WEBVTT
Kind: captions
Language: en-US

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Hello. This is Sam Johnson
of the USGS Pacific Coastal

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and Marine Science Center.

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I’ll be talking about the northern
San Andreas Fault from a coastal

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and marine perspective.
The San Andreas Fault forms –

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is the master fault in the boundary
between the Pacific Plate and this

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North American microplate that
extends for about 450 kilometers

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from the San Juan Bautista area on
the southeast up to the south flank of

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Cape Mendocino. About 40% of
the fault is offshore in the ocean.

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That occurs in three different areas –
offshore of San Francisco, north of

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Point Reyes here, and between
Point Arena and Point Delgada.

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The entire fault ruptured in
a magnitude 7.9 earthquake in 1906,

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after which there has been continuing
intensive investigations of onshore

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parts of the fault. In contrast,
detailed studies of the offshore areas

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has only occurred in about the
last 20 years offshore of San Francisco

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in the early 2000s. And, in these two
northern areas, in the last decade.

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In my limited time today, I’ll be
talking about these northern two areas.

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I’ll be – I’ll briefly discuss our mapping,
the importance of fault strike

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on geomorphology, the significance
for earthquake hazards and

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hazard assessments, and how
the fault ends to the north.

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All of the data and interpretations
have been published – the map and

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data sets for the California Seafloor
Mapping Program and these two journal

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articles authored by me and
my colleague, Jeff Beeson.

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Just want to review a few
basics of strike-slip faulting.

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Where slip along this structure
is parallel to the plate boundary,

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that should result in
pure strike-slip faulting.

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Where the strike of the fault has
a more westerly trend, that should

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result in oblique contraction or
transpression, higher elevations,

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pop-ups, a combination of
strike-slip and reverse faulting.

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Where the fault trends more northerly,
that should result in transtensional

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deformation and lower elevations,
perhaps basin formation.

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That’s validated on this plot,
which shows geography on the X axis –

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this is San Juan Bautista to the
southeast up to Point Delgada

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on the northwest – against the strike
of the NSAF on the Y axis.

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360 degrees, or pure north,
on the bottom of the plot.

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West-northwest strike of
300 degrees on the top of the plot.

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The curvy line is the fault strike.
It’s measured at 2-kilometer intervals,

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smoothed over 5 intervals
to produce a rolling average.

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The on-land measurements we used
are from the Jennings compilation.

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The offshore measurements
are from our work.

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The green line is the
vector of plate motion.

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So anything above the line
should result in transpression,

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or anything below the line
should be transtensional areas.

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And the offshore areas of the fault
are shown by the blue shading and

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the red parts of the curvy line.
And it’s consistent with the lower

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elevations occurring in transtensional
areas and upland areas in

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transpressional areas.
Also this dip here – this marine area

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coincides with this transtensional
bend that you see right here.

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So we’re going to begin by looking at
the Tomales Point to Salt Point area.

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This rectangle here is expanded
on the right and rotated.

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This plot shows – this map shows our
high-resolution bathymetry in orange.

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Our track lines – these were
collected at 1-kilometer line spacing.

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In most of the area, 500-meter
line spacing inside the water body

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of Bodega Bay.
It’s distinct from the town

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of Bodega Bay, which is up
a little bit farther to the north.

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The map on the right is an
onshore/offshore geology map.

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The blue line is the shoreline.
The pink shows granitic basement

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of the Salinian Block.
The green shows metasedimentary

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basement of the Franciscan terrane.
And that’s separated by the northern

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San Andreas Fault,
which extends through here.

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Okay, this is the
Bodega Bay area.

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Again, the elevated rim on
the western margin of the bay.

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Blue line is the shoreline.

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Three different seismic
reflection profiles.

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These are vertically
exaggerated 12-1/2 to 1.

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Red lines are the faults.
Orange lines – orange and yellow

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are inferred Pleistocene
and Holocene deposits.

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The mapping reveals that
the fault in Bodega Bay

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is a distributed shear zone
with shallow deformed valley fill.

