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Seismic Hazards in the Klamath Falls Area
Prepared By Gretchen Boyer, ES 473 Environmental
Geology, Spring 2009
SEISMIC MONITORING
KLAMATH FALLS EARTHQUAKES
INTRODUCTION
ABSTRACT
In September 1993, Klamath Falls was struck by
the largest earthquake to hit Oregon since 1873.
The quakes were felt as far away Eugene to the
north, and Chico, California to the south. A
foreshock had a magnitude of 3.9, and the first
of the two main shocks registered 5.9, which was
followed by the largest shock that registered a
magnitude 6.0. A few people reported feeling
intensities of VII on the Mercalli Intensity
Scale, but most reported an intensity of about
VI. The earthquakes caused landslides
throughout an area of about 162 square miles
surrounding the epicenter. One rock slide
occurred near highway 97 and crashed through the
barrier, hitting a vehicle and killing the
driver. The earthquakes also caused extensive
damage to the county courthouse and over a
thousand residences and businesses suffered
damages involving cracked walls, broken windows,
collapsed chimneys, and damaged plumbing. All
totaled, the damage was upwards of 10 million
dollars.
The Klamath earthquakes were followed by many
aftershocks by October, more than 400 that had
had been recorded. Initially, individual
earthquake hypocenters were poorly located due to
a lack of permanent seismographs in the area, so
20 portable seismographs were rapidly deployed by
from local Universities. By October, the U.S.
Geological Survey installed four permanent
seismographs in the epicentral region. Data from
these instruments is now telemetered to the
University of Washington, where it is recorded as
part of the UW seismic network. The UW is now
able to precisely locate aftershocks as small as
magnitude 0.3. In addition to the 3 sensitive
stations, There is one "strong motion"station,
KFAL at Klamath Falls, and that will stay "on
scale" and record large ground motions in high
fidelity in case of a large earthquake. This
equipment won't allow scientists to predict
earthquakes, but it does allow them to keep a
very close eye on fault movement, which allows
for a greater understanding of slip and fault
displacement in the region that ultimately cause
earthquakes.
Because of its dynamic underlying fault
structure, the Klamath Falls region in Southern
Oregon experiences unique seismic activity that
comes with a multitude of potential hazards that
pose many different types of threats to
residents. Inhabitants and local government are
faced with the daunting task of preparing for
earthquakes that can be sporadic and that can
vary in intensity from mild to severe. Due to its
multiple faults, an earthquake could happen in
any number of locations, so seismologists keep a
watchful eye for any indication of seismic
disturbances. Klamath Falls does have a slight
potential for volcanic earthquakes up to
magnitude 5, and an even smaller potential for a
Cascadia earthquake of up to magnitude 8-9,
however, shallow tectonic earthquakes are more
common in the region and are of the most
immediate concern.
Mount Mazama and the Crater Lake caldera lies at
the north end of the fault bounded basin that is
known as the Klamath Graben. This structural
low lies at the northwestern end of a set of
complex northwest-trending fault-block mountains,
located on the western edge of the Basin and
Range province. The graben is bounded on the west
and east sides by the active West Klamath Lake
and East Klamath Lake fault zones, respectively.
While both of the margin-bounding fault zones are
active, there is more data available for the West
Klamath Lake portion. This data provides the
scientific framework that allows understanding of
seismic hazards in the Klamath Falls area. The
West Klamath lake fault zone terminates to the
south, near the epicenter of the September 1993
Klamath Falls Earthquakes. The two main shocks
that struck the Klamath Falls area during that
time registered magnitudes 5.9 and 6.0, causing
landslides, significant building damage and two
deaths. As a result of the 1993 earthquakes,
closer seismic monitoring of the Klamath region
was implemented in order to better understand the
dynamics of fault activity. This paper focuses
on earthquake mechanics and hazards monitoring
programs associated with the Klamath Falls area.
FAULT STRUCTURE
Klamath Falls lies on a series of faults and a
structure known as the Klamath Graben. Horst and
graben refer to regions that lie between normal
faults, and a graben is a block that has dropped
down. The Klamath graben is one of the major
structures within the Cascades Basin and Range
transgression of northern California and southern
Oregon. The Klamath graben is moving towards the
northwest, where it meets the High Cascades
volcanic arc that is trending north-south. The
graben is bounded by the West Klamath Lake and
East Klamath lake fault zones, which are both
seismically active. West Klamath Lake Fault Zone
The West Klamath Lake fault zone (WKLFZ) is
composed of several individual faults with
lengths of up to 15 kilometers an an overall
length of 60 km, and has been mapped up to Crater
Lake National Park. The WKLFZ faults trend north
south, and shows displacement of about 0.3
millimeters per year. The consistency in age and
displacement suggests that the WKLFZ merges at
depth into a single through going structure. One
of the constituent faults, the Annie Spring
fault, near Crater Lake has been responsible for
a few minor earthquakes, however, the Southern
boundary of the WKLFZ has historically been the
site of more earthquakes. This region is marked
by a change in strike faults from north to
north-northwest near the Epicenter of the 1993
Klamath Falls earthquakes, which registered as
high as magnitude 6.
CONCLUSION
Seismic monitoring and public education efforts
in Klamath Falls are ongoing. Some progress has
been made by enacting a dangerous building
ordinance and retrofitting some buildings, but
efforts will need to continue in order to be
prepared for future earthquakes. A recent state
report found that many Klamath county schools and
public safety buildings could collapse during a
large earthquake. Voters recently defeated a 20
million dollar bond that included seismic
upgrades. Seismologists closely monitor Klamath
falls, but residents must also prepare and
educate themselves about earthquakes.
Figure 1 Map showing Faults active in the past
few million years and earthquake epicenters
through September 1997.
Figure 2 Isoseismal map indicating the combined
effects of the two strongest quakes on September
20th, 1993.
REFERENCES
1.Bacon, C.R, 1997, Volcano and Earthquake
Hazards in the Crater Lake Region, Oregon U.S
Department of the Interior, U.S Geological
Survey. 2.Bacon, C.R, 1999, Late Quaternary
Slip Rate and Seismic Hazard of the West Klamath
Lake Fault Zone near Crater Lake, Oregon
Cascades Geology, Volume 27, no. 1, p.43-46.
3.Beach, L., 2007, Schools Will Not Withstand
Earthquake, Herald and News 4.Wiley, T.A et
al., 1993, Klamath Falls Earthquakes. September
20th 1993 Including the Strongest Quake ever
measured in Oregon Oregon Geology, Volume 55
Number 6.
Figure 4 Structural damage during the 1993
earthquakes.
Figure 3 Location of Seismic Stations
Near Klamath Falls.