Hello. I’m Tyler Ladinsky with the California Geological Survey
Seismic Hazards Program. And today I’ll provide a brief discussion
of the current understanding of activity on onshore upper-plate faults behind
the Redwood Curtain in Humboldt County in southern Cascadia.
Special thanks to Harvey Kelsey and Gary Simpson who provided
critical feedback and discussions for this talk.
Okay, so let’s look at the talk outline. We’ll start by reviewing the geodetic
budget of the southern Cascadia subduction zone and Mendocino
Triple Junction. Then we’ll do a brief overview and introduction
to upper-plate faults in the region. From there, we’ll transition to review
modeling of the megathrust interface and discuss the 6.2 Petrolia earthquake.
Then we’ll spend time discussing two upper-plate faults – the Mad River
Fault Zone and the Little Salmon. And I’ll review and compare the
paleoseismic records of the Little Salmon and the megathrust.
So starting with the geodetics in this region, looking at the image in the
center of the screen, the tectonic map of the Pacific Northwest,
outlined in red is southern Cascadia and the Mendocino Triple Junction.
Now, if we zoom in on that red box, we see the plate and block model
from Williams et al. on the left.
The non-colored area labeled MDZ reflects the Mendocino deformation
zone, which is accommodating deformation from three primary
stresses – eastward subduction of the Gorda Plate,
west-northwest translation of the Sierra Nevada Great Valley block,
and north-northwest translation of the San Andreas Fault system.
Eastward conversion for the Gorda Plate
is estimated to be about 34 millimeters a year.
Now, this image on the right is also from Williams et al. and it shows
GPS residual velocities with the effects of subduction removed.
And at the latitude of Cape Mendocino or the Mendocino Triple Junction,
it’s estimated to be around 22 millimeters a year of distributed
dextral strain across an 80-kilometer- wide swath noted by that red rectangle.
At the latitude of Humboldt Bay in the green rectangle, it’s estimated about
6 to 10 millimeters a year of San Andreas Fault parallel compression.
So overall, we can see that southern Cascadia is being affected not only
by subduction but by northward migration of the Mendocino Triple Junction.
Okay, so let’s talk brief introduction and overview to the location and
orientation of upper-plate faults in southern Cascadia.
Note the insert map of the upper plate – note the insert map in the upper left
for geographic location. I’m going to go through these pretty fast
because there’s a lot of them. So, in the interest of time,
from north to south we have the Grogan Fault, which is there.
The Big Lagoon/ Bald Mountain Fault there.
The Mad River Fault zone, which we’ll talk more about in a minute is there.
Freshwater Fault.
Little Salmon Fault and Goose Lake Fault there,
which we’ll talk more about as well. Eaton Roughs Fault.
Russ Fault.
Bear River Fault zone.
Then the Honeydew Fault zone.
Now, listed on the right is the modeled slip rate for each fault
based off UCERF3. Notably, the Mad River and
Little Salmon contribute 6.4 and 4.5 millimeters of slip,
which is about 86% of the total budget.
Total modeled slip budget in the area is about 12-1/2 millimeters a year.
Okay, so let’s talk a little bit about the megathrust interface.
So, on the image on the left, we have the Gorda Slab interface
as modeled by McCrory et al.
Notice along southern Cascadia, the slab flattens significantly.
The cross-section across the black line is shown there.
This image is modified from McCrory et al. to include low-frequency
earthquakes from Plourde et al., as shown by the red circles,
to depict a location of transition from seismic to aseismic deformation
along the megathrust [inaudible].
So that inverted brown triangle represents the deformation front.
This blue triangle is the coastline.
So, overall – and the magenta line, I should say, is the top of the slab as well.
So what we can see is that the seismogenic zone extends about
140 kilometers inland from the deformation front.
The depth of the megathrust underneath the location
of these upper-plate faults is around 20 kilometers.
So all this to say that the upper-plate faults have ample possibility to connect
with the locked zone of the megathrust. Example of a 45-degree dipping fault
at the coastline shown there. And, if we were to go 20 kilometers inland
from the coast and have a 30-degree dipping fault, it’s shown there.
Okay, so let’s talk briefly about the Petrolia event.
It seems like it fits so well into fault connectivity,
I wanted to throw in a little bit, but in the interest of time,
I’m going to kind of just make a couple observations.
So the image on the left shows a foreshock, main shock,
and aftershock location to the 6.2 Petrolia earthquake.
