SHIPS Publications Sorted By Topic -- 9/30/2005

Ground Amplification Studies

Barberopoulou, A., A. Qamar, T.L. Pratt, K.C. Creager, and W.P. Steele,
2004, Local amplification of seismic waves from the Mw7.9 Alaska
earthquake and damaging seiches in Lake Union, Seattle, Washington,
*Geophys. Res. Lett*., v. 31, L03607, doi: 10.1029/2003GL018569.

The Mw7.9 Denali, Alaska earthquake of 3 November, 2002, caused minor
damage to at least 20 houseboats in Seattle, Washington by initiating
water waves in Lake Union. These water waves were likely initiated
during the large amplitude seismic surface waves from this earthquake.
Maps of spectral amplification recorded during the Denali earthquake on
the Pacific Northwest Seismic Network (PNSN) strong-motion instruments
show substantially increased shear and surface wave amplitudes
coincident with the Seattle sedimentary basin. Because Lake Union is
situated on the Seattle basin, the size of the water waves may have been
increased by local amplification of the seismic waves by the basin.
Complete hazard assessments require understanding the causes of these
water waves during future earthquakes.

Brocher, T.M., T. L. Pratt, C.S. Weaver, C.M. Snelson, and A.D. Frankel,
2002, Implosion, earthquake, and explosion recordings from the 2000
Seattle Kingdome Seismic Hazards Investigation in Puget Sound (SHIPS),
Washington, *U.S. Geological Survey Open-File Report 02-123*, 29 p.
<http://geopubs.wr.usgs.gov/open-file/of02-123/>

This report describes seismic data obtained in Seattle, Washington,
March 24-28, 2000, during a Seismic Hazards Investigation of Puget Sound
(SHIPS). The seismic recordings obtained by this SHIPS experiment,
nicknamed Kingdome SHIPS, were designed to (1) measure site responses
throughout Seattle and to (2) help define the location of the Seattle
fault. During Kingdome SHIPS, we recorded the Kingdome implosion, four
150-lb (68-kg) shots, and a M~w~ = 7.6 teleseism using a dense network
of seismographs deployed throughout Seattle. The seismographs were
deployed at a nominal spacing of 1 km in a hexagonal grid extending from
Green Lake in the north to Boeing Field in the south.

The Seattle Kingdome was a domed sports stadium located in downtown
Seattle near the Seattle fault. The Seattle Kingdome was imploded
(demolished) at 8:32 AM local time (16:32 UTC) on March 26 (JD 086),
2000. The seismic energy produced by implosion of the Kingdome was
equivalent to a local earthquake magnitude of 2.3. Strong impacts
produced by the implosion of the Kingdome generated seismic arrivals to
frequencies as low as 0.1 Hz. An mpeg movie of the ground motions
recorded during the demolition of the Kingdome may be downloaded from
the following website:
<http://groundmotion.cr.usgs.gov/html/movies.shtml>. This movie
documents longer shaking durations in the Duwamish River valley, as
expected for the low shear wave velocities found in these youthful
alluvial deposits along the river.

Although the shots varied in their quality, useful seismic refraction
data were acquired from all four shot points, located in the corners of
our temporary array. Two shots located north of the Seattle fault, where
the charges were detonated within the ground water column (Discovery and
Magnuson Parks), were much more strongly coupled than were the two shots
to the south of the Seattle fault, where the shots were detonated above
the water table (Lincoln and Seward Parks).

Thirty-eight RefTek stations, scattered throughout Seattle, recorded the
M~w~ = 7.6 Japan Volcano Islands earthquake (22.4˚N, 143.6˚E, 104 km
depth) of 28 March 2000 (JD 088). This teleseism produced useful signals
for periods between 4 and 7 seconds. Only a few recordings of small
magnitude local earthquakes were made, and these recordings are not
presented.

In this report, we describe the acquisition of these data, discuss the
processing and merging of the data into common shot gathers, and
illustrate the acquired data. We also describe the format and content of
the archival tapes containing the SEGY-formatted, common-shot gathers.

Brocher, T.M., Pratt, T.L., and the Dry SHIPS Working Group, 2001, Dry
SHIPS Recordings of the Chi-Chi earthquake, Seattle, Washington, *Bull.
Seismol. Soc. Amer*., v. 91, p. 1395 (with CD-supplement of seismic
data).

Dry SHIPS was a seismic refraction study of the Seattle basin,
Washington State, involving 29 chemical explosions recorded by 1008
seismographs, conducted in September 1999. The E-W trending Dry SHIPS
line ran eastward from the Olympic Peninsula, through the Seattle basin,
to the foothills of the Cascade mountains. Twenty-eight REFTEKs (DAS
Model 07G; 24-bit) deployed along the Dry SHIPS line at 4 km intervals
were programmed to record data continuously from 3-component sensors
(Table 1). These 28 stations recorded the main shock and several
aftershocks of the Mw = 7.6 Chi-Chi earthquake (Shin et al., 2000;
Brocher et al., 2000a).

The recordings were made with REFTEKs described by PASSCAL (1991). The
three-component Mark Products (copyright) L-28 4.5-Hz geophones were
oriented such that the longitudinal (N-S) component was directed to
magnetic north. \[The eastward declination of magnetic north relative to
true north in Seattle is about 20 degrees.\] The REFTEKs were equipped
with Global Positioning System (GPS) receivers to synchronize the
internal timing on the individual REFTEKs to satellite timing. The
REFTEKs recorded continuously at a sample rate of 250 samples/sec (4
msec sample interval) but have been resampled here to a 20 msec sample
rate after the application of an anti-alias filter. The original records
are available from the IRIS Data Management Center in Seattle. On this
CD the stations are identified by their Data Acquisition System (DAS)
number (Table 1).

The azimuth of propagation to the Dry SHIPS array was about 37 degrees
for the Chi-Chi signals. The stations were located at distances between
88.5 and 89.5 degrees from the Chi Chi epicenter. The Dry SHIPS
recordings of the Chi-Chi mainshock provide useful signals from 50 Hz
down to 10 second periods, although at higher frequencies the Chi-Chi
signals are not discernible due to the ambient noise. A further caveat
is that these data contain abrupt offsets in amplitude caused by voltage
steps associated with writing data to disk. At high frequencies these
VOLTAGE steps are lost in the noise and pose no problems, but at longer
periods (e.g., greater than 4 seconds) these steps may cause serious
problems on all three channels.

The recordings document a significant (factor between 5 and 10)
amplification of compressional- and shear-wave energy in the Seattle
basin at periods between 1 and 2 seconds relative to BEDROCK sites east
and west of the basin (Brocher et al., 2000b). Signal durations in the
Seattle basin were also substantially increased relative to BEDROCK
sites in the Olympic peninsula and Cascade foothills, approaching 100
seconds in the vicinity of Seattle.

Hartzell, S., Carver, D., Cranswick, E., and A. Frankel, 2000,
Variability of site response in Seattle, Washington, *Bull. Seism. Soc.
Am., 90*, 1237-1250.

Ground motion from local earthquakes and the SHIPS (Seismic Hazards
Investigation in Puget Sound) experiment is used to estimate site
amplification factors in Seattle. Earthquake and SHIPS records are
analyzed by two methods: (1) spectral ratios relative to a nearby site
on Tertiary sandstone, and (2) a source/site spectral inversion
technique. Our results show site amplifications between 3 and 4 below 5
Hz for West Seattle relative to Tertiary rock. These values are
approximately 30% lower than amplification in the Duwamish Valley on
artificial fill, but significantly higher than the calculated range of 2
to 2.5 below 5 Hz for the till-covered hills east of downtown Seattle.
Although spectral amplitudes are only 30% higher in the Duwamish Valley
compared to West Seattle, the duration of long-period ground motion is
significantly greater on the artificial fill sites. Using a
three-dimensional displacement response spectrum measure that includes
the effects of ground-motion duration, values in the Duwamish Valley are
2 to 3 times greater than West Seattle. These calculations and estimates
of site response as a function of receiver azimuth point out the
importance of trapped surface-wave energy within the shallow,
low-velocity, sedimentary layers of the Duwamish Valley. One-dimensional
velocity models yield spectral amplification factors close to the
observations for till sites east of downtown Seattle and the Duwamish
Valley, but underpredict amplifications by a factor of 2 in West
Seattle. A two-dimensional finite-difference model does equally well for
the till sites and the Duwamish Valley and also yields duration
estimates consistent with the observations for the Duwamish Valley. The
two-dimensional model, however, still underpredicts amplification in
West Seattle by up to a factor of 2. This discrepancy is attributed to
3D effects, including basin-edge--induced surface waves and
basin-geometry--focusing effects, caused by the proximity of the Seattle
thrust fault and the sediment-filled Seattle basin.

Li, Q., W. Wilcock, T.L. Pratt, C.M. Snelson, and T.M. Brocher, 2005,
Seismic attenuation structure of the Seattle basin using Dry SHIPS
Experiment data, in press.

We used waveform data from the 1999 SHIPS (Seismic Hazard Investigation
of Puget Sound) seismic refraction experiment to constrain the
attenuation structure of the Seattle basin, Washington State. We
inverted the spectral amplitudes of compressional- and shear-wave
arrivals for source spectra, site responses, and one- and
two-dimensional *Q^-1^* models at frequencies between 1 and 40 Hz for
*P*-waves and 1 and 10 Hz for *S*-waves. We also obtained *Q^-1^* models
from *t\** values calculated from the spectral slopes of *P*-waves
between 10 and 40 Hz. One-dimensional inversions show that *Q*~p~ at the
surface is 22 at 1 Hz, 130 at 5 Hz and 390 at 20 Hz. The corresponding
values at 18 km depth are 100, 440 and 1900. *Q*~s~ at the surface is 16
and 160 at 1 Hz and 8 Hz, respectively, increasing to 80 and 500 at 18
km depth. The *t*\* inversion yields a *Q~P~* model that is consistent
with the amplitude inversions at 20 and 30 Hz. The basin geometry is
clearly resolved in the *t\** inversion, but the amplitude inversions
only imaged the basin structure after removing anomalously high
amplitude shots near Seattle. When these shots are removed, we infer
that *Q*^-1^ values may be \~30% higher in the center of the basin than
the one-dimensional models predict. We infer that seismic attenuation in
the Seattle basin will significantly reduce ground motions at
frequencies at and above 1 Hz, partially countering amplification
effects within the basin.

Pitarka, A., R. Graves, and P. Somerville, 2004, Validation of a 3D
velocity model of the Puget Sound region based on modeling ground motion
from the February 28, 2001 Nisqually earthquake, *Bull. Seism. Soc.
Am*., 94, 1670-1689.

In this study we prepared a 3D velocity model suitable for modeling
long-period wave propagation in the Puget Sound region. The model is
based on products of the Seismic Hazard Investigation in Puget Sound
(SHIPS) and geophysical information from other studies of the region.
The adequacy of the velocity model was evaluated based on analyses of
goodness of fit between recorded and simulated ground-motion velocity
from the *M* 6.8 Nisqually earthquake. The earthquake was located about
60 km south of Seattle with a hypocentral depth of 59 km. The analyses
were performed in the frequency range of 0.02--0.5 Hz, using data from
40 stations. Although our model covers a wide area of the Puget Sound
region, its quality is assessed in the Seattle region in which the
distribution of stations that recorded the Nisqually earthquake was
denser. Our 3D finite-difference ground-motion modeling suggests that
the propagation of long-period waves (periods longer than 3 sec) in the
Seattle basin is mostly affected by the deep basin structure. The
tomographic velocity model of Parsons *et al.* (2001), combined with the
model of depth to the basement of the Seattle basin of Blakely *et al.*
(1999), was essential in preparing and constraining geometrical features
of the proposed velocity model.

Pratt, T.L., K.L. Meagher, T.M. Brocher, T. Yelin, R. Norris, L.
Hultgrien, E. Barnett, and C.S. Weaver, 2003, Earthquake recordings from
the 2002 Seattle Seismic Hazard Investigation of Puget Sound (SHIPS),
Washington State, *U.S. Geological Survey Open-File Report* 03-361, 72
p. http://geopubs.wr.usgs.gov/open-file/of03-361/

This report describes seismic data obtained during the fourth Seismic
Hazard Investigation of Puget Sound (SHIPS) experiment, termed Seattle
SHIPS. The experiment was designed to study the influence of the Seattle
sedimentary basin on ground shaking during earthquakes. To accomplish
this, we deployed seismometers over the basin to record local
earthquakes, quarry blasts, and teleseisms during the period of January
26 to May 27, 2002. We plan to analyze the recordings to compute
spectral amplitudes at each site, to determine the variability of ground
motions over the basin. During the Seattle SHIPS experiment,
seismometers were deployed at 87 sites in a 110-km-long east-west line,
three north-south lines, and a grid throughout the Seattle urban area
(Figure 1). At each of these sites, an L-22, 2-Hz velocity transducer
was installed and connected to a REF TEK Digital Acquisition System
(DAS), both provided by the Program for Array Seismic Studies of the
Continental Lithosphere (PASSCAL) of the Incorporated Research
Institutes for Seismology (IRIS). The instruments were installed on
January 26 and 27, and were retrieved gradually between April 18 and May
27. All instruments continuously sampled all three components of motion
(velocity) at a sample rate of 50 samples/sec. To ensure accurate
computations of amplitude, we calibrated the geophones in situ to obtain
the instrument responses. In this report, we discuss the acquisition of
these data, we describe the processing and merging of these data into
1-hour long traces and into windowed events, we discuss the geophone
calibration process and its results, and we display some of the
earthquake recordings.

Pratt, T.L., T.M. Brocher, C.S. Weaver, K.C. Creager, C.M. Snelson, R.S.
Crosson, K.C. Miller, and A.M. Tréhu, 2003, Amplification of seismic
waves by the Seattle basin, *Bull. Seism. Soc. Am*., 93, 533-545.

