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Greetings. My name is Michael Oskin.
And today I’m presenting our work on the active tectonics of Napa Valley.
What’s shown here in the cover slide is a photograph of a lovely vineyard
near St. Helena. And what you’re seeing in the middle ground there
is a step in the landscape, which is likely a fault scarp.
The question motivating our work here is simply,
how does the West Napa Fault end? From that information,
what is the maximum magnitude expectation we might have for
earthquakes in Napa Valley? And what is the slip rate
and earthquake frequency on this fault, for which
we have no geologic information published to date?
What I’m showing here in this slide is the Jennings 2010 active fault map
for the Napa Valley region. And what you see here is that
Quaternary faults – Quaternary active faults are only defined in the south end.
And then there’s a fault cutting Quaternary and late Pliocene
materials to the north. And then nothing at the
very farther northern end of Napa Valley.
Napa River here is shown to illustrate the position of the valley.
Now, Napa Valley has been quite active the last couple of decades,
most recently with the 2014 magnitude 6.0 South Napa earthquake with the
epicenter shown as the star and the red line as the surface rupture.
Can we expect larger events on a longer fault that has
been mapped to date in Napa Valley?
Next I’m going to show you a hillshade image of some Lidar from the northern
part of Napa Valley here just north of the town of St. Helena.
This is a hillshade image of the Lidar point cloud with vegetation
removed from the 2003 Napa River watershed Lidar survey.
The Napa River is this line here on the east side of the valley.
And this is pretty typical for the Napa River.
It is pushed over to the east side of the valley probably because
the sediment flux coming off the Mayacamas Mountains to the west
is greater than the sediment flux off the Vaca Mountains to the east.
You could also see that the mountain front is highly embayed
on the Vaca Mountains side and much more linear to cuspate
on the Mayacamas Mountains side.
Now, in detail, the first thing that strikes me about this image is this
alignment of hills here in the middle of the valley and a very subtle scarp
right where the blue arrow is. And then these hills are cut by a series
of water gaps, probably from Mill Creek.
Next I’m going to show you a scarp in the landscape here,
right along Mill Creek. And then we’re going to spend some
time looking at field photographs, mapping, and geochronology data
from a portion of Bothe-Napa State Park, located here.
And, for reference, St. Helena is just to the south,
and Calistoga is just to the northwest.
This is a photograph of the Napa Valley Grist Mill,
which was built in 1846 right across this fault scarp at the
base of the Mayacamas Mountains here, 3 meters high.
I think this was built here because the step in the landscape provided
additional elevation for the very large water wheel
and flume that was constructed here.
This is a picture of that scarp in Bothe-Napa State Park.
That’s Tim Dawson for scale. Here it’s approximately 4 meters high.
It’s very obscured by the trees, but back behind there, the landscape levels out,
and there’s a bench along the mountain front in the – what turns out to be
the hanging wall of the northern continuation of the West Napa Fault.
This is a geologic map showing Quaternary units of the
Bothe-Napa State Park area. That picture I showed you
was just from right over here.
And what we mapped in this area is a flight of alluvial terraces
sourced from Ritchey Creek that grade into alluvial fans
emptying into the northern part of Napa Valley.
The lower-most terrace in light yellow here is inset by the
current course of Napa Creek. And then there are a series of terraces
in a flight up the east side of the Mayacamas Mountains here.
And these terraces are quite distinct because they contain large clasts of
basaltic andesite that were produced upstream of the field locality.
This is a picture of the Qt2 terrace. Here’s the terrace fill here.
And the orange line denotes the contact between the terrace fill
and the underlying Sonoma volcanics bedrock, which,
in this area, is rhyolite lava flows.
And you can see in this terrace here the boulders, primarily of basaltic
andesite from upstream, that make those terraces so easy to distinguish.
This is one of the better outcrops of the terrace materials, and we worked
here for several days to try to find material for carbon-14 dating,
ultimately yielding four samples, the dates of which are shown over here,
and the youngest to about 14,000 years.
This is a photograph of the T1 terrace where it’s exposed
in the bank of Ritchey Creek. It’s only about a meter thick, cobbly.
It’s resting on the Sonoma volcanics here. And we found two samples for
carbon dating from terrace materials, both around 6,000 years in age.
So our mapping shows that the Qt2 terrace is cut by a branch
of the West Napa Fault. And that fault, as I’ll show you
in a moment, also cuts the younger Qt1 terrace.
