WEBVTT
Kind: captions
Language: en-US

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Okay. Good afternoon, everyone.
My name is Bob McPherson,

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and I’m going to talk about
where broken plates meet.

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We have the Pacific, the North
American, and the Gorda Plate

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all meeting at the triple junction.
And the Gorda Plate and the

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North American Plate are suffering
internal deformation and faulting,

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whereas the Pacific Plate
is the strongest of the three,

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and it’s inflicting
all of the punishment.

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What I’m going to show you
is the offshore Gorda Plate –

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or, offshore of the Gorda Plate
is caught in a tectonic die,

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and in response to being
squeezed in that die,

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it’s failing along left-lateral faults.
And those earthquakes are influencing

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the daily strain accumulation
in the – in the locked zone.

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So that’s what I’m
going to speak to you about.

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[silence]

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To understand this concept of
tectonic die, I’m going to show you

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that the Blanco trend – the trend of
the Blanco Transform Fault and the

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trend of the Mendocino Fault converge,
thus pinching the offshore Gorda

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in what Denlinger referred to as
a tectonic die that it has to extrude

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through moving under North America.
And the result of that puts a strong

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north-south compression within
the Gorda Plate – consistent.

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And also a result of that
squeezing has deformed the plate

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considerably and –
especially in the south.

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… come from a paper – a
draft paper, and these are

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the people that helped
me with the ideas.

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[silence]

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All right. Try to
move quickly here.

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So we have five hazards that
we have to deal with on the

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North Coast here that you
may or may not already know.

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And Lori Dengler and Gary Carver
and I wrote about them.

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Here’s the corner of the Pacific Plate –
the cutting edge of the – the southern

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edge of the cutting die.
And so, south of it, it’s a transform

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boundary as the corner of the
Pacific Plate migrates northwest

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relative to stable North America.
And, as it does that, it drags with it

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slivers by the Maacama Fault
and the Bartlett Springs Fault.

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And then it’s dragging also
the northern Sierra Nevada block.

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And all of those are being scraped off
in our region here – the Mendocino.

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The second hazard is the
Mendocino Fault, which I’ll show you

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what it can deliver
in ’94 earthquake.

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And so the Pacific Plate is imparting a
transform push along the Mendocino

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Fault and pinching the Gorda Plate
in between the Juan de Fuca,

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which I already referred to
as a tectonic die.

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And here’s the result of that is these
left-lateral vertical faults out in

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the southern Gorda Plate.
And they tend to be magnitude 7s,

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and they can break, oh, 2 meters
average slip – left-lateral slip.

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And they can break underneath the
western edge of North America.

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And I call these the roving
Gorda Plate earthquakes.

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Moving on to the North American
Plate is our fourth hazard – these

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reverse faults. And the fifth hazard
is the subduction zone itself.

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The corner of the Pacific Plate.
Here’s the San Andreas Fault.

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Here’s the end of the Mendocino
Fault activity – the yellow circles

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end right at the corner here.
I just want to show you that the effect

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of this corner goes all the way into
Oregon and creates this cloud.

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And, in this cloud of earthquakes,
you can see the descending

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Gorda Plate in the orange
and reds and blues and purples.

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And, over – so east of the corner,
all those earthquakes are east-west

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down-slab tension. And west of it,
it’s north-south compression.

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And then, in the
lower part of this slide,

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you can see the Maacama
and the Bartlett Springs trends.

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[silence]

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So the point to be made in this slide,
which is a screen capture of a poster,

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here is the corner
of the Pacific Plate.

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You can see these big
Pacific Plate vectors moving.

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And, as you cross the Mendocino Fault,
they rotate, slow down – and, by the

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time you get up into northern Oregon,
they’re rotated around almost to the

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convergence direction between
Juan de Fuca and North America.

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If I remove some of the vectors so we
can see, again, here is the transform

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boundary shown by the decreasing –
the dragging of the slivers.

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Here is the Nevada block.
And then, over towards the coast,

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this is going to be an important
station here, south Humboldt Bay,

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P162, I’m going to show data on.
But notice these vectors as they rotate

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and slow down and the collision
over our reverse faults.

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Notice that there’s still a very strong
northerly component that I would argue

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is from the corner of the Pacific
Plate and goes clear into Oregon.

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And notice that these stations that
are colored green are going down.

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In the ’70s – ’60s and ’70s, we had
focal mechanisms, and we knew that

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the solutions to the offshore
earthquakes were either on

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a northwest plane right-lateral or
on a northeast plane left-lateral.