Recordings of the 1999 *M*w 7.6 Chi-Chi (Taiwan) earthquake, two local
earthquakes, and five blasts show seismic-wave amplification over a
large sedimentary basin in the U.S. Pacific Northwest. For weak ground
motions from the Chi-Chi earthquake, the Seattle basin amplified 0.2- to
0.8-Hz waves by factors of 8 to 16 relative to bedrock sites west of the
basin. The amplification and peak frequency change during the Chi-Chi
coda: the initial *S*-wave arrivals (0--30 sec) had maximum
amplifications of 12 at 0.5--0.8 Hz, whereas later arrivals (35--65 sec)
reached amplifications of 16 at 0.3--0.5 Hz. Analysis of local events in
the 1.0- to 10.0-Hz frequency range show fourfold amplifications for
1.0-Hz weak ground motion over the Seattle basin. Amplifications
decrease as frequencies increase above 1.0 Hz, with frequencies above 7
Hz showing lower amplitudes over the basin than at bedrock sites.
Modeling shows that resonance in low-impedance deposits forming the
upper 550 m of the basin beneath our profile could cause most of the
observed amplification, and the larger amplification at later arrival
times suggests surface waves also play a substantial role. These results
emphasize the importance of shallow deposits in determining ground
motions over large basins.

Pratt, T.L., and T.M. Brocher, 2005, Attenuation within sedimentary
basins and the shapes of site response curves in the Puget Lowland,
Washington State, *Bull. Seism. Soc. Am.*, in press.

Simple spectral ratio (SSR) and horizontal-to-vertical (H/V) site
response estimates at 47 sites in the Puget Lowland of Washington State
document significant attenuation of 1.5 to 20 Hz shear waves within
sedimentary basins there. Amplitudes of the horizontal components of
shear-wave arrivals from three local earthquakes were used to compute
SSRs with respect to the average of two bedrock sites, and H/V spectral
ratios with respect to the vertical component of the shear-wave arrivals
at each site. SSR site response curves at thick basin sites show peak
amplifications of 2 to 6 at frequencies of 3 to 6 Hz, and decreasing
spectral amplification with increasing frequency above 6 Hz. SSRs at
non-basin sites show a variety of shapes and larger resonance peaks. We
attribute the spectral decay at frequencies above the amplification peak
at basin sites to attenuation within the basin strata. Computing the
frequency-independent, depth-dependent attenuation factor (*Q~s,int~*)
from the SSR spectral decay between 2 to 20 Hz gives values of 5 to 40
for shallow sedimentary deposits and about 250 (error range of 90-1000)
for the deepest sedimentary strata. H/V site responses show less
spectral decay than the SSR responses but contain many of the same
resonance peaks. We hypothesize that the H/V method yields a flatter
response than SSRs across the frequency spectrum because the H/V
reference signal (vertical component of the shear-wave arrivals) has
undergone a similar degree of attenuation as the horizontal component
recordings. Correcting the SSR site responses for attenuation within the
basins improvesagreement between SSR and H/V estimates.

Pratt, T.L., M.E. Templeton, R. Frost, and A. Pierson, 2005, Variation
in the response of IRIS L-22 Geophones from Field and Laboratory
Calibrations, *Seism. Res. Lett*., in review.

We calibrated 72 Mark Products, L-22 geophones from the Program for
Array Seismic Studies of the Continental Lithosphere (PASSCAL) using the
signal-coil method. Two sets of calibrations were carried out: one *in
situ* and one in the laboratory immediately after the experiment.
Repeated calibration of ten geophones gives results that are consistent
to within 5%. The average resonant frequency, percent damping, and
damped generator constant were found to average several percent higher
than the manufacturer's specifications, but it is not clear whether this
is a systematic difference in the geophone characteristics or a bias in
the calibration method. When converted to velocity sensitivity, the
average geophone parameters showed an amplitude response up to 2.4%
higher than specification at frequencies above 4 Hz, but as much as 7%
lower at frequencies of less than 4 Hz. For the vertical component, the
geophone response varied by less than 10% for most geophones, but
several geophones showed anomalously large differences from
specification. For the horizontal components, the velocity sensitivity
was generally within 10% of specification at frequencies above the
fundamental frequency, but sensitivities varied by as much as 30% at
lower frequencies. These results are consistent with the horizontal
components being more sensitive to the geophone placement than the
vertical component. The *in* situ and laboratory calibrations for most
geophones showed agreement to within±10% for the vertical component at
all frequencies and for the horizontal component at frequencies above
resonant frequency, but at lower frequencies there were large
differences (often \>30%) in the velocity sensitivity in the two
calibrations. Our results demonstrate that *in* situ calibrations are
required if horizontal ground velocities are to be known to within an
accuracy of 10%.

Upper Mantle Structure and Subducting Slab Geometry

Blakely, R.J., T.M. Brocher, and R.E. Wells, 2005, Subduction zone
magnetic anomalies and implications for hydrated forearc mantle,
*Geology*, 33, 445-448.

Continental mantle in subduction zones is hydrated by release of water
from the underlying oceanic plate. Magnetite is a significant byproduct
of mantle hydration, and forearc mantle, cooled by subduction, should
contribute to long-wavelength magnetic anomalies above subduction zones.
We test this hypothesis with a quantitative model of the Cascadia
convergent margin, based on gravity and aeromagnetic anomalies and
constrained by seismic velocities, and find that hydrated mantle
explains an important disparity in potential-field anomalies of
Cascadia. A comparison with aeromagnetic data, thermal models, and
earthquakes of Cascadia, Japan, and southern Alaska suggests that
magnetic mantle may be common in forearc settings and thus magnetic
anomalies may be useful in mapping hydrated mantle in convergent margins
worldwide.

Brocher, T.M., T. Parsons, A.M. Tréhu, C.M. Snelson, and M.A. Fisher,
2003, Seismic evidence for widespread serpentinized forearc upper mantle
along the Cascadia margin, *Geology*, 31, no. 3, 267-270.

Petrologic models suggest that dehydration and metamorphism of
subducting slabs release water that serpentinizes the overlying forearc
mantle. To test these models, we use the results of controlled-source
seismic surveys and earthquake tomography to map the upper mantle along
the Cascadia margin forearc. We find anomalously low upper-mantle
velocities and/or weak wide-angle reflections from the top of the upper
mantle in a narrow region along the margin, compatible with recent
teleseismic studies and indicative of a serpentinized upper mantle. The
existence of a hydrated forearc upper-mantle wedge in Cascadia has
important geological and geophysical implications. For instance,
shearing within the upper-mantle, inferred from seismic reflectivity and
consistent with its serpentinite rheology, may occur during aseismic
slow slip events on the megathrust. In addition, progressive dehydration
of the hydrated mantle wedge south of the Mendocino triple junction may
enhance the effects of a slab gap during the evolution of the California
margin.

Calvert, A.J., M.A. Fisher, K. Ramachandran, and A.M. Tréhu, 2003,
Possible emplacement of crustal rocks into the forearc mantle of the
Cascadia subduction zone, Geophys. Res. Lett., 30(23), 2196, SDE 3-1 to
3-4, doi:10.1029/2003GL018541.

Seismic reflection profiles shot across the Cascadia forearc show that a
5--15 km thick band of reflections, previously interpreted as a lower
crustal shear zone above the subducting Juan de Fuca plate, extends into
the upper mantle of the North American plate, reaching depths of at
least 50 km. In the extreme western corner of the mantle wedge, these
reflectors occur in rocks with P wave velocities of 6750--7000 m/s.
Elsewhere, the forearc mantle, which is probably partially
serpentinized, exhibits velocities of approximately 7500 m/s. The rocks
with velocities of 6750--7000 m/s are anomalous with respect to the
surrounding mantle, and may represent either: (1) locally high mantle
serpentinization, (2) oceanic crust trapped by backstepping of the
subduction zone, or (3) rocks from the lower continental crust that have
been transported into the uppermost mantle by subduction erosion. The
association of subparallel seismic reflectors with these anomalously low
velocities favours the tectonic emplacement of crustal rocks.

Calvert, A.J., 2004, Seismic reflection evidence for two megathrusts in
the northern Cascadia subduction zone, *Nature*, 428, 163-167.

At convergent continental margins, the relative motion between the
subducting oceanic plate and the overriding continent is usually
accommodated by movement along a single, thin interface known as a
megathrust. Great thrust earthquakes occur on the shallow part of this
interface where the two plates are locked together. Earthquakes of lower
magnitude occur within the oceanic plate, and have been linked to
geochemical dehydration reactions caused by the plate's descent. Here I
present deep seismic reflection data from the northern Cascadia
subduction zone which show that the inter-plate boundary is up to 16 km
thick, and comprises two megathrusts that bound a \>5 km-thick, \~110
km-wide region of imbricated crustal rocks. Earthquakes within the
subducting plate occur predominantly in two geographic bands where the
dip of the plate is inferred to increase as it is forced around the
edges of the imbricated inter-plate boundary zone. This interpretation
implies that seismicity in the subducting slab is controlled primarily
by deformation in the upper part of the plate. Slip on the shallower
megathrust, which may occur by aseismic slow slip, will transport
crustal rocks into the upper mantle above the subducting oceanic plate,
and may, in part, provide an explanation for P wave velocities as low as
6800 m/s that are observed there.

Creager, K.C., L.A. Preston, R.L. Crosson, T. van Wagoner, A. Tréhu, and
the SHIPS Working Group, 2002, Three-dimensional reflection image of the
subducting Juan de Fuca plate, *U.S. Geological Survey Open-File Report*
02-328 and *Geological Survey of Canada Open File 4350*, p. 37-42.
[[http://geopubs.wr.usgs.gov/open-file/of02-328/]{.underline}](http://geopubs.wr.usgs.gov/open-file/of02-425/)

The dynamics and environment under which intraslab earthquakes occur
have been debated during the past several decades. Earthquake nucleation
dynamics proposed for the shallow crust, involving brittle fracture when
shear stresses overcome normal stresses, are difficult to extend to the
high lithostatic pressures encountered in subduction zones. A process
known as transformational faulting has been invoked to explain
catastrophic faulting of ice at high confining pressure as ice undergoes
a phase transition to ice II *\[Kirby,* 1987\]. The olivine to spinel
phase transition exhibits similar behavior in the laboratory, offering a
reasonable explanation for deep earthquakes \[e.g., *Green and Burnley*,
1989; *Houston and Green*, 1995; *Kirby*, 1995; *Kirby et al.*, 1991\].
An important phase transformation in the shallowest 100 km of subduction
zones is the transition of the basalt in the subducting oceanic crust to
eclogite. Dehydration embrittlement, faulting associated with the
release of fluids as a result of this phase transition, provides an
especially attractive explanation for these earthquakes \[*Kirby*,
1995\]. The depth range over which this reaction proceeds is strongly
temperature dependent because the equilibrium mineral assemblages depend
strongly on the temperature \[*Peacock and Wang*, 1999\] and because the
transition may be thermally inhibited in a cold, fast moving slab
\[*Kirby*, 1995\]. For example, the oceanic crust of the Pacific plate
subducting under northeast Japan is very old and fast moving.
Earthquakes apparently in the subducted crust occur to depths as great
as 100 km, as well as in a more active zone clearly within the mantle
part of the subducted plate that extends deeper. In contrast, the
Philippine Sea plate subducting below southwest Japan is young and slow,
and contains shallow Wadati--Benioff seismicity down to only 60 km.
Geotherms calculated by thermal modeling of these two subduction zones
can explain the first order differences in observed seismicity in terms
of basalt to eclogite phase transitions in equilibrium \[*Peacock and
Wang*, 1999\].

An obvious test of the hypothesis that earthquakes are associated with
the basalt to eclogite phase transition is to determine whether the
earthquakes occur within the subducted crust or the subducted mantle.
For example, do the earthquakes occur above or below seismic reflectors
associated with the subducted oceanic crustmantle boundary, or with the
interplate interface? *P*--to-- *S* and *S*--to--*P* conversions from
local earthquakes to seismometers in northern Honshu, Japan show
convincing evidence for a strong discontinuity at the upper edge of the
seismicity, probably associated with the interplate interface \[e.g.,
*Hasegawa et al.*, 1978; *Matsuzawa et al.*, 1990; *Zhao et al.*,
1997\]. They infer that earthquakes occur within both the crustal and
mantle portions of the old subducted Pacific plate.

In contrast, the width of the Cascadia seismogenic zone is much
narrower, perhaps as narrow as five kilometers. Our goal is to determine
the relative position of intraslab earthquakes to reflector surfaces
estimated using data from the 1998 active-source Wet SHIPS experiment
\[*Fisher et al.*, 1999\]. We observe strong secondary arrivals
(Figure 1) with mid-points under a broad area of the northern Olympic
Peninsula (Figure 2). The amplitudes and polarities of reflected
arrivals vary systematically over spatial scales as short as one to two
kilometers (Figure 1). Beyond offsets of 80 km, the reflected arrivals
are typically larger than direct arrivals. We picked approximately 500
travel times from stacks of reflected arrivals after applying a low-pass
filter at 14 Hz. Using three-dimensional ray tracing through a
three-dimensional tomography model obtained from SHIPS and earthquake
first arrival data \[*Crosson et al.*, 1999\], we determine the locus of
all point scatterers which would correspond to an observed travel time
for a given source and receiver. This defines an approximately
ellipsoidal surface from which a ray could reflect and produce the
observed travel time. For coherent arrivals, a reflector must be locally
tangent to this ellipsoid. We invert for a smooth reflective surface
that optimally is tangent to each of the data ellipsoids.

Nedimovic, M.R., R.D. Hyndman, K. Ramachandran, and G.D. Spence, 2003,
Reflection signature of seismic and aseismic slip on the northern
Cascadia subduction thrust, *Nature*, 424, 416-420.

