WEBVTT
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Language: en-US

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[silence]

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Greetings. My name
is Michael Oskin.

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And today I’m presenting our work on
the active tectonics of Napa Valley.

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What’s shown here in the cover slide
is a photograph of a lovely vineyard

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near St. Helena. And what you’re
seeing in the middle ground there

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is a step in the landscape,
which is likely a fault scarp.

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The question motivating
our work here is simply,

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how does the West Napa Fault end?
From that information,

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what is the maximum magnitude
expectation we might have for

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earthquakes in Napa Valley?
And what is the slip rate

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and earthquake frequency
on this fault, for which

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we have no geologic
information published to date?

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What I’m showing here in this slide
is the Jennings 2010 active fault map

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for the Napa Valley region.
And what you see here is that

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Quaternary faults – Quaternary active
faults are only defined in the south end.

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And then there’s a fault cutting
Quaternary and late Pliocene

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materials to the north.
And then nothing at the

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very farther northern
end of Napa Valley.

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Napa River here is shown to
illustrate the position of the valley.

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Now, Napa Valley has been quite
active the last couple of decades,

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most recently with the 2014 magnitude
6.0 South Napa earthquake with the

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epicenter shown as the star and
the red line as the surface rupture.

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Can we expect larger events
on a longer fault that has

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been mapped to date 
in Napa Valley?

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Next I’m going to show you a hillshade
image of some Lidar from the northern

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part of Napa Valley here just north
of the town of St. Helena.

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This is a hillshade image of the
Lidar point cloud with vegetation

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removed from the 2003 Napa
River watershed Lidar survey.

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The Napa River is this line here
on the east side of the valley.

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And this is pretty typical
for the Napa River.

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It is pushed over to the east side
of the valley probably because

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the sediment flux coming off the
Mayacamas Mountains to the west

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is greater than the sediment flux
off the Vaca Mountains to the east.

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You could also see that the
mountain front is highly embayed

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on the Vaca Mountains side
and much more linear to cuspate

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on the Mayacamas
Mountains side.

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Now, in detail, the first thing that
strikes me about this image is this

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alignment of hills here in the middle
of the valley and a very subtle scarp

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right where the blue arrow is.
And then these hills are cut by a series

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of water gaps, probably
from Mill Creek.

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Next I’m going to show you
a scarp in the landscape here,

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right along Mill Creek.
And then we’re going to spend some

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time looking at field photographs,
mapping, and geochronology data

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from a portion of Bothe-Napa
State Park, located here.

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And, for reference,
St. Helena is just to the south,

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and Calistoga is
just to the northwest.

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This is a photograph of the
Napa Valley Grist Mill,

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which was built in 1846 right
across this fault scarp at the

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base of the Mayacamas
Mountains here, 3 meters high.

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I think this was built here because
the step in the landscape provided

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additional elevation for
the very large water wheel

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and flume that was
constructed here.

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This is a picture of that scarp
in Bothe-Napa State Park.

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That’s Tim Dawson for scale.
Here it’s approximately 4 meters high.

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It’s very obscured by the trees, but back
behind there, the landscape levels out,

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and there’s a bench along the mountain
front in the – what turns out to be

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the hanging wall of the northern
continuation of the West Napa Fault.

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This is a geologic map showing
Quaternary units of the

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Bothe-Napa State Park area.
That picture I showed you

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was just from
right over here.

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And what we mapped in this area
is a flight of alluvial terraces

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sourced from Ritchey Creek
that grade into alluvial fans

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emptying into the northern
part of Napa Valley.

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The lower-most terrace in light
yellow here is inset by the

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current course of Napa Creek.
And then there are a series of terraces

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in a flight up the east side of the
Mayacamas Mountains here.

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And these terraces are quite distinct
because they contain large clasts of

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basaltic andesite that were produced
upstream of the field locality.

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This is a picture of the Qt2 terrace.
Here’s the terrace fill here.

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And the orange line denotes the
contact between the terrace fill

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and the underlying Sonoma
volcanics bedrock, which,

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in this area,
is rhyolite lava flows.

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And you can see in this terrace here
the boulders, primarily of basaltic

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andesite from upstream, that make
those terraces so easy to distinguish.

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This is one of the better outcrops of
the terrace materials, and we worked

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here for several days to try to find
material for carbon-14 dating,

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ultimately yielding four samples,
the dates of which are shown over here,

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and the youngest
to about 14,000 years.

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This is a photograph of the
T1 terrace where it’s exposed

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in the bank of Ritchey Creek.
It’s only about a meter thick, cobbly.

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It’s resting on the Sonoma volcanics
here. And we found two samples for

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carbon dating from terrace materials,
both around 6,000 years in age.

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So our mapping shows that the
Qt2 terrace is cut by a branch

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of the West Napa Fault.
And that fault, as I’ll show you

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in a moment, also cuts
the younger Qt1 terrace.

