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July 31,2003
Tigard-Tualatin School District 23J •
6960 SW Sandburg Street
Tigard, Oregon 97223
Attn: Mr. Stephen Poage
Director of Capital Projects
Re: Seismic Site Hazard Investigation
Alberta Rider Elementary School
Tigard-Tualatin School District 23J
Tigard, Oregon
URS Job No: 25695391.10001
Dear Mr. Poage:
We are pleased to submit herewith our report entitled "Seismic Site Hazard Investigation,
Alberta Rider Elementary School, Tigard-Tualatin School District 23J, Tigard, Oregon." This
report formally documents our conclusions and recommendations regarding the proposed project.
It has been our pleasure to assist you with this project. Should you have any questions regarding
the contents of this report,please call us at your convenience.
Yours very truly,
URS Corporation
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Bryan J. Duevel, PE (30N Tian M. Wil man, Ph.D., P.E.
Project Engineer 414)44)-,7 ,tea ,p Manager, Geotechnical Engineering
•
EXPTREs: ' 1'
URS Corporation
111 SW Columbia, Suite 900
Portland, OR 97201-5814
Tel: 503.222.7200
Fax: 503.222.4292
TABLE OF CONTENTS
Section 1 Introduction 1-1
1.1 General 1-1
1.2 Scope of Work 1-1
Section 2 Geologic Setting
2-1
2.1 Site Description 2-1
2.2 Regional Geologic Structure 2-1
2.3 Site Geology 2-1
2.4 Site Hydrogeology 2-2
Section 3 Seismic Conditions 3-1
3.1 Earthquake Effects-General 3-1
3.2 Historical Seismicity 3-1
3.2.1 Significant Earthquakes 3-1
3.2.2 Summary 3-3
3.3 Earthquake Sources 3-3
3-4 Fault Descriptions 3-4
3.4.1 Helvetia Fault Zone 3-5
3.4.2 Newberg Fault 3-5
3.4.3 Portland Hills Fault 3-5
3.4.4 Bolton Fault 3-6
3.4.5 Mt. Angel Fault 3-6
3.4.6 East Bank Fault 3-7
3.4.7 Oatfield Fault 3-7
3.4.8 Clackamas River Fault Zone 3-7
3.4.9 Grant Butte, Damascus, Tickle Creek Fault Zone 3-8
3.4.10 Other Fault Zones 3-8
3.4.10.1 Sherwood Fault 3-8
' 3.4.10.2 Dairy Creek Fault 3-8
3.4.10.3 Beaverton Fault 3-9
3.4.11 Cascadia Subduction Zone 3-9
3.4.11.1 Megathrust 3-9
Section 4 Design Ground Motion 41
4.1 Ground Motion Analyses 4-1
4.1.1 Geomatrix 1995 Probabilistic Study 4-1
4.1.2 URS 2001 Probabilistic Study 4-1
4.1.3 URS/DOGAMI 2000 Portland Metropolitan Study 4-1
4.1.4 1998 OSSC Zonation 4-1
4.1.5 Results Comparison 4-2
4.2 Recommended Design Ground Motions 4-2
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TABLE OF CONTENTS
Section 5 Seismic Hazard Evaluation 5-1
5.1 Anticipated Foundation Design 5-1
5.2 Seismic Hazards 5-1
5.2.1 Liquefaction Hazard 5-1
5.2.2 Tsunami/Seiche Hazard 5-1
5.2.3 Seismic Slope Stability Hazard 5-1
5.2.4 Surface Rupture Hazard ' 5-2
5.2.5 Ground Shaking Amplification Hazard 5-2
Section 6 Closure 6-1
Section 7 References 7-1
List of Tables •
Table 1 Comparison of Peak Ground Accelerations 4-2
List of Figures
Figure 1 Vicinity Map
Figure 2 Site Map
Figure 3 Tectonic Structures f the Tualatin Basin
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SECTIONONE introduction
1.1 GENERAL
This report presents the results of our seismic site hazard investigation for the proposed Alberta
Rider Elementary School in Tigard, Oregon. This work was completed in accordance with our
proposal to Tigard-Tualatin School District 23J dated May 16, 2003. The project site is located
approximately as shown on the Vicinity Map, Figure 1. The project involves the construction of
a new elementary school with a footprint of approximately 40,000 square feet, as well as an
entrance drive, parking lots and play fields. The Site Map presented on Figure 2 shows a
preliminary plan layout of the site.
The proposed school is considered to be a "special occupancy structure" under Oregon Revised
Statutes (ORS) 455.447. As such, a seismic site hazard investigation is required per Oregon
Structural Specialty Code (OSSC) Section 1804.1. This report is prepared in accordance with
OSSC Section 1804.3.2. The purpose of this report was to evaluate the surface and subsurface
conditions at the site and to evaluate the potential seismic hazards of the proposed school. This
report is a companion to the geotechnical report entitled"Foundation Investigation, Alberta Rider
Elementary School,Tigard-Tualatin School District, Tigard, Oregon." This report was submitted
to the Tigard-Tualatin School District in July 2003.
1.2 SCOPE OF WORK
The scope of this investigation included completion of the following:
1. Description of the site geologic setting including regional geology, site topography,
subsurface stratigraphy and groundwater.
2. Description of the seismic setting including the regional tectonic framework, historical
seismicity, and potential earthquake sources.
3. Probabalistic and deterministic analyses to assess design earthquake ground motions.
4. Evaluation of seismic hazards including landslides, liquefaction, regional
subsidence/collapse, fault surface rupture, and tsunami/seiche inundation.