At the northern Cascadia margin, the Juan de Fuca plate is
underthrusting North America at about 45 mm/yr, resulting in the
potential for destructive great earthquakes. The downdip extent of
coupling between the two plates is difficult to determine because the
most recent such earthquake (thought to have been in 1700) occurred
before instrumental recording. Thermal and deformation studies indicate
that, off southern Vancouver Island, the interplate interface is
presently fully locked for a distance of 60km downdip from the
deformation front. Great thrust earthquakes on this section of the
interface (with magnitudes of up to 9) have been estimated to occur at
an average interval of about 590 yr. Further downdip there is a
transition from fully locked behaviour to aseismic sliding (where high
temperatures allow ductile deformation), with the deep aseismic zone
exhibiting slow-slip thrust events. Here we show that there is a change
in the reflection character on seismic images from a thin sharp
reflection where the subduction thrust is inferred to be locked, to a
broad reflection band at greater depth where aseismic slip is thought to
be occurring. This change in reflection character may provide a new
technique to map the landward extent of rupture in great earthquakes and
improve the characterization of seismic hazards in subduction zones.

Preston, L. A., 2003, Simultaneous inversion of 3D velocity structure,
hypocenter Locations, and reflector geometry in Cascadia, Ph. D. thesis,
University of Washington, Seattle, 135 pp.

We present results from a non-linear inversion of direct and wide-angle
reflection travel times for 3-D P-wave velocity structure, earthquake
hypocenters, and reflector geometry under NW Washington focusing on the
structure of the subducting Juan de Fuca plate. The first-arrival travel
times are derived from both active-source experiments and from local
earthquakes. The reflection arrivals were picked from data collected
during the 1998 Wet SHIPS active-source experiment, which consisted of
air-gun sources detonated within the inland waterways of NW Washington
and SW British Columbia to land-based stations. As part of this
research, we have developed a method of incorporating the reflection and
first-arrival travel times into a simultaneous non-linear iterative
inversion scheme for reflector geometry, 3-D velocities and earthquake
relocations. This procedure reduces the well-known trade-off between
reflector position and the velocities above it by including independent
first-arrival information. Results indicate the wide-angle reflector to
be the Moho of the subducting Juan de Fuca slab. The relocated intraslab
earthquakes separate into two groups: those located up-dip of the 45km
reflector depth contour generally lie below the reflector in the
subducting mantle, while those down-dip of this contour primarily occur
within the subducted oceanic crust. These results are consistent with
the subducted mantle events being associated with serpentine dehydration
embrittlement and the subducted crustal events being associated with the
basalt to eclogite transformation. Error and resolution analyses
demonstrate we have the necessary resolvability and can distinguish the
relative locations among the velocities, reflector, and intraslab
hypocenters within the subducting slab with sufficient precision to make
our interpretations. Our results have important implications for our
general understanding of the causes of intraslab earthquakes, earthquake
hazards, and fluid processes within the shallowest portion of a warm
subduction zone.

Preston, L.A., K.C. Creager, R.S. Crosson, T.M. Brocher, and A.M. Tréhu,
2003, Intraslab earthquakes: Dehydration of the Cascadia slab,
*Science*, 302, no. 5648, p. 1197-1200.

We simultaneously invert travel times of refracted and wide-angle
reflected waves for three-dimensional compressional-wave velocity
structure, earthquake locations, and reflector geometry in northwest
Washington state. The reflector, interpreted to be the crust-mantle
boundary (Moho) of the subducting Juan de Fuca plate, separates
intraslab earthquakes into two groups, permitting a new understanding of
the origins of intraslab earthquakes in Cascadia. Earthquakes up-dip of
the Moho's 45-kilometer depth contour occur below the reflector, in the
subducted oceanic mantle, consistent with serpentinite dehydration;
earthquakes located down-dip occur primarily within the subducted crust,
consistent with the basalt-to-eclogite transformation.

Tréhu, A.M., T.M. Brocher, K. Creager, M. Fisher, L. Preston, G. Spence,
and the SHIPS98 Working Group, 2002, Geometry of the subducting Juan de
Fuca plate: New constraints from SHIPS98, *U.S. Geological Survey
Open-File Report* 02-328 and *Geological Survey of Canada Open File
4350*, p. 25-32. <http://geopubs.wr.usgs.gov/open-file/of02-328/>

We have processed seismic reflection profiles acquired in the Strait of
Juan de Fuca, inverted travel times of first arrivals from onshore
recordings of the offshore shots to determine the velocity of the upper
crust in this region and inverted the travel times of first and
secondary arrivals that undershoot the central core of the Olympic
Mountains. The seismic reflection profiles show a pattern of crustal
reflectivity similar to that recorded beneath Vancouver Island.
Inversion of first arrivals, interpreted to be diving waves through the
upper and mid-crust indicate a 5--7 kilometer-deep linear,
northwest-trending basin beneath the southwestern shore of the Strait
and uplift of the basement Crescent terrane rocks beneath the
northwestern shore. Velocities beneath the central Olympic Peninsula at
15--25 kilometers depth show strong lateral variation, with higher
velocities underlying the surface exposures of the Crescent terrane and
lower velocities at these depths beneath the Olympic core rocks.
Inversion of secondary seismic arrivals interpreted to be reflections
from the base of the crust of the subducted Juan de Fuca plate indicate
that the Moho is at a depth of about 34 kilometers beneath the western
Strait of Juan de Fuca and dips about 7o to the east, reaching a depth
of about 46 kilometers beneath the eastern boundary of Olympic National
Park. Correlating these results with results of previous experiments on
Vancouver Island and in southwestern Washington confirm earlier
interpretations of an arch in the subducted slab beneath the Olympic
peninsula but indicate that the arch is asymmetric and less pronounced
than previously thought. We attribute both the northwest-trending
folding of the Crescent terrane and the asymmetry in the shape of the
subducting slab to resistance to the northward motion of the
Paleocene-age Cascadia forearc terrane by the thick lithosphere of the
pre-Tertiary terranes of British Columbia.

Seattle Fault and Basin

Brocher, T.M., T. Parsons, K. C. Creager, R. S. Crosson, N. P. Symons,
G. Spence, B.C. Zelt, P.T.C. Hammer, R. D. Hyndman, D.C. Mosher, A. M.
Tréhu, K. C. Miller, U.S. ten Brink, M.A. Fisher, T. L. Pratt, M.G.
Alvarez, B.C. Beaudoin, and C.S. Weaver, 1999, Wide-angle seismic
recordings from the 1998 Seismic Hazards Investigation in Puget Sound
(SHIPS), western Washington and British Columbia, *U.S. Geological
Survey Open-File Report 99-314*, 110 p.
<http://geopubs.wr.usgs.gov/open-file/of99-314/>

This report describes the acquisition and processing of deep-crustal
wide-angle seismic reflection and refraction data obtained in the
vicinity of Puget Lowland, the Strait of Juan de Fuca, and Georgia
Strait, western Washington and southwestern British Columbia, in March
1998 during the Seismic Hazards Investigation of Puget Sound (SHIPS). As
part of a larger initiative to better understand lateral variations in
crustal structure along the Cascadia margin, SHIPS participants acquired
1000 km of deep-crustal multichannel seismic-reflection profiles and
1300 km of wide-angle airgun shot lines in this region using the **R/V
Thompson** and **R/V Tully**. The **Tully** was used to record airgun
shots fired by the **Thompson** in two different geometries: (1)
expanding spread profiles (ESPs) and (2) constant offset profiles
(COPs). Prior to this reflection survey, we deployed 257 Reftek and 15
ocean-bottom seismic recorders to record the airgun signals at far
offsets. All data were recorded digitally on large-capacity hard disks.
Although most of these stations only recorded the vertical component of
motion, 95 of these seismographs recorded signals from an oriented
3-component seismometer. By recording signals generated by the
**Thompson\'s** marine air gun array, operated in two differing
geometries having a total volume of 110 and 79 liters (6730 and 4838 cu.
in.), respectively, the arrays of wide-angle recorders were designed to
(1) image the crustal structure, particularly in the vicinity of crustal
faults and Cenozoic sedimentary basins, (2) determine the geometry of
the Moho, and (3) image the subducting Gorda and Juan de Fuca plates.
Nearly 33,300 air gun shots were recorded along several seismic lines.
In this report, we illustrate the expanding spread profiles acquired
using the **Thompson** and **Tully**, describe the land and ocean-bottom
recording of the air gun signals, discuss the processing of the land
recorder data into common receiver gathers, and illustrate the processed
wide-angle seismic data collected using the Refteks and ocean-bottom
seismometers. We also describe the format and content of the archival
tapes containing the SEGY-formated, common-receiver gathers for the
Reftek data. Data quality is variable but SHIPS appears to have
successfully obtained useful data from almost all the stations deployed
to record the airgun shots. Several interesting arrivals were observed:
including refractions from the sedimentary basin fill in several basins,
refractions from basement rocks forming the upper crust, Pg, refractions
from the upper mantle, Pn, as well as reflections from within the crust
and from the top of the upper mantle, PmP. We separately archived more
than 30 local earthquakes recorded by the Reftek array during our
deployment.

Brocher, T.M., T. Parsons, M.A. Fisher, A.M. Tréhu, G.D. Spence, and the
SHIPS Working Group, 2000, Three-dimensional tomography in the eastern
Strait of Juan de Fuca: Preliminary results from SHIPS, the 1998 Seismic
Hazards Investigation in Puget Sound, in Mosher, D.C., and S.Y. Johnson
(eds.), Rathwell, G.J., R.B. King, and S.B. Rhea (compilers),
Neotectonics of the eastern Strait of Juan de Fuca; a digital geologic
and geophysical atlas, *Geological Survey of Canada Open File Report*
*3931*.

Brocher, T.M., T. L. Pratt, K.C. Miller, A.M. Tréhu, C.M. Snelson, C.S.
Weaver, K. C. Creager, R.S. Crosson, U.S. ten Brink, M.G. Alvarez, S.H.
Harder, and I. Asudeh, 2000, Report for explosion and earthquake data
acquired in the 1999 Seismic Hazards Investigation in Puget Sound
(SHIPS), Washington, *U.S. Geological Survey Open-File Report 00-318*,
85 p. <http://geopubs.wr.usgs.gov/open-file/of00-318/>

This report describes the acquisition, processing, and quality of
seismic reflection and refraction data obtained in the Seattle basin,
central Puget Lowland, western Washington, in September 1999 during the
Seismic Hazards Investigation of Puget Sound (SHIPS). As a sequel to the
1998 SHIPS air gun experiment (also known as "Wet SHIPS"), the 1999
experiment, nicknamed "Dry SHIPS", acquired a 112-km-long east-west
trending multichannel seismic-reflection and refraction line in the
Seattle basin. One thousand and eight seismographs were deployed at a
nominal spacing of 100 meters and 29 shot points were detonated at
approximately 4 km intervals along the seismic line. The wide-angle
seismic profile was designed to (1) determine the E-W geometry of
Seattle basin, (2) measure the seismic velocities within the basin, and
(3) define the basement structure underlying the Seattle basin. In this
report, we describe the acquisition of these data, discuss the
processing and merging of the data into common shot gathers, and
illustrate the acquired profiles. We also describe the format and
content of the archival tapes containing the SEGY-formatted, common-shot
gathers. Data quality is variable but useful data were acquired from all
29 shot points fired along the Dry SHIPS seismic line. The data show
pronounced travel time delays associated with the low velocity
sedimentary rocks filling the Seattle basin.

Thirty-five REFTEK stations, deployed at 4 km intervals along the Dry
SHIPS line, recorded 26 regional earthquakes and blasts and 17
teleseismic events, including the main shock and several aftershocks of
the M~w~ = 7.6 Chi-Chi (Taiwan) earthquake of 9/20/1999. The teleseismic
recordings of the Chi-Chi (Taiwan) mainshock provide useful signals down
to 10 second periods. They document a significant (factor between 5 and
10) focusing of compressional- and shear-wave energy by the Seattle
basin at periods between 1 and 2 seconds relative to "bedrock" sites
east and west of the basin. Signal durations in the Seattle basin were
also substantially increased relative to "bedrock" sites in the Olympic
peninsula and Cascade foothills.

Brocher, T.M., T. L. Pratt, C.S. Weaver, A.D. Frankel, A.M. Tréhu, C.M.
Snelson, K.C. Miller, S.H. Harder, U.S. ten Brink, K. C. Creager, R.S.
Crosson, and W. P. Steele, 2000, Urban seismic experiments investigate
the Seattle fault and basin, *Eos, Trans. AGU*, v. 81, no. 46, p. 545,
551-552.

In the past decade earth scientists have come to recognize the seismic
hazards posed by crustal faults and sedimentary basins to Seattle,
Washington (Figure 1). In 1998, the U.S. Geological Survey and its
collaborators initiated a series of urban seismic studies of the upper
crust to better map seismogenic structures and sedimentary basins in the
Puget Lowland. We call these studies the Seismic Hazard Investigations
of Puget Sound (SHIPS).

In March 1998 we conducted our first SHIPS study, an investigation of
the upper crustal structure of the Puget Lowland, nicknamed Wet SHIPS,
using marine airgun sources and land recorders \[Fisher et al., 1999\].
In September 1999, we obtained a seismic refraction line to study the
upper crustal structure in the Seattle area, in a landbased study
nicknamed Dry SHIPS \[Brocher et al., 2000\]. In March 2000, we recorded
the demolition of the Seattle Kingdome sports stadium using a dense
array of seismic recorders for a detailed site response study, nicknamed
Kingdome SHIPS.

Brocher, T.M., T. Parsons, R.A. Blakely, N.I. Christensen, M.A. Fisher,
R.E. Wells, and the SHIPS Working Group, 2001, Upper crustal structure
in Puget Lowland, Washington: Results from the 1998 Seismic Hazards
Investigation in Puget Sound, *J. Geophys. Res*., *106*, 13,541-13,564.

