- Hello, my name's Lisa McNeill
and I'm a member of staff
in the School of Ocean and Earth Science
here in Southampton.
One of the things that
I work on in my research
is working on subduction zones
and I'm gonna tell you a little bit
about some of the methods that we use
to measure and monitor them
and what we've understood
about the processes occurring at them
and hopefully it'll be
something new for you
on top of your studies in school.
So this is an area that quite a lot of us,
quite a large group here in
Southampton are working on.
So it's something that
you would hear about
if you were to come here and study.
So you, and we, know that
earthquakes around the world
are actually marking the boundaries
between the tectonic plates
and we can see that on this map here.
The subduction zones are where
the largest earthquakes occur
and they're also where the
deepest earthquakes occur.
So you can see these with the
blue and yellow dots here.
So these are where the
subduction zones are located.
So they're around the Pacific Ocean
but, also, others are
located in the Indian Ocean,
Mediterranean, and parts of the Atlantic.
This is what a cross section
of a subduction zone would look like
if we were to image beneath the seafloor.
So a subduction zone is
where one tectonic plate
is going underneath another
and the boundary between those plates
is marked by the red line
that you can see here
and this is the plate boundary fault
and it's the movement on this
that generates these
really large earthquakes
and, ultimately, the tsunami.
And it goes from the trench,
which you can see on the right,
which is where the two plates meet,
and extends all the way
and, in fact, beyond where
the volcanic arc is located.
So there are other hazards here
as well as earthquakes and tsunamis.
So why are earthquakes at
subduction zones the biggest?
Well, earthquakes occur
in the part of the crust
that is cool enough, as in cold enough,
for rocks to break brittlely.
Temperature increases
with depth in the earth
and it's only in the
upper 10s of kilometres
that we can have this
brittle kind of behaviour.
The subduction zone fault
between the two plates,
which is the red line here,
is dipping at a really
quite shallow angle.
In fact, it's quite sort of
squashed and exaggerated here
and in places it's pretty
close to horizontal.
And so what that means is you end up with
a large area of the subduction zone fault
sitting within this upper
shallow, brittle zone,
and the larger of the area
of the fault that can move,
the larger the earthquake.
Hence why these are
the largest earthquakes
that we have on earth.
So what sort of methods can we use?
Well, there's a whole range,
and really what we're trying to do
is image and reach materials
deep beneath the seafloor
and then have monitoring
techniques that we can use
over a period of 10s of years
and, ultimately, longer if possible.
So this is just showing
you an example of that.
This is a scientific drill
ship called the Chikyu,
which is Japanese,
and here it is sailing in
front of Mount Fuji in Japan.
So the first thing that we need to do
is image at the seafloor
and below the seafloor
and we use geophysics to do that.
So we use acoustic techniques,
seismic techniques,
and this allows us to reconstruct
what the layers of sediments and rocks
and the faults look like
beneath the seafloor.
So we can see an example of this here
from the Nankai subduction zone in Japan.
This is a location where there's been
a really major project for
the last 20 years or so
where there's an attempt
to try to both image and to sample
the faults that are generating
these large earthquakes.
So the sampling process
takes place through drilling.
This is where we drill large boreholes
or deep boreholes beneath the seafloor
and then we can actually
reach, in particular,
the fault zones that are
generating the earthquakes.
So that's usually our first step,
is the imagining part of it.
And then, as I said before,
we can also use scientific ocean drilling.
So here's that Chikyu drill ship
that I mentioned from Japan.
This ship is more than 200 metres long.
So I've had the privilege
of sailing on this ship a couple of times,
and although it's enormous,
there's not a lot of space
because basically every
part of the ship is used
and taken up by equipment.
So we use this to drill boreholes
that could be hundreds of metres
to kilometres beneath the seafloor
and we can take back cores
from within that borehole
and hopefully a relatively
continuous sequence
of those sediments and rocks.
And we can also put
instruments down the borehole
and we can use those to measure
the physical and the electrical properties
of the rocks and the sediments
and the faults as well.
In addition, we can
also put in instruments
and take in situ measurements.
So, for example, we might want to measure
the pressure or the temperature
within these fault zones
and so we've got an example
of an instrument like that on the right.
We can also use some techniques on shore.
So one method that's
really quite important
is to try and work out
how fast parts of the
earth's surface are moving
and moving relative to each other.
So we use GPS for that.
So you all know GPS from
your satnav in your car
or from your phone,
but it's a technique
that's used scientifically
and pretty widespread in earth science
and it's more precise and more accurate.
But we basically have a number of stations
dotted around the area
and then they're constantly measuring
their relative position to each other
and we can put that together
and measure long-term motion
of different parts of the earth's surface
but also over a short-term,
as in, what happened during an earthquake?
And so the map on the right
is showing you some of the data from GPS
that was taken during the
2004 Indian Ocean earthquake
that occurred in Sumatra
and further north.
So the arrows that you can see,
the coloured green and red arrows,
are showing you how
much movement there was,
particularly of places
like Thailand and Malaysia,
to the west after the earthquake,
or during the earthquake.
