Transcript Research Poster 36 x 48 - Western Oregon University

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INTRAPLATE CRUSTAL FAULTING
AND EARTHQUAKE SOURCES IN THE PACIFIC NORTHWEST
Presented by: Matthew T. Moore, ES473 Environmental Geology, Spring 2009
ABSTRACT
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 earth’s 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 northnortheast. 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.
CONTACT
Name: Matthew Moore
Organization: Western Oregon University
Email: [email protected]
Phone: 503-851-6252
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.
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.
•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.
STRUCTURAL CHARACTERISTICS
•Although interpolate crustal faulting generates
earthquakes with maximum magnitudes between 5
and 7 on the Richter scale, they can still cause
considerable damage.
•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.
Figure 1. Oregon Earthquake Faults.
•For future construction, building materials and
geological aspects of ground on which construction
occurs are being more heavily regulated.
•Southeastern Oregon’s faults are characterized by
normal faulting and are the result of the spreading of
the basin and range province.
CONCLUSIONS
•Fracture zones can rupture creating earthquake foci
at depths anywhere between the Earth’s surface and
15 meters below.
EFFECTS
•Crustal faults produce frequent seismic activity with
magnitude 1 to 2 earthquakes occurring every day.
•In efforts to reduce destruction, programs to
augment public awareness are being implemented.
Figure 2. Map of Selected Earthquakes for Oregon.
•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.
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. Recent
advances 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 danger
zones, the Northwest’s inhabitants can better
prepare for the inevitable.
REFERENCES
Hill, John, Lee Graham, Robert Henry. "Wide-Area Topogarphic Mapping and
Applications Using Airborne Light Detection and Ranging (LIDAR)
Technology."(2000): 908-915.
•Structures developed on unconsolidated soil are
susceptible to amplification of seismic velocity.
Madin, I.P., & Mabey, M.A., 1996, Earthquake Hazard Maps for Oregon:
Department of Geology and Mineral Industries.
•Structures built upon solid bedrock resist and
dampen the effects of
seismic waves.
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.
Figure 3. Faulting Types.
Wang, Yumei. "Environmental, Groundwater and Engineering Geology: Applications
from Oregon." (1998): 325-342.