### A new three-dimensional (3-D) model shows seismic velocities beneath the Puget Lowland to a depth of 11 km. The model is based on a tomographic inversion of nearly one million first-arrival travel times recorded during the 1998 Seismic Hazards Investigation in Puget Sound (SHIPS), allowing higher-resolution mapping of subsurface structures than previously possible. The model allows us to refine the subsurface geometry of previously proposed faults (e.g., Seattle, Hood Canal, southern Whidbey Island, and Devils Mountain fault zones) as well as to identify structures (Tacoma, Lofall, and Sequim fault zones) that warrant additional study. The largest and most important of these newly identified structures lies along the northern boundary of the Tacoma basin; we informally refer to this structure here as the Tacoma fault zone. Although tomography cannot provide information on the recency of motion on any structure, Holocene earthquake activity on the Tacoma fault zone is suggested by seismicity along it and paleoseismic evidence for abrupt uplift of tidal marsh deposits to its north. The tomography reveals four large, west to northwest trending low-velocity basins (Tacoma, Seattle, Everett, and Port Townsend) separated by regions of higher velocity ridges that are coincident with fault-bounded uplifts of Eocene Crescent Formation basalt and pre-Tertiary basement. The shapes of the basins and uplifts are similar to those observed in gravity data; gravity anomalies calculated from the 3-D tomography model are in close agreement with the observed anomalies. In velocity cross sections the Tacoma and Seattle basins are asymmetric: the basin floor dips gently toward a steep boundary with the adjacent high-velocity uplift, locally with a velocity "overhang" that suggests a basin vergent thrust fault boundary. Crustal fault zones grow from minor folds into much larger structures along strike. Inferred structural relief across the Tacoma fault zone increases by several kilometers westward along the fault zone to Lynch Cove, where we interpret it as a zone of south vergent faulting overthrusting Tacoma basin. In contrast, structural relief along the Seattle fault zone decreases west of Seattle, which we interpret as evidence that the N-S directed compression is being accommodated by slip transfer between the Seattle and Tacoma fault zones. Together, the Tacoma and Seattle fault zones raise the Seattle uplift, one of a series of east-west trending, pop-up structures underlying Puget Lowland from the Black Hills to the San Juan Islands. {#a-new-three-dimensional-3-d-model-shows-seismic-velocities-beneath-the-puget-lowland-to-a-depth-of-11-km.-the-model-is-based-on-a-tomographic-inversion-of-nearly-one-million-first-arrival-travel-times-recorded-during-the-1998-seismic-hazards-investigation-in-puget-sound-ships-allowing-higher-resolution-mapping-of-subsurface-structures-than-previously-possible.-the-model-allows-us-to-refine-the-subsurface-geometry-of-previously-proposed-faults-e.g.-seattle-hood-canal-southern-whidbey-island-and-devils-mountain-fault-zones-as-well-as-to-identify-structures-tacoma-lofall-and-sequim-fault-zones-that-warrant-additional-study.-the-largest-and-most-important-of-these-newly-identified-structures-lies-along-the-northern-boundary-of-the-tacoma-basin-we-informally-refer-to-this-structure-here-as-the-tacoma-fault-zone.-although-tomography-cannot-provide-information-on-the-recency-of-motion-on-any-structure-holocene-earthquake-activity-on-the-tacoma-fault-zone-is-suggested-by-seismicity-along-it-and-paleoseismic-evidence-for-abrupt-uplift-of-tidal-marsh-deposits-to-its-north.-the-tomography-reveals-four-large-west-to-northwest-trending-low-velocity-basins-tacoma-seattle-everett-and-port-townsend-separated-by-regions-of-higher-velocity-ridges-that-are-coincident-with-fault-bounded-uplifts-of-eocene-crescent-formation-basalt-and-pre-tertiary-basement.-the-shapes-of-the-basins-and-uplifts-are-similar-to-those-observed-in-gravity-data-gravity-anomalies-calculated-from-the-3-d-tomography-model-are-in-close-agreement-with-the-observed-anomalies.-in-velocity-cross-sections-the-tacoma-and-seattle-basins-are-asymmetric-the-basin-floor-dips-gently-toward-a-steep-boundary-with-the-adjacent-high-velocity-uplift-locally-with-a-velocity-overhang-that-suggests-a-basin-vergent-thrust-fault-boundary.-crustal-fault-zones-grow-from-minor-folds-into-much-larger-structures-along-strike.-inferred-structural-relief-across-the-tacoma-fault-zone-increases-by-several-kilometers-westward-along-the-fault-zone-to-lynch-cove-where-we-interpret-it-as-a-zone-of-south-vergent-faulting-overthrusting-tacoma-basin.-in-contrast-structural-relief-along-the-seattle-fault-zone-decreases-west-of-seattle-which-we-interpret-as-evidence-that-the-n-s-directed-compression-is-being-accommodated-by-slip-transfer-between-the-seattle-and-tacoma-fault-zones.-together-the-tacoma-and-seattle-fault-zones-raise-the-seattle-uplift-one-of-a-series-of-east-west-trending-pop-up-structures-underlying-puget-lowland-from-the-black-hills-to-the-san-juan-islands. .Abstract}

Brocher, T.M., C.S. Weaver, and R.S. Ludwin, 2003, Assessing hypocentral
accuracy and lower magnitude completeness in the Pacific Northwest using
seismic refraction detonations and cumulative frequency-magnitude
relationships, *Seism. Res. Lett.*, 74, 773-790.

Absolute hypocentral location errors are traditionally estimated from
the errors in the location of quarry blasts. Catalog completeness has
been evaluated by examining cumulative frequency-magnitude relationships
(e.g., Rydelek and Sacks, 1989). In the Pacific Northwest, however, the
number of quarries with well-defined blasting schedules is relatively
sparse (e.g., Benson et al., 1992). Seismic refraction detonations
provide an independent assessment of actual location errors of surface
events in the Pacific Northwest and elsewhere. Because the detonation
yields are also known, their reported magnitudes can be used to
investigate the detection and location thresholds for low magnitude
earthquakes. As the number and distribution of seismic stations in the
Pacific Northwest expanded (Figure 1), the location accuracy of the
networks and the completeness of their catalogs has improved with time.
Because the station coverage is not uniform geographically, these
network properties vary with location.

In this note we use 72 refraction detonations listed in the Advanced
National Seismic System (ANSS) and Pacific Northwest Seismic Network
(PNSN) catalogs for the Pacific Northwest to investigate the hypocentral
accuracy and completeness of the earthquake catalog as a function of
time and location since 1984. These reported detonations had an average
charge size of 939 kg and yielded an average coda magnitude of 1.65.
Applying magnitude-versus-charge-size relationships to 64 detonations
not listed in the ANSS and PNSN catalogs permits us to extend our
analysis back to 1965. Brocher (2003a) noted several reasons why the
location errors for surface detonations may not be fully representative
of location errors for tectonic earthquakes. Nonetheless, quarry blasts
and refraction detonations represent seismic sources whose origin times
and locations are precisely known, and they provide an independent
measure of the quality of location solutions by the network.
Furthermore, the regional variation in the ability of a seismic network
to locate surficial detonations, which can be measured, presumably
reflects the regional variation in the ability of the network to locate
shallow (near surface to a few km depth) earthquakes, which cannot be
measured. We sought to determine whether regional variations exist in
the capability of the Pacific Northwest Seismic Network (PNSN) to locate
earthquakes in Oregon and Washington.

Brocher, T.M., R.J. Blakely, and R.E. Wells, 2004, Interpretation of the
Seattle Uplift, Washington, as a passive roof duplex, *Bull. Seism. Soc.
Am*., 94, 1379-1401.

We interpret seismic lines and a wide-variety of other geological and
geophysical data to suggest that the Seattle uplift is a passive-roof
duplex. A passive roof duplex is bounded top and bottom by thrust faults
with opposite senses of vergence that form a triangle zone at the
leading edge of the advancing thrust sheet. In passive-roof duplexes the
roof thrust slips only when the floor thrust ruptures. The Seattle fault
is a south-dipping reverse fault forming the leading edge of the Seattle
uplift, a 40-km-wide fold-and-thrust belt. The recently discovered,
north-dipping Tacoma reverse fault is interpreted as a back thrust on
the trailing edge of the belt, making the belt doubly vergent. Floor
thrusts in the Seattle and Tacoma fault zones are interpreted as blind
faults that flatten updip into bedding plane thrusts. Shallow monoclines
in both the Seattle and Tacoma basins are interpreted to overlie the
leading edges of thrust-bounded wedge tips advancing into the basins.
Across the Seattle uplift, seismic lines image several shallow,
short-wavelength folds exhibiting Quaternary or late Quaternary growth.
From reflector truncation, several north-dipping thrust faults are
inferred to core these shallow folds and to root into a shallow roof
thrust. Although currently aseismic, some of the shallow thrusts above
the triangle zone have ruptured to the surface in the late Holocene.
Ages from offset soils in trenches across the fault scarps and from
abruptly raised shorelines indicate that both the floor and roof thrusts
of the Seattle and Tacoma faults ruptured about 1100 years ago.

Brocher, T.M., R. J. Blakely, R. E. Wells, B. L. Sherrod, and K.
Ramachandran , 2005, The Transition Between N-S and NE-SW Directed
Crustal Shortening in the Central and Northern Puget Lowland: New
Thoughts on the Southern Whidbey Island Fault, Eos, Trans. Amer.
Geophys. Union, in press.

We hypothesize that the southern Whidbey Island fault (SWIF) is a NW-SE
oriented fold and thrust belt accommodating NE-directed crustal
shortening. The SWIF has been considered a dextral strike-slip fault
based largely on two interpretations: (1) its northwest orientation in a
region believed to be undergoing dominantly N-S compression, and (2)
interpretation of industry seismic-reflection data across the SWIF as a
flower structure, suggestive of transpressional faulting. Both
interpretations require reconsideration based on evidence outlined
below.

Recent GPS studies (e.g., Miller et al., 2001) have shown that the Puget
Lowland is a zone of transition between N-directed compression to the
south and NE-SW directed compression (parallel to the plate-convergence
vector) to the north. While N-S compression provides an adequate
explanation for the E-trending Seattle and Tacoma thrust faults to the
south, recent paleoseismic and geophysical studies suggest that NE-SW
compression producing NE-directed tectonic wedging (passive roof
duplexing) dominates at the SWIF. Evidence for a SW-dipping floor thrust
forming the base of the tectonic wedge is provided by gravity and
seismic tomography models demonstrating higher structural relief of
basement rocks to the south of the SWIF than to its north. Aeromagnetic
anomalies, lidar studies, and paleoseismic evidence indicate a broader
(\~25 km wide) zone of abundant NE-side-up shallow reverse faults
parallel to the SWIF than previously recognized. We interpret these
faults as evidence for a zone of NW-oriented, NE-dipping splay faults
soling into a shallow (3 to 4 km deep), NE-dipping detachment surface
forming the top of the tectonic wedge. We re-examined oil industry
seismic-reflection profiles across the SWIF, previously seen as evidence
for transpressional faults, and find them more compatible with shallow
thrust folds associated with shallow (upper 3 to 4 km) splay faults.

In sum, these observations are consistent with a blind NE-vergent wedge
tip, accompanied by shallow, NE-dipping roof and splay thrusts of late
Holocene age. Available data do not rule out a small component of
transpressional faulting along the SWIF. Indeed, paleoseismic
excavations at the Utsulady fault, which lies subparallel to and north
of the SWIF, reveal late Holocene sinistral slip. Thus, the SWIF may
represent the southernmost crustal fault in the central Puget Lowland
that responds to plate convergence rather than northward migration of
the forearc. To the north, along southern Vancouver Island, other fold
and thrust belts (e.g., the Cowichan Fold and Thrust Belt) are clearly
oriented perpendicular to NE-directed plate convergence as measured by
GPS studies.

Calvert, A.J., M.A. Fisher, and SHIPS Working Group, 2001, Imaging the
Seattle fault zone with high-resolution seismic tomography, *Geophys.
Res. Lett., 28*, 2337-2340.

The Seattle fault, which trends east-west through the greater Seattle
metropolitan area, is a thrust fault that, around 1100 years ago,
produced a major earthquake believed to have had a magnitude greater
than 7. We present the first high resolution image of the shallow P-wave
velocity variation across the fault fault obtained by tomographic
inversion of first arrivals recorded on a seismic reflection profile
shot through Puget Sound adjacent to Seattle. The velocity image shows
that above 500 m depth the fault zone extending beneath Seattle
comprises three distinct fault splays, the northernmost of which dips to
the south at around 60 degrees. The degree of uplift of Tertiary rocks
within the fault zone suggests that the slip-rate along the northernmost
splay during the Quaternary is 0.5 mm/yr, which is twice the average
slip-rate of the Seattle fault over the last 40 Ma.

Calvert, A.J., M.A. Fisher, S.Y. Johnson, and the SHIPS Working Group,
2003, Along-strike variations in the seismic velocity structure of the
Seattle fault zone: Evidence for fault segmentation beneath Puget Sound,
*J. Geophys. Res.*, 108(B1), doi:10.1029/2001JB001703.

Around 1100 years ago, the Seattle fault, which trends east-west beneath
Puget Sound and the greater Seattle metropolitan area, experienced a M
\> 7 earthquake. We present high-resolution images of the shallow P wave
velocity variation across the fault zone. These images were obtained by
tomographic inversion of the first arrivals recorded along two
north-south oriented seismic reflection lines shot within Puget Sound
near Seattle. Just beneath the seafloor, the fault zone includes
uplifted Tertiary rocks with seismic velocities in the range of 2300 to
2600 m s 1. These velocities contrast markedly with values of 1600 m s 1
in shallow Holocene sediments. South of the Seattle fault zone volcanic
rocks of the Crescent Formation, which exhibit velocities \>3700 m s 1,
are identified at depths of only 900 m. Seismic velocities of around
2600 m s 1, which represent Oligocene rocks, are found in the hanging
wall of the Seattle fault beneath eastern Puget Sound. In the west,
lower, 2300 m s 1 seismic velocities occur, probably due to the presence
of Miocene rocks, which are not found in the east. Along-strike velocity
variations arise from the folding of Tertiary rocks and the presence of
distinct fault splays, including a north striking tear fault
characterized by depressed seismic velocities that was intersected by
the eastern seismic line. Along-strike differences in the uplift of
Tertiary rocks beneath Puget Sound are likely associated with the
existence of a segment boundary of the Seattle fault system.