Note the 5.7 foreshock occurred offshore on the Mendocino fracture
zone at a depth of around 16 kilometers, followed very closely by the onshore
6.2 main shock, which occurred at a depth of around 27 kilometers.
So, on the image on the left, we have the aftershock sequence,
and we have these blue lines that show the contour intervals of the slab
interface from McCrory et al., and we can see that the slab contour that goes –
that projects towards these aftershock sequences is 20 kilometers’ depth.
Now, most of the aftershocks were predominantly greater than
20 kilometers’ depth, which is consistent with that a lot of these
aftershock sequences were confined to the lower plate.
Now, what’s really interesting is that, while the aftershock pattern seems to
be located in the lower plate, it roughly aligns with the
west-northwest-trending Bear River Fault Zone in the upper plate,
which indicates that these plates may be interacting with each other.
Focal mechanism in the main shock was consistent
with west-northwest dextral motion as well.
Okay, so let’s move on and talk about some of these upper-plate faults.
So the Mad River Fault Zone located there.
I’m going to zoom up on the Mad River Fault Zone.
So the Mad River Fault Zone is a southwest-verging fold and thrust belt
composed of five principal imbricate thrust faults – the Trinidad, Blue Lake,
McKinleyville, Mad River, and Fickle Hill.
So, from north to south, Trinidad, Blue Lake’s down here,
McKinleyville, the Mad River strand proper, and then Fickle Hill.
And you can see that there’s numerous of other strands that aren’t named.
And then you can see the image on the – on the bottom here shows
basically the geometry of this – the Mad River Fault Zone,
which is approximately a cross-section through this white line here.
Minimum shortening across the Mad River is about 3-1/2 kilometers a year.
It’s around 40 kilometers long. There’s an area of well-preserved
scarps, particularly out on the marine terrace here, forming these late
Pleistocene marine terraces with scarps greater than 10 meters high.
So very well-preserved scarps. The recurrence interval is considered
to be greater than or equal to about 5,000 years.
So clearly the recurrence interval appears to be an order of magnitude
greater than the megathrust. Also paleoseismic data is
limited around here. We have evidence on three strands
in the Mad River Fault Zone, which is the Blue Lake –
or, I’ll start here actually – the McKinleyville in the north;
the Mad River, which is also known as the School site;
and then the Blue Lake here.
So we’re going to focus a little bit on the School Road site here.
So there’s some conflicting paleoseismic data.
Carver and Burke noted multiple earthquakes, about six events during the
Late Pleistocene and early Holocene based off of event-derived colluvium.
Here’s their trench log on the right. A couple things to note.
The thick black line represents the abrasion platform that’s been overturned.
And then they have a sequence of six stacked colluvial wedges here as well.
Now, there was sparse radiocarbon data collected, and it overall
provides pretty broad constraints – timing of deformation.
And then subsequent trenching adjacent to School Road by
Gary Simpson was not able to corroborate these number of events.
And there’s his – there’s his interpreted trench log on the left.
He was able to identify two paleoearthquakes, two colluvial
wedges – or, event-derived colluvium. And he also identified other colluvial
packages that were interpreted as interseismic deposition.
So many questions remain about the Mad River Fault Zone.
I would say there’s more we don’t know about it than maybe we do.
It appears southwest-stepping. What traces are active?
What traces are inactive?
Is a rupture out of sequence? There’s a lot of strands we really
don’t have any information on. And, for the major strands, I would say
the mid- to late Holocene slip rate recurrence interval and
earthquake chronology remains unknown.
So let’s move on, talk about the Little Salmon Fault.
There’s the Little Salmon. Zooming in on the blue triangle now.
So the Little Salmon Fault is thought to be the primary upper-plate fault
in the region with a slip rate of about 3 to 12 millimeters a year.
Now, in this image on the left, we can see that it’s basically –
it’s outlined here in red, first off. And you can see that it’s been
broadly categorized by its orientation with the three different segments.
So the northwestern segment trends north-northwest.
The central segment has a little more westerly component to it.
And then the southeastern segment trends really west-northwest.
The green circles here represent paleoseismic studies that have been
completed on paleoseismic data points that we have on the Little Salmon Fault.
And you can see that they’re generally confined to the northwest segment.
In the northwest segment, they were able to conclude
there’s been about three earthquakes in the last 1,700 years.
So recent work that I was involved with is down here in the central segment.
