WEBVTT
Kind: captions
Language: en-US

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Hello. I’m Tyler Ladinsky with
the California Geological Survey

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Seismic Hazards Program. And
today I’ll provide a brief discussion

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of the current understanding of activity
on onshore upper-plate faults behind

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the Redwood Curtain in Humboldt
County in southern Cascadia.

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Special thanks to Harvey Kelsey
and Gary Simpson who provided

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critical feedback and
discussions for this talk.

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Okay, so let’s look at the talk outline.
We’ll start by reviewing the geodetic

00:00:24.720 --> 00:00:27.360
budget of the southern Cascadia
subduction zone and Mendocino

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Triple Junction. Then we’ll do
a brief overview and introduction

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to upper-plate faults in the region.
From there, we’ll transition to review

00:00:34.480 --> 00:00:39.176
modeling of the megathrust interface
and discuss the 6.2 Petrolia earthquake.

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Then we’ll spend time discussing two
upper-plate faults – the Mad River

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Fault Zone and the Little Salmon.
And I’ll review and compare the

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paleoseismic records of the Little
Salmon and the megathrust.

00:00:49.280 --> 00:00:53.520
So starting with the geodetics in this
region, looking at the image in the

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center of the screen, the tectonic
map of the Pacific Northwest,

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outlined in red is southern Cascadia
and the Mendocino Triple Junction.

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Now, if we zoom in on that red box,
we see the plate and block model

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from Williams et al.
on the left.

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The non-colored area labeled MDZ
reflects the Mendocino deformation

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zone, which is accommodating
deformation from three primary

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stresses – eastward
subduction of the Gorda Plate,

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west-northwest translation of the
Sierra Nevada Great Valley block,

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and north-northwest translation
of the San Andreas Fault system.

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Eastward conversion
for the Gorda Plate

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is estimated to be about
34 millimeters a year.

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Now, this image on the right is also
from Williams et al. and it shows

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GPS residual velocities with the
effects of subduction removed.

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And at the latitude of Cape Mendocino
or the Mendocino Triple Junction,

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it’s estimated to be around
22 millimeters a year of distributed

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dextral strain across an 80-kilometer-
wide swath noted by that red rectangle.

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At the latitude of Humboldt Bay in the
green rectangle, it’s estimated about

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6 to 10 millimeters a year of San
Andreas Fault parallel compression.

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So overall, we can see that southern
Cascadia is being affected not only

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by subduction but by northward migration
of the Mendocino Triple Junction.

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Okay, so let’s talk brief introduction
and overview to the location and

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orientation of upper-plate
faults in southern Cascadia.

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Note the insert map of the upper plate –
note the insert map in the upper left

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for geographic location. I’m going to
go through these pretty fast

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because there’s a lot of them.
So, in the interest of time,

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from north to south we have
the Grogan Fault, which is there.

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The Big Lagoon/
Bald Mountain Fault there.

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The Mad River Fault zone, which we’ll
talk more about in a minute is there.

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Freshwater Fault.

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Little Salmon Fault and
Goose Lake Fault there,

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which we’ll talk more about as well.
Eaton Roughs Fault.

00:02:51.440 --> 00:02:53.736
Russ Fault.

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Bear River Fault zone.

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Then the
Honeydew Fault zone.

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Now, listed on the right is the
modeled slip rate for each fault

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based off UCERF3.
Notably, the Mad River and

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Little Salmon contribute
6.4 and 4.5 millimeters of slip,

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which is about
86% of the total budget.

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Total modeled slip budget in the area
is about 12-1/2 millimeters a year.

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Okay, so let’s talk a little bit
about the megathrust interface.

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So, on the image on the left,
we have the Gorda Slab interface

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as modeled
by McCrory et al.

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Notice along southern Cascadia,
the slab flattens significantly.

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The cross-section across
the black line is shown there.

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This image is modified from McCrory
et al. to include low-frequency

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earthquakes from Plourde et al.,
as shown by the red circles,

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to depict a location of transition
from seismic to aseismic deformation

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along the
megathrust [inaudible].

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So that inverted brown triangle
represents the deformation front.

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This blue triangle
is the coastline.

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So, overall – and the magenta line, I
should say, is the top of the slab as well.

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So what we can see is that the
seismogenic zone extends about

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140 kilometers inland
from the deformation front.

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The depth of the megathrust
underneath the location

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of these upper-plate faults
is around 20 kilometers.

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So all this to say that the upper-plate
faults have ample possibility to connect

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with the locked zone of the megathrust.
Example of a 45-degree dipping fault

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at the coastline shown there. And,
if we were to go 20 kilometers inland

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from the coast and have a 30-degree
dipping fault, it’s shown there.