5. Preparation of five copies of this report describing the results of this investigation.
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•
SECTIONTWO Geologic Setting
2.1 SITE DESCRIPTION
• The site is located near the crest of Bull Mountain as shown on the Vicinity Map, Figure 1. The
topography across most of the site is relatively gentle, with elevations ranging from a high of
approximately 574 feet above Mean Sea Level (MSL) along the northwestern boundary to the •
site to 515 feet above MSL in the far southeast corner of the site. The maximum slopes present
onsite approach 25% at the northwest edge of the site. Elsewhere, maximum slopes are
approximately 15/0, located in the northwest and southeast portions of the site
2.2 REGIONAL GEOLOGIC STRUCTURE
The site is located in the northern Willamette Valley physiographic province, an elongate,
roughly north-south trending alluvial valley that lies between the Coastal Range and Cascade
Mountains to the west and east, respectively. Marine sedimentary rocks and basalt are found
below the alluvial sediments (Orr, et. al., 1992). The Northern Willamette Valley has undergone
substantial structural deformation since the Eocene, resulting in the Portland fold belt as defined
by Unruh et al. (1994). The tectonic underpinnings of the Portland Fold Belt are not well
understood and complicated by the fact that this area lies in a transition zone between the rotating
forearc block and the continental interior(Wells et al, 1998).
Specifically, the project site is located in the Tualatin Basin, a northwest trending synclinal
subbasin to the Willamette Valley basin (Unruh et al., 1994). The Tualatin Basin is fault bound
by structurally controlled, northwest-trending highlands, specifically along its northeastern
margin by the Portland Hills and on the southwestern edge by the Chehalem Mountians (Madin,
1990). The two highlands are parallel to mapped regional faults including the East Bank fault,
the Portland Hills fault, the Oatfield fault, the Mollala-Canby fault, the Gales Creek fault, the
Newberg fault, and the Mt. Angel fault.
Internal structure to the basin includes the faulting that has resulted in the formation of the Bull
Mountain and Cooper Mountain anticlines. The site is located immediately south of the anticline
axis as mapped by Madin.
. 2.3 SITE GEOLOGY
Subsurface investigation of the site was performed in June 2003. The investigation was
comprised of 15 test pits and 3 soil/rock core borings. The locations of these explorations is
shown in Figure 2.
The site is underlain by approximately 5 to 9 feet of medium stiff brown clay. This clay is
weathered late Quaternary windblown silt. Underlying the weathered silts is 2 to 6 feet of stiff
• reddish brown lean clay. This clay is basalt bedrock residuum that grades to extremely to highly
weathered basalt at depths ranging from 8 to 16 feet below ground surface. The basalt bedrock is •
Miocene-aged Columbia River Basalts. The highly weathered basalt is very weak (indicating it
can be pealed with a pocketknife) and highly fractured. The degree of weathering gradually
decreases with depth. The rock grades to moderately weathered, moderately strong (requiring a
hammer blow to break a sample)basalt at depths between 21 and 26 feet bgs.
MIS
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SECTIONTWO Geologic Setting
2.4 SITE HYDROGEOLOGY
Groundwater was not encountered during the subsurface investigation. URS conducted a review
of water well logs publicly available from the Oregon Water Resources Department. Static
groundwater levels reported on well logs are in excess of 150 feet bgs in the vicinity of the site.
Perched groundwater may be present within the fine-grained soils during the winter months.
However, discharge from these perched systems is anticipated to be minimal.
•
•
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SECTIONTHREE Seismic Conditions
3.1 EARTHQUAKE EFFECTS - GENERAL
Several factors control the level and character of earthquake ground shaking at a site. Generally,
these factors are: (1) rupture dimensions, geometry, and orientation of the causative fault; (2)
distance from the causative fault; (3) magnitude of the earthquake; (4) the rate of attenuation of
the seismic waves along the propagation path from the source to site; and (5) site factors
including the effects of near-surface geology particularly from soils and unconsolidated
sediments. Other factors, which vary in their significance depending on specific conditions,
include slip distribution along the fault, rupture process, footwall/hanging-wall effects, and the
effects of crustal structures such as basins.
3.2 HISTORICAL SEISMICITY
Historically, the Portland region has been characterized by a moderate level of seismicity with
the largest earthquakes not exceeding magnitude (M) 6 (Bott and Wong, 1993). A historical
earthquake catalog of all known events in northwestern Oregon and southwestern Washington
for the period 1841 to 2000 was compiled. Earthquake data were acquired from: a catalog
compiled by Woodward-Clyde Consultants (URS) for DOE Hanford; Ludwin (1991); University
of Washington; National Earthquake Information Center; Stover, Reagor and Algermissen; the
Decade of North American Geology; and the Council of the National Seismic System earthquake
catalog. This catalog contains over 18,000 events, a large percentage of which are associated with
the St. Helens seismic zone. Only 5 earthquakes are M 6.0 or larger and these all occurred at
distances greater than 80 km from the proposed school site. Approximately 38 earthquakes in the
catalog have magnitudes between M 5.0 to 5.9, the largest of which is the 1993 moment
magnitude (Mw) 5.6 Scotts Mills earthquake.
In characterizing earthquake occurrence, historical earthquakes can generally be divided into pre-
instrumental and instrumental periods. Prior to adequate seismographic coverage,the detection of
earthquakes was generally based on direct observation and felt reports. Thus results are strongly
dependent on population density and distribution. This part of the Pacific Northwest is typical of
much of the western United States, and was sparsely populated in the 1800's. Therefore the
detection of pre-instrumental earthquakes shows varying degrees of completeness. The pre-
instrumental historical record is estimated to be complete for earthquakes of Richter local
magnitude (ML) 5 and larger since about 1850 for the Portland region. Seismograph stations were
established in 1906 in Seattle and 1944 in Corvallis, but adequate seismographic coverage of
small events (M < 3.0) did not begin in northwest Oregon until about 1980 when the University
of Washington expanded its regional network. The historical record is complete for ML 2.5 and
greater only since 1980 (Bott and Wong, 1993).
3.2.1 Significant Earthquakes
Significant earthquakes and earthquakes greater than M 6.0 in the region are discussed below.
Earthquakes are described with the modified Mercalli intensity (MMI) that rates intensity from I
(lowest—generally not felt)to XII(highest—total damage) (Kramer, 1996).
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SECTIONTHREE Seismic Conditions
1872 North Cascades Earthquake
On 15 December 1872, a large earthquake occurred in the wilderness of central Washington with
an approximate ML 7.4 (Malone and Bor, 1979). The exact source of the earthquake is unknown.
The event generated an approximate intensity of MM IV-V in the region of the proposed school
(Woodward-Clyde Consultants, 1992).