However, at this position, the Qt1 terrace is either faulted or
buttressed against the scarp in Qt2. And you can tell from the gradient of
that terrace that it is different than Qt2. So the scarp height is essentially
a minimum. Because we don’t have Qt2 exposed in the footwall.
And the center of the scarp here is 7.6 meters high. And, depending on
where you place the fault, the throw could be lower or higher.
Based on the carbon-14 dates for Qt2, I assign an age of
14,000 years to this terrace. And, with this much uplift,
we get an uplift rate slightly more than a half millimeter per year.
Now, we don’t have good control on the dip of the West Napa Fault,
but there is good evidence from the regional geology and
gravity data that it does dip. So, assuming a dip of 45 degrees,
the slip rate here would be a minimum of 0.8 plus or minus 0.1 millimeters
per year in a reverse sense. And that doesn’t include
any kind of lateral slip.
Walking along strike, where the Qt1 terrace is now
inset along Ritchey Creek, we find that the fault also cuts Qt1.
The scarps are very subtle. In the scale photograph here,
next to Veronica’s feet, you can see a very slight rise in the ground surface
to the right. And that is a scarp. It’s about 30 to 40 centimeters high.
On the right side of the slide here, we took a terrestrial Lidar survey
of this region. And this is showing a sequential series of scarp profiles,
each taken 1 meter apart, with a 5-to-1 vertical exaggeration.
And the gray lines through here highlight the position of two scarps
cutting the Qt1 terrace. And summing these along each of
these profiles, the average is about 0.8 meters, plus or minus
0.3 meters, of vertical. This is indicating at least one event
cutting the Qt1 terrace, and therefore, in the last 6,000 years.
Stepping out, this is a regional geologic map of the active tectonic
framework for Napa Valley. And it’s based on a number of
prior published maps, references shown down here.
The blue line are the recognized active faults from various studies,
going back to Fox et al. 1973 to the Wesling and Hanson report in 2008,
McLaughlin’s mapping in 2004 up here in Knights Valley, and connecting
these areas, the work in this study, showing active faulting between
Calistoga and St. Helena. All these faults cut Sonoma volcanics,
Quaternary gravels, and are bounding or cutting through
the late Quaternary basin fill.
Overall, Napa Valley appears to be an active transpressional basin,
transitioning from strike-slip in the south to transpression and
reverse faulting in the north. And, in this area, you can see
that the faults change strike to a more westerly orientation.
Also, there’s a gravity low defining the greater northern part of the Napa basin.
And that gravity low is centered below the foot of the Mayacamas Mountains,
consistent with overthrusting of this part of the basin.
At right here is a cross-section interpretation along A to A-prime
showing that the Mayacamas Mountains is a faulted anticline
with three thrust branches here cutting the Miocene and Pliocene
Sonoma volcanics. And then the West Napa Fault
bounding the Napa Basin and, as I’ll discuss in a moment,
some evidence for a concealed thrust that may underlie the basin here.
Zooming in, this is the northern part of Napa Valley.
Here is the northern reverse strand of the West Napa Fault, stepping over
to the more strike-slip strands from St. Helena southward.
And then these backthrusts here – Petrified Forest thrust,
Mount St. John thrust, which cut the Sonoma volcanics
and could quite possibly be active as well.
In this region here – this is where that Lidar scene I showed you earlier
in the talk sits. Here’s an interpreted slope map of that same scene
showing the water gaps in that ridge that was interpreted now
as an active fault-related fold. The scarps across Mill Creek.
And, as we look south along the east side of Napa Valley,
there are additional areas of uplifted terraces and even
an anticline at the area of – location of Glass Mountain here.
And steeply dipping to folded outcrops of the Sonoma volcanics.
So we interpret this as an additional component of the active system,
the East Napa Valley active fold belt, which is quite possibly detached on
serpentine here at the base of the Great Valley sequence and then
likely roots into the West Napa Fault underneath the valley.
So these are the conclusions we have so far.
The West Napa Fault does span the entire length of Napa Valley.
With this length, it is capable of producing at least a magnitude 6.7
earthquake, which would produce strong ground shaking
throughout the valley.
We find a reverse slip rate of a minimum of 0.8, plus or minus 0.1,
millimeters per year at Bothe-Napa State Park and evidence for at least
one event in the past 5,000 to 6,000 years,
cutting the lowest-most terrace at that locality.
And lastly, active folding is present along the east side
of Napa Valley in addition to slip on the West Napa Fault.
Thank you.
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