Okay. Good afternoon, everyone. My name is Bob McPherson,
and I’m going to talk about where broken plates meet.
We have the Pacific, the North American, and the Gorda Plate
all meeting at the triple junction. And the Gorda Plate and the
North American Plate are suffering internal deformation and faulting,
whereas the Pacific Plate is the strongest of the three,
and it’s inflicting all of the punishment.
What I’m going to show you is the offshore Gorda Plate –
or, offshore of the Gorda Plate is caught in a tectonic die,
and in response to being squeezed in that die,
it’s failing along left-lateral faults. And those earthquakes are influencing
the daily strain accumulation in the – in the locked zone.
So that’s what I’m going to speak to you about.
[silence]
To understand this concept of tectonic die, I’m going to show you
that the Blanco trend – the trend of the Blanco Transform Fault and the
trend of the Mendocino Fault converge, thus pinching the offshore Gorda
in what Denlinger referred to as a tectonic die that it has to extrude
through moving under North America. And the result of that puts a strong
north-south compression within the Gorda Plate – consistent.
And also a result of that squeezing has deformed the plate
considerably and – especially in the south.
… come from a paper – a draft paper, and these are
the people that helped me with the ideas.
[silence]
All right. Try to move quickly here.
So we have five hazards that we have to deal with on the
North Coast here that you may or may not already know.
And Lori Dengler and Gary Carver and I wrote about them.
Here’s the corner of the Pacific Plate – the cutting edge of the – the southern
edge of the cutting die. And so, south of it, it’s a transform
boundary as the corner of the Pacific Plate migrates northwest
relative to stable North America. And, as it does that, it drags with it
slivers by the Maacama Fault and the Bartlett Springs Fault.
And then it’s dragging also the northern Sierra Nevada block.
And all of those are being scraped off in our region here – the Mendocino.
The second hazard is the Mendocino Fault, which I’ll show you
what it can deliver in ’94 earthquake.
And so the Pacific Plate is imparting a transform push along the Mendocino
Fault and pinching the Gorda Plate in between the Juan de Fuca,
which I already referred to as a tectonic die.
And here’s the result of that is these left-lateral vertical faults out in
the southern Gorda Plate. And they tend to be magnitude 7s,
and they can break, oh, 2 meters average slip – left-lateral slip.
And they can break underneath the western edge of North America.
And I call these the roving Gorda Plate earthquakes.
Moving on to the North American Plate is our fourth hazard – these
reverse faults. And the fifth hazard is the subduction zone itself.
The corner of the Pacific Plate. Here’s the San Andreas Fault.
Here’s the end of the Mendocino Fault activity – the yellow circles
end right at the corner here. I just want to show you that the effect
of this corner goes all the way into Oregon and creates this cloud.
And, in this cloud of earthquakes, you can see the descending
Gorda Plate in the orange and reds and blues and purples.
And, over – so east of the corner, all those earthquakes are east-west
down-slab tension. And west of it, it’s north-south compression.
And then, in the lower part of this slide,
you can see the Maacama and the Bartlett Springs trends.
[silence]
So the point to be made in this slide, which is a screen capture of a poster,
here is the corner of the Pacific Plate.
You can see these big Pacific Plate vectors moving.
And, as you cross the Mendocino Fault, they rotate, slow down – and, by the
time you get up into northern Oregon, they’re rotated around almost to the
convergence direction between Juan de Fuca and North America.
If I remove some of the vectors so we can see, again, here is the transform
boundary shown by the decreasing – the dragging of the slivers.
Here is the Nevada block. And then, over towards the coast,
this is going to be an important station here, south Humboldt Bay,
P162, I’m going to show data on. But notice these vectors as they rotate
and slow down and the collision over our reverse faults.
Notice that there’s still a very strong northerly component that I would argue
is from the corner of the Pacific Plate and goes clear into Oregon.
And notice that these stations that are colored green are going down.
In the ’70s – ’60s and ’70s, we had focal mechanisms, and we knew that
the solutions to the offshore earthquakes were either on
a northwest plane right-lateral or on a northeast plane left-lateral.
