Transcript Seismic Hazards in Mid-Willamette Valley

Seismic Hazards in the Mid-Willamette Valley
Sarah Johnson
ES473 Environmental Geology – Spring 2009
Abstract
Seismic Hazards
The Willamette Valley is subjected to many hazards due to its geologic setting.
Volcanic activity, landslides, floods, and earthquakes are all realistic considerations
when living near an active plate boundary. Oregon and Washington are both located
at the convergent zone of the North American and Juan de Fuca plates, with a long
history of oblique subduction of the latter throughout the Cenozoic. This ongoing
tectonic process generates magma in the Cascade arc and greatly influences the
geologic structure of the Pacific Northwest.
The largest and most widespread earthquakes are those generated from subduction
zones. They tend to range in magnitude 8-9 on the Richter scale and can cause
severe damage and fatalities along the entire range of the subduction zone.
Although, Oregon hasn’t had subduction zone earthquake in historic times, several
are thought to have occurred in the past few thousand years (Wang, 1996). Luckily,
subduction zone earthquakes are far less frequent then crustal earthquakes usually
occurring 200-1,000 years apart. Unluckily, evidence suggests that the Cascadia
subduction zone hasn’t had one since the 1700s, so we need to be prepared for the
possibility that one may occur within the next century. Evidence such as deformation
along the coast range and lack of movement at the subduction zone is concerning
because it suggests that the plates may be stuck together causing pressure to build.
If this is the case, the plates will eventually release that pressure and snap back
causing a possible 9+ magnitude earthquake. If such an earthquake were to occur,
several hazards associated with groundshaking such as landslides, liquefaction
(solid ground becoming “quicksand” like), tsunamis, fires, hazardous material spills,
floods, and building damage would cause millions of dollars worth of damage to all of
the western US. One big concern for the Willamette Valley is the possibility of a dam
breaking during an earthquake. Figure 3 shows the Hills Creek dam in Eugene. If
this dam were to rupture, it would send a 40 ft wall of water over the EugeneSpringfield area. The aftermath of this would be similar to that of Hurricane Katrina in
New Orleans.
Much of Oregon’s diverse landscape, including the Mid-Willamette Valley, is due to
Cascadia subduction processes. This forearc region has several fault zones that
pass through, including the Mt Angel, the Canby-Molalla and the Gales Creek.
These fault patterns are a manifestation of regional tectonic stress fields and are
potential sources of crustal seismic activity. Historic seismicity, such as the 1993
earthquake, is an indicator that inland crustal faults in the mid-Willamette Valley are
capable of causing widespread damage. This paper presents a summary of past
seismic activity in the Willamette Valley and risk potential for catastrophic
earthquakes in the future.
Introduction
Over the millennia, seismic activity has done much to shape the present physical
character of the mid-Willamette Valley. Below the weathered surface, which seems
calm and stable, run fault lines that can drastically change those serene features in
thirty seconds time. Roads, bridges and buildings, seemingly well-constructed but
too often with little thought of the power of a major earthquake, lend a false sense of
security to the dramatic destructive capability of earthquakes that not only can, but
will, occur in this area. This poster discusses the seismic character of the midWillamette Valley, reviews the causes and effects of recent and historical
earthquakes and volcanoes and presents suggested actions we can take to prevent
or reduce some of the damage that typically occurs from earthquakes.
Fig. 2 Important faults include the Gales Creek fault, Mt. Angel fault,
and those within the Portland Hills fault zone. Note the numerous
earthquakes that fall between the Portland Hills-Clackamas River and
Mt. Angel-Gales Creek zones.
