Week 9 Disaster and Disaster Risk
Week 9 Disaster and Disaster Risk
Week 9 Disaster and Disaster Risk
Reduction
Week no. : 9
Earthquake Hazards
Learning Targets
h s
E a r t q u a k e are probably the most frightening naturally occurring hazard
encountered. Why? Earthquakes typically occur with little warning. There is no escape
from an earthquake! Earthquakes have
devastating effects, resulting in hundreds to
thousands of deaths and injuries, and millions to
billions of dollars’ worth of property damage. The
earthquake's location, magnitude of the
earthquake, surface geology, and population
density are major factors contributing to
earthquake damage.
Types of faults
Earthquakes result from movement along a
fault. Faults and earthquakes are cause and effect. The sense of motion on faults
describes how the block move relative to each other. Faults may move along preexisting
fracture or may form a new one. There are 3 basic types of faults: normal, reverse, and
strike-slip. Normal and reverse faulting result in vertical slip, while strike-slip faulting
results in horizontal slip. In nature, motion is seldom absolutely along one direction. There
Normal faults
Normal faults are associated with extension. A good example of normal faulting is the
Basin and Range topography of the western United States. The western part of the North
American plate has been pulled apart into a series of "blocks". Most Basin and Range
structures result from the tilting of these blocks. A major Basin and Range fault zone is the
Wasatch Fault zone, which is 220 miles long (360 kilometers) and extends from Utah into
Idaho.
A. Watkins diagram
Reverse faults
Reverse faults are associated with compressional forces- 2 plates or fault blocks pushing
towards each other. One side ends up on top! Thrust faults are reverse faults that move
up a shallower angle than ordinary reverse faults.
Strike-slip faults
Strike-slip faults are associated with shear stresses. One side of the fault "slides" past
the other. "Sometimes" it is fairly easy to recognize where movement on a strike-slip fault
has occurred. The photo below shows a creek located along the San Andreas Fault. The
Compare the photo of the San Andreas Fault with the strike-slip fault diagram. The San
Andreas Fault is a right-lateral strike-slip fault.
Earthquake processes
Rupturing rocks release huge amounts of energy. The sudden release of energy is what
is felt in an earthquake. Earthquake energy is in the form of seismic waves. The seismic
waves radiate out from a central point, called the focus or hypocenter, like ripples moving
outward from a pebble tossed into a lake. The location directly above the hypocenter, on
the earth's surface, is called the epicenter.
Seismic waves
Four types of seismic waves are generated when faulting triggers an earthquake. All the
seismic waves are generated at the same time, but travel at different speeds and in
different ways. Body waves penetrate the earth and travel through it, while surface waves
travel along the surface of the ground.
Primary and secondary waves are body waves. Primary waves (P-waves) travel the
fastest and can move through solids and liquids. The P-wave energy causes the ground to
move in a compressional motion in the same direction that the wave is traveling.
Secondary waves (S-waves) are slower and travel only through solids. The S-wave
energy causes the ground to move in a shearing motion perpendicular to the direction of
wave movement.
Rayleigh and Love waves are the two types of surface waves. Rayleigh wave energy
causes a complex heaving or rolling motion, while Love wave energy causes a sideways
movement. The combination of Rayleigh and Love waves results in ground heave and
swaying buildings. Surface waves cause the most devastating damage to buildings,
bridges, and highways.
The Mercalli Scale defines intensity. Intensity is rated by how much damage was caused
by an earthquake and how it affected people.
The UNR Seismological Laboratory Page is full of interesting information for earthquake
enthusiasts.
Landslides
Seismic vibration is a common triggering mechanism for landslides. In hilly or
mountainous regions, landslides can have particularly devastating effects. Damages can
range from debris-covered roadways to extensive property damage and numerous
casualties.
Liquefaction
How much can surface and subsurface material contribute to
earthquake damages? Like many other physical phenomena, the
answer is, "It depends." Thick sequences of unconsolidated
sediments, such as sand, mud, and artificial fill, greatly magnify
ground shaking during an earthquake. Ground shaking transmits
forces to building that most buildings are not designed and
constructed to endure. Ground shaking results in extensive property damage. Bedrock is
less likely to be affected by ground shaking than is unconsolidated material. Buildings
constructed on bedrock sustain far less damage than those built on unconsolidated
material. Other dangers also come from the ground during an earthquake. Buildings
constructed on sandy soil prone to water saturation have the greatest potential for
complete destruction, because water-saturated sandy soil is subject to a phenomena
called liquefaction. During liquefaction, water-saturated soil behaves as a fluid rather than
as a solid. It becomes incapable of supporting much weight. (Remember the soil module
and the section of soil strength?)
Fires
Earthquakes cause fires. Even moderate ground shaking can
break gas and electrical lines, sever fuel lines, and overturn
stoves. Water pipes rupture, making it impossible to fight the
earthquake-caused fires. The famous San Francisco earthquake
in 1906 ruptured the city's main water pipes. Extensive fire
damage was the result!
References:
Rimando, R., & Belen, J, Phil. (2016). Disaster Readiness And Risk Reduction: Basic Concept of Disaster and
Disaster Risk (First Edition). REX.
http://geology.isu.edu/wapi/EnvGeo/EG5_earthqks/eg_mod5.htm#:~:text=Examples%20include%20landslides%2C
%20tsunami%2C%20liquefaction,the%20greater%20the%20damage%20potential.