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Historic Seismicity, Neotectonics and Active
Faulting in Oregon Ryan Stanley, Earth and
Physical Sciences Department, Western Oregon
University, Monmouth, Oregon, 97361 email
rstanley06_at_wou.edu
Neotectonics
Abstract
Active Faulting
The instrumental period of earthquake
monitoring is brief in the context of geologic
time. Therefore current earthquake source models
(Fig. 2) do not definitively represent Oregons
seismic hazards. Multiple lines of geologic
evidence indicate a successive history of
megathrust earthquakes along the Cascadia
Subduction Zone, yet the historical record lacks
such events. Buried soils in coastal intertidal
lowlands suggest a land subsidence event caused
by rupture along the CSZ (Atwater, 1987).
Locations along the Oregon coast yield evidence
for tectonic activity, including tsunami sands,
buried peats, and trees killed by salt-water
(Wong Bott, 1995). Surmounting evidence
suggests the most recent megathrust earthquake
occurred A.D. 1700 with a moment magnitude of 9.
From 1841 to 1994, nearly 6,000 earthquakes
ranging from magnitude 1 to 6.75 have occurred in
Oregon (Wong Bott, 1995). Until the 1840s,
written documentation of earthquakes had not been
recorded. In 1906 the first Pacific Northwest
seismograph station was installed in Seattle, but
Oregon did not receive one until OSU installed an
instrument in 1946 (Wong Bott, 1995).
Significant improvements in seismic sensing began
in 1979 when UW expanded its network into Oregon.

Earthquakes result from sudden displacement
along a fault. The area of a rupture dictates
the amount of seismic energy released. Some
events, such as the 1993 Scotts Mills earthquake,
rupture along blind faults. Oregons climate and
geography ultimately inhibit fault visibility
through increased erosion, weathering, and
vegetation. Known Oregon faults have been mapped
(Fig. 4), but few historical events are
associated with known active faults, notably the
Milton-Freewater, Scotts Mills, and Klamath Falls
earthquakes. Those three episodes may have been
associated with the Wallula or Hite faults, Mount
Angel fault, and Lake of the Woods fault zone,
respectively. Seismicity in the Portland region
is attributed to the right lateral, strike-slip
Portland Hills and Frontal fault zones.
Evidence of active faulting and seismicity
in the Pacific Northwest has been documented
through analysis of stratigraphic relationships
and use of modern instrumentation. Crustal,
intraplate, and subduction zone earthquakes
represent three types of Cascadia deformation,
each with unique seismic characteristics.
Earthquake monitoring utilizes specialized
instruments that collect information to
understand sizes, locations, frequency of
occurrence, and types of earthquakes. Written
documentation of historical events did not appear
in Oregon until the 1840s, and seismograph
stations were not established in the Pacific
Northwest until 1906. The discovery of
multiple buried soils in coastal intertidal
lowlands suggests that great megathrust
earthquakes have repeatedly occurred within the
Cascadia subduction zone. Multiple lines of
evidence indicate an average megathrust
earthquake recurrence interval of 500 years,
with the most recent occurring A.D. 1700. Since
1841 more than 6,000 earthquakes have occurred in
Oregon. Oregon's largest earthquakes include the
1873 Crescent City, 1936 Milton-Freewater, 1962
Portland, 1993 Scotts Mills, and 1993 Klamath
Falls earthquakes. This paper provides a
synopsis of historic seismicity and seismic
sources in Oregon, with implications for hazard
planning throughout the state.
Pacific Northwest Seismic Network
A primary goal of earthquake monitoring is
to detect and locate sources with specialized
instruments and acquire enhanced understanding of
potential sources. Seismic instruments collect
information to understand sizes, locations,
frequency of occurrence, and types of earthquakes
and their effects. The Pacific Northwest Seismic
Network (PNSN), connected through University of
Washington and the USGS, covers a large part of
the Pacific Northwest (Fig. 3). The Network
provides real-time information on earthquakes and
maintains strong motion, short-period, and
broadband seismic instruments (Wang et al.,
1998). Stations provide instantaneous
calculation of magnitude and location. Three
types of seismic instruments specialize in the
recording of earthquakes. Strong motion
instruments measure strong levels of shaking over
a wide range of frequencies. Such strong motion
seismographs record shaking that might severely
impact engineered structures. Short-period
instruments measure low levels of shaking, but
only at higher frequencies. Broadband
instruments measure low levels of shaking over a
wide range of frequencies. A real-time
monitoring network called Rapid Alert of Cascadia
Earthquakes (RACE) automatically telemeters
earthquake data to facilities in Seattle where an
algorithm calculates epicenter location (Wang et
al., 1998). The goal is to relay information to
nearby communities before onset of damaging
shaking. DOGAMIs instrumentation program focuses
on improving the regional seismic network.
Introduction
The Pacific Northwest experiences seismicity
generated from three different sources. Crustal,
intraslab, and subduction zone earthquakes (Fig.
1) result from stress associated with the
Cascadia Subduction Zone (CSZ). Subduction zone
earthquakes are less frequent yet release larger
amounts of energy than other seismic sources.
Crustal earthquakes result from active faulting
in the overriding North American plate, and deep
earthquake hypocenters appear in the subducting
Juan de Fuca slab. Seismograph networks and
other seismic instruments allow real-time
tracking of seismicity and active faulting in the
Pacific Northwest.
Conclusion
Geologic and historical records both
document Oregons seismic activity. Modern
seismic instrumentation, such as that employed by
the PNSN, now allows real-time tracking of
earthquakes in the Pacific Northwest. Primarily
crustal earthquakes account for seismic moments,
although the 1873 6.75-magnitude earthquake was
possibly from an intraplate source. Paleoseismic
data from the Oregon coast points to massive
subduction zone earthquakes. Additionally, Late
Quaternary faults in eastern Oregon reveal the
occurrence of magnitude 7 earthquakes (Pezzopane
Weldon, 1993). Increased seismic monitoring
and paleoseismic studies will quantify Oregons
earthquake potential, thereby improving upon the
states incomplete historical record.
Historical Earthquakes
Six earthquakes larger than magnitude 5 have
occurred in Oregons recorded history (Table 1).
On December 2, 1841, the first earthquake in
Oregons record was felt near Fort Vancouver with
an intensity of MM III (Wong Bott, 1995). An
1893 Umatilla earthquake that destroyed a stone
buildings wall might be Oregons largest event,
but little is known about the episode. The
Portland region is the most seismically active
area in Oregon, having experienced a total of 17
earthquakes magnitude 4 and greater. Such
activity may be explained by the Portland
pull-apart basin that involves two fault zones
(Yelin Patton, 1991). The 1962 Portland
earthquake was felt over an area of 70,000 km2 in
northwestern Oregon, causing chimneys, windows,
and tile ceilings to break (Wong Bott, 1995).
The first deaths from an Oregon earthquake
occurred in Klamath Falls, where two moderate
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Table 1. This list outlines the most
significant historical earthquake events in
Oregon (Wong Bott, 1995).
sized earthquakes caused 7.5 million in damage.
Aftershock distribution and focal mechanisms
indicate the Lake of the Woods fault zone as a
primary source (Braunmiller et. al., 1995). A
magnitude 4.0 event occurred at Mount Hood in
1974, demonstrating the possibility of relatively
shallow earthquakes associated with Cascade
volcanism. In eastern Oregon, the Pine Valley
graben-Cuddy Mountain area is the most
seismically active (Wong Bott, 1995). The
regions first recorded earthquake in 1913 was
assigned an intensity of MM VI and damaged
furniture and windows in Landore, Idaho.