Willamette Valley History of Seismic Activity
Historic seismicity in western Oregon has been centered between Salem and
Portland, which is the northern Willamette Valley. One of the most recent
earthquakes in the Pacific Northwest was the 5.7 magnitude Scotts Mills crustal
earthquake of 1993. The Scotts Mills earthquake generated ground motion felt from
Roseburg, Oregon in the south all the way to Seattle, Washington and caused $28
million of damage. The main-shock epicenter in Scotts Mills, which is located about
56 km south of Portland, had a depth of 15.1 km. Figure 2 shows the extensive
faulting in this area, which is an indicator of past seismic activity. The main fault
running through this area is the Mount Angel Fault (MAF). The MAF is a reverse fault
approximately 25 km in length dipping to the northeast. It parallels the Portland Hills
fault zone. The large number of fault lines in these areas are a result of tectonic
stress caused by the larger Pacific plate dragging the Juan de Fuca plate in the
opposite direction of the North American causing oblique subduction as shown in
figure 1. Analysis of the MAF suggests that it and possibly other crustal faults in the
Pacific Northwest may be capable of generating earthquakes through about half the
thickness of the crust. “Therefore, the seismic hazard posed by the MAF, or related
faults, may be higher than previously estimated” (Thomas, et al, 1996).
Fig. 1
Tectonic Setting of the Willamette Valley
Deep cracks or faults develop in the crust from the seismic activity. The largest fault
system in the Willamette Valley runs through the Corvallis-Turner-Waldo Hills region.
The northern basin of the valley is cut by the Portland Hills-Clackamas and Gales
Creek-Mount Angel strike-slip faults. Of all these faults running through the valley,
only the Portland Hills fault is considered active (Yeats, 2002). The faulting in the
Willamette Valley is the result of the active plate margin along the Pacific Coast
where the Juan de Fuca (JDF) plate is converging with the North American (NA)
plate causing the JDF plate to subduct underneath the NA plate. When a plate
subducts, seismic activity may occur in one of three ways: intraplate (inside the JDF
plate), interplate subduction (earthquake vibrations radiating from subduction zone),
and crustal (inside the NA plate). The Willamette Valley has the potential to
experience any of these possible earthquakes. The 1993 Scotts Mills earthquake
was a crustal earthquake that occurred in the Willamette Valley. Although we have
no record of the other two types happening in Oregon, there is evidence that a large
magnitude subduction earthquake has occurred in the past.
Hazard Mitigation
Although we cannot do anything to prevent an earthquake, we can try to minimize the
damage and the risk of loss of lives through preparation of what to do before and after
an earthquake. Some actions we can take to prepare include:
 Educating residents about the dangers and encourage them to have a plan
 Reducing the risk to essential public buildings such as schools, hospitals, and
police and fire stations
 Retrofitting older buildings that are at risk
 Reinforcing transportation structures such as roads, bridges, and railroads
 Preparing tsunami evacuation routes and educating people about their location
Conclusion
When assessing seismic risks for the western US, Oregon tends to be forgotten
behind California and Washington because we have had few earthquakes
comparatively. However, there is significant risk for us especially in the valley and
coastal range, so we need to be prepared. Unfortunately, if Oregon were to
experience a large magnitude earthquake today, the damage would be very
extensive. After the damage of the Scotts Mills earthquake was seen, Oregonians
have been more proactive about attempting to prepare for a larger earthquake. In
Portland, they have started reinforcing older buildings and bridges, but this can be
a slow process when funds are limited. Perhaps one of the best actions we can
take is to spread awareness.
References Cited
Yeats, Robert S. Tectonic Setting of the Willamette Valley. Dept. of Geosciences,
Oregon State University. <gsa.confex.com>
Thomas, G. C. et al. 1996. The 25 March 1993 Scotts Mills, Oregon, Earthquake and
Aftershock Sequence: Spatial Distribution, Focal Mechanisms, and the Mount Angel
Fault. Bulletin of the Seimological Society of America, Vol. 86. pp. 925-935.
Fig 3 Hills Creek dam
Wang, Yumei and Leonard, J. William. 1996. Relative Earthquake Hazard Maps of the
Salem East and Salem West Quadrangles, Marion and Polk Counties, Oregon.
Oregon Dept. of Geology and Mineral Industries.