1873 Crescent City Earthquake
On 23 November 1973 an earthquake of estimated Mw 7.3 (Bakun, 2000) occurred near the
Oregon-California border east-southeast of Brookings, though there are large uncertainties as to
its exact location. This earthquake may be a rare example of an intraslab event in western Oregon
(Ludwin et al, 1991; Wong, 1997). The event had a maximum intensity of MM VIII, and an
intensity of MM 1II-IV in the region of the proposed school (Toppozada et al., 1981).
1877 Portland Earthquake
The earliest known historical earthquake in the Portland region occurred on 12 October 1877.
Two events were actually reported on this day, one which probably occurred near Cascades,
Washington and had a maximum intensity of MM III. The other event occurred near Portland and
had a maximum intensity of MM VII. The larger of the two events, it has an estimated magnitude
of ML 53/4 (Bott and Wong, 1993). At the Alberta Rider Elementary site, the intensity was
estimated to be MM IV (Bott and Wong, 1993).
1939 Southern Puget Sound Earthquake
On 13 November 1939, an earthquake of surface wave magnitude (Ms) 53/4 occurred in southern
Puget Sound. It had a maximum intensity of MM VII and an intensity of MM IV in the region of
the school site (Stover and Coffman, 1993).
1949 Puget Lowland Earthquake
On 13 April 1949, the largest historic event in the Puget Sound region occurred northeast of
Olympia, Washington, with a body wave magnitude (mb) of 7.1. The event occurred at a depth
of 54 km within the Juan de Fuca plate. Eight people were killed, many injured and property
damage was sustained at a loss of$25 million. The intensity in the region of the school site was
MM VI-VII(Thorsen, 1986).
1962 Portland Earthquake
On 6 November 1962, an earthquake occurred 15 km northeast of downtown Portland with a
magnitude of ML 5.2 to 5.5, a depth of 16 km, and a maximum intensity of MM VII. This
earthquake was felt throughout northwest Oregon and southwest Washington. The intensity in
the region of the school was MM V-VI (Wong and Bott, 1995). This is the second largest
earthquake known to have originated in the Portland region (Bott and Wong, 1993).
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SECTIONTHREE seismic Conditions
1965 Puget Lowland Earthquake
4
On 29 April 1965, the second largest known event to date in the southern Puget Sound occurred
north of Tacoma 6 , , al th
60 km and was widelywitha mb felt. .5.SixThe people
eventwerean killed
intraslab and damageearthquakereached an estiafocmateddep$12.5of
million. The earthquake hada maximum intensity of MM VIII and a probableoccurred atintensity of MM
Vin the vicinity of the proposed school (Thorsen, 1986).
1981 Elk Lake Earthquake
On 14 February 1981, the largest known earthquake associated with the St. Helens seismic zone
occurred with a mb 5.1. The aftershock zone delineates a fault zone 5 to 12 km in depth. The
maximum intensity of MM VI was reported for the epicentral region and an intensity of MM V
•
for the school vicinity(Bott and Wong, 1993).
1993 Scoffs Mills Earthquake
On 25 March 1993, an earthquake occurred near Scotts Mills in western Oregon with a
magnitude of M,,, 5.6, a depth of 16 km, maximum intensity of MM VII, and an intensity MM V-
VII in the school vicinity. It caused over $28 million in property damage. This earthquake is
thought to have occurred on the Mount Angel fault. Through 1994, over 300 aftershocks had
been recorded (Thomas et al., 1996).
2001 Nisqually Earthquake
On 28 February 2001 at 18:54 GMT, an earthquake occurred approximately 17 km northeast of
Olympia, Washington. The earthquake had a magnitude of MW 6.8 at a depth of 52.4 km.
Damage from the earthquake was widely reported throughout the Seattle and Olympia areas. The
earthquake had a maximum intensity of MM III-IV in the vicinity of the new school (University
of Washington).
3.2.2 Summary
•
The strongest ground shaking that the area of the proposed school has historically experienced
appears to be MM VI-VII in the 1949 earthquake and MM V-VII in the 1993 Scotts Mills event.
A MM VII intensity is roughly equivalent to a peak horizontal acceleration of 0.18 to 0.34g
(Wald et al., 1999).
3.3 EARTHQUAKE SOURCES
The Pacific Northwest has four types of seismic sources due to the presence of the Cascadia
subduction zone. These sources include (1) the subduction zone megathrust, which represents the
boundary (interface) between the downgoing Juan de Fuca plate and the overriding North
American plate; (2) faults located within the Juan de Fuca plate (referred to as the intraplate or
intraslab region); (3) crustal faults principally in the North American plate; and (4) volcanic
sources beneath the Cascade Range (Wong and Silva, 1998).
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3-3
SECTIONTHREE Seismic Conditions
In the past two decades, significant geologic, seismologic, and geophysical studies have been
undertaken to investigate seismic sources in the Pacific Northwest. Such studies, particularly
along the coast of the Pacific Northwest, have been the key to our understanding of the
earthquake processes within the Cascadia subduction zone. Few paleoseismic studies to
investigate crustal faults, however, have been performed west of the Cascades because of dense
vegetation, relatively rapid erosion rates, and past glaciation, which makes it difficult to find
evidence of young faulting (e.g., Pezzopane, 1993). Alternative approaches such as subsurface
imaging are now being carried out in the Portland area (e.g., Blakely et al., 1995) and the Puget
Sound region.(e.g., Johnson et al., 1999).
In the following section, we identify and characterize the seismic sources that are significant to
seismic hazards near the school. As specified in OSSC 1804.2.1.1, the probable source faults
must all be individually examined for contribution to site hazards. For this analysis, the
earthquakes need to be defined for each seismic source considered in the seismic hazard
assessments by their the Maximum Credible Earthquake (MCE). The MCE is commonly defined
as "the largest earthquake that is capable of being produced from a source, structure, or region,
under the currently known tectonic framework. It is a rational and believable event that can be
supported by all known geologic and seismologic data. An MCE is determined by judgment
considering the geologic evidence of past movement and the recorded seismic history of the
area."We adopt this definition in these assessments.