Crosson, R.S., N.P. Symons, K.C. Creager, L.A. Preston, T. Van Wagoner,
T.M. Brocher, M. Fisher, and the SHIPS Working Group, 2002,
Three-dimensional structure of the Cascadia forearc region from SHIPS
active experiment and earthquake observations: tomographic inversion
provides a high-resolution view into the core of the Cascadia forearc
complex, *U.S. Geological Survey Open-File Report* 02-328 and
*Geological Survey of Canada Open File 4350*, p. 33-34.
[[http://geopubs.wr.usgs.gov/open-file/of02-328/]{.underline}](http://geopubs.wr.usgs.gov/open-file/of02-425/)

Phase I of SHIPS (Seismic Hazards in Puget Sound) was a large-scale
airgun deployment in the inland waterways of western Washington State
and western British Columbia. The coincidence of SHIPS travel time data
in the Puget Sound region with earthquake arrival time data from the
Pacific Northwest Seismograph Network has allowed us to carry out
high-resolution joint active source and earthquake travel time inversion
(tomography) for *P* wave velocity structure to depths of approximately
60 km over a portion of the Cascadia forearc margin. A total of about
70,000 *P* wave arrival observations were used in our initial effort;
approximately 1000 earthquakes comprise 45% of the total number of *P*
wave observations. Excellent constraint of the shallow structure by
SHIPS data allows greatly improved resolution of the deep structure
through earthquake observations, compared to inversion using earthquake
data alone. The three-dimensional velocity model fits both airgun and
earthquake picks to approximately 0.1 second root-mean-squared (*rms*)
overall. Simple variance reduction from a best fitting onedimensional
model to our preferred three-dimensional model exceeds 90%.

After inversion, earthquake hypocenters migrate in general to slightly
shallower depths relative to depths computed with a best fitting
one-dimensional starting model. Increasing the average velocity of the
one-dimensional model could bring the one-dimensional and
three-dimensional depths into closer agreement; however, because of very
significant three-dimensional effects, such a model may still not
provide a satisfactory one-dimensional average earth model. Earthquakes
in the Wadati--Benioff zone are moved an average of about three
kilometers shallower but generally maintain the average dip of the zone
(about 14°). The scatter of Wadati--Benioff zone earthquakes about a
plane is also reduced by a factor of about two in standard deviation in
going from one-dimensional to three-dimensional models. Mafic
Crescent/Siletz volcanic rocks form the basement of the Puget basin
region with *P* wave velocity exceeding 7 km/s at 20 km depth. In the
central Puget basin, Crescent/Siletz is in turn underlain at a depth of
about 30 km by lenses of lower-velocity, subducted accretionary prism
rocks which are gradational with rocks of the Olympic core further to
the west. This suggests that the Olympic core rocks are mechanically
emplaced by subduction beneath the Crescent/Siletz eastward of the
Olympic Mountains, producing a local low velocity zone in the deep crust
beneath the Puget basin (although in the conventional sense "crust" may
be ill-defined in this region). The *P* wave velocity reversal produced
by this emplacement process is typically about 0.5 km/s. The
relationship between Olympic core rocks and the Crescent/Siletz terrane
is consistent with models of "unroofing" of the Olympic core \[e.g.,
*Brandon et al.,* 1998\].

Fisher, M.A., T. M. Brocher, R. D. Hyndman, A. M. Tréhu, C. S. Weaver,
K. C. Creager, R. S. Crosson, T. Parsons, A. K. Cooper, D. Mosher, G.
Spence, B. C. Zelt, P. T. Hammer, U. ten Brink, T. L. Pratt, K. C.
Miller, J. R. Childs, G. R. Cochrane, S. Chopra, and R.Walia, 1999,
Seismic survey probes urban earthquake hazards in Pacific Northwest,
*EOS, Trans. Amer. Geophys. Un., v. 80*, no. 2, p. 13-17.

Fisher, M.A., R.D. Hyndman, S.Y. Johnson, R.S. Crosson, U.S. ten Brink,
A.J. Calvert, T.M. Brocher, R.E. Wells, and the SHIPS Working Group,
2003, Crustal structure and earthquake hazards of the Subduction Zone in
the Central Part of Cascadia, *U.S.* *Geological Survey Prof. Paper
1661-X*, in press.

Liberty, L. H., 2005, Seismic Profiling of the Seattle Fault, Lake
Sammamish region, Washington, NEHRP Contract Report, Project Award
Number: 03HQGR0132.

This report summarizes the results from vibroseis seismic reflection
profiling across the Seattle fault zone (SFZ) during the summer 2004
field season. I collected two seismic transects consisting of 13
north-south profile segments. Each transect crosses the southern
portions of the Seattle Basin, extends across the main strand of the SFZ
and Seattle monocline, and terminates on the Seattle uplift. I sited
both transects east of Lake Washington, where a published seismic
profile defines the character of the SFZ. One vibroseis transect extends
10 km through Bellevue and Newcastle, Washington, with the profile
segments located 5-10 km east of Lake Washington. The second transect is
located 5-7 km east of Lake Sammamish and extends more than 4 km in a
north-south direction. The survey objectives were to characterize the
eastern reaches of the SFZ to better estimate fault length, to
characterize the style of deformation in the upper one km where
Quaternary-age glacial sediments appear, and to identify fault strands
for future paleoseismic studies to further constrain fault history.

My results show that the SFZ extends beneath my eastern profile, which
is further east than it has previously been imaged. This clearly
demonstrates that the SFZ extends at least 7 km east of Lake Sammamish
for a total fault length of more than 50 km. Both transects show a
similar reflection character that generally matches the reflection
character observed on the Lake Washington profile. The similarity of the
fault zone on these profiles suggests that the eastern portion of the
SFZ is not segmented, and that the fault may rupture along its entire
length during a large event. My observations also support a tectonic
model where back thrusts accommodate near-surface faulting along the
SFZ, constistent with a passive roof duplex system.

The Seattle monocline defines the southern boundary of the Seattle Basin
and is characterized by uplifted Tertiary sediments. Eocene and
Oligocene beds within this monocline dip to the north at approximately
40 degrees west of Lake Sammamish and at about 20 degrees east of Lake
Sammamish. The Quaternary sediments that lie upon the Seattle monocline
contain deformed Pliocene and younger glacial, marine, and nonmarine
sediments. The continuous, but dipping Tertiary-age strata of the
Seattle monocline, with overlying short-wavelength Quaternary folding,
suggests south-directed bedding-plane slip may accommodate near-surface
shortening within the SFZ. The deformation front extends approximately 2
km north of the main strand of the Seattle fault, as defined by a
lineament on LIDAR imagery and associated paleoseismic trenches. This
scarp defines a fault segment that last ruptured A.D. 900 during a M7
event. North of the deformation front, I do not observe deformation in
the upper one km of the Seattle Basin. The Tertiary strata that I image
south of the deformation front show a complex fault history. These
strata are tilted, and km-scale folding prevails. Faults cut these
broad, regionally continuous folds; however, tectonic timing within the
Tertiary package is unclear. The decrease in Seattle monocline dip to
the east is consistent with a broadening of the magnetic lineament
associated with Miocene-age volcanic sediments. It is unclear whether
this decrease in structural dip is associated with a decrease in uplift
rates along the eastern reaches of the Seattle fault. Further studies
are needed if we are to characterize the SFZ east of our existing
profiles.

Miller, K. C., and C. M. Snelson, 2000, Structure and geometry of the
Seattle basin, Washington State-Results from Dry SHIPS (Seismic Hazards
Investigations of Puget Sound) '99: Collaborative Research (USGS, OSU,
UTEP), NEHRP Contract Report 99HQGR0054).

The overall objectives of the Seismic Hazards Investigation of Puget
Sound (SHIPS) are to understand the distribution and size of earthquake
sources (fault locations and geometry) as well as earthquake effects
(the influence of the regional velocity structure on strong ground
motion) in this urban area. Over tens of millions of years, oblique
convergence in the Pacific Northwest has led to the development of a
series of strike-slip faults as well as east-west trending reverse
faults within the Puget - Willamette Lowland (e.g., Riddihough, 1984;
Johnson, 1985). This system of faults has created a series of deep fault
bounded basins (e.g., Johnson and others, 1994; Pratt and others, 1997).
The city of Seattle overlies one of these deep basins, which may focus
energy and enhance ground shaking when an earthquake occurs (Frankel and
others, 1999). The Dry SHIPS phase of this project is designed to assess
the seismic hazard in the Seattle region through acquisition and
analysis of wide-angle seismic reflection/refraction data along a
transect from the Olympic Peninsula through the city of Seattle in to
the foothills of the Cascades (Figure 1). In this report we present our
preliminary velocity model for the Seattle basin based on analysis of
these data and our plans for future work.

Parsons, T., R.J. Blakely, and T.M. Brocher, 2001, A simple algorithm
for sequentially incorporating gravity observations in seismic
traveltime tomography*, International Geology Review, 43*, 1073-1086.

The geologic structure of the Earth's upper crust can be revealed by
modeling variation in seismic arrival times and in potential field
measurements. We demonstrate a simple method for sequentially satisfying
seismic traveltime and observed gravity residuals in an iterative 3-D
inversion. The algorithm is portable to any seismic analysis method that
uses a gridded representation of velocity structure. Our technique
calculates the gravity anomaly resulting from a velocity model by
converting to density with Gardner's rule. The residual between
calculated and observed gravity is minimized by weighted adjustments to
the model velocity-depth gradient where the gradient is steepest and
where seismic coverage is least. The adjustments are scaled by the sign
and magnitude of the gravity residuals, and a smoothing step is
performed to minimize vertical streaking. The adjusted model is then
used as a starting model in the next seismic traveltime iteration. The
process is repeated until one velocity model can simultaneously satisfy
both the gravity anomaly and seismic traveltime observations within
acceptable misfits. We test our algorithm with data gathered in the
Puget Lowland of Washington State, USA (Seismic Hazards Investigation in
Puget Sound (SHIPS) experiment). We perform resolution tests with
synthetic traveltime and gravity observations calculated with a
checkerboard velocity model using the SHIPS experiment geometry and show
that the addition of gravity significantly enhances resolution. We
calculate a new velocity model for the region using SHIPS traveltimes
and observed gravity, and show examples where correlation between
surface geology and modeled subsurface velocity structure is enhanced.

Pratt, T.L., Weaver, C.S., Brocher, T.M., Parsons, T., Fisher, M.A.,
Creager, K.C., Crosson, R.S., Hyndman, R.D., Spence, G., Trehu, A.M.,
Miller, K.C., and ten Brink, U.S., 2002, Understanding the
seismotectonics of the Cascadia subduction zone: Overview and recent
seismic work, *in* Fujinawa, Y., and Toshida, A., editors,
Seismotectonics in Convergent Plate Boundary, Terra Scientific
Publishing, Tokyo, p. 25-36.

Snelson, C.M., 2001, Investigating crustal structure in western
Washington and in the Rocky Mountains: Implications for seismic hazards
and crustal growth, Ph.D. thesis, Univ. Texas El Paso, 234 p.

This dissertation consists of two seismic studies, one in western
Washington State and one in the Rocky Mountains. The study in western
Washington State is one component of the SHIPS (Seismic Hazards
Investigations of Puget Sound) experiments, a continuing effort to
define Cenozoic basin and fault geometry beneath the densely populated
Puget Lowland. In September 1999, the U. S. Geological Survey and a
number of university collaborators collected the "Dry" SHIPS seismic
profile across the Seattle basin of western Washington State. The
objectives of the "Dry" SHIPS study were to define the geometry of the
Seattle basin in an E-W direction and to determine the structure of the
eastern and western boundaries of the basin. In addition, the experiment
was designed to test the hypothesis that N-S trending faults lie beneath
Puget Sound or the adjacent Lowland.

One of these faults may form the eastern boundary of the Siletz terrane.
The "Dry" SHIPS data are characterized by travel time advances
associated with the Siletz terrane to the west and the Cascades to the
east and by delays of as much as 2 s in the Seattle basin. P-wave 3-D
tomographic results show that the basin is about 70 km wide and contains
sedimentary strata with velocities increasing gradually from 1.8 - 4.5
km/s. The contact with underlying basement rocks is characterized by a
rapid increase in velocity from 4.5 to 5.0 km/s. At its center, the
basin is 6 - 7 km deep along this profile. This result is consistent
with results from a N-S trending reflection line collected in 1998
during the "Wet" SHIPS phase of the project that is tied to well
control. The symmetry of the Seattle basin is consistent with thrust
loading as the major contributor to the formation of the basin. The
lower velocities within the upper part of the basement found east of the
Puget Sound may be indicative of pre-Tertiary basement rocks of the
Cascades. This change is probably an expression of the Coast Range
Boundary fault, which has previously been interpreted from gravity and
magnetic data. Density modeling tied to the velocity model shows that
the Olympic accretionary wedge is indistinguishable from surrounding
rocks below a depth of about 20 km. The contact between the Siletz and
Pre-Tertiary basement rocks is a subtle contact as inferred from the
velocity and gravity models.