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We recognize two main strands.
Strand 2 – the onshore continuation

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is where fissuring and rupturing were
identified in the Lawson report for the –

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for the 1906 earthquake.
This map – this inset is shown

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in the block over on the
lower right where we show

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significant offset on another
strand of the fault labeled Fault 1.

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Here’s a close-up, and you can see
clearly 75 meters of right-lateral

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offset on the back
edge of this sandbar,

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which extends across
the width of Bodega Bay.

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We interpret the bar as an early
Holocene analog of the modern

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sand spit and shoreface that now
forms the natural boundary of –

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southern boundary of Bodega Harbor.
The blue line shows sea level at minus

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18 meters in here – the depth at which
we think this sandbar probably formed.

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On the seismic profile,
it shows this sandbar –

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the seismic profile here shows
the sand spit and shoreface here.

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Another probable shoreface a little bit
farther to the north, and then it extends

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up to the modern shoreface
coming off of the modern sand spit.

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The depth at which we think this
sand spit formed suggests an age of

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7,500 to 8,700 years before present,
based on matching a sea level curve.

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That slip – amount of slip and that
timing suggests a slip rate of

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8.6 to 10 millimeters per year,
which is significantly less than

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the 24 millimeters a year
from the NSAF based on GPS

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and paleoseismology
in other areas.

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So the slip must be divided
between two different strands.

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[silence]

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This – apart from its local relevance,
this emphasizes the importance

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of fault zone mapping for
paleoseismology work.

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By not mapping an entire fault zone,
one could miss the complete

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event history or part of
the slip rate by working on

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one strand in a
multi-strand zone.

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Let’s talk about late
Pleistocene paleogeography.

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It’s important to realize that,
for about 85% of the last 70,000 years,

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the prominent strike valley of
Tomales Bay is actually

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about 15 kilometers longer.
It extended beyond Bodega Bay –

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the town and
the water body.

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50 kilometers long and 1 to 2
kilometers wide – straight as an arrow.

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It’s actually – and it was non-marine
during this period of lower sea level.

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It’s very similar to what one sees now
in the non-marine valley of the Garcia

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and Gualala Rivers, like in this narrow,
straight-as-an-arrow section

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of the fault trace. And what’s pretty
remarkable is the similarity in fault

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strike between the two different areas –
a clear indication of the importance

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of fault strike on
geomorphology.

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I now want to turn your attention
to these two odd polygons offshore

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of the Russian River here.
Here and here – they’re each

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about 5 to 6 kilometers in area.
This is what they look like.

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Here and here – they consist of chutes
and lobes that are being – appear to be

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being draped now by recent sediment.
They have very low relief, as seen on

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this seismic reflection profile.
And our hypothesis is that these

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represent liquefaction-generated
slope failure associated with

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the 1906 earthquake.
In arriving at this interpretation,

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there’s a very important comparison
with coseismic 1980 slope failure

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offshore of the Klamath River
associated with the magnitude 7.2

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earthquake. The upper four rows in this
table show the similarities between the

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two slope failures in terms of the
width of the zone, the length of runout,

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the water depth,
and sediment thickness.

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The lower rows shown in yellow font
actually point out why slope failures

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on the – offshore of the Russian River
should be much more common.

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Specifically, coarser grain size
suggests an easier flow, steeper slopes,

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proximity to earthquakes of much
greater magnitude, and much –

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in much greater proximity
to the rupturing fault.

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So what this tells me is that these kinds
of slope failures should be associated

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with every large earthquake event
on the northern San Andreas Fault.

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[silence]

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In our paper, we also describe how
onshore relief and uplift rates of marine

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terraces increase to the north from the
Bodega Bay area, where they’re

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about 0.1 millimeters per year,
to about 0.5 millimeters per year,

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and we associate that with
this transtensional bend

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in the nearby offshore
northern San Andreas Fault.

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This is just a couple of Google Earth
images showing the higher relief of

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the terraces in the north at the mouth
of the Russian River compared to

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the lower elevations to the south.
This is the sand spit of Bodega Bay.