3.4 FAULT DESCRIPTIONS
Because of their proximity, crustal faults are possibly the most significant seismic sources to
inland sites. Studies by Pezzopane (1993) and Geomatrix Consultants (1995) show that at least
70 crustal faults that may have earthquake potential exist in Oregon. Many of these faults were
unknown or not recognized as being seismogenic a decade ago. Although the largest known
crustal earthquake in western Oregon is only about Mw 6 (Wong and Bott, 1995), potential exists
for events of Mw 6' or greater along several recognized faults including the Portland Hills and
the recently discovered East Bank faults in Portland and the Gales Creek-Mt. Angel fault zone
(Wong et al., 1999). As discussed earlier, the Mt. Angel fault is the possible source of the 1993
Scotts Mills Mw 5.6 earthquake.
Several crustal faults occur in the vicinity of the proposed school site that are either active or
potentially active. There has not been a historic surface rupture earthquake on any fault within
northwest Oregon and, to date, paleoseismic investigations of the regional faults has been
limited. However, historical seismicity in the region appears, in a few cases, to be associated with
mapped faults. In addition, some regional seismotectonic studies have been conducted that
provide preliminary data regarding the potential activity of these faults.
The major fault features that have an effect on seismic hazards within the Tualatin basin as
identified in the Unruh et al (1994) report are the Portland Hills Fault Zone (which includes the
East Bank and Oatfield Faults), the Newberg Fault, the Grant Butte Fault, and the Bolton Fault.
These features are shown on Figure 3. Several fault features that should be considered in a
seismic hazard assessment, but are not labeled on the Unruh et al (1994) map include the
Clackamas River Fault Zone and the Helvetia Fault.
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•
•
SECTIONTHREE Seismic Conditions
Seismic source characterization is concerned with, three fundamental elements: (1) the
identification of significant sources of earthquakes; (2) the maximum size of these earthquakes;
and (3) the rate at which they occur. The faults described below all could potentially effect the
new school site. All faults are dominantly Lateral-slip faults and are assumed to extend to the full
extent of the seismogenic crust (approximately 15 to 25 km; Wong, 1997). Maximum
earthquake magnitudes were estimated based on the empirical relationship of Wells and
Coppersmith (1994) between moment magnitude (MW) and surface rupture lengths for all types
of faults. Length estimates were taken from mapped fault lengths.
3.4.1 Helvetia Fault Zone
The north-northwest-striking Helvetia fault is located about 21.6 km from the proposed school.
The fault is about 10 km as measured in the subsurface, and is identified from seismic reflection
images and water well logs. The fault possibly displaces overlying Miocene-Pliocene to
Pleistocene sediments down-to-the-west by as much as 20 m (Yeats et al., 1991). The Helvetia
fault is not exposed at the surface, however, based on a lack of evidence disputing the activity of
the fault. Geomatrix Consultants (1995) considered the fault to be potentially active.
3.4.2 Newberg Fault
The Newberg fault trends northwest and has a subsurface length of about 8 km. The fault is
located approximately 17 km to the southwest of the proposed school. The fault displaces the
top of Columbia River Basalt by about 250 meters down-to-the-southwest. Anomalous
aeromagnetic and gravity gradients also indicate the presence of the fault (Geomatrix
Consultants, 1995; Yeats et al., 1991). Silvio Pezzopane (U.S. Geological Survey) documented
lineaments in fluvial terraces and bedrock notches along the projection of the Newberg fault
(Geomatrix Consultants, 1995). No seismicity is recorded along the trend of the Newberg fault,
and although there is no direct evidence for activity of the Newberg fault, the along-strike
proximity of the fault to the seismically-active Mt. Angel fault suggests that it may be potentially
active. Although the maximum measured subsurface length of the fault is 8 km, we consider the
fault to have a longer potential surface rupture of 17 km. This length is based on a minimum
magnitude of Mw 6.5 that appears to be necessary to produce surface rupture in western Oregon.
3.4.3 Portland Hills Fault
The Portland Hills fault zone includes a series of northwest-trending subsurface faults that extend
for a distance of about 40 km along the eastern margin of the Portland Hills (Geomatrix
Consultants, 1995; Madin, 1990). Extension of the fault toward the southeast, beyond the
Portland Hills, based on aeromagnetic gravity (Blakely et al., 1995) and high-resolution seismic
reflection imaging (Pratt et al., 2001), provides a total estimated fault length of about 62 km. The
closest approach of the Portland Hills fault to the proposed school site is approximately 13.2 km.
Several interpretations have been proposed to describe the style of faulting and the kinematic
setting of the Portland Hills fault. Based on the interpretation of surface geology,
geomorphology, gravity data, and seismicity, Beeson et al. (1985; 1989) have described the
Portland Hills fault as a structurally complex dextral strike slip zone with minor normal faulting.
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SECTIONTHREE Seismic Conditions
Yelin and Patton (1991) consider the Portland Hills fault to be an active right-lateral strike-slip
fault within an en echelon, releasing step of a large dextral slip zone. In contrast, the Portland
Hills are thought by some researchers (Beeson et al., 1989; Unruh et al., 1994) to be the surface
expression of an anticline associated with the hanging wall of a southwest-dipping thrust fault.
The age of the most recent event along the Portland Hills fault is not clearly understood. Recent
investigation of the southern Portland Hills fault using high-resolution seismic reflection
methods, provides evidence for faulting of Missoula flood deposits. The flood sediments are
faulted at least several meters down-to-the-east along the fault. Although no direct estimates for
the age of the sediments are available, the most recent catastrophic floods in the area occurred
about 15.5 to 13 ka (Madin, 1990). This suggests that the Portland Hills fault has been active in
about the past 13 ka. A swarm of M < 3.5 earthquakes occurred at the northern end of the
Portland Hills fault in 1991 (Blakely et al., 1995). Focal mechanisms from the largest event
suggest a mixed right-lateral and reverse mechanism for the fault (Blakely et al., 1995). Based
on a maximum estimated length of 62 km (Wong et al., 2000), which includes projection of the
fault to the south of the Portland Hills, an estimated MCE of Mw 7.2 is calculated for the
Portland Hills fault.
3.4.4 Bolton Fault
The Bolton fault appears at the surface as a 9-km-long northwest-striking structure located
between the northern Willamette Valley and the Portland Basin. At its closest approach the fault
is about 9.6 km from the proposed school site. Beeson et al. (1989) map the fault as a high-
angle, down-to-the-northeast structure that displaces late Pleistocene (11 to 14 ka) flood deposits.