And it wasn’t until 1980 and the breakup of the Gorda Plate by the
Humboldt Bay Seismic Network and my team down there,
Stewart Smith and Niles Severy, that we discussed these left-lateral faults.
Like, for instance, this series of three earthquakes.
And I would argue that this is about as big as the earthquakes can get,
this 1980 7.2, because of the limits of the strength of the Gorda Plate.
It can only store so much energy – strain energy before it gives it
back as an earthquake. And so I think that the strength of
the Gorda Plate limits how big these earthquakes can be offshore.
[silence]
If we – so this is a figure from Dziak and Chaytor papers, and I just want to
show quickly – again, we have this deformation separating
north from south. And, in the north, it seems to be
intact and moving with the Juan de Fuca Plate.
And also I would argue, in the north, you can actually
draw a convergent vector because the plate is acting like a plate.
But, in the south, we got all kinds of deformation, and then we have these
left-lateral faults responding to the north-south compression.
And I’ve only drawn the arrows on the south of the fault indicating that
the triangle south of the fault is actually being shoved under
North America, which I’ll show. And I’ll also show a earthquake along
the Mendocino Fault – interplate – between-the-plate earthquake.
… 2005, a little earlier, Todd Williams and others installed all these permanent
GPS stations in our area. And Todd pointed out to me in 2005,
in an article we wrote for the Friends of the Pleistocene guidebook in 2006,
that this 2005 earthquake actually perturbed the locked zone –
could be seen in the locked zone in the GPS stations.
And so we wrote an article. And that’s available
on Cascadia GeoSciences. Ask Jay Patton about it.
So these three earthquakes – intraplate earthquakes I’m going to show you
data for – from, and also, I’ll look at this ’94 interplate earthquake.
[silence]
All right. Here’s the data for P162, south Humboldt Bay.
And hopefully you can see three steps in the easting –
the northing above it, the vertical below it, in the panels.
So here’s 2005.
Here’s 2010.
And 2014.
And, if I draw some lines through the trends of the data, you can see that
the majority of the strain accumulation was in the same direction as the
daily secular strain. And the majority of it, it was caused by
these offshore earthquakes. And also the station is going down
because of its proximity to the trench – close to the trench and above the
locked zone, which makes perfect sense to me.
So, looking at the data, these data were provided by Todd Williams.
If I draw a line between unloading and loading, as far as I interpret it,
south of the fault, the triangular piece of Gorda is being shoved under the
North American Plate, and these motions all moving to the east are
consistent with that interpretation. And it’s bringing the south –
well, it’s bringing this portion closer to failure.
Much closer to the coast, this data, supplied by Svarc and Murray,
and all the mechanisms are supplied by Doug Dreger – the moment tensors
mechanisms are supplied by Doug, and, again, if I draw a line between
what I call unloading and loading, you can see, south of the fault,
it’s consistent with bringing the southernmost Cascadia closer to failure.
[silence]
And the third intraplate earthquake that was recorded on the continuous
UNAVCO stations was the 6.8. Again, separating unloading from
loading, this portion was brought closer to failure by this offshore earthquake.
If I look at the interplate earthquake along the Mendocino Fault, again,
this was a campaign done by Mark Murray, and these data –
notice the scale change. So all of these moved greater than any of the intraplate
earthquakes, and, again, from above Trinidad south,
the southern Cascadia was brought closer to failure.
So, in summary, using Chris Rollins’ slide as a background, I would argue
from Oregon to the Mendocino Fault, all this area adjacent to these offshore
earthquakes should be considered its own segment.
Up in here, you get a mixed bag of unloading.
By the time you get into the gold, it’s primarily loading towards failure.
And, in the red, no matter what earthquake occurs offshore,
this zone comes closer to failure.
And, to me, it’s no coincidence that the three largest North American
Plate earthquakes are in this red zone –
the 1954, the 1992, and the 1991 Honeydew.
So, with that, I close.
Offshore earthquakes are influencing the loading in southern Cascadia.
And I thank the workshop for putting together – I thank the people that put
together this workshop, and I thank Keith for inviting me to tell my story.
Thank you very much, and good afternoon.