Maya Tolstoy – TRANSCRIPT
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.