Unruh et al. (1994) were unable to confirm displacement in stream exposures of Miocene
Columbia River Basalts or Plio-Pleistocene conglomerates. Instead, they suggest that scarps
along the fault may be the result of erosion. Unruh et al. (1994) and Geomatrix Consultants
(1995) both considered the Bolton fault to be potentially active. Although the maximum mapped
surface length of the Bolton fault is 9 km, the estimated minimum magnitude earthquake that we
consider sufficiently large to produce surface rupture is Mw 6.5. This MCE corresponds with an
associated surface rupture length of 17 km.
3.4.5 Mt. Angel Fault
The Mount Angel fault is a 24- to 32-km-long northwest-trending fault located approximately
25.3 km to the southwest of the proposed school site. The fault strikes northwest and dips
steeply to the northeast. The fault is mapped at the subsurface based on seismic reflection lines,
water well logs, and seismicity(Geomatrix Consultants, 1995; Yeats et al., 1991). The top of the
Columbia River Basalt group and Mio-Pliocene fluvio-lacustrine deposits are displaced by the
fault. In 1993, the Mw 5.6 Scotts Mills earthquake occurred about 8 km south of the mapped
extent of the Mt. Angel fault (Geomatrix Consultants, 1995).
It is still unclear whether the earthquake occurred along the Mt. Angel fault. The focal
mechanism for the earthquake suggests that the earthquake involved a northwest-striking fault
and suggests subequal right- and reverse slip (Geomatrix Consultants, 1995). Recent
investigations along the Mt. Angel fault suggest that faulting has occurred in Missoula flood
deposits (Liberty et al., 1996). Recent high-resolution seismic reflection and refraction imaging
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SECTIONTHREE Seismic Conditions
suggests that possible Holocene deposits may also be displaced (Ian Madin, DOGAMI, personal
communication, 2000). Based on potential historic seismicity, displaced Missoula flood
deposits, and a surface scarp in Holocene deposits, we consider the Mt. Angel fault to be active.
The maximum surface rupture length ascribed to the fault is 32 km (Wong et al., 2000) which
corresponds with a MCE of Mw 6.8.
•
3.4.6 East Bank Fault
The East Bank fault has been defined based on the presence of an aeromagnetic signature
(Blakely et al., 1995) and based on high-resolution seismic imaging (Pratt et al., in press). The
fault is shown as a magnetic lineament to the northeast and parallel to the Portland Hills fault
(Blakely et al., 1995). The fault is believed to be a major structural feature because the
pronounced aeromagnetic anomaly is consistent with vertical displacements of at least 1 km of
the basement volcanic rocks (Blakely et al., 1995). The East Bank fault is entirely concealed
beneath Quaternary deposits (Madin, 1990). The fault was previously mapped based on an
apparent< 200 m vertical offset of the volcanic basement (Blakely et al., 1995). The East Bank
fault may serve as a significant component of the eastern margin of the Portland Basin. Pratt et
al. (2001)reports that high-resolution seismic imaging indicates that the East Bank fault has had
late Pleistocene and possibly Holocene activity. The data suggest that paleochannels of the
Willamette River have been faulted, and that the river channel might be fault controlled.
Because of the geophysical evidence available for the East Bank fault, we consider the fault to be
active. At its closest projection, the East Bank fault is located about 17.3 km from the school site.
The mapped trace of the fault is not well-constrained, however, estimates for the fault length
range from 40 to 55 km (Wong et al., 2000), which corresponds with an MCE of MW 7.1.
3.4.7 Oatfield Fault
The Oatfield fault is recognized on the basis of aeromagnetic anomalies and possible association
with historic seismicity (Blakely et al., 1995). The fault is located along the western flank of the
Portland Hills and may be structurally associated with the Portland Basin. The school is located
12.4 km west of the Oatfield fault (Figure 5.9). The fault is mapped by Madin (1990) based on
water well data. No definitive surface trace of the fault has been mapped. Blakely et al. (1995)
suggest that the northern projection of the Oatfield fault may intersect the 1991 swarm of M<3.5
earthquakes that were also considered to be associated with the Portland Hills fault. As with the
Portland Hills fault, the style of deformation of the Oatfield fault is not understood. The
associated historical seismicity indicates oblique faulting dominated by right-lateral slip with
lesser reverse motion (Blakely et al., 1995). Because of its potential structural and kinematic
association with the Portland Hills fault and Portland Basin, and the nearby presence of historical
seismicity, we consider the Oatfield fault to be potentially active. The length of the Oatfield fault
is not well-known, but best estimates suggest that it may be up to 40 km long (Wong et al., 2000)
which corresponds to an MCE of MW 6.9.
3.4.8 Clackamas River Fault Zone
The Clackamas River fault zone includes a series of northwest-trending oblique-slip faults
mapped south of Estacada, Oregon along the Clackamas River (Geomatrix Consultants, 1995).
\\Ror6\projects\25695391 TTSD Alberta Rider\TTSD_seismic_hazard.doc131-JUL-03
3-7
SECTIONTHREE Seismic Condit-
ions
The maximum length of faults in the zone is 22 km. Faults within the zone have documented
right-lateral and normal displacement (Hammond et al., 1980). The faults displace middle
Miocene (ca. 15 Ma) Grande Rhonde and Wanapum Basalts. Late Pliocene to early Pleistocene
lavas do not appear to be deformed (Priest et al., 1983; Sherrod and Conrey, 1988). A gravel
terrace estimated to be approximately 1 Ma crosses the fault and does not appear to be displaced
(Geomatrix Consultants, 1995). Also, no evidence for Quaternary activity was documented
during photogeologic analyses for the U.S. Bureau of Reclamation (Geomatrix Consultants,
1995). However, the Clackamas River fault has a similar orientation to the Oak Grove - Lake
Harriet fault zone to the south and Geomatrix Consultants suggest that there may be a structural
association between the two fault zones. Geomatrix Consultants report that some faults within
the Oak Grove - Lake Harriet fault zone may have had Quaternary activity. Because of this
possible association between the two fault zones, we consider the Clackamas River fault zone to
be potentially active. Geomatrix Consultants estimate a maximum surface rupture length of 22
km for the fault zone which corresponds with an MCE of Mw 6.6.