And we were trying to see if we can also corroborate the number
of events that they were seeing in the northwestern segment.
A couple other facts about the Little Salmon, then we’ll get into
the Quail site – Quail trench site in a minute.
40 kilometers long with about 4 kilometers of shortening across it.
And I wanted to note that the eastern segment where there’s been
no work done, the relatively recent Lidar does provide evidence for robust
scraps in the very steep, rugged terrain, as shown here.
So it suggests that it continues to be very active
as it comes through the southeastern segment.
Okay. So the Quail trench – Quail site. So this was the most recent work
done on the Little Salmon. It was a project I was involved with.
And we referred to it as the Quail site, as I mentioned.
So this image here shows the site map, and then this lower image on the right
is our interpreted trench log. And just briefly, you know, a series
of shallow, low-angle thrust faults. Strong evidence for the most recent
earthquake and moderate evidence for two additional earthquakes.
Overall sediments in the hanging wall were folded around the fault tip
and thrust over the footwall.
And going on to our earthquake chronology, our preferred OxCal model
for the MRE, or most recent earthquake, based off of C-14 and OSL,
constrains it be between 950 and 1540 BP. So, it appears the central segment
of the Little Salmon has less-frequent earthquakes
than the northwest segment. It’s on a couple caveats, which is,
the single trench, stratigraphic resolution of the trench was moderate,
and there was bioturbation – we did struggle with bioturbation
in the upper part of the trench.
So now let’s kind of bring this all together in the Little Salmon
and megathrust. So this image on the right basically has, in blue here,
the earthquake chronology of the megathrust at the latitude
of Humboldt Bay. This work was done by Jay Padgett, and it’s shown here.
We have Event A, C, D, and E.
And I should say that his work was completed using
subsidence stratigraphy and high-resolution C-14 dating.
Also I want to say that Event A is correlated to the 1700 A.D. event.
So these next columns here, we have the Swiss Hall site, which is on the
northwest segment; the Little Salmon Valley, which is also on the northwest
segment; and then the site we were just discussing about the Quail site.
Also included the turbidite data from Goldfinger et al. as another
point of comparison. So starting out with the Quail site
on the central segment of the Little Salmon, our most recent
earthquake could correlate with Event D or Event E on the megathrust.
However, we did not find evidence for the 1700 A.D., or Event A, event.
Now, looking at the Swiss Hall site by Witter et al., the MRE they identified
does correlate with the 1700 A.D. Similarly, work done by Carver and
Burke was also able to corroborate or correlate with the 1700 A.D. event.
Another important point is that both the Swiss Hall and Little Salmon Valley
sites were able to identify about three earthquakes within the last 1,700 years.
So it does seem that the paleoseismic record allows the possibility for
synchronous rupture between the Little Salmon Fault and the megathrust,
at least looking at the northwestern segment.
A couple quick things just to note. Most of these studies relied
on radiometric analysis. And, while there’s no problem
with that, there are some limitations with precision and inherited ages
with C-14 in some of these paleoseismic sites.
And, when you combine that with the short recurrence interval of the
megathrust, which down here can be as short as 200 years, maybe 500 years,
it really makes it challenging to get unequivocal evidence of synchronous
rupture through the earthquake chronology record.
Other thing to – important to note, the paleoseismic data doesn’t require
the Little Salmon to always rupture with the megathrust.
So it could also seem to rupture independently.
Okay, so wrapping up. Numerous upper-plate faults
with potential for Holocene activity. The majority of them are poorly
characterized. The slab depth model indicate these faults have
the potential to connect into the locked zone of the megathrust.
We have historic examples of splay faulting and complex interactions
with the megathrust upper-plate faults. 1964 Anchorage 9.2 earthquake
with the Patton Bay Splay Fault is well-documented.
And the 2016 Kaikoura event also shows a lot of similarities between
southern Cascadia subduction zone. We have the paleoseismic record
of the northwestern segment of Little Salmon that suggests there’s
potential correlation between the megathrust and the Little Salmon.
So overall, evidence suggests connectivity and complex interactions
between lower-plate, megathrust, and upper-plate faults are plausible.
I would strongly suggest that more field data is needed to evaluate and
characterize faults and deformation in this region, particularly the Lidar
currently ends basically at Fortuna, and it was found that the Lidar has
been helpful to identify numerous faults that were unmapped and to also try and
get a better sense of characterization of deformation in the region.
Thanks a lot.