WEBVTT
Kind: captions
Language: en-US

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The work that I’m going to show you
today is part of various collaborative

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efforts among multiple
government agencies,

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academic and private institutions,
and many colleagues across the USGS,

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coastal and marine, and
earthquake hazards programs.

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Today I will briefly summarize
published results of systematic

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margin-wide characterization of
Cascadia morpho-tectonic variability

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and hazard implications that have
recently been published in Geosphere

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with my colleague Danny Brothers.
And then I will present some

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preliminary work offshore northern
California using multi-scale geophysics

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to image and characterize Late
Pleistocene and Holocene deformation

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along both cross-shore faults in the
shelf and along the deformation front.

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Cascadia lacks an instrumental record
of large megathrust earthquakes,

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making estimates of past and future
rupture parameters difficult.

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However, we can learn from other
seismically active subduction zones

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where studies suggest that spatial
variations in frictional properties

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of the megathrust are linked to
tectonic and morphological evolution

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of the upper plate and rupture
behavior along strike.

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We were particularly inspired by the
work in Sumatra where they noted that

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along-strike changes in prism geometry
and vergence coincide with the rupture

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boundaries of the 2004 and 2005
megathrust earthquakes.

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We took a similar approach in Cascadia,
focusing on the outer wedge, located

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offshore between the deformation
front and the outer arc high.

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The reason we focused here is
because this is the portion of

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the marine floor where destructive
tsunamis are generated.

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Statistical analysis of outer wedge
width and steepness reveal four

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distinct regions along the Cascadia
margin, denoted by the thick

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dashed black lines
in the map to the right.

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Each of these regions have unique
patterns of outer wedge geometry,

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structural vergence,
as denoted in the middle panel,

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and mean profile shape,
as denoted in the far right panel.

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I want to highlight the distinct end
members of this analysis here as

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Regions 2 offshore Washington,
where we have a large area of landward

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vergence, denoted in the red, as well as
an area of extension at the shelf break.

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And also Region 4 offshore southern
Oregon and northern California,

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where we have numerous
cross-shore faults

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and a very steep and
narrow outer wedge.

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According to critical tapered theory,
variations in the geometry and

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shape of the wedge reflect
changes in the strength

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of the wedge and the strength
of the décollement.

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Since we can’t get at the strength
of the décollement with a megathrust

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[inaudible] using existing data,
we decided to map out first-order

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material strength boundaries within
the wedge in the form of backstops,

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denoted on the right
in the map with the

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bright green lines and
the bright red line.

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The arrows indicate the dip
direction of those boundaries.

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Notice that each morpho-tectonic
region has a unique backstop

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configuration in terms of distance from
the trench, lithology, and geometry.

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These backstop boundaries are often
associated with megasplay faults on

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other margins, such as that activated in
the 1964 Alaska megathrust earthquake.

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We postulate in this manuscript that the
morpho-tectonic variability that we see

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in Cascadia reflects both geometric
and rheological heterogeneity

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within the megathrust system
that may control megathrust

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behavior by modulating pore
pressure and effective stress.

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The backstop characterization here
provides first-order estimation of

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strength contrasts within the wedge
and highlights potential megasplay

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fault structures that we know
can amplify tsunami waves

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and decrease travel
time to the coast.

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In addition, this along-strike variability
suggests that the earthquake and

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tsunami source characteristics likely
vary along the margin and need to

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be assessed on a regional basis.
For example, if you look at our end

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members again, perhaps offshore
Washington we might expect a scenario

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similar to Tohoku, where you have
extension in the upper plate and then

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slip to the trench further out in
the prism. While offshore northern

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California, we might expect a scenario
similar to that in Kaikoura, where we

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have complex rupture of
upper plate and the megathrust.

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This margin-wide characterization
provides the framework for more

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detailed work offshore northern
California that I will talk about now,

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focusing on preliminary results from
geophysical imaging to characterize

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Late Pleistocene and
Holocene deformation.

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So here is a 3D perspective view
of southern Cascadia looking north

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from the Mendocino Triple Junction.
The bathymetry is shown

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in a rainbow of colors. Along this
part of the margin, we have complex

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tectonic interaction involving east-west
compression along the deformation

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front, north-south compression related
to the northward-migrating triple

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junction, and northwest-southeast-
directed transpression associated with

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a rotating forearc interacting
with the northern extent of

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transform motion along the
San Andreas Fault system.

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As a reminder, this is Region 4, one
of the end members in our morpho-

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tectonic analysis, that is characterized
by very a steep and narrow outer wedge.

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This outer wedge is separated from
the inner wedge by the outer arc highs

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denoted with the white line.
The backstops, denoted in black,

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and the outer arc high within the wedge
bend eastward as they approach the

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Mendocino Triple Junction and
come onshore in the Humboldt area.

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In order to better characterize the
Late Pleistocene/Holocene deformation

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in this region, we have collected new
high-resolution seismic reflection data

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shown in the black lines, sediment
cores shown in the blue circles,

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and we’ve installed two seafloor
GPS-A stations straddling

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the deformation front
offshore Crescent City,

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shown by the orange circles.
Today I will highlight preliminary

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work along both the shelf and
the deformation front.

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So here is a map view perspective
of that same area that we saw

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in the last slide.
This is showing a preliminary map

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of Quaternary faults and folds,
denoted in black.

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The mapping is based on integration
of new high-resolution multi-channel

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sparker and CHIRP imaging,
combined with legacy crustal-scale

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data and a new 30-meter bathymetric
DEM as a base map.