3.4.9 Grant Butte, Damascus, Tickle Creek Fault Zone
Madin (1990) mapped an east-northeast-trending fault within the Portland Basin. A series of
randomly oriented faults were mapped in an excavation within Troutdale Formation gravel on
Grant Butte and comprise the informally-named Grant Butte fault (Geomatrix Consultants,
1995). The Damascus-Tickle Creek fault zone displaces Pliocene and possible Pleistocene
sediments near Boring, Oregon (Madin, 1990). The northwest-striking fault zone is defined by
relatively short (less than 7 km) faults that comprise a zone approximately 17 km long. The
combined fault zone is located approximately 32.4 km from the proposed pipeline corridor. The
maximum estimated rupture length of 17 km reported by Madin (1990) can be used to calculate
an MCE of Mw 6.5.
3.4.10 Other Fault Zones
Several faults that are near the school site are discussed below. Based on either a lack of data
indicating that these faults are active, or a preponderance of information suggesting that they are
not active, we do not include them in the hazard analysis.
3.4.10.1 Sherwood Fault
The proposed site lies approximately 3.8 km south of the school site. Geomatrix Consultants
(1995) assessed the Sherwood fault for its seismogenic potential and concluded that there was no
evidence for Quaternary activity. URS (2001) also performed a photogeologic study in this area,
finding no evidence for recent movement. Based on the conclusions of Geomatrix Consultants
(1995) and our photogeologic analysis, we do not consider the Sherwood fault to be potentially
active.
3.4.10.2 Dairy Creek Fault
The Dairy Creek fault is a relatively short structure that has been mapped on the basis of
subsurface geophysical anomalies (Ian Madin, DOGAMI, personal communication, 2000). There
\\Por6\projects\25695391 TTSD Alberta RideATTSD_seismic_hazard.do631-JUL-03
3-8
SECTIONTHREE Seismic Conditions
is no evidence for surface expression of the fault, and it does not appear to be structurally or
kinematically associated with any nearby faults, and it has not been associated with historic
seismicity. Based on a lack of information suggesting that the fault is potentially active, we do
not consider it in the hazard analysis.
3.4.10.3 Beaverton Fault
The west-southwest-striking Beaverton fault is located to the north of the proposed school. The
fault has been located on the basis of geophysical anomalies. No additional information
regarding the activity or seismic potential of the fault is currently available, thus we did not
consider it in the hazard analysis.
3.4.11 Cascadia Subduction Zone
The megathrust and intraslab region in the subducting Juan de Fuca plate represent two very
different seismic sources within the Cascadia subduction zone. Due to the duration and distance
effects on ground motion, the megathrust rupture will be considered in this hazard analysis.
3.4.11.1 Megathrust
Paleoseismic evidence (e.g., Atwater et al., 1995) and historic tsunami studies (Satake et al.,
1996) indicate that the most recent megathrust earthquake in 1700 probably ruptured the full
length of the Cascadia subduction zone and was about M 9 in size. Thus, seismic hazard
evaluations need to consider future earthquakes of this magnitude, although data cannot preclude
the possibility that smaller events have occurred in the past along the megathrust.
A significant factor that will control the ground-shaking hazards posed by the Cascadia
subduction zone revolves around the location of the megathrust zone. The eastern edge of the
megathrust is allowed to vary from about 25 miles offshore to beneath the Coast Ranges with a
preferred location beneath the coastline. This results in a source-to-site distance of
approximately 120-km to the site. We adopt a range of maximum magnitudes from M 8 to 9
with the latter given the highest weight.
URS
\\Por6\projects\25695391 TTSD Alberta Rider\TTSDseisrryc_hazard doc\31-JUL-03 3-9
SECTIONFOUR Design Ground Motion
4.1 GROUND MOTION ANALYSES
Several factors control the level and character of earthquake ground shaking. These factors are in
general: (1) rupture dimensions, geometry, and orientation of the causative fault; (2) distance
from the causative fault; (3) magnitude of the earthquake; (4) the rate of attenuation of the
seismic waves along the propagation path from the source to site; and (5) site factors including
the effects of near-surface geology particularly from soils and unconsolidated sediments. Other
factors, which vary in their significance depending on specific conditions, include slip
distribution along the fault, rupture process, footwall/hanging-wall effects, and the effects of
crustal structure such as basin effects.
In this section, probabilistic analyses have been reviewed to evaluate the ground shaking hazard
at the site. This data is being reviewed because insufficient knowledge exists regarding seismic
sources in the site region to reliably estimate ground motions associated with the MCE using
deterministic methods.
4.1.1 Geomatrix 1995 Probabilistic Study
The probabilistic seismic hazard analysis conducted for the 1995 Geomatrix Report produced
maps for given design return periods for the State of Oregon. The peak horizontal acceleration
experienced at the site for a"500-year" return period on bedrock is 0.19 g.
4.1.2 URS 2001 Probabilistic Study
The probabilistic seismic hazard analysis conducted by URS in the Tualatin Valley (URS, 2001)
produced peak ground accelerations anticipated for given design return periods for the State of
Oregon. The peak horizontal acceleration experienced at the new school site for a "500-year"
return period was selected to be similar to peak ground acceleration value calculated for a site
with similar subsurface conditions in the 2001 study. This peak ground acceleration is
anticipated to be 0.22 g.
4.1.3 URS/DOGAMI 2000 Portland Metropolitan Study
The probabilistic seismic hazard analysis conducted by URS for the Oregon Department of
Geology and Mineral Industries produced maps for given design return periods for the Portland
Metropolitan Area (Wong et al, 2000). The peak horizontal acceleration experienced at the site
for a "500-year"return period for bedrock is modeled to be 0.20 to 0.25 g.