The study in the Rocky Mountains is one component of the Continental
Dynamics - Rocky Mountains Project (CD-RoM \'99), a collaborative
interdisciplinary study involving 14 American universities and the
University of Karlsruhe, Germany that focuses on Precambrian features
and their effects on Phanerozoic deformation. One of the major field
efforts in the CD-RoM project took place during August, 1999. The
University of Texas at El Paso and the University of Karlsruhe, with the
assistance of several other institutions, collected data along a \~ 950
km long seismic refraction/wide-angle reflection profile extending from
Fort Sumner, New Mexico to the Gas Hills, Wyoming. Station spacing was
nominally 800 m using \~ 600 instruments during two deployments. Eleven
shots were fired ranging in size from 167.2 - 4540.9 kg and were
nominally spaced at \~ 100 km intervals along the profile. The profile
crosses major structural features of the continent including the Jemez
lineament, the Colorado mineral belt, and the Cheyenne belt (a prominent
Proterozoic suture).

Velocity modeling, employing several techniques, indicates that crustal
thicknesses ranges from \~ 45 to 55 km in New Mexico and Colorado. In
northern Colorado, the crust begins to thin from \~ 50 and reaches \~ 40
km in Wyoming, north of the Cheyenne belt. A mid-crustal interface is
very prominent within the data and can be thought of as the Conrad
discontinuity. This interface falls at depths of about 25 to 30 km and
is a discontinuity below which velocities increase to about 6.8 km/s. A
high-velocity lowermost crustal layer with a thickness ranging from 5 to
10 km is evident in the Southern Rocky Mountains - Great Plains (SRM-GP)
portion of the model. The velocity of this layer ranges from 7.0 to 7.4
km/s, a value that is consistent with a composition of mafic garnet
granulite. One interpretation of this high-velocity lower crustal layer
is that it originally formed during assembly of the Proterozoic
terranes. Magmatic underplating at 1.4 Ga may have increased the
thickness of this layer beneath the SRM-GP. This is not to say that the
depth to the Moho has not been locally modified during Phanerozoic
events, but that major modification took place during the Precambrian.

Snelson, C. M., T.M. Brocher, K.C. Miller, T.L. Pratt, and A.M. Tréhu,
2005, Seismic amplification within the Seattle basin, Washington:
Insights from SHIPS seismic tomography experiments, *Bull. Seism. Soc.
Am.*, in journal review.

Recent observations indicate that the Seattle sedimentary basin,
underlying Seattle and other urban centers in the Puget Lowland,
Washington, amplifies long period (1 to 5 s) weak ground motions by
factors of 10 or more. We computed east-trending P- and S-wave velocity
models across the Seattle basin from Seismic Hazard Investigations of
Puget Sound (SHIPS) experiments in order to better characterize the
seismic hazard the basin poses. The 3-D tomographic models, which
resolve features to a depth of 10 km, for the first time define the P-
and S-wave velocity structure of the eastern end of the basin. The
basin, which contains sedimentary rocks of Eocene to Holocene age, is
broadly symmetric in east-west section and reaches a maximum thickness
of 6 km along our profile beneath north Seattle. A comparison of our
velocity model with coincident amplification curves for weak ground
motions produced by the 1999 Chi-Chi earthquake suggests that the
distribution of Quaternary deposits, and reduced velocity gradients in
the upper part of the basement east of Seattle, have significance in
forecasting variations in seismic wave amplification across the basin.
Specifically, eastward increases in the amplification of 0.2 to 5 Hz
energy correlate to locally thicker unconsolidated deposits and a change
from Crescent Formation basement to pre-Tertiary Cascadia basement.
Amplification curves at 7 to 9 s periods mirror the E-W symmetry of the
basin. Seismicity within the Seattle basin along the profile lines up
with this inferred basement contact and coincides with proposed
strike-slip faults of Eocene age.

Symons, N., 1998, Seismic velocity structure of the Puget Sound region
from 3-D non-linear tomography, Ph. D. thesis, Univ. Washington, 168 p.

ten Brink, U.S., P.C. Molzer, M.A. Fisher, R. J. Blakely, R.C. Bucknam,
T. Parsons, R.S. Crosson, and K.C. Creager, 2002, Subsurface geometry
and evolution of the Seattle fault zone and the Seattle Basin,
Washington, *Bull. Seism. Soc. Am.*, 92, 1737-1753.

The Seattle fault, a large, seismically active, east--west-striking
fault zone under Seattle, is the best-studied fault within the
tectonically active Puget Lowland in western Washington, yet its
subsurface geometry and evolution are not well constrained. We combine
several analysis and modeling approaches to study the fault geometry and
evolution, including depth-converted, deep-seismic-reflection images,
*P*-wave-velocity field, gravity data, elastic modeling of shoreline
uplift from a late Holocene earthquake, and kinematic fault restoration.
We propose that the Seattle thrust or reverse fault is accompanied by a
shallow, antithetic reverse fault that emerges south of the main fault.
The wedge enclosed by the two faults is subject to an enhanced uplift,
as indicated by the boxcar shape of the shoreline uplift from the last
major earthquake on the fault zone. The Seattle Basin is interpreted as
a flexural basin at the footwall of the Seattle fault zone. Basin
stratigraphy and the regional tectonic history lead us to suggest that
the Seattle fault zone initiated as a reverse fault during the middle
Miocene, concurrently with changes in the regional stress field, to
absorb some of the north--south shortening of the Cascadia forearc.
Kingston Arch, 30 km north of the Seattle fault zone, is interpreted as
a more recent disruption arising within the basin, probably due to the
development of a blind reverse fault.

Trehu, A. M., 2005, Seismic Hazards of Puget Sound (SHIPS):
Collaborative Research with U.S. Geological Survey, Oregon State Un.,
Un. of Texas at El Paso, Un. of British Columbia, Un. of Washington, Un.
of Victoria, Pacific Geoscience Center, NEHPR Contract Report, Project
Number: 1434HQ98GR00041.

The heavily populated Puget Basin is underlain by thick sequences of
Cenozoic sedimentary rocks that amplify and focus seismic energy, thus
increasing ground shaking during an earthquake. During SHIPS (Seismic
Hazards Investigations of Puget Sound), controlled-source seismic data
were acquired in order to model and map areas of expected strong ground
shaking and to better determine the regional velocity structure and
tectonic framework of the Puget Sound region, including the location,
configuration, and seismic properties of faults that cross this urban
region. This project represents the first phase of SHIPS. Data were
acquired during March, 1998, and include marine multichannel seismic
data, expanding spread profiles, and ocean-bottom on onshore/offshore
large-aperture recording of marine airgun shots. Over 30,000 airgun
array shots were recorded on a multichannel streamer, 257 Refteks, and
15 ocean-bottom seismometers. Work at Oregon State University (OSU) has
focused on the structure beneath the Straits of Juan de Fuca and beneath
the core of the Olympic Mountains. This study provides new constraints
on deformation of the subducting and overriding plates. While this
region is west of the zone of greatest earthquake activity,
understanding the deformation here is critical for tying the structure
beneath Puget Sound to that beneath the well-imaged continental margin
and for interpreting the tectonic causes of seismicity further east.

Van Wagoner, T.M., R.S. Crosson, K.C. Creager, G.F. Medema, L.A.
Preston, N.P. Symons, and T.M. Brocher, 2002, Crustal structure and
relocated earthquakes in the Puget Lowland, Washington from high
resolution seismic tomography, *J. Geophys. Re*s., 107(B12), 2381,
doi:10.10129/2001JB000710.

The availability of regional earthquake data from the Pacific Northwest
Seismograph Network (PNSN), together with active source data from the
Seismic Hazards Investigation in Puget Sound (SHIPS) seismic
experiments, has allowed us to construct a new highresolution 3-D, P
wave velocity model of the crust to a depth of about 30 km in the
central Puget Lowland. In our method, earthquake hypocenters and
velocity model are jointly coupled in a fully nonlinear tomographic
inversion. Active source data constrain the upper 10--15 km of the
model, and earthquakes constrain the deepest portion of the model. A
number of sedimentary basins are imaged, including the previously
unrecognized Muckleshoot basin, and the previously incompletely defined
Possession and Sequim basins. Various features of the shallow crust are
imaged in detail and their structural transitions to the mid and lower
crust are revealed. These include the Tacoma basin and fault zone, the
Seattle basin and fault zone, the Seattle and Port Ludlow velocity
highs, the Port Townsend basin, the Kingston Arch, and the Crescent
basement, which is arched beneath the Lowland from its surface exposure
in the eastern Olympics. Strong lateral velocity gradients, consistent
with the existence of previously inferred faults, are observed, bounding
the southern Port Townsend basin, the western edge of the Seattle basin
beneath Dabob Bay, and portions of the Port Ludlow velocity high and the
Tacoma basin. Significant velocity gradients are not observed across the
southern Whidbey Island fault, the Lofall fault, or along most of the
inferred location of the Hood Canal fault. Using improved earthquake
locations resulting from our inversion, we determined focal mechanisms
for a number of the best recorded earthquakes in the data set, revealing
a complex pattern of deformation dominated by general arc-parallel
regional tectonic compression. Most earthquakes occur in the basement
rocks inferred to be the lower Tertiary Crescent formation. The
sedimentary basins and the eastern part of the Olympic subduction
complex are largely devoid of earthquakes. Clear association of
hypocenters and focal mechanisms with previously mapped or proposed
faults is difficult; however, seismicity, structure, and focal
mechanisms associated with the Seattle fault zone suggest a possible
high-angle mode of deformation with the north side up. We suggest that
this deformation may be driven by isostatic readjustment of the Seattle
basin.

Georgia Basin and Vancouver Island

Brocher, T.M., T.L. Pratt, G.D. Spence, M. Riedel, and R.D. Hyndman,
2003, Wide-angle seismic recordings from the 2002 Georgia Basin
Geohazards Initiative, Northwestern Washington and British Columbia,
*U.S. Geological Survey Open-File Report 03-160*, 34 p.
http://geopubs.wr.usgs.gov/open-file/of03-160/

This report describes the acquisition and processing of shallow-crustal
wide-angle seismic-reflection and refraction data obtained during a
collaborative study in the Georgia Strait, western Washington and
southwestern British Columbia. The study, the 2002 Georgia Strait
Geohazards Initiative, was conducted in May 2002 by the Pacific
Geoscience Centre, the U.S. Geological Survey, and the University of
Victoria. The wide-angle recordings were designed to image shallow
crustal faults and Cenozoic sedimentary basins crossing the
International Border in southern Georgia basin and to add to existing
wide-angle recordings there made during the 1998 SHIPS experiment. We
recorded, at wide-angle, 800 km of shallow penetration multichannel
seismic-reflection profiles acquired by the **Canadian Coast Guard Ship
(CCGS) Tully** using an air gun with a volume of 1.967 liters (120 cu.
in.). Prior to this reflection survey, we deployed 48 Refteks onshore to
record the airgun signals at wide offsets. Three components of an
oriented, 4.5 Hz seismometer were digitally recorded at all stations.
Nearly 160,300 individual air gun shots were recorded along 180 short
seismic reflection lines. In this report, we illustrate the wide-angle
profiles acquired using the **CCGS** **Tully**, describe the land
recording of the air gun signals, and summarize the processing of the
land recorder data into common-receiver gathers. We also describe the
format and content of the archival tapes containing the SEGY-formated,
common-receiver gathers for the Reftek data. Data quality is variable
but the experiment provided useful data from 42 of the 48 stations
deployed. Three-fourths of all stations yielded useful first-arrivals to
source-receiver offsets beyond 10 km: the average maximum
source-receiver offset for first arrivals was 17 km. Six stations
yielded no useful data and useful first-arrivals were limited to offsets
less than 10 km at five stations. We separately archived our recordings
of 86 local and regional earthquakes ranging in magnitude from 0.2 to
4.3 and 16 teleseisms ranging in magnitude 5.5 to 6.5.

Dash, R.K., G.D. Spence, M. Riedel, R.D. Hyndman, T.M. Brocher, and T.L.
Pratt, 2003, Crustal velocity structure from first arrival seismic
tomography in the Strait of Georgia, British Columbia and Washington,
***Eos Trans. AGU*,** 84(46), Fall Mtg. Suppl., Abs. S42A-0150.

Seismic refraction/wide-angle reflection data were collected in May 2002
in the Strait of Georgia, southwestern British Columbia and northwestern
Washington. The experiment, part of the Georgia Basin Geohazards
Initiative, provides knowledge about potential geohazards associated
with near-surface crustal faulting in the strait. The last significant
earthquake (M = 4.6 in 1997) within the strait reflects the regional
seismic hazard. The seismic survey was aimed at studying the
three-dimensional (3D) velocity structure of the shallow upper crust
particularly the Cenozoic sedimentary basins, the Outer Island fault and
the Lummi Island fault. The data were acquired using 48 Reftek stations
deployed onshore to record the signals from a 120 cu. in. airgun fired
at a nominal interval of 5 sec or 12 m. Single channel reflection data
were collected simultaneously. The experiment provided useful data from
42 of the 48 stations deployed. The data quality at many receivers was
very good. The average maximum source-receiver offset for first arrivals
was 17 km with a maximum offset of 35 km. Preliminary 2D velocity
modeling indicates three layers in the shallow upper crust down to a
maximum depth of 3 km. The uppermost layer in the strait represents
recent unconsolidated sediments. The second layer consists of basin fill
sediments (velocity 4.1- 4.4 km/s) associated with the Upper Cretaceous
Nanaimo Group. These sediments occur at the surface within the Canadian
Gulf Islands but at a depth of about 700 m within the strait. The
underlying basement layer (velocity 5.3 - 5.7 km/s) at a depth of 1.7 km
possibly represents the volcanic Wrangellia terrane, extending from
Vancouver Island beneath the strait. The wide distribution of receivers
in the islands and shots in the surrounding waters provides excellent
constraints for 3D inversion of first arrival travel times and hence for
a high-resolution 3D image of the shallow structure.

Graindorge, D., G. Spence, P. Charvis, J.Y. Collot, R.D. Hyndman, and
A.M. Tréhu, 2003, Crustal structure beneath Juan de Fuca Strait and
southern Vancouver Island from seismic and gravity analyses, *J.
Geophys. Res*., 108, B10, doi:10.1029/2002JB001823, EPM 5-1 to 5-23.