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It is now published and publicly
available on ScienceBase.

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On the next slide,
I will focus in on the

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cross-shore faults along the shelf
in the area of the black box.

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Here I’m showing a Google Earth
image in the background looking

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to the west and offshore where our
sparker seismic data has beautifully

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captured Neogene and younger strata
to sub-seafloor depths of greater than

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500 meters that are locally folded
and faulted along a series of

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cross-shore fault zones. I’ve highlighted
two sub-seafloor horizons here.

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The blue marking the 500,000-year-old
Hookton nonconformity and the

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orange marking the transgressive
surface that records erosion and

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sea level rise following the
last glacial maximum

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that occurred approximately
21,000 years ago.

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We used straddle geometry on
CHIRP profiles to identify and

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map the regional transgressive
surface using a strain marker

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for characterizing for
offshore fault deformation.

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The stars here denote structures
that are offset and/or deformed

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this transgressive surface.
And I know that surface is

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difficult to see here in this view,
so we’ll zoom in here.

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Here I’m zooming in on the
transgressive surface of erosion on that

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same profile and showing the modern
seafloor in black for context.

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We see two modes of deformation here.
One, long-wavelength features that

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likely reflect active folding.
And punctuated vertical offset ranging

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from 5 to 10 meters coincident
with fault traces of the Table Bluff,

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Mad River, Bald Mountain/Big Lagoon,
and Lost Man Fault zones

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imaged in MCS and
CHIRP seismic data.

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Now, the transgressive surface ages
at depths between 50 and 100 meters

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below sea level along this seismic
profile are estimated to be between

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11,000 and 15,000 years old,
based on sea level curves,

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which result in a maximum vertical
deformation rate along these –

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on these structures between
0.3 and 0.9 millimeters per year.

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It should be noted that these rates are
less than those estimated on land

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and potentially reflect a change
from compressional to transpressional

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tectonics across the shoreline
as these faults either die off

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and/or bend to the north
along the shelf break.

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Now I’m going to take a closer look at
deformation in the outer wedge along

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the deformation front in
the area of the black box.

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Here I am showing a sparker profile
that crosses the deformation front

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along the red line denoted
in the map on the left.

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It crosses the deformation front,
the dynamic backstop boundary,

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and the outer arc high.
Thinning of the most recent sediment

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package onto the anticlines,
denoted by the red arrow,

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suggest active uplift along these
structures and the underlying thrust

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faults, including the most seaward
thrust fault along the deformation front.

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In the next slide, I will zoom into this
portion of the line to take a better look.

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So here, again, we’re looking
at our sparker seismic data

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at the deformation front.
You see the flat-line strata

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above the incoming oceanic plate
on the left and the onset of

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deformation along the
interpreted faults in black.

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The thinning of the most recent
sediment package onto this first

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anticline suggests activity along
the underlying thrust fault.

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Now, we ask, could this represent
geologic evidence potentially

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of trench-breaching rupture?
That would depend on whether the

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deformation we see was the result of
aseismic creep or coseismic movement.

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Wouldn’t it be nice to take a closer
look at this region in much more

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detail and get a better look
at the style of deformation?

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Well, thanks to our colleagues at
MBARI, we were able to take

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a much closer look at this part of the
deformation front at 1 meter –

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using 1-meter resolution
AUV bathymetry

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and CHIRP
sub-bottom imaging.

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So here is a map view of that
part of the deformation front

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in our 30-meter
bathymetric compilation.

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Now, in case you can’t see it,
the deformation front

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is marked by
the blue arrow.

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Here is the same view with
1-meter AUV bathymetry.

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You can now clearly see
the deformation front.

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You may also see the beautiful
landslide head scarps and runout

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along the deformation front –
the giant hill discussed earlier.

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We talk about
their significance.

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But I am more interested in
linear features – these spark-like

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lineations denoted
by the red arrows.

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Here I’m showing a preliminary
geologic interpretation based on

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the AUV imagery and associated
CHIRP sub-bottom imagery.

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The white line in the map
shows the location of the

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CHIRP profile shown below.
What we see here is that those

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lineaments in the bathymetry
reflect scarps that formed by

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both compressional and
extensional structures

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imaged in that
CHIRP imagery.

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And, since I know you can’t really
see anything at this scale, if I zoom in,

00:13:19.200 --> 00:13:22.960
you can clearly see vertical
deformation along thrust faults

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within the first anticline and normal
faults in the second anticline.

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And the apparent constant
throw suggests single-event

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offset of between 2 to 4 meters
occurred in the final anticline.

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So now, zooming back out,
with the sparker data to

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provide context, we suggest that
the combination of thinning

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of recent strata onto the anticline
and brittle faulting within the anticline

00:13:53.360 --> 00:13:59.655
suggest coseismic slip occurred
along the underlying thrust fault,

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supporting the suggestion that slip to
the trench has occurred in this region.

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We look forward to combining
the detailed geologic observations

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in the previous slide with GPS-A
seafloor measurements in the coming

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years in the same location to get
a better, more comprehensive picture

00:14:20.320 --> 00:14:23.760
of shallow fault behavior in
southern Cascadia, which is inferred

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to be locked based on the
latest modeling by Lindsey et al.

00:14:27.280 --> 00:14:30.456
using onshore
GPS data [inaudible].

00:14:30.480 --> 00:14:37.460
So stay tuned for more observations and
more results coming soon. Thank you.