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However, at this position,
the Qt1 terrace is either faulted or

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buttressed against the scarp in Qt2.
And you can tell from the gradient of

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that terrace that it is different than Qt2.
So the scarp height is essentially

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a minimum. Because we don’t
have Qt2 exposed in the footwall.

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And the center of the scarp here is
7.6 meters high. And, depending on

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where you place the fault,
the throw could be lower or higher.

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Based on the carbon-14 dates
for Qt2, I assign an age of

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14,000 years to this terrace.
And, with this much uplift,

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we get an uplift rate slightly more
than a half millimeter per year.

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Now, we don’t have good control
on the dip of the West Napa Fault,

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but there is good evidence from
the regional geology and

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gravity data that it does dip.
So, assuming a dip of 45 degrees,

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the slip rate here would be a minimum
of 0.8 plus or minus 0.1 millimeters

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per year in a reverse sense.
And that doesn’t include

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any kind of lateral slip.

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Walking along strike,
where the Qt1 terrace is now

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inset along Ritchey Creek,
we find that the fault also cuts Qt1.

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The scarps are very subtle.
In the scale photograph here,

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next to Veronica’s feet, you can see
a very slight rise in the ground surface

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to the right. And that is a scarp.
It’s about 30 to 40 centimeters high.

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On the right side of the slide here,
we took a terrestrial Lidar survey

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of this region. And this is showing
a sequential series of scarp profiles,

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each taken 1 meter apart,
with a 5-to-1 vertical exaggeration.

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And the gray lines through here
highlight the position of two scarps

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cutting the Qt1 terrace.
And summing these along each of

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these profiles, the average is
about 0.8 meters, plus or minus

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0.3 meters, of vertical.
This is indicating at least one event

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cutting the Qt1 terrace, and
therefore, in the last 6,000 years.

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Stepping out, this is a regional
geologic map of the active tectonic

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framework for Napa Valley.
And it’s based on a number of

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prior published maps,
references shown down here.

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The blue line are the recognized
active faults from various studies,

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going back to Fox et al. 1973 to the
Wesling and Hanson report in 2008,

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McLaughlin’s mapping in 2004 up
here in Knights Valley, and connecting

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these areas, the work in this study,
showing active faulting between

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Calistoga and St. Helena.
All these faults cut Sonoma volcanics,

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Quaternary gravels, and are
bounding or cutting through

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the late Quaternary basin fill.

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Overall, Napa Valley appears to
be an active transpressional basin,

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transitioning from strike-slip in
the south to transpression and

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reverse faulting in the north.
And, in this area, you can see

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that the faults change strike
to a more westerly orientation.

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Also, there’s a gravity low defining the
greater northern part of the Napa basin.

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And that gravity low is centered below
the foot of the Mayacamas Mountains,

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consistent with overthrusting
of this part of the basin.

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At right here is a cross-section
interpretation along A to A-prime

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showing that the Mayacamas
Mountains is a faulted anticline

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with three thrust branches here
cutting the Miocene and Pliocene

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Sonoma volcanics.
And then the West Napa Fault

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bounding the Napa Basin and,
as I’ll discuss in a moment,

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some evidence for a concealed thrust
that may underlie the basin here.

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Zooming in, this is the
northern part of Napa Valley.

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Here is the northern reverse strand
of the West Napa Fault, stepping over

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to the more strike-slip strands
from St. Helena southward.

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And then these backthrusts here –
Petrified Forest thrust,

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Mount St. John thrust,
which cut the Sonoma volcanics

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and could quite possibly
be active as well.

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In this region here – this is where
that Lidar scene I showed you earlier

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in the talk sits. Here’s an interpreted
slope map of that same scene

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showing the water gaps in that
ridge that was interpreted now

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as an active fault-related fold.
The scarps across Mill Creek.

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And, as we look south along
the east side of Napa Valley,

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there are additional areas of
uplifted terraces and even

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an anticline at the area of –
location of Glass Mountain here.

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And steeply dipping to folded
outcrops of the Sonoma volcanics.

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So we interpret this as an additional
component of the active system,

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the East Napa Valley active fold belt,
which is quite possibly detached on

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serpentine here at the base of the
Great Valley sequence and then

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likely roots into the West Napa
Fault underneath the valley.

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So these are the conclusions
we have so far.

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The West Napa Fault does span
the entire length of Napa Valley.

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With this length, it is capable of
producing at least a magnitude 6.7

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earthquake, which would
produce strong ground shaking

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throughout the valley.

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We find a reverse slip rate of a
minimum of 0.8, plus or minus 0.1,

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millimeters per year at Bothe-Napa
State Park and evidence for at least

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one event in the past
5,000 to 6,000 years,

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cutting the lowest-most
terrace at that locality.

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And lastly, active folding is
present along the east side

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of Napa Valley in addition to
slip on the West Napa Fault.

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Thank you.

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[silence]