Marine seismology with Robert Weekly

Introducing our very own Robert Weekly – that’s right, he’s the first science+comics interviewee from within our “quakes and whales” lab group!

Name: Robert Weekly
Job: Graduate student – Marine Geology & Geophysics, University of Washington School of Oceanography
Research: Marine geophysics, mid-ocean ridge processes

Fifty-some years ago, if you’d assumed that the seafloor was a vast and immovable backdrop to the rolling sea above, you wouldn’t have been alone. It wasn’t until the mid-1960s that scientists started seeing evidence that tectonic plates were shifting entire continents and ocean basins. Part of the reason that it wasn’t more obvious before was the incredibly slow rate of new crust formation: we’re talking a few centimeters per year, about the same speed that your fingernails grow.

globalMOR

Oceanic crust is constantly being created and destroyed at plate boundaries. Many of us who live near subduction zones are familiar with the effects of oceanic lithosphere plunging under continental or oceanic crust – and the resulting large volcanoes and devastating earthquakes that put many people at risk. The creation of new seafloor, on the other hand, happens along a vast mid-ocean ridge system, snaking its way around the globe, like stitches on a really messed up baseball (see figure above). At these mid-ocean ridges, hot material from deep within the earth’s mantle leaks towards the surface and pools into magma chambers just below the seafloor. This magma inevitably forces its way up through cracks in the seafloor, ever so slowly forcing entire slabs of lithosphere apart. This intruded magma eventually cools and hardens, becoming the newest addition to the ocean crust.

MORdiagram

In a lot of ways, the seafloor is similar to a desert: barren, stark landscapes without much in the way of life-sustaining features. Mid-ocean ridges, on the other hand, are basically an oasis – an explosion of life driven by a heat source from below. But, although we are learning more all the time, no one really understands how the underlying magma “plumbing” helps drive the system.

That’s where Robert’s research comes in. In a nutshell, Robert is trying to figure out what’s going on underneath mid ocean ridges and how that contributes to the formation of new oceanic crust. He does that by looking at how seismic waves travel through the seafloor – you can think of it like a giant CT scan for the earth – except, WAY harder because you can only look at it from one side, and it’s HUMONGOUS. Imagine a CT scanner that needed to be a hundred kilometers long. Woah.

There are two ways that he can take a peek under the seafloor (you know, without going down and digging a massive hole): using active or passive seismic experiments. And he’s done a bit of both up at the Endeavour segment of the Juan de Fuca Ridge, which is a hydrothermally active chunk of mid ocean ridge that’s just a couple hundred kilometers off the coast of Vancouver Island.

Passive-source seismology
Rob spent a long time looking at one of the largest seismic datasets out there: three years worth of continuously recorded data on eight different seismometers on the seafloor. By listening for earthquakes, it was possible to put together a picture of what was happening, in fairly high resolution, near the center of the Endeavour segment. There is a magma chamber directly under the Endeavour, feeding the hydrothermal system and supplying magma to the ridge. What you’d expect is that the magma would spread outward from that central chamber. But instead, Robert saw an unexpected intrusion of magma coming down from another ridge segment to the north!

Passive_seis_600px

Active-source seismology
This research gave some fascinating results, but ultimately brought up even more questions. And to answer those, Robert needed to look at a larger area. For that, he used an active-source seismic dataset collected in September 2009 over the same region. Active-source seismology means that instead of listening for a source, you generate your own source. In this case, it meant a sound source on a ship.

Here’s a diagram to describe how it works:

Active_seis_600px

First, an airgun is set off near the sea surface, which sends a sound wave through the water column and then down into the seafloor. The waves travel through the seafloor, taking different paths depending on the structure of the underlying crustal material. Robert can then use the signals recorded on each of the instruments to create an image of the subsurface.

All of this work brings us a little bit closer to understanding the complex geologic processes controlling the formation of oceanic crust and the accompanying biological and chemical processes that produce incredible mid-ocean ridge environments.

References

Look for Rob’s paper coming out in a special edition of G-Cubed in the next few months!

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