Abstract

1
Earthquake Risks I Ground Shaking Hazards in
Oregon Presented by Thomas VanNice Earth Science
Major ES473 Environmental Geology Spring 2009
Ground Motion Hazards Ground motion is mainly
caused by the Love waves. The hazards that result
from ground motion are endless but most commonly
are liquefaction, fault scarp formation, slope
failure, debris flows, and extensive
infrastructure damage. Liquefaction would cause
the most damage in the heavily populated areas,
like Portland for example. Portland is built on
many feet of loose unconsolidated saturated
alluvium, gravels and sands. The shaking from the
Love waves would be magnified in areas like this
resulting in liquefaction of the soil that the
buildings are built on. Some of the other hazards
like slope failure and debits flows would be
caused by semi-saturated slops that are then
shaken off their bedrock foundations. These
events would occur mainly in the mountainous
regions of Oregon. Fault scarp formation can
occur anywhere along a fault and would result in
an uplift or down drop from one side of the fault
to another.
Abstract Earthquakes are one of the most
powerful natural disasters that occur in the
world. Ground motion and shaking during an
earthquake can be detected hundreds of miles away
from the source. The ground motion and shaking
that accompany an earthquake are produced by the
waves that are a result of the motion derived
from brittle failure of consolidated rock
materials. There are three main types of waves,
S-waves, P-waves, and surface waves, each of
which results in different styles of ground
motion. The intensity and the duration of the
shaking will be determined by the amount of
energy released during the earthquake event. The
strength of an earthquake is measured from
seismograms using a moment-magnitude calculation
(e.g. Richter scale). Earthquakes with magnitudes
3.0 and above can be felt by humans depending on
material mechanics and other variables. Shaking
and ground motion are associated with a
significant portion of infrastructure damage and
resulting deaths in an earthquake
event. Ground-motion modeling is used to
construct seismic hazards maps in the Pacific
Northwest. Critical input parameters include
seismic sources, wave travel paths, and
composition of subsurface materials. This
project focuses on ground-shaking processes and
hazards mapping in Western Oregon.
Figure 3.

Wave types There are three main types of waves
that result from an earthquake P-waves, S-waves,
and surface waves. There are two types of
surface waves Love and Rayleigh waves. The
P-wave is the first wave to register on the
seismograph. The P-wave is a compression wave,
this allows it to travel fast through the earth.
The next wave to arrive at the seismograph
station is the S-wave. The S-wave is know as a
sin wave for the way it resembles a sin function
wave as it travels through the earth. The S-wave
is slower then the P-wave but both are faster
than surface waves. Because both wave types are
body waves they travel all of the way through
the earth. This is good because it gives us a
chance to be able to triangulate the earthquakes
epicenter and apply help where needed. The
surface waves can be broken into two types Love
and Rayleigh waves. Love waves are faster than
Rayleigh waves. This causes them to be more
destructive than Rayleigh waves. Love waves are
the waves that you are most likely to feel during
an earthquake. Rayleigh waves are the slowest of
all the wave types. They are formed by backwards
ellipses that roll. This causes the wavelength to
be larger and less frequent. This causes less
damage than Love waves.
Figure 1. Effects of the 1906 San Francisco
Earthquake caused 524 million dollars in
infrastructure damage. Magnitude 7.8 on the
Richter scale.
Figure 2. Effects of the 2002 Alaskan
Earthquake caused extensive ground and slop
damage. Magnitude 7.9 on the Richter scale.
Figure 5. Ground motion acceleration model of
Oregon in three different sized earthquakes.
Figure 4. Basic seismograph reading of an
earthquake.
Conclusion Oregon is a great place to live and
people will continue to live in Oregon. However,
as Oregonians we need to take the proper steps in
developing a system to help predict the size and
occurrence interval of earthquakes. We also need
to better prepare ourselves in the area of
highest risk that we have concluded from the
ground motion models that we created, as well as
develop a plan that we could set in motion if a
large earthquake was to rock the Pacific
Northwest.
Introduction Oregon has many natural hazards
one of the worst being earthquakes. A subduction
zone, mega thrust, earthquake could result in a
lot of damage due to ground motion and movement.
The ground motion would be felt extremely well in
the Willamette Valley due to the loss
unconsolidated soil. Some of the hazards would be
liquefaction, uplift and down drop, and would
cause intense infrastructure damage. The motion
would also be felt in the both the Cascade and
Coast Range Mountains. Some of the hazards that
would occur in the mountains would be land
slides, debris flows, and fault scarp formation.
These potential hazards have caused geologists to
develop and produce ground motion and hazard maps
for populated areas in Oregon. These maps aid the
development of urban areas and business
districts.
Ground Motion Modeling Ground motion can have a
large impact when it comes to infrastructure
damage. This is why geologists have developed
ground motion acceleration models. Geologists
create them by determining things like
composition both regolith and bedrock. They also
take the moisture or ground water into
consideration as well as things like slope angles
and preexisting faults. The moment magnitude
scale is used as a ground motion model
classification mechanism and is used to determine
the severity of an earthquake.

• References
• Earthquake Maps for Oregon (DOGAMI GMS100, 1996)
• Earthquake Hazards in the Pacific Northwest
  (Rogers and Priest, 19xx)
• Potential for EQ damage in Oregon (Wang and
  Clark, 1999)
• Projected Risk and Losses due to Earthquake
  Damage in Oregon (Wang, 1998)
• Earthquake Risks in Oregon (Wang, 1998)