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And it wasn’t until 1980 and the
breakup of the Gorda Plate by the

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Humboldt Bay Seismic Network
and my team down there,

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Stewart Smith and Niles Severy, that
we discussed these left-lateral faults.

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Like, for instance,
this series of three earthquakes.

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And I would argue that this is about
as big as the earthquakes can get,

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this 1980 7.2, because of the limits
of the strength of the Gorda Plate.

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It can only store so much energy –
strain energy before it gives it

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back as an earthquake.
And so I think that the strength of

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the Gorda Plate limits how big
these earthquakes can be offshore.

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[silence]

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If we – so this is a figure from Dziak
and Chaytor papers, and I just want to

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show quickly – again, we have
this deformation separating

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north from south.
And, in the north, it seems to be

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intact and moving with
the Juan de Fuca Plate.

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And also I would argue,
in the north, you can actually

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draw a convergent vector because
the plate is acting like a plate.

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But, in the south, we got all kinds of
deformation, and then we have these

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left-lateral faults responding to
the north-south compression.

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And I’ve only drawn the arrows on
the south of the fault indicating that

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the triangle south of the fault is
actually being shoved under

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North America, which I’ll show.
And I’ll also show a earthquake along

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the Mendocino Fault – interplate –
between-the-plate earthquake.

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… 2005, a little earlier, Todd Williams
and others installed all these permanent

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GPS stations in our area.
And Todd pointed out to me in 2005,

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in an article we wrote for the Friends
of the Pleistocene guidebook in 2006,

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that this 2005 earthquake actually
perturbed the locked zone –

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could be seen in the locked
zone in the GPS stations.

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And so we wrote an article.
And that’s available

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on Cascadia GeoSciences.
Ask Jay Patton about it.

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So these three earthquakes – intraplate
earthquakes I’m going to show you

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data for – from, and also, I’ll look at
this ’94 interplate earthquake.

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[silence]

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All right. Here’s the data for
P162, south Humboldt Bay.

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And hopefully you can see three
steps in the easting –

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the northing above it,
the vertical below it, in the panels.

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So here’s 2005.

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Here’s 2010.

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And 2014.

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And, if I draw some lines through the
trends of the data, you can see that

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the majority of the strain accumulation
was in the same direction as the

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daily secular strain. And the
majority of it, it was caused by

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these offshore earthquakes.
And also the station is going down

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because of its proximity to the trench –
close to the trench and above the

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locked zone, which makes
perfect sense to me.

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So, looking at the data, these data
were provided by Todd Williams.

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If I draw a line between unloading
and loading, as far as I interpret it,

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south of the fault, the triangular piece
of Gorda is being shoved under the

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North American Plate, and these
motions all moving to the east are

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consistent with that interpretation.
And it’s bringing the south –

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well, it’s bringing this
portion closer to failure.

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Much closer to the coast, this data,
supplied by Svarc and Murray,

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and all the mechanisms are supplied
by Doug Dreger – the moment tensors

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mechanisms are supplied by Doug,
and, again, if I draw a line between

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what I call unloading and loading,
you can see, south of the fault,

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it’s consistent with bringing the
southernmost Cascadia closer to failure.

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[silence]

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And the third intraplate earthquake
that was recorded on the continuous

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UNAVCO stations was the 6.8.
Again, separating unloading from

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loading, this portion was brought closer
to failure by this offshore earthquake.

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If I look at the interplate earthquake
along the Mendocino Fault, again,

00:13:28.480 --> 00:13:34.000
this was a campaign done by
Mark Murray, and these data –

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notice the scale change. So all of these
moved greater than any of the intraplate

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earthquakes, and, again,
from above Trinidad south,

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the southern Cascadia was
brought closer to failure.

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So, in summary, using Chris Rollins’
slide as a background, I would argue

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from Oregon to the Mendocino Fault,
all this area adjacent to these offshore

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earthquakes should be
considered its own segment.

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Up in here, you get a
mixed bag of unloading.

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By the time you get into the gold,
it’s primarily loading towards failure.

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And, in the red, no matter what
earthquake occurs offshore,

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this zone comes
closer to failure.

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And, to me, it’s no coincidence
that the three largest North American

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Plate earthquakes are
in this red zone –

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the 1954, the 1992,
and the 1991 Honeydew.

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So, with that, I close.

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Offshore earthquakes are influencing
the loading in southern Cascadia.

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And I thank the workshop for putting
together – I thank the people that put

00:15:04.560 --> 00:15:10.296
together this workshop, and I thank
Keith for inviting me to tell my story.

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Thank you very much,
and good afternoon.