Research Poster 36 x 48 - A

1
INTRAPLATE CRUSTAL FAULTING AND EARTHQUAKE
SOURCES IN THE PACIFIC NORTHWEST
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Presented by Matthew T. Moore, ES473
Environmental Geology, Spring 2009
ABSTRACT
INTRODUCTION
DETECTION AND MITIGATION

• Many intraplate crustal faults are difficult to
  identify due to unconformities caused by glacial
  erosion and deposits, as well as vegetative
  cover.
• LIDAR Light Distance and Ranging makes
  identification of fault scarps previously
  unidentified by surveyors identifiable up to 15
  cm.
• LIDAR can detect fault scarps as small as 14m in
  length.
• Although interpolate crustal faulting generates
  earthquakes with maximum magnitudes between 5 and
  7 on the Richter scale, they can still cause
  considerable damage.
• In efforts to reduce destruction, programs to
  augment public awareness are being implemented.
• For future construction, building materials and
  geological aspects of ground on which
  construction occurs are being more heavily
  regulated.

Intraplate crustal faulting is a result of active
tectonic motion in the Pacific Northwest. Seismic
analysis indicates that fault depths can extend
15 km, or deeper, into the earths crust. Unlike
Cascadia megathrust slip events, these types of
structures rupture quite frequently, usually with
low magnitudes (M 1 to 2) that are only observed
by seismographs. Despite the low magnitudes of
the average crustal fault earthquake, they
periodically produce sizeable events capable of
causing considerable shaking and damage. The
Scotts Mills Earthquake of 1962 (M 5.5), the
Spring Break Earthquake of 1993 (M 5.6), and late
Holocene evidence of a M-7.0 event along the
Tacoma Fault Zone are examples. Different types
of crustal faults are associated with varying
stress regimes across the Pacific Northwest. The
region extending from northwestern Oregon to
Puget Sound, is characterized by
northwest-striking reverse faults. In contrast,
southeast and central Oregon is associated with
Late Cenozoic normal faults that are generally
oriented to the north-northeast. Due to extensive
surficial cover and vegetation in the western
portions of Washington and Oregon, many surface
faults have remained undetected. Recent advances
with LIDAR (Light Distance and Ranging)
technology have permitted identification of
previously hidden scarps, giving a more detailed
perspective on intraplate faulting in the Pacific
Northwest.
Intraplate crustal faulting is evident all
throughout the pacific coasts of Oregon,
Washington, and Northern California. Earthquakes
associated with these faults occur frequently,
but on average, produce seismic activity below
Richter magnitudes of 2. Nonetheless, they are
capable of causing considerable harm.
STRUCTURAL CHARACTERISTICS

• Intraplate crustal faulting is ultimately caused
  by the subduction of the Juan de Fuca plate
  beneath the North American plate.
• Crustal faulting from this process occurs in
  offshore and nearshore coastal regions with
  reverse faults striking to the Northwest.
• Southeastern Oregons faults are characterized by
  normal faulting and are the result of the
  spreading of the basin and range province.
• Fracture zones can rupture creating earthquake
  foci at depths anywhere between the Earths
  surface and 15 meters below.

Figure 1. Oregon Earthquake Faults.
CONCLUSIONS
Although the effects of intraplate crustal
earthquakes may not be as devastating as a
Cascadia megathrust earthquake, they still
disrupt and cause great burdens for many Pacific
Northwesterners. Recentadvances in
identification of possible hazard zones have
greatly assisted in informing builders of the
safety of their area. With public education
informing people about the effects specific to
intraplate crustal faulting and increased
awareness of specific dangerzones, the
Northwests inhabitants can better prepare for
the inevitable.
EFFECTS

• Crustal faults produce frequent seismic activity
  with magnitude 1 to 2 earthquakes occurring every
  day.
• Magnitude 5 earthquakes occur less frequently. An
  example of this is the Scotts Mills earthquake of
  1993. Magnitude 7 earthquakes from crustal faults
  occur very infrequently. The last one occurred
  around Seattle between 900 and 930 AD.
• Structures developed on unconsolidated soil are
  susceptible to amplification of seismic
  velocity.
• Structures built upon solid bedrock resist and
  dampen the effects ofseismic waves.

Figure 2. Map of Selected Earthquakes for Oregon.
REFERENCES
Hill, John, Lee Graham, Robert Henry. "Wide-Area
Topogarphic Mapping and Applications Using
Airborne Light Detection and Ranging (LIDAR)
Technology."(2000) 908-915. Madin, I.P.,
Mabey, M.A., 1996, Earthquake Hazard Maps for
Oregon Department of Geology and Mineral
Industries. Niewendorp, C.A., Neuhaus, M.E.,
2003, Map of Selected Earthquakes for Oregon,
1841 through 2002 Department of  Geology and
Mineral Industries. Sherrod, Brian, Thomas
Brocher, Craig Weaver, Robert Bucknam,
Richard Blakely, Harvey Kelsey, Alan Nelson,
Ralph Haugerud. "Holocene Fault Scarps Near
Tacoma, Washington, USA." (2004) 9-12. Wang,
Yumei. "Environmental, Groundwater and
Engineering Geology Applications from Oregon."
(1998) 325-342.
CONTACT
Name Matthew Moore Organization Western
Oregon University Email MatthewTMs_at_Gmail.com Phon
e 503-851-6252
Figure 3. Faulting Types.