4.1.4 1998 OSSC Zonation _
The site lies within Seismic Zone 3 as defined by the 1998 version of the Oregon Structural
Specialty Code (OSSC). Based on the soils encountered during the exploration program, OSSC
Soil Type Sc (very dense soil and soft rock) represents the closest approximation to the site
conditions and is recommended for use in design. The seismic response coefficients that
URS
\\Por6\projects\25695391 TTSD Alberta Rider\TTSD_seismic_hazard.doc\31-JUL-03 4-1
SECTIONFOUR Design Ground Motion
corresponds with Z=0.3 and Sc are Ca=0.33 and C,=0.45 and were obtained from tables 16-Q
and 16-R of the UBC,respectively.
4.1.5 Results Comparison
The probabilistic seismic hazard analyses reviewed above are compared in Table 1 below.
TABLE 1: COMPARISON OF PEAK GROUND ACCELERATIONS
Portland
2001 URS"500" Metropolitan OSSC 1998"500"
Reference 1995 Geomatrix °500"year event
"500"year event year event year event(Ca)
(Wong et al, for Site S,
2000)
Peak Ground
Acceleration 0.19g 0.22g 0.20—0.25g 0.33g
(gravity)
6
•
4.2 RECOMMENDED DESIGN GROUND MOTIONS
As indicated above in Table 1, the OSSC 1998 peak ground acceleration and associated spectrum
generally encompass the spectral response of the school site for the "500-year" return period. It is
URS opinion that application of the 1998 OSSC is conservative at this site, and would be
appropriate.
Should the project team express an interest in exploring the site-specific response spectrum for
the site, URS should be contacted to conduct the analyses required to generate the spectrum in
accordance with Section 1631.2.2 of the 1998 OSSC. Should this approach be taken, it may be
possible to reduce the design base shear for structural members by up to 20% in accordance with
Section 1631.5.4 of the 1998 OSSC.
•
URS > \\Por6\projects\25695391 TTSD Alberta Rider\TTSD_seismic_hazard.doc\31-JUL-03 4-2
SECTIONFIVE Closure
5.1 ANTICIPATED FOUNDATION DESIGN
Based on the soil conditions present at the site, it is anticipated that conventional continuous or
isolated shallow foundations will be used to support the proposed structure. Footings will likely
be founded on shallow silts or on bedrock. Detailed discussion of foundation design, allowable
bearing capacity, expected settlements, and construction considerations are included in• the
companion URS geotechnical report entitled "Foundation Investigation, Alberta Rider
Elementary School, Tigard-Tualatin School District, Tigard, Oregon." This report was submitted
to the Tigard-Tualatin School District in July 2003.
5.2 SEISMIC HAZARDS
Seismic hazards for the purposes of this report include liquefaction, tsunami/seiche inundation,
seismically-induced landslides, surface rupture and ground ampliflication. These are evaluated
and discussed separately in the following sections.
5.2.1 Liquefaction Hazard
Liquefaction is the drastic loss of soil strength that can accompany ground shaking during a
moderate to strong seismic event. During ground shaking, cyclic earthquake loading on the soil
increases pore water pressure to a point where the effective stress on the soil is zero or even
negative, resulting in suspension of soil particles in the water. Loose, granular soils located
below the water table are generally susceptible to liquefaction.
It should be noted that soil liquefaction, in and of itself, does not pose a risk to buildings and
infrastructure. It is the phenomena accompanying liquefaction that can severely damage
structures situated in or on the soil. These phenomena include settlement, lateral spreading, flow
failures, and bearing capacity failure, which are discussed in the following sections.
Based on the clay soils and shallow bedrock present at the site, it is URS opinion that there is not
a liquefaction hazard at this site. The site is not at risk for the liquefaction-related phenomena of
seismically induced settlement, lateral spreading, or bearing capacity failure.
5.2.2 Tsunami/Seiche Hazard
URS understands that this site is not located near any body of water that is susceptible to tsunami
or seiche. Therefore, it is URS opinion that tsunami or seiche hazard at this site does not exist.
5.2.3 Seismic Slope Stability Hazard
Because of the gentle slopes, the soil conditions and shallow rock found at the site, it is URS
opinion that seismic slope instability is not a hazard at this site.
URS
UPor6\projects\25695391 TTSD Alberta Rider\TTSD_seisn¢c_hazard.doc\31-JUL-03 5_1
•
SECTIONFNE closure
5.2.4 Surface Rupture Hazard
Review of available geologic mapping indicates that no known fault trace passes beneath the
proposed facility. Therefore, it is URS opinion that hazards from ground rupture at this site does
not exist.
5.2.5 Ground Shaking Amplification Hazard
Because of the soil conditions and the shallow rock found at the site, it is URS opinion that there
is low risk for ground shaking amplification at this site.
URS
\\Por6\projects\25695391 TTSD Alberta Rider\TTSD_seismic_hazard.doc\31-JUL-03
5-2
SECTIONSIX Closure
The analyses, conclusions and recommendations presented in this report are based on site
conditions as they existed at the time of our field exploration and the state of practice at the time
- of this report. This report was prepared for the exclusive use of the Tigard-Tualatin School
District and its agents and consultants.
LIRS
\\Por6\projects\25695391 TTSD Alberta RideATTSD_seismic hazard.doc\31-JUL-03 6-1
SECTIONSEVEN References
Atwater, B.F., Nelson, A.R., Clague, J.J., Carver, G.A., Yamaguchi, D.K., Bobrowsky, P.T.,
Bourgeois, J., Darienzo, M.E., Grant, W.C., Hemphill-Haley, E., Kelsey, H.M. Jacoby, G.C.,
Nishenko, S.P., Palmer, S.P., Peterson, C.D., and Reinhart, M.A., 1995, Summary of coastal
geologic evidence for past great earthquakes at the Cascadia subduction zone: Earthquake
•
Spectra, v. 11, p. 1-18.
Beeson, M.H., Fecht, K.R., Reidel, S.P., and Tolan, T.L. (1985). Regional Correlations Within
the Frenchman Springs Member of the Columbia River Basalt Group: New Insights Into the
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Beeson, M.H., Tolan, T.L., and Anderson, J.L., (1989) The Columbia River Basalt Group in
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Blakely, R.J., Wells., R.E., Yelin, T.S., Madin, I.P., and Beeson, M.H. (1995). "Tectonic setting
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Geomatrix Consultants, Inc., 1995, "Seismic Design Mapping State of Oregon: Final Report",
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Hammond, P.E., Anderson, J.L., and Manning, K.J. (1980). "Guide to the Geology of the Upper
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Johnson, S. Y., Dadisman, S. V., Childs, J. R., and Stanley, W. D. (1999) "Active Tectonics of
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New Jersey.