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Okay, so let’s talk briefly
about the Petrolia event.

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It seems like it fits so well
into fault connectivity,

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I wanted to throw in a little bit,
but in the interest of time,

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I’m going to kind of just
make a couple observations.

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So the image on the left shows
a foreshock, main shock,

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and aftershock location to
the 6.2 Petrolia earthquake.

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Note the 5.7 foreshock occurred
offshore on the Mendocino fracture

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zone at a depth of around 16 kilometers,
followed very closely by the onshore

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6.2 main shock, which occurred at
a depth of around 27 kilometers.

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So, on the image on the left,
we have the aftershock sequence,

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and we have these blue lines that
show the contour intervals of the slab

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interface from McCrory et al., and we
can see that the slab contour that goes –

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that projects towards these aftershock
sequences is 20 kilometers’ depth.

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Now, most of the aftershocks
were predominantly greater than

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20 kilometers’ depth, which is
consistent with that a lot of these

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aftershock sequences were
confined to the lower plate.

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Now, what’s really interesting is that,
while the aftershock pattern seems to

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be located in the lower plate,
it roughly aligns with the

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west-northwest-trending Bear
River Fault Zone in the upper plate,

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which indicates that these plates
may be interacting with each other.

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Focal mechanism in the
main shock was consistent

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with west-northwest
dextral motion as well.

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Okay, so let’s move on and talk about
some of these upper-plate faults.

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So the Mad River
Fault Zone located there.

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I’m going to zoom up
on the Mad River Fault Zone.

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So the Mad River Fault Zone is a
southwest-verging fold and thrust belt

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composed of five principal imbricate
thrust faults – the Trinidad, Blue Lake,

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McKinleyville,
Mad River, and Fickle Hill.

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So, from north to south, Trinidad,
Blue Lake’s down here,

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McKinleyville, the Mad River
strand proper, and then Fickle Hill.

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And you can see that there’s numerous
of other strands that aren’t named.

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And then you can see the image
on the – on the bottom here shows

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basically the geometry of this –
the Mad River Fault Zone,

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which is approximately a cross-section
through this white line here.

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Minimum shortening across the Mad
River is about 3-1/2 kilometers a year.

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It’s around 40 kilometers long.
There’s an area of well-preserved

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scarps, particularly out on the marine
terrace here, forming these late

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Pleistocene marine terraces with
scarps greater than 10 meters high.

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So very well-preserved scarps.
The recurrence interval is considered

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to be greater than or equal to
about 5,000 years.

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So clearly the recurrence interval
appears to be an order of magnitude

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greater than the megathrust.
Also paleoseismic data is

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limited around here.
We have evidence on three strands

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in the Mad River Fault Zone,
which is the Blue Lake –

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or, I’ll start here actually –
the McKinleyville in the north;

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the Mad River, which is
also known as the School site;

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and then the
Blue Lake here.

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So we’re going to focus a little bit
on the School Road site here.

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So there’s some conflicting
paleoseismic data.

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Carver and Burke noted multiple
earthquakes, about six events during the

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Late Pleistocene and early Holocene
based off of event-derived colluvium.

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Here’s their trench log on the right.
A couple things to note.

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The thick black line represents the 
abrasion platform that’s been overturned.

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And then they have a sequence of six
stacked colluvial wedges here as well.

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Now, there was sparse radiocarbon
data collected, and it overall

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provides pretty broad constraints –
timing of deformation.

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And then subsequent trenching
adjacent to School Road by

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Gary Simpson was not able to
corroborate these number of events.

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And there’s his – there’s his
interpreted trench log on the left.

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He was able to identify two
paleoearthquakes, two colluvial

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wedges – or, event-derived colluvium.
And he also identified other colluvial

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packages that were interpreted
as interseismic deposition.

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So many questions remain
about the Mad River Fault Zone.

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I would say there’s more we don’t
know about it than maybe we do.

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It appears southwest-stepping.
What traces are active?

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What traces are inactive?

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Is a rupture out of sequence?
There’s a lot of strands we really

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don’t have any information on.
And, for the major strands, I would say

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the mid- to late Holocene slip rate
recurrence interval and

00:09:09.360 --> 00:09:12.970
earthquake chronology
remains unknown.

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So let’s move on, talk about
the Little Salmon Fault.

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There’s the Little Salmon.
Zooming in on the blue triangle now.

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So the Little Salmon Fault is thought
to be the primary upper-plate fault

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in the region with a slip rate of
about 3 to 12 millimeters a year.

