Transcript Research Poster 36 x 48 - Western Oregon University

Seismic Hazards in the Klamath Falls Area
Prepared By Gretchen Boyer, ES 473 Environmental Geology, Spring 2009
ABSTRACT
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.
INTRODUCTION
KLAMATH FALLS 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.
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.
FAULT STRUCTURE
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.
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.
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.
CONCLUSION
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.
SEISMIC MONITORING
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.
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.
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:
Figure 3: Location of Seismic Stations Near
Klamath Falls.
Figure 4: Structural damage during the
1993 earthquakes.
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.