Liberty, L.M., Trehu, A.M., Dougherty, M.D., and Blakely, R.J. (1996). "High-Resolution
Seismic-Reflection Imaging of the Mt. Angel/Gales Creek Fault System Beneath the
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Neotectonics of North America, D.B. Slemmons, E.R. Engdahl, M.D. Zoback, and D.D.
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Malone, S.D. and Bor, S.S. (1979). "Attenuation Patterns in the Pacific Northwest Based on
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URNS
\\Por6\projects125695391 TTSD Alberta Rider\TTSD_seisrrtic_hazard.doc\31-JUL-03
7-1
•
SECTIONS EVEN References
Ordonez-Comparini, Gustavo A., 2000, SHAKE2000, A Computer Program for the 1-D Analysis
of Geotechnical Earthquake Engineering Problems, Ameritech Engineering.
Orr, E.L., On, W.N. and Baldwin, E.M., 1992, Geology of Oregon, Fourth Edition,
Kendall/Hunt Publishing Company,Dubuque, Iowa.
Pezzopane, S.K., 1993,Active Faults and Earthquake Ground Motions in Oregon, Ph.D. Thesis,
University of Oregon, 208 p.
Pratt, T.L, Odum, J., Stephenson, W., Williams, R., Dadisman, S., Holmes, M., and Haug, B.
(2001). High-resolution seismic images of the late Pleistocene unconformity, ancestral
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Priest, G.R., Woller, N.M., Black, G.L., and Evans, S.H. (1983). Overview of the Geology of the
Central Oregon Cascade Range, Chapter 2, in Priest, G.R., and Vogt, B.F. (eds.), Geology
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Satake, K., Shimazaki, K., Tsuji, Y., and Ueda, K., 1996, Time and size of a giant earthquake in
Cascadia inferred from Japanese tsunami records of January 1700: Nature, v. 379, p. 246-
249.
Sherrod, D.R. and Conrey, R.M. (1988). "Geologic Setting of the Breitenbush-Austin Hot
Springs Area, Cascade Range, North-Central Oregon," in Sherrod, D.R. (ed.), Geology and
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Counties, Oregon: Oregon Department of Geology and Mineral Industries Open-File Report
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Stover, C.W., and Coffman, J.L. (1993). Seismicity of the United States, 1568-1989 (Revised),
U.S. Geological Survey Profressional Paper 1527, 415 p.
Thomas, G c., Crosson, R.S., Carver, D.L., and Yelin, T.S. (1996). "The 25 March 1993 Scotts
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Thorsen, G.W. ed. (1986). The Puget Lowland Eqrthquakes of 1949 and 165, Washington
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Toppozada, T.R., Real, C.R., and Parke, D.L. (1981). Preparation of Isoseismal Maps and
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Associates and Woodward-Clyde Federal Services, unpublished final report prepared for the
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URS
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•
SECTIONSEVEN References
Wald, D.J., Quitoriano, V., Heaton, T.H., and Kanomori, H. (1999). "Relationships Between
Peak Ground Acceleration, Peak Ground Velocity, and Modified Mercalli Intensity in
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Wells, R.E., Weaver, C.S., and Blakely, R.J., 1998, Forearc migration in Cascadia and its
neotectonic significance: Geology, v. 26, p. 759-762.
Wong, I.G., 1997,The historical earthquake record in the Pacific Northwest: Applications and
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Wong, I.G. and Bott, J.D.J., 1995, A look back at Oregon's earthquake history, 1981-1994:
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Wong, I.G. and Silva, W.J., 1998, Earthquake ground shaking hazards in the Portland and
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the Portland, Oregon, Metropolitan Area, Oregon Department of Geology and Mineral
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Yeats, R.S., Graven, K., Werner, C., Goldfinger, p y
C., and Po owsk , T. (1991). Tectonics of the
Willamette Valley, Oregon; U.S. Geological Survey Open-File Report, 91-441-P, 47 p.
Yelin, T.S. and Patton, H.J., (1991). "Seismotectonics of the Portland, Oregon, Region,"
Bulletin of the Seismological Society of America, v. 81, p. 109-130.
•
URS
\\Por6\projects\25695391 TTSD Alberta Rider\TTSD_seismic_hazard.doc\31-JUL-03 7-3
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G ,' ____ SITE MAP
TTSD Alberta Rider Elementary School
'.._._. ._.. -.- _... _.... _i -._. ......_ ...... ... July 2003 Seismic Site Hazard Investigation
25695391 Tigard,Oregon
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FIGURE 2
LEGEND
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—1—--?-- Tertiary fault,dashed where approximately,ocated
dotted where concealed(barb on downih own block). •
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Queried traces have not been veidied by detailed �
studies tt - - j ;,
----••• Faun with known or suspected Quaternary movement, - • I=
dashed where approximately located:coned where .00 •�' - - *Vancouver,
ccncealetl{barb on downihrown block) O • -
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_ _
Anticline,arrow=^ows directicr•-'plunge dir •
a
t Syncline:arrow shows duction of plunge • ,• .. _ i
y '• :tib - .
n'w-n'T Late Neogene or younger t-rust fault:teem on - / C RTLA
hanging wail /// ''O S4.
I `Iomocl nal flexure - '�. BASIN ..
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* Site showing evidence of one or more rapid Holocene ' S - _
coastal subsidence events(Darienzo and Peterson. , •� • -
'9e Atwater,1992) •.�
_ \e >T ` '.
.eGr6ve T(JALATINoh �
BASIN-. mo : ' -
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•
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• 4swego--l .
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4 ITE
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% \ . `�a \ ;ee• 1 \ 'r ,,; \'t
• •tim ,
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REFERENCE: UNRUH ET. Al:., 1994
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TECTONIC STRUCTURES OF THE TUALATIN BASIN
1
USD Alberta Rider Elementary School
July 2003 Seismic Site Hazard Investigation
ei 1411111S 25695391 Tigard,Oregon
FIGURE 3
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