Wide-angle and vertical incidence seismic data from Seismic Hazards
Investigations in Puget Sound (SHIPS), gravity modeling, and seismicity
are used to derive two dimensional crustal models beneath the Strait of
Juan de Fuca. The Eocene volcanic Crescent-Siletz terrane is
significantly thicker than previously recognized and extends from near
the surface to depths of 22 km or greater. For the northern strait, a
weak midcrustal reflector, dipping east from 12- to 22-km depth, is
inferred from wide-angle reflections. A stronger deeper reflector,
dipping eastward from 23- to 36-km depth, is associated with the top of
''reflector band E,'' a zone of high reflectivity on coincident
Multichannel Seismic (MCS) data, interpreted as a shear zone. A
high-velocity zone (7.60 ± 0.2 km s 1) between these reflectors is
interpreted as a localized slice of mantle accreted with the overlying
Crescent-Siletz terrane. For the southern strait, no deep high-velocity
layer is observed and the E-band reflectivity is weaker than to the
north. A strong deep reflector, interpreted as the oceanic Moho dips
eastward from 35 to 42 km. Seismicity within the subducting slab occurs
mainly above the inferred oceanic Moho. Gravity modeling, constrained by
the wide-angle seismic models and seismicity, is consistent with the
inferred large thickness of Crescent-Siletz and high-density rocks (3030
kg m 3) in the lower crust.

Hayward, N., Nedimovic, M. Cleary, and A.J. Calvert, Identifying faults
and their recent motion in eastern Strait of Juan de Fuca, NEHRP
Contract Report 02HQGR0055,35 pp., 2003.

Multi-channel seismic reflection data, acquired in the SHIPS (Seismic
Hazards Investigation in Puget Sound) survey are used to interpret the
faulting and structure of the eastern Strait of Juan de Fuca. A number
of major fault zones, including the Devils Mountain, a left-lateral
transpressional fault zone, and the Southern Whidbey Island fault zone,
underlie the region of large prehistoric earthquakes. Numerous
Pleistocene glaciations and associated erosion and deposition have
resulted in the surface masking of faults, which are now most easily
investigated using seismic data. In this study, first arrival
tomographic velocity models derived from the seismic data are used to
accurately characterise the shallow P-wave velocity structure across
fault zones and aid in the identification of potentially active faults,
which may pose a significant seismic hazard to local communities.

Seismic reflection data from SHIPS lines JDF-1, JDF-2, JDF-3. JDF-4
JDF-5, JDF-6, PS-2 and SG-1 were reprocessed, using variable shot
spacing geometry, to improve the quality of seismic images and allow for
more detailed interpretation of the near surface. First arrival
tomographic inversion velocities were calculated using an iterative
two - dimensional inversion algorithm based on a finite-difference
solution to the eikonal equation. With far offsets of approximately 2600
m, a high density of subsurface raypaths and a velocity grid spacing of
25 m, a high-resolution estimate of P-wave velocity structure is
calculated for depths in the range of 500-1200 m. These P-wave velocity
models provide information on the variation of physical properties with
depth and across faults, which when overlain directly upon seismic
profiles significantly aid in the interpretation.

Seismic reflection profiles, of the Devils Mountain fault, suggest that
primary pre-Quaternary motion to the east of 122.95° W was transferred
to a large fault scarp identified on profiles south of the mapped E-W
trend on the DMF. This fault scarp appears to be related to the westward
extension on the Utsalady Point fault, which shows a similar, although
slightly smaller scarp west of Whidbey Island. More recent deformation
has been experienced on the eastward continuance of the DMF, which east
of 122.95° W affects a shallow pre-Tertiary basement and thin overburden
far north of the primary fault scarp. However, Quaternary deformation
associated with the primary fault scarp appears to be of as large a
magnitude as faulting on the DMF to the northeast.

Hayward, N., M. Nedimovic, M. Cleary, and A.J. Calvert, Structural
variation along the Devils Mountain fault zone, northwestern Washington,
*Bull. Seism. Soc. Am*., in review.

The eastern Juan de Fuca Strait is currently subject to primarily
northward tectonic stresses resulting from subduction of the Juan de
Fuca plate, and motion of the Oregon block within the North American
plate. Seismic data, acquired during the SHIPS (Seismic Hazards
Investigation in Puget Sound) survey are used, with coincident first
arrival tomographic velocities, to interpret structural variation along
the Devil\'s Mountain fault zone in the eastern Juan de Fuca Strait.

The Primary fault of the Devil\'s Mountain fault zone developed during
south-directed Tertiary thrusting, forming the northern boundary of the
Everett basin. The structural similarity of the Tertiary Primary fault
of the Devil\'s Mountain fault to the west of \~ 122.95° W and the
Utsalady Point fault, suggests that they represent the main thrust of
the Tertiary Devil\'s Mountain fault. The Tertiary Primary fault west of
\~ 122.95° W was divided from faults to the east (Utsalady Point,
Devil\'s Mountain, and another to the south), by a Tertiary aged oblique
NNE trending transfer fault.

Left-lateral transpression controlled the Quaternary evolution of the
Devil\'s Mountain fault zone. Quaternary offsets on the Primary fault
are smaller to the east of \~ 122.95° W, suggesting that stress here was
in part accommodated by the prevalent oblique compressional structures
to the north. Holocene deformation has focussed on the Devil\'s Mountain
fault, and the Utsalady Point and Strawberry Point faults to the east of
\~ 122.8° but has not affected the Utsalady Point fault to the west of
\~ 122.8° W.

Johnson, S.Y., Dadsman, S.V., Mosher, D.C., Blakely, R.J., and Childs,
J.R., 2001, Active tectonics of the Devils Mountain fault and related
structures, northern Puget Lowland and eastern Strait of Juan de Fuca
region, Pacific Northwest, *U.S. Geological Survey Professional Paper
1643* (65 p. text, 2 tables, 38 figs., 2 plates).
<http://pubs.usgs.gov/pp/p1643/> or
http://onlinepubs.er.usgs.gov/djvu/PP/pp_1643.djvu

Information from marine high-resolution and conventional
seismic-reflection surveys, aeromagnetic mapping, coastal exposures of
Pleistocene strata, and lithologic logs of water wells is used to assess
the active tectonics of the northern Puget Lowland and eastern Strait of
Juan de Fuca region of the Pacific Northwest. These data indicate that
the Devils Mountain fault and the newly recognized Strawberry Point and
Utsalady Point faults are active structures and represent potential
earthquake sources.

Lowe, C., S. A. Dehler, and B. C. Zelt (2003). Basin architecture and
density structure beneath the Strait of Georgia, British Columbia, Can.
J. Earth Sci., 40 (7), 965-981.

Georgia Basin is located within one of the most seismically active and
populated areas on Canada's west coast. Over the last decade, geological
investigations have resolved important details concerning the basin's
shallow structure and composition. Yet, until recently, relatively
little was known about deeper portions of the basin. In this study, new
seismic velocity information is employed to develop a 3-dimensional
density model of the basin. Comparison of the calculated gravity
response of this model with the observed gravity field validates the
velocity model at large scales. At smaller scales, several differences
between model and observed gravity fields are recognized. Analysis of
these differences and correlation with independent geoscience data
provide new insights into the structure and composition of the
basin-fill and underlying basement. Specifically, four regions with
thick accumulations of unconsolidated Pleistocene and younger sediments,
which were not resolved in the velocity model, are identified. Their
delineation is particularly important for studies of seismic
ground-motion amplification and offshore aggregate assessment. An
inconsistency between the published geology and the seismic structure
beneath Texada and Lasqueti Islands in the central Strait of Georgia is
investigated; however, the available gravity data cannot preferentially
validate either the geologic interpretation or the seismic model in this
region. We interpret a northwest-trending and relatively linear gradient
extending from Savory Island in the north to Boundary Bay in the south
as the eastern margin of Wrangellia beneath the basin. Finally, we
compare Georgia Basin with the Everett and Seattle basins in the
southern Cascadia fore arc. This comparison indicates that while a
single mechanism may be controlling present-day basin tectonics and
deformation within the fore arc this was not the case for most of the
Mesozoic and Tertiary time periods.

Mosher, D.C., Cassidy, J.F., Lowe, C., Mi, Y., Hyndman, R.D., Rogers,
G.C., and M.A. Fisher (2000). Neotectonics in the Strait of Georgia:
Tentative correlation of seismicity with shallow geologic structure in
southwestern British Columbia, *Geological Survey of Canada, Current
Research 2000-A22*, 1-9.

Multichannel seismic reflection data within the Strait of Georgia
provide the first tentative correlation of geological structure with
recent seismicity. Reflection data show broad folding of sedimentary
rocks in the southernmost part of the strait. To the north are two broad
(5-10 km wide) deformation zones with interpreted normal faults. Both
zones have an associated magnetic anomaly. The northern deformation
zone, 30 km west of Vancouver, correlates with the location of a number
of recent shallow earthquakes. Detailed analyses of an M=4.6 event at
this site (June 1997) describe a shallow (2-4 km depth) reverse thrust
on a northern dipping (\~50°), east-trending fault plane. The fault dip
direction and strike agree with seismic reflection interpretations and
the magnetic anomaly trend. These data are required to provide
fundamental geological information for addressing issues of strain
partitioning in the Cascadia forearc.

Mosher, D.C., and S.Y. Johnson (eds.), Rathwell, G.J., R.B. King, and
S.B. Rhea (compilers), 2000, Neotectonics of the eastern Strait of Juan
de Fuca; a digital geologic and geophysical atlas, *Geological Survey of
Canada Open File Report 3931*.

Ramachandran, K., 2001, Velocity structure of S.W. British Columbia, and
N.W. Washington, from 3-D non-linear seismic tomography, Ph.D. thesis,
Univ. of Victoria, B.C., 198 pp.

This thesis applies three-dimensional (3-D) non-linear seismic
tomography to image crustal/upper mantle structure of S.W. British
Columbia and N.W. Washington. Two tomographic inversions are carried out
including high-resolution imaging of upper crustal structure using
controlled source data, and deeper imaging by simultaneous inversion of
controlled source and earthquake data.

Non-linear first arrival travel-time tomography is applied to controlled
source data from the Seismic Hazards Investigation of Puget Sound
(SHIPS) experiment conducted in 1998. Nearly 175,000 first arrival
travel-times are inverted to obtain a minimum structure upper crustal
velocity model to a depth of 12 km with a cubical cell size of 1 km.
Results from checkerboard tests for this velocity model indicate a
lateral resolution of 20 km and above. The main geological and
structural features in the study area are well defined by this velocity
model. The structural outline of the sedimentary basins in the Straits
of Georgia and Juan de Fuca are distinctly mapped. The Crescent Terrane
is mapped beneath southern Vancouver Island with velocities up to 7 km/s
that correlate well with the presence of gabbro in the subsurface. The
northwest-southeast structural trend observed in the Strait of Georgia
correlates with the observed seismicity. Shallow seismicity observed at
the southern tip of Vancouver Island correlates with the location of the
Leech River Fault.

An earthquake tomography algorithm was developed for joint estimation of
hypocentral and velocity parameters, and tested on a synthetic data set.
Using this algorithm, tomographic inversion was performed simultaneously
on earthquake and controlled source data from southwestern British
Columbia and northwestern Washington. Approximately 15,000 first
arrivals from 1,400 earthquakes and 40,000 first arrivals from the SHIPS
experiment were simultaneously inverted for hypocentral parameters and
velocity structure. Model resolution studies indicate a lateral
resolution of 30 km and above. Upper-crustal earthquakes close to
southern Vancouver Island correlate with the velocity contrasts
associated with the Leech River, Southern Whidbey Island, and
Darrington-Devils Mountain faults. Three mafic to ultramafic high
velocity units are identified at approximately 25 km depth beneath the
Crescent Terrane and above the subducting Juan de Fuca crust. The
continental crust and subducting Juan de Fuca crust and mantle are well
mapped. The transition zone to continental mantle occurs at 35 km depth
beneath the eastern Strait of Georgia. The slab seismicity beneath the
Strait of Georgia at depths *\>* 65 km lies below a low velocity zone
mapped in the mantle wedge at depths of about 45--55 km. This low
velocity zone may be indicative of the presence of fluids released
during the phase change from basalt/gabbro to eclogite in the subducting
slab.

Ramachandran, K., S.E. Dosso, C.A. Zelt, G.D. Spence, R.D. Hyndman, and
T.M. Brocher, 2004, Upper crustal structure of southwestern British
Columbia from the 1998 Seismic Hazards Investigation in Puget Sound, *J.
Geophy. Res*., 109, B9, B09303, doi: 10.1029/2004JB003092.

This paper applies nonlinear three-dimensional travel time tomography to
refraction data recorded during the 1998 Seismic Hazards Investigation
in Puget Sound (SHIPS) to derive the first large-scale, high-resolution
upper crustal velocity model for southwestern British Columbia. A
minimum structure P wave velocity model is constructed using 175,000
first arrival travel times picked from data recorded by 58 temporary
onshore stations. The model details forearc crustal structures related
to terrane accretion and subsequent basin formation to a depth of about
10 km. The Metchosin igneous complex (correlative with the Eocene
Crescent-Siletz Terrane in Washington) is imaged as a laterally
extensive WNW trending high-velocity anomaly underlying southernmost
Vancouver Island and much of the Strait of Juan du Fuca. Northeast of
the Strait of Georgia, the southwesterly dip of the contact between the
Wrangellia terrane rocks of Vancouver Island and the Coast Plutonic
Complex suggests Wrangellia rocks are downfaulted against the plutonic
complex. At the southwestern end of the Strait of Juan de Fuca, the 50
km long WNW trending Clallam basin has a maximum thickness of 5--6 km.
Near the eastern end of the Strait of Juan de Fuca, Port Townsend basin
has an inferred thickness of approximately 4--5 km. The southern end of
the 9 km thick Georgia basin is bounded by a high-velocity basement
ridge. Beneath the Strait of Georgia, clusters of well-located
earthquakes have a prominent NW trend and coincide spatially with rapid
lateral velocity changes. Clusters of microearthquakes there are
associated with the intersection of several east trending structural
highs within this NW trend.

