran earthquake,
2004, 8-10 minutes shaking. Tohoku,
2011, 6 minutes shaking.

• Surface rupture along faults.

Question 4

What were the three acts passed as a result of the 1971 San Fernando earthquake

Answer 4

The Alquist-Priolo Act
prohibits the construction of most human-occupied structures within 20 metres of an active fault.

The Dam Safety Act
requires evaluation of the safety of existing dams in California.

The Hospital Safety Act called
for the strengthening of
construction standards for
hospitals.

Question 5

Compare Haiti 2011 and Dartfield earthquake 2010

Answer 5

Mw 7 Haiti earthquake 12 January 2010,
16.53 local time, focal depth 13 km,
230000 killed.

Mw 7.1 Darfield earthquake, New
Zealand, 4 September 2010, 04.35 local
time, focal depth 10 km, no deaths.

The Haiti earthquake had a significantly higher death count due to the building quality - shantytown dwellings.

Question 6

Describe the secondary/indirect effects of an earthquake

Answer 6

• Seismically triggered landsliding.
• Liquefaction.
• Flooding from breached dams, as well as
  from landslide-dammed lakes.
• Urban fires and explosions.
• Tsunami and seiches.

Question 7

Describe Liquefaction

Answer 7

During earthquakes, saturated unconsolidated granular sediments (especially sands capped by relatively impermeable strata) temporarily lose strength and behave as a viscous fluid.

This happens because ground shaking causes the grain-to-grain contact stress, which maintains the strength of the sediment, to be momentarily transferred to the water filled pore spaces.

Question 8

Give some examples of Urban fires and explosions due to earthquakes

Answer 8

• 1906 San Francisco
• 1923 Tokyo (140000 killed, most by fire)
• 1989 Loma Prieta
• 2011 Tohoku, Japan. Fukashima nuclear
  power plant.

Question 9

What can be done to reduce earthquake-related deaths?

Answer 9

Earthquake forecasting?
* Increasing resilience?
* Limited resources, how to prioritise?
* GEM (Global Earthquake Model)
* Other national and international initiatives

GEM framework is aimed at constructing seismic risk models,
and is organised to include the full spectrum of factors: