Translator: Denise RQ Reviewer: Lena Clemente
My story is one of earthquakes on the deep seafloor,
it's also the story of the fundamental value of basic research;
research into tiny earthquakes that's helping us understand
the forces producing the devastating larger earthquakes and tsunamis,
forces that are constantly pushing and pulling on our little blue planet.
The deep seafloor is a dark and mysterious place.
We know more about the surface of Mars than we know about our own planet
because so much of it is shrouded in water.
This is actually what the deep seafloor looks like
because sunlight doesn't reach it.
In this environment,
sound travels much more efficiently than light does
as whales have learned when they fill the ocean with their songs;
so we can learn a lot from just listening to the seafloor.
I've spent much of my career
developing the instrumentation and utilizing it to do just that.
We hear amazing things down there
from the calls of the baleen whales
to the creeks of icebergs in Antarctica,
the sounds of which reach all the way up to the Equator,
and then to the cracking of the seafloor
as it deforms from tectonic and volcanic processes.
But one of the eeriest sounds that I've ever heard
is that of the Great Sumatra - Andaman earthquake
ripping the crust apart on December 26, 2004.
This is the sound of it sped up ten times
to be audible to the human ear.
About a decade before this earthquake, as I was finishing my PhD,
I became fascinated with the tiny earthquakes
that are happening unseen on the deep seafloor.
This is a map of the topography of the seafloor
with the red showing areas at a shallower water
and the blue showing the deeper water;
but the areas I became fascinated with
are yellow green areas that run down in the middle of the oceans.
These are mid-ocean ridges,
they're chains of seafloor volcanoes where the plates are pulling apart
and a new surface of our planet is constantly forming.
At these locations, fresh lava is regularly erupted onto the seafloor,
and it's quickly quenched by the overlying ocean
to form these pillar-like formations.
There's occasional slow-moving life
in what's a low oxygen, very cold environment,
but mostly, it's a barren seascape.
But then, as you get close to the mid-ocean ridge,
somewhat paradoxically,
earthquakes are actually helping life to thrive.
So, at the ridge axis,
the earthquakes are cracking the seafloor
and allowing the ocean water to penetrate deep into the crust
where it picks up heats and nutrients
that come gushing out of the seafloor at these hydrothermal vent systems
and enable these bizarre, chemo-synthetic ecosystems
to thrive there;
but to truly understand the geophysics of these hydrothermal systems,
we need seafloor instrumentation and in particular, we need
ocean bottom seismographs or OBSs as we refer to them
as pictured here.
These instruments are simply craned over the side of the ship,
and dropped, and they fall up to five kilometers to the seafloor,
and they sit there, and do their thing, and record their data.
They have an autonomous recording package
where the data is stored until we recover them;
they have a small acoustic transponder package
that allows us to do some rudimentary communication with it,
from the ship;
there's an anchor plate, and then glass sphere flotations
in the yellow cases
that when we signal it to drop the anchor,
the instrument will become positively buoyant,
float back up to the sea surface,
and we'll go find it with our ship,
pick it up, and get our data back.
Now we can routinely deploy these instruments for over a year,
but in the early 90s, a few weeks was the maximum.
In 1994, as we were extending that deployment lengths
to about two months,
I had the opportunity to deploy a fleet of these
at a site called axial volcano in the Northeast Pacific.
It was showing signs that it was nearing an eruption
so we thought it would be an exciting place to go.
This actually caused me a few sleepless nights
because my thesis adviser had left me in charge of the experiment design,
and I put half a million dollars of his equipment
into the caldera of an active volcano.
Fortunately, it didn't erupt that summer;
we got our instruments back,
and I was allowed to graduate in the fall.
Shortly thereafter, I began to look at these data.
This was a vast amount of data
compared to what I was used to looking at,
so I decided to tackle it
by making daily plots of the data like the one shown here,
where each line is an hour's worth of data
in a given day.
So, as I was sitting
flicking through this pile of daily plots,
I started to notice something rather strange:
there were these noisy bursts of activity
that were occurring around the same time each day.
At first, I thought maybe I'd made a mistake
and I plotted the same day twice,
but there were enough differences to rule that out,
so I'd been looking for signs of magma movement
in the restless volcano,
but what I'd found was even more fascinating.
I realized that these noisy bursts were occurring at tidal intervals
so this was really my Eureka moment;
we were seeing evidence of tidal triggering of earthquakes
a long-postulated but never convincingly-observed phenomenon.
Let me just take a moment
to explain the history of tidal triggering.
Tides, as we all know, are caused
by the gravitational attraction of the Moon
and to a lesser extent, the Sun on our planet
that causes it to bulge out slightly toward the Moon and Sun,
and that bulge rotates as the Earth spins on its axis.
In addition to the ocean tides that you can see at the shore,
where the water bulges up by meters,
the solid Earth actually deforms with the tidal forces as well
only bulging up a few centimeters because it doesn't deform as easily,
but the crust is actually moving with the tides, too.
On the deep ocean floor,
you have that movement of the seafloor
from the crust deforming,
but you also have that extra weight of the oceans going up and down
and so this makes for very complex forces on the seafloor
acting on a daily basis.
Tidal triggering of earthquakes
was hypothesized back as early as the 17th century,
but at the dawn of the 21st century, it was largely dismissed,
and some people had even written that it was theoretically impossible
because the forces of the tides were too small.
But here, on the deep seafloor,
where we had that extra weight of the ocean, pushing up and down,
we were finally seeing the long-postulated tidal triggering.
We were seeing it
even though we weren't looking for it;
it was as though the seafloor was actually breathing
with the tides, with water gushing in and out of the cracks;
but we found it
because we finally had the instrumentation
to record data for long enough to see these trends,
and also, as a recent graduate who'd worked mostly on other things,
I didn't know it wasn't supposed to be there.
This observation has now been observed
in many other places on the deep seafloor
and even in a few places in-land.
In fact, recently, using just data from land stations
the Japanese scientist, Sachiko Tanaka
has made some remarkable geophysical observations.
What she has found is that the earthquakes
in the area of the Great Sumatra-Andaman earthquake
that happened before the big earthquake itself, the smaller ones,
were showing signs of tidal triggering
as it led up to the big earthquake itself,
What this plot shows
is the statistical significance of the tidal triggering in that area
that finally ruptured.
In the early 80s and into the very beginning of the 90s,
it basically wasn't statistically significant;
and then, in the decade [after] it,
it started to become increasingly significant
until the earthquake itself - which happened at the dashed line -
and then it became less significant again.
Just to be clear, tidal triggering doesn't mean
that tides are actually causing earthquakes
so the earthquakes will happen anyway because of the tectonic forces;
so in this case, in what's called a subduction-zone environment,
one plate is diving under another,
and it's those tectonic forces that are causing the earthquakes.
What tides do is they provide an extra little nudge
or they inhibit the movement slightly depending on the state of stress.
So, when tides are at their peak stress,
or what we call the time of encouraging stress,
it makes those earthquakes
just slightly more likely to happen at that time.
What we're seeing here with this change
in the strength of the tidal triggering
is that as the whole fault system of this large magnitude nine earthquake
as it becomes critically stressed, as we call it,
as it's building up to its big earthquake,
it becomes more and more sensitive to those little nudges from the tides;
it's actually similar to the way you and I behave
when we get critically stressed -
we become a lot more sensitive to little aggravations and things
that normally wouldn't bother us, but now, might make us snap.
The Earth is responding in the same way
as this large fault zone is becoming critically stressed;
it's becoming more and more sensitive to those little tidal nudges.
This is incredibly exciting
in terms of the ability to improve forecasts
for these large tsunami-generating earthquakes.
Note I use the term forecast rather than predict;
so to predict means to foretell
the time, location, and size of an earthquake very precisely.
What we talk about in earthquake hazards is forecasting,
similar to a weather forecast;
when you talk about a percent risk.
So we might say
there's a 30% probability of a magnitude 7
on a certain section of fault over the next 30 years;
but with this tidal triggering data, it's exciting,
because it provides the possibility
that we could perhaps shrink that forecast window
because of the decadal scale warning sign
and improve the certainty
and do better than 30%, hopefully,
of whether or not it's going to happen within that time window.
One of the drawbacks is that to date,
this method has only been used in hind casting mode
that's yet to be used to actually forecast an earthquake.
Also, because they're using only stations from land,
Dr. Tanaka has to use 3,000 day averages
to get enough earthquakes to get accurate statistics
so that's much easier to do in a hind-cast mode
when you know where the earthquake's going to be.
What we need is more instruments on the seafloor
to improve this
so that we can shrink those error bars of 3,000 days.
One of the exciting things about my work is
I get to help develop the technology
that we need to address the questions we want to address.
With seafloor instruments,
we already actually have very sophisticated capabilities
on the seafloor to measure earthquakes,
but in this environment, there are couple of challenges.
First of all, at the subduction zones,
we have the deepest waters in the world,
like the Mariana Trench;
and also some very shallow waters which tend to be heavily troll fished
and that's a danger for our instruments which are very expensive.
We also want to develop slightly cheaper lower-cost instruments
to be able to do this on a large scale.
So at Lamont Doherty Earth Observatory Ocean Bottom Seismology Lab,
we've been working on these problems
and with funding from the US National Science Foundation,
we've just developed a prototype
a full-ocean depth, ocean-bottom seismograph
- the one shown here - that's capable of going
all the way to the deepest spots on the planet.
This picture it's just our prototype;
as it was being deployed in the Mariana Trench.
It certainly needs a lot of aesthetic modification,
but this one was successfully deployed in February, 2013
down to 8,500 meters
which is the deepest that an OBS has ever gone,
and more importantly, it came back again
which is really always the critical test of your instrument.
We actually utilize the syntactic foam
that James Cameron developed
for his historic Mariana Trench dive in 2012,
and we even borrowed his vertical torpedo design
so that the instrument would go up and down
through the water column more quickly
which is important
when you're dealing with such deep water depths.
We're also working on a shallow-water instrument
called a TRM
- this stands for troll resistant mount -
and you can see it being deployed here.
This weighs about 1,400 pounds,
and you can tell its dome shape is designed
to stop troll nets from being caught on it.
We used a very sophisticated kick method
to get it overboard; it worked.
So here is one already deployed on the seafloor,
and as we're preparing to recover it with a remotely operated vehicle.
The vehicle is approaching the TRM here
and it's been recording data for over a year;
it successfully survived the heavy trolling in the area;
it's also served as a crab and fish habitat,
somewhat unintentionally,
This instrument is actually much more sophisticated
than we need;
this was developed for different seismological purposes;
it's much more sophisticated than we need
for simple tidal triggering monitoring,
so we want to make a cheaper,
lighter, easier to deploy low-cost version of this,
and we already have prototype sketches.
Our long-term goal is to develop a fleet of these instruments
that are capable of being deployed around the globe
at these subduction zones
so that we can look for signs
of tidal triggering that may warn us
that areas are building toward their state of critical stress.
We proposed to begin this monitoring in Alaska
off the coast of the Aleutian Islands
where there's a segment that's considered by some overdue
for a magnitude nine earthquake
and modeling has shown that when that segment goes,
it may produce a tsunami
that would be aimed directly at Southern California.
So we don't yet know
where the next tsunami-generating earthquake
is going to occur on the deep seafloor,
but somewhere out there,
the next fault is building towards its state of critical stress,
and it may be sending us out warning signals;
it's urgent that we prepared to listen to it.
Thank you.
(Applause)