Ramachandran, K., S.E. Dosso, C.A. Zelt, G.D. Spence, R.D. Hyndman, and
T.M. Brocher, 2005, Forearc structure beneath southwestern British
Columbia: A 3-D tomography velocity model, *J. Geophy. Res*., 110,
B02303, doi:10.1029/2004JB003258.

This paper presents a three-dimensional compressional wave velocity
model of the forearc crust and upper mantle and the subducting Juan de
Fuca plate beneath southwestern British Columbia and the adjoining
straits of Georgia and Juan de Fuca. The velocity model was constructed
through joint tomographic inversion of 50,000 first-arrival times from
earthquakes and active seismic sources. Wrangellia rocks of the accreted
Paleozoic and Mesozoic island arc assemblage underlying southern
Vancouver Island in the Cascadia forearc are imaged at some locations
with higher than average lower crustal velocities of 6.5--7.2 km/s,
similar to observations at other island arc terranes. The mafic Eocene
Crescent terrane, thrust landward beneath southern Vancouver Island,
exhibits crustal velocities in the range of 6.0--6.7 km/s and is
inferred to extend to a depth of more than 20 km. The Cenozoic Olympic
Subduction Complex, an accretionary prism thrust beneath the Crescent
terrane in the Olympic Peninsula, is imaged as a low-velocity wedge to
depths of at least 20 km. Three zones with velocities of 7.0--7.5 km/s,
inferred to be mafic and/or ultramafic units, lie above the subducting
Juan de Fuca plate at depths of 25--35 km. The forearc upper mantle
wedge beneath southeastern Vancouver Island and the Strait of Georgia
exhibits low velocities of 7.2--7.5 km/s, inferred to correspond to 20%
serpentinization of mantle peridotites, and consistent with similar
observations in other warm subduction zones. Estimated dip of the Juan
de Fuca plate beneath southern Vancouver Island is 11 , 16 , and 27 at
depths of 30, 40, and 50 km, respectively.

Riedel, M., V. Barrie, P. Hill, T.M. Brocher, T.L. Pratt, and R.D.
Hyndman, 2002, Mapping of near Surface Active Faults in Georgia Strait,
Eos Trans. AGU, 83(47), Fall Meet. Suppl., Abstract S22B-1031, 2002.

The Georgia Basin is an area of strong crustal seismicity but few fault
traces have been identified. The last significant crustal earthquake, M
= 4.6, occurred in 1997 at a depth of 3- 4 km, about 30 km west of
Vancouver. Focal mechanism solutions and joint hypocenter relocation of
the earthquake and aftershocks delineated an east-west striking fault
that dips north at about 53 degrees. High-resolution seismic surveys and
multibeam seafloor swath mapping have delineated an active fault zone
and associated seafloor pockmarks in an adjacent area west of the
epicenter. A high-resolution 3D seismic survey conducted in May 2002
covered the area of the pockmark field to further study the occurrence
and significance of near surface faults close to urban centers such as
Vancouver, Victoria and Nanaimo. One hundred inlines spaced at 50 m was
shot perpendicular to the main chain of pockmarks (oriented NW-SE), with
multichannel and single channel seismic, and high-resolution Huntec
data. The main chain of pockmarks is associated with a Holocene fault,
which pinches out at the seafloor. It appears to have Holocene movement
of up to 40 m based on the displacement of major seismic horizons mapped
across the fault. The pockmarks are underlain by gas-rich sediments that
mask deeper reflectivity especially in the high-frequency Huntec data.
The area of the epicenter itself was covered with several seismic lines
crossing the epicenter location in multiple directions. No clear
evidence for near surface tectonic deformation was found in the
immediate area. Deeper sub-basement structures, imaged by the earlier
conventional multichannel SHIPS project data, were tentatively
correlated to the site of the 1997 earthquake, but these data do not
provide a detailed image of the faults and deformation zones. The marine
seismic data acquisition was complemented by a detailed land-based
seismic study to further investigate the structure of the Georgia Basin.
Fifty Reftek (IRIS-PASSCAL) stations were deployed along the Southern
and Northern Gulf Islands and on the Vancouver mainland side. Several
regional seismic lines shot in the Georgia Strait will be used for a
high-resolution first-arrival time tomography study to delineate the
P-wave velocity structure in the upper few kilometers of sediments of
the Georgia Basin.

Zelt, B.C., R.M. Ellis, C.A. Zelt, R.D. Hyndman, C. Lowe, G.D. Spence,
and M.A. Fisher, 2001, Three-dimensional crustal velocity structure
beneath the Strait of Georgia, British Columbia*, Geophys. J. Int.*,
144, 695-712.

The Strait of Georgia is a topographic depression straddling the
boundary between the Insular and Coast belts in southwestern British
Columbia. Two shallow earthquakes located within the strait (M=4.6 in
1997 and M=5.0 in 1975) and felt throughout the Vancouver area
illustrate the seismic potential of this region. As part of the 1998
Seismic Hazards Investigation of Puget Sound (SHIPS) experiment, seismic
instruments were placed in and around the Strait of Georgia to record
shots from a marine source within the strait. We apply a tomographic
inversion procedure to first-arrival travel-time data to derive a
minimum-structure 3-D P-wave velocity model for the upper crust to about
13 km depth. We also present a 2-D velocity model for a profile
orientated across the Strait of Georgia derived using a
minimum-parameter traveltime inversion approach.

This paper presents the first detailed look at crustal velocity
variations with the major Cretaceous to Cenozoic Georgia Basin, which
underlies the Strait of Georgia. The 3-D velocity model clearly
delineates the structure of the Georgia Bain. Taking the 6 km/s
isovelocity contour to represent the top of the underlying basement, the
basin thickens from between 2 and 4 km in the northwestern half of the
strait to between 8 and 9 km at the southeastern end of the study
region. Basin velocities in the northeastern half are 4.5 to 6 km/s and
primarily represent the Upper Cretaceous Nanaimo Group. Velocities to
the south are lower (3-6 km/s) because of the additional presence of the
overlying Tertiary Huntingdon Formation and more recent sediments,
including glacial and modern Fraser River deposits. In contrast to he
relatively smoothly varying velocity structure of the basin, velocities
of the basement rocks, which comprise primarily Paleozoic to Jurassic
rocks of the Wrangellia Terrane and possibly Jurassic to mid-Cretaceous
granitic rocks of the Coast Belt, show significantly more structure,
probably an indication of the varying basement rock lithologies. The 2-D
velocity model more clearly reveals the velocity layering associated
with the recent sediments, Huntingdon Formation and Nanaimo Group of the
southern Georgia Basin, as well as the underlying basement. We interpret
lateral variations in sub-basin velocities of the 2-D model as a
transition from Wrangellian to Coast Belt basement rocks. The effect of
the narrow, onshore-offshore recording geometry of the seismic
experiment on model resolution was tested to allow a critical assessment
of the validity of the 3-D velocity model. Lateral resolution throughout
the model to a depth of 3-5 km below the top of basement is generally
10-20 km.

Tacoma Fault and Basin

Hiett, B., 2001, Structure of the Tacoma Basin, M.S. thesis, Univ. Texas
El Paso.

Within the highly populated Puget Lowland of western Washington, local
faulting and the subduction of the Juan de Fuca plate pose a seismic
threat that is compounded by the potential for amplification of ground
shaking within basins that underlie the region. Studies of other basins
on the west coast of the United States (e.g., in San Francisco and Los
Angeles) show that they have the propensity to amplify seismic energy up
to 15 times. Although the Pacific Northwest has been relatively aseismic
compared to other subduction zones, studies have found that this region
has M7.0+ earthquake potential on more than one seismogenic structure.
The objective of the Seismic Hazards Investigation in Puget Sound
(SHIPS) in March 1998 was to produce a 3-D velocity and structural model
of the Puget Lowland to help identify and resolve structures not
previously studied. Over 300 data acquisition systems were deployed in
the Puget Lowland and recorded airgun shots every 16 seconds for an
18-day period. This study presents the results of tomographic inversion
of SHIPS data in and around the Tacoma basin. Inversion of over 140,000
arrivals from 61 recorders produces a velocity model that shows that the
Tacoma basin is made up of 3 subbasins that trend northwest-southeast.
Assuming velocities of 5.5 km/s as a proxy for the top of the Crescent
Formation, depths of the subbasins vary from 4.5 to 6 km. Previous
studies had estimated the depth of the Tacoma basin to be 3 to 4.5 km
depth. The Tacoma basin may have formed as the result of relief created
by thrusting up the Seattle uplift. The subbasins may have originated as
pull-apart basins in an Eocene from north-south strike-slip regime.

Johnson, S.Y., R.J. Blakely, W.J. Stephenson, S.V. Dadisman, and M.A.
Fisher, 2004, Active shortening of the Cascadia forearc and implications
for seismic hazards of the Puget Lowland: Tectonics, v. 23, TC1011,
doi:10.1029/2003TC001507, 27 pages.

Margin-parallel shortening of the Cascadia forearc is a consequence of
oblique subduction of the Juan de Fuca plate beneath North America.
Strikeslip, thrust, and oblique crustal faults beneath the densely
populated Puget Lowland accommodate much of this north-south
compression, resulting in large crustal earthquakes. To better
understand this forearc deformation and improve earthquake hazard
assessment, we here use seismic reflection surveys, coastal exposures of
Pleistocene strata, potential-field data, and airborne laser swath
mapping to document and interpret a significant structural boundary near
the City of Tacoma. This boundary is a complex structural zone
characterized by two distinct segments. The northwest trending, eastern
segment, extending from Tacoma to Carr Inlet, is formed by the broad
(11.5 km), southwest dipping (11--20 degree) Rosedale monocline. This
monocline raises Crescent Formation basement about 2.5 km, resulting in
a moderate gravity gradient. We interpret the Rosedale monocline as a
fault-bend fold, forming above a deep thrust fault. Within the Rosedale
monocline, inferred Quaternary strata thin northward and form a growth
triangle that is 4.1 to 6.6 km wide at its base, suggesting 2--3 mm/yr
of slip on the underlying thrust. The western section of the
\>40-km-long, north dipping Tacoma fault, extending from Hood Canal to
Carr Inlet, forms the western segment of the Tacoma basin margin.
Structural relief on this portion of the basin margin may be several
kilometers, resulting in steep gravity and aeromagnetic anomalies.
Quaternary structural relief along the Tacoma fault is as much as
350--400 m, indicating a minimum slip rate of about 0.2 mm/yr. The
inferred eastern section of the Tacoma fault (east of Carr Inlet)
crosses the southern part of the Seattle uplift, has variable geometry
along strike, and diminished structural relief. The Tacoma fault is
regarded as a north dipping backthrust to the Seattle fault, so that
slip on a master thrust fault at depth could result in movement on the
Seattle fault, the Tacoma fault, or both.

Sherrod, B.L., A.R. Nelson, H.M. Kelsey, T.M. Brocher, R.J. Blakely,
C.S. Weaver, N.K. Rountree, S.B. Rhea, and B.S. Jackson, 2003, The
Catfish Lake scarp, Allyn, Washington: Preliminary field data and
implications for earthquake hazards posed by the Tacoma fault, *U.S.
Geol. Surv. Open-File Rept.* 03-455, 11 pp., 1 sheet.
http://pubs.usgs.gov/of/2003/of03-455.

The Tacoma fault bounds gravity and aeromagnetic anomalies for 50 km
across central Puget lowland from Tacoma to western Kitsap County.
Tomography implies at least 6 km of post-Eocene uplift to the north of
the fault relative to basinal sedimentary rocks to the south.

Coastlines north of the Tacoma fault rose about 1100 years ago during a
large earthquake. Abrupt uplift up to several meters caused tidal flats
at Lynch Cove, North Bay, and Burley Lagoon to turn into forested
wetlands and freshwater marshes. South of the fault at Wollochet Bay,
Douglas-fir forests sank into the intertidal zone and changed into
saltmarsh. Liquefaction features found beneath the marsh at Burley
Lagoon point to strong ground shaking at the time of uplift.

Recent lidar maps of the area southwest of Allyn, Washington revealed a
4 km long scarp, or two closely spaced en-echelon scarps, which
correspond closely to the Tacoma fault gravity and aeromagnetic
anomalies. The scarp, named the Catfish Lake scarp, is north-side-up,
trends east-west, and clearly displace striae left by a Vashon-age
glacier. A trench across the scarp exposed evidence for postglacial
folding and reverse slip. No organic material for radiocarbon dating was
recovered from the trench. However, relationships in the trench suggest
that the folding and faulting is postglacial in age.

Sherrod, B.L., T.M. Brocher, C.S. Weaver, R.C. Bucknam, R.J. Blakely,
H.M. Kelsey, A.R. Nelson, and R. Haugerud, 2004, Holocene fault scarps
near Tacoma, Washington, *Geology*, 32, 9-12.

Airborne laser mapping confirms that Holocene active faults traverse the
Puget Sound metropolitan area, northwestern continental United States.
The mapping, which detects forest-floor relief of as little as 15 cm,
reveals scarps along geophysical lineaments that separate areas of
Holocene uplift and subsidence. Along one such line of scarps, we found
that a fault warped the ground surface between A.D. 770 and 1160. This
reverse fault, which projects through Tacoma, Washington, bounds the
southern and western sides of the Seattle uplift. The northern flank of
the Seattle uplift is bounded by a reverse fault beneath Seattle that
broke in A.D. 900--930. Observations of tectonic scarps along the Tacoma
fault demonstrate that active faulting with associated surface rupture
and ground motions pose a significant hazard in the Puget Sound region.