(cat meowing)
And that was my cat. I apologise.
So we can do something similar
to that on the seafloor
but we have to use a different technique.
So making those sorts of measurements
is possible by using acoustics.
So we can put instruments on the seafloor
and, again, measure their distance
by measuring travel time
of a signal between them.
So that technique is a little
bit more in its infancy
but it's moving closer to
becoming more mainstream.
We can also make some measurements
over a longer time period
beneath the seafloor as well.
So in addition to seafloor measurements
and the sort of direct in
situ measurements we make,
we can also put instruments
down in the borehole
and leave them there
for a number of years.
So we can measure things
like pressure and temperature
but we can also put seismometers
down those boreholes
and measure for earthquakes directly
and it's a much less noisy,
a much more quiet environment
down there in the borehole
than on the seafloor.
So this is a whole range of techniques
that we have available to us.
There are obviously others.
Let's have a look at an example
and see how some of those
techniques have been used
and also how some new
ones are being introduced.
So you probably remember the
Japan earthquake in March 2011,
known scientifically as
the Tohoku-oki earthquake.
This was a really important earthquake.
It was obviously devastating
in terms of loss of life
and impact on the economy
in Japan and worldwide,
but it was also important
record-breaking earthquake.
First of all, the fault slipped
all the way, pretty much,
to the oceanic trench.
Now, that's pretty unusual and
we didn't really expect that.
So it tells us something
about the properties
of the fault materials
that we didn't really understand before.
And then the second part is
how much the fault slipped.
So it moved up to about 50 to 80 metres,
so nearly a hundred metres.
I always think of this as
running a hundred metres,
haven't done that fast for a long time,
but if you think about that in your mind
and how far that is
and then picture a
fault, a tectonic fault,
moving that amount in a matter
of seconds in one location,
it's quite incredible to think about.
This was the largest slip ever recorded
on a single fault in a single earthquake.
We knew that these sorts of
faults could move 10s of metres
but to move nearly a hundred metres
was probably a little bit
beyond what we were thinking.
So there were two methods
that we used during this earthquake,
or before and after this earthquake,
that hadn't been used before
and I think at least one of them
is gonna become much more widespread.
First of all, they used measurements
of the seafloor water depth,
or topography or bathymetry,
before and after the earthquake
to work out out how much it had moved.
So, basically, you take a (indistinct)
of the seafloor depth before and after,
subtract one from the other,
and, ultimately, how
much the seafloor moves
is a function of how much
the fault have moved.
And they were able to use this
to confirm this amount of more
than 50 metres on the fault
in the outer part near the trench.
This was only really possible here
because the slip was so large.
So, basically, the data are fairly noisy
and you've got to get to
a slip of a certain amount
before you'd actually
be able to resolve it.
Another method that was used here,
really fortuitously
because the instruments
happened to be on the seafloor on the time
but could be used much more widespread,
was using pressure gauges.
So these were instruments
sitting on the seafloor
and they're measuring the
pressure at the seafloor,
which is basically a function
of the height of the water
column overlying them.
Now, as a fault moves
during an earthquake,
even if it's fairly close to horizontal,
there's still gonna be some
vertical motion of the seafloor
and that will reduce or increase
the height of the water column
and it will change the
pressure at the seafloor.
And these gauges are quite sensitive
and so they can be used
to work out how much the seafloor moved.
And you can see on the
plot on the top right
a few of these instruments
and what there motions were
at the time of this March 11th earthquake.
And so you can see,
relative to the motion
before the earthquake,
it's really quite
distinct and quite abrupt.
And here, the pressure
has then been translated
to how much the seafloor moved.
And so they're then able to model that
and work out, again, how
much the fault moved,
and, again, came up with
this value of 80 metres.
So this is a really important technique.
These instruments are
not terribly expensive.
They could be distributed in areas
where we think there
might be an earthquake
and they will be really important
for helping to reconstruct
what happened during that event
and allow us to learn more.
So I think these are gonna
become more common in the future.
So what have we learnt?
We've learnt that every
earthquake generates surprises.
We still have much to learn.
These sorts of very large earthquakes
only happen, really, once every decade,
so we don't have too many opportunities
to learn from them.
Now, of course, they do
bring great destruction
and loss of life,
but the positive is that we
learn a huge amount from them
and we can then use that
to reduce their impact in the future.
We learnt how much slip can occur
on one fault at one time,
which is quite incredible.
We learnt that these
subduction zone earthquakes
can rupture much more of
the fault than we thought,
which increases the size of
the earthquake and the tsunami,
and I think we've also
learnt how important it is
to have instruments on
the seafloor just in case
to record what happens.
So I'll leave you with a
lovely sunset over the ocean.
You for, hopefully,
enjoying this short lecture.
I hope that you choose to
study geology, geophysics,
geoscience, environmental geoscience,
one of these subjects.
They're fantastic.
It's amazing to learn about
how the earth operates
and the impacts it has on our
lives on earth and vice versa.
So I hope you choose
to take it as a subject
and hopefully choose
to come to Southampton.