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Now, in this image on the left,
we can see that it’s basically –

00:09:35.440 --> 00:09:40.320
it’s outlined here in red, first off.
And you can see that it’s been

00:09:40.320 --> 00:09:43.440
broadly categorized by its orientation
with the three different segments.

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So the northwestern segment
trends north-northwest.

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The central segment has a little
more westerly component to it.

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And then the southeastern segment
trends really west-northwest.

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The green circles here represent
paleoseismic studies that have been

00:09:58.400 --> 00:10:02.960
completed on paleoseismic data points
that we have on the Little Salmon Fault.

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And you can see that they’re generally
confined to the northwest segment.

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In the northwest segment,
they were able to conclude

00:10:10.240 --> 00:10:13.496
there’s been about three earthquakes
in the last 1,700 years.

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So recent work that I was involved with
is down here in the central segment.

00:10:17.440 --> 00:10:20.240
And we were trying to see if we
can also corroborate the number

00:10:20.240 --> 00:10:23.416
of events that they were seeing
in the northwestern segment.

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A couple other facts about the
Little Salmon, then we’ll get into

00:10:25.920 --> 00:10:28.616
the Quail site –
Quail trench site in a minute.

00:10:28.640 --> 00:10:32.000
40 kilometers long with about
4 kilometers of shortening across it.

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And I wanted to note that the
eastern segment where there’s been

00:10:34.880 --> 00:10:39.520
no work done, the relatively recent
Lidar does provide evidence for robust

00:10:39.520 --> 00:10:43.430
scraps in the very steep,
rugged terrain, as shown here.

00:10:43.735 --> 00:10:47.255
So it suggests that it
continues to be very active

00:10:47.280 --> 00:10:50.456
as it comes through the
southeastern segment.

00:10:50.480 --> 00:10:58.160
Okay. So the Quail trench – Quail site.
So this was the most recent work

00:10:58.160 --> 00:11:01.360
done on the Little Salmon.
It was a project I was involved with.

00:11:01.360 --> 00:11:03.680
And we referred to it
as the Quail site, as I mentioned.

00:11:03.680 --> 00:11:07.360
So this image here shows the site map,
and then this lower image on the right

00:11:07.360 --> 00:11:10.720
is our interpreted trench log.
And just briefly, you know, a series

00:11:10.720 --> 00:11:15.360
of shallow, low-angle thrust faults.
Strong evidence for the most recent

00:11:15.360 --> 00:11:18.776
earthquake and moderate evidence
for two additional earthquakes.

00:11:18.800 --> 00:11:22.320
Overall sediments in the hanging
wall were folded around the fault tip

00:11:22.320 --> 00:11:24.136
and thrust over
the footwall.

00:11:24.160 --> 00:11:28.880
And going on to our earthquake
chronology, our preferred OxCal model

00:11:28.880 --> 00:11:33.360
for the MRE, or most recent earthquake,
based off of C-14 and OSL,

00:11:33.360 --> 00:11:39.040
constrains it be between 950 and 1540 BP.
So, it appears the central segment

00:11:39.040 --> 00:11:40.800
of the Little Salmon has
less-frequent earthquakes

00:11:40.800 --> 00:11:44.320
than the northwest segment.
It’s on a couple caveats, which is,

00:11:44.320 --> 00:11:48.000
the single trench, stratigraphic
resolution of the trench was moderate,

00:11:48.000 --> 00:11:51.600
and there was bioturbation –
we did struggle with bioturbation

00:11:51.600 --> 00:11:54.230
in the upper
part of the trench.

00:11:55.200 --> 00:11:58.960
So now let’s kind of bring this
all together in the Little Salmon

00:11:58.960 --> 00:12:05.016
and megathrust. So this image on
the right basically has, in blue here,

00:12:05.040 --> 00:12:08.640
the earthquake chronology of
the megathrust at the latitude

00:12:08.640 --> 00:12:15.280
of Humboldt Bay. This work was done
by Jay Padgett, and it’s shown here.

00:12:15.280 --> 00:12:18.800
We have
Event A, C, D, and E.

00:12:18.800 --> 00:12:20.960
And I should say that his
work was completed using

00:12:20.960 --> 00:12:25.400
subsidence stratigraphy and
high-resolution C-14 dating.

00:12:27.040 --> 00:12:32.635
Also I want to say that Event A is
correlated to the 1700 A.D. event.

00:12:33.440 --> 00:12:36.160
So these next columns here, we have
the Swiss Hall site, which is on the

00:12:36.160 --> 00:12:40.000
northwest segment; the Little Salmon
Valley, which is also on the northwest

00:12:40.000 --> 00:12:43.040
segment; and then the site we were
just discussing about the Quail site.