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Now we want to jump to the north
and show our – some data from

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the area between Point Arena
and Point Delgada.

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We crossed the fault 120 times
with line spacing of 1 kilometer.

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Here’s a couple of examples of the –
from our beautiful data set showing

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the fault as a
multi-strand feature.

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The area is – the fault is farther
offshore, receives less sediment,

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and therefore multibeam bathymetry
actually shows some exciting intra-fault

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zone geomorphology,
including scarps, shallow depressions,

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these intra-fault zone uplifts.
This is an area where we think

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we’re actually seeing slip transferred
from an eastern strand of the fault

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to a western strand of the fault.
And, in the process, capturing material

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and transferring it from the Pacific
to the North American Plate.

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On this map, it shows a significant
9-degree bend in the northern

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San Andreas Fault that’s
associated with the maximum

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divergence from the plate vector
as much as 27 degrees.

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Also the maximum distance
offshore – 20 kilometers.

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And the minimum elevation
is minus 200 meters below sea level

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for the occurrence
of the fault zone.

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That bend is also associated with
development of an asymmetric

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lazy-Z style sedimentary basin
11 kilometers wide, 37 kilometers long.

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I want to talk – now I want to
talk about how the fault ends.

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We know that it goes onshore here at
Point Delgada in the Shelter Cove area.

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It’s never been conclusively mapped
onshore that led these geologists

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to suggest, very speculatively,
that the fault could actually extend

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offshore just below the –
just offshore of the King Range.

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Our grid of seismic reflections
effectively rules out a fault

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in this location. There’s a couple of
the seismic profiles on which we

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don’t see that kind of a fault.
We now think that the fault actually,

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in a very ad hoc interpretation –
one where we have no data,

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but we think that the fault could
actually occur in the area between

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the shore – the shoreline and the
northern end of our seismic profiles,

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shown in this location here.
Again, it’s ad hoc,

00:14:30.480 --> 00:14:33.896
but it’s supported by
a few lines of evidence.

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There’s significant structural relief
between the top of the basement

00:14:38.720 --> 00:14:43.040
in the King Range in our –
on our offshore seismic profiles.

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We know that the King Range
is actually going up at

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very, very high rates – as much as
5 millimeters per year.

00:14:49.600 --> 00:14:54.160
Whereas, we see – know from our
seismic profiles that the offshore

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is going down, so there’s
a real contrast there.

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The strike of the fault is actually
not anomalous, even though it

00:15:00.720 --> 00:15:05.440
goes through this big bend,
you know, from the maximum

00:15:05.440 --> 00:15:11.496
transtensional spot associated
with the Noyo Basin.

00:15:11.520 --> 00:15:18.240
But this strike was actually similar to
the strike of the northern San Andreas

00:15:18.240 --> 00:15:22.560
Fault and the Santa Cruz Mountains.
And it’s a very viable interpretation

00:15:22.560 --> 00:15:28.936
to extend it to the northwest to continue
into the Mattole Canyon Fault.

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And this just basically shows
the Mattole Canyon Fault as

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a significantly – juxtaposing different
panels on seismic – different panels

00:15:40.560 --> 00:15:44.198
with different properties
on seismic reflection data.

00:15:45.200 --> 00:15:49.229
Okay, so what to take
home from this talk.

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One, that the offshore northern
San Andreas Fault is now

00:15:54.320 --> 00:15:58.000
completely mapped.
Two, that the fault controls coastal

00:15:58.000 --> 00:16:01.976
and marine geomorphology
at regional to local scales.

00:16:02.000 --> 00:16:07.096
Three, that it’s important to understand
and map multi-strand fault zones.

00:16:07.120 --> 00:16:10.640
Four, that massive near-shore
slope failures are probably a

00:16:10.640 --> 00:16:14.456
common and underappreciated
earthquake hazard.

00:16:14.480 --> 00:16:17.360
Thanks very much
for your attention.