00:12:43.040 --> 00:12:46.000
Also included the turbidite data
from Goldfinger et al. as another

00:12:46.000 --> 00:12:49.440
point of comparison.
So starting out with the Quail site

00:12:49.440 --> 00:12:52.240
on the central segment of the
Little Salmon, our most recent

00:12:52.240 --> 00:12:58.216
earthquake could correlate with
Event D or Event E on the megathrust.

00:12:58.240 --> 00:13:04.320
However, we did not find evidence
for the 1700 A.D., or Event A, event.

00:13:04.320 --> 00:13:10.080
Now, looking at the Swiss Hall site by
Witter et al., the MRE they identified

00:13:10.080 --> 00:13:15.120
does correlate with the 1700 A.D.
Similarly, work done by Carver and

00:13:15.120 --> 00:13:20.320
Burke was also able to corroborate or
correlate with the 1700 A.D. event.

00:13:20.320 --> 00:13:23.440
Another important point is that both the
Swiss Hall and Little Salmon Valley

00:13:23.440 --> 00:13:29.983
sites were able to identify about three
earthquakes within the last 1,700 years.

00:13:30.160 --> 00:13:36.160
So it does seem that the paleoseismic
record allows the possibility for

00:13:36.160 --> 00:13:41.120
synchronous rupture between the
Little Salmon Fault and the megathrust,

00:13:41.120 --> 00:13:44.698
at least looking at the
northwestern segment.

00:13:46.720 --> 00:13:50.480
A couple quick things just to note.
Most of these studies relied

00:13:50.480 --> 00:13:54.240
on radiometric analysis.
And, while there’s no problem

00:13:54.240 --> 00:13:58.720
with that, there are some limitations
with precision and inherited ages

00:13:58.720 --> 00:14:02.080
with C-14 in some of
these paleoseismic sites.

00:14:02.080 --> 00:14:05.600
And, when you combine that with
the short recurrence interval of the

00:14:05.600 --> 00:14:09.760
megathrust, which down here can be
as short as 200 years, maybe 500 years,

00:14:09.760 --> 00:14:12.880
it really makes it challenging to get
unequivocal evidence of synchronous

00:14:12.880 --> 00:14:17.089
rupture through the earthquake
chronology record.

00:14:19.040 --> 00:14:22.880
Other thing to – important to note,
the paleoseismic data doesn’t require

00:14:22.880 --> 00:14:26.136
the Little Salmon to always
rupture with the megathrust.

00:14:26.160 --> 00:14:29.968
So it could also seem to
rupture independently.

00:14:31.440 --> 00:14:34.800
Okay, so wrapping up.
Numerous upper-plate faults

00:14:34.800 --> 00:14:37.600
with potential for Holocene activity.
The majority of them are poorly

00:14:37.600 --> 00:14:41.120
characterized. The slab depth
model indicate these faults have

00:14:41.120 --> 00:14:46.136
the potential to connect into the
locked zone of the megathrust.

00:14:46.160 --> 00:14:49.280
We have historic examples of splay
faulting and complex interactions

00:14:49.280 --> 00:14:54.560
with the megathrust upper-plate faults.
1964 Anchorage 9.2 earthquake

00:14:54.560 --> 00:14:57.680
with the Patton Bay Splay Fault
is well-documented.

00:14:57.680 --> 00:15:01.840
And the 2016 Kaikoura event also
shows a lot of similarities between

00:15:01.840 --> 00:15:06.080
southern Cascadia subduction zone.
We have the paleoseismic record

00:15:06.080 --> 00:15:09.976
of the northwestern segment of
Little Salmon that suggests there’s

00:15:10.000 --> 00:15:15.496
potential correlation between the
megathrust and the Little Salmon.

00:15:15.520 --> 00:15:20.640
So overall, evidence suggests
connectivity and complex interactions

00:15:20.640 --> 00:15:25.496
between lower-plate, megathrust,
and upper-plate faults are plausible.

00:15:25.520 --> 00:15:28.880
I would strongly suggest that more
field data is needed to evaluate and

00:15:28.880 --> 00:15:34.160
characterize faults and deformation
in this region, particularly the Lidar

00:15:34.160 --> 00:15:39.360
currently ends basically at Fortuna,
and it was found that the Lidar has

00:15:39.360 --> 00:15:44.480
been helpful to identify numerous faults
that were unmapped and to also try and

00:15:44.480 --> 00:15:49.176
get a better sense of characterization
of deformation in the region.

00:15:49.200 --> 00:15:50.640
Thanks a lot.