Digging for Earthquakes

I’m a seismologist. Most of my research involves sitting in front of the computer writing code to manipulate waves on my screen, and then using particular characteristics of those waves to infer properties of the Earth’s interior. The waves are recordings of earthquakes, which send vibrations radiating out through the Earth.

The recordings are made by specialised instruments called seismometers, which are sufficiently sensitive that they can theoretically detect vibrations smaller than a billionth of a metre. In fact, they’re so sensitive that we have to cover them with heat-shields so tiny changes in temperature don’t cause their parts to minutely expand or contract, messing with our signals. We don’t put them too near trees because as trees sway in the wind their roots tug at the ground around them, causing soil and rock and seismometers to tilt, ever so slightly. Then there is so-called “cultural noise” caused by pesky humans with their cars and trains and pneumatic drills. In order to minimise all the sources of surface noise, we bury these instruments beneath the surface: around a metre down. This means seismologists (or their paid field assistants) have a side-career as semi-professional hole diggers.

Installing a sensor near Mt. St. Helens, WA in 2014. Gina is helping me dig the ~1m deep hole in which we will bury the sensor. 

There are some high-quality permanent seismic stations that are professionally installed. These are often housed in special heat/noise/everything-proof “vaults” in particularly quiet locations. For instance, the 150+ stations of the Global Seismographic Network (GSN) have provided 24/7 data for decades. The global network was first established in the 1960s and has provided a live-stream of global earthquakes ever since. That’s not actually what the stations were put there for, though - they were (and are) utilised by international bodies responsible for enforcing bans on nuclear weapons testing. Earthquakes have provided most of their stimulation since the last Chinese and Pakistani tests in 1998, but North Korea have recently helped keep nuclear monitoring relevant. Thanks, Kim Jung-Un, for keeping us employed!

The Global Seismographic Network (GSN) is a 150+ station, globally distributed, state-of-the-art digital seismic network that provides free, realtime, open access data through the IRIS DMC. Link.

Okay, but what happens if a seismologist like me wants to study a particular area in detail? Well, if I’m lucky enough to get funding, I gather up a whole load of instruments, put some beers in a cooler, and venture into the ‘field’. I then try to distribute the seismometers over the whole area of interest, ideally within reach of roads (but not too close). Finally, I wait for earthquakes around the world to show up on those sensors and tell me about the Earth underneath my little array of instruments.

The process of actually installing the sensors is pretty laborious. First, we have to negotiate with land owners to ask permission to bury complicated-looking sensors in their back gardens - you can imagine how those conversations go. Luckily, Americans are NEVER paranoid about being monitored by the government and they all feel kindly towards liberal university elites… Actually, people are almost always lovely and interested and happy to help.

Once we know where the sensors are going, it’s time to start digging! We generally dig about a metre down, pour concrete in the bottom of the hole for the sensor to stand on, and then fashion a complicated contraption of pipes and wires and big plastic bins to keep the whole thing fairly waterproof. We cover it with heatproof foam and bury the whole thing again. At the surface, we keep a box with the digital recorder and power. Since these guys can sit out there for several years, we have to build a solar panel array to keep their battery charged. Then we try to hide the solar panels so no one makes off with them (dooming the station as a byproduct of their larceny). The beers are for after the digging is finished.

Installing a station near Mt. St. Helens, WA, in 2014. In the background Mark is securing the solar panel, while in the foreground Dylan is filling back in the hole with the sensor (attached via blue cables to the power/digitizer package in the middle).

Installing a station near Mt. St. Helens, WA, in 2014. In the background Mark is securing the solar panel, while in the foreground Dylan is filling back in the hole with the sensor (attached via blue cables to the power/digitizer package in the middle).

So, yeah - seismological fieldwork is basically a series of DIY projects with mild landscaping thrown in. It’s tremendous fun. Also, while there are a few ‘best practice’ techniques for sensor installation, everyone does them a little differently and people are always coming up with new tips and tricks. Field seismology is also an exercise in patience and faith - you bury these little packages of electronics for years at a time, hoping they stay alive through the cold and the damp. We try to go and check up on them every few months, but it’s too expensive and energy-intensive to get them to send permanent streams of data. So it’s not uncommon (but it is depressing) to return to a station after 6 months to find that it died 5 months ago and there is no data! That’s why PhDs take so bloody long…

So next time you’re out hiking and you come across a small solar panel, a little mound of earth, and a sign that says “Earthquake monitoring equipment - do not disturb”, tread lightly, and wish us luck!

The Magic Magnetic Profile

This week the Lamont-Doherty Earth Observatory (where I work) held a symposium marking the 50th anniversary of the theory of Plate Tectonics. This paradigm shift was such an important breakthrough that we even describe other scientific advances in terms that rely upon it: “tectonic shift in thinking…”, “groundbreaking idea…”, “plate-deformingly great”. Okay, I made the last one up, but you get my drift (ahem).

Anyway, this week Lamont turned into a walking hall of fame for the geosciences, as giants in the field - at least one of whom used to sit at the desk I now occupy - returned to pay homage to the institution that housed so many breakthroughs. They reminisced about the good old days when computers filled entire rooms, poring over nautical charts with your advisor between 10pm and 2am was a normal day’s work (including whisky after 11pm), and no one wore shirts on the scientific cruises (let alone hard hats or shoes…). I particularly enjoyed the story of a Lamont ship that got lost off Bermuda and safely navigated to shore by periodically dropping dynamite off the ship. With a wrist watch to measure the time it took for the ‘bang’ to reflect off the seafloor, they echo-sounded their way up-slope, towards land. People were awesome back then.

Tellingly, only one of the decorated Professors Emeriti was a woman (the inimitable Tanya Atwater); in those days women were considered unlucky on ships and in 1963 the first woman to ever board a US scientific vessel was actually a Soviet scientist (Elena Lubimova) who was reluctantly accommodated to help smooth relations following a small diplomatic incident in Cuba

The symposium was convened explicitly to celebrate the breakthroughs in the mid 1960’s and early 1970’s that solidified the theory of Plate Tectonics. At the turn of 1960, almost all Earth scientists were ‘fixists’: unmoved (!) by Wegener’s outmoded theory of Continental Drift. Their major gripe was that no mechanism existed to plough the continents through the hard volcanic rock that they knew underlay the Earth’s oceans.

But their adversaries, the ‘mobilists’, had an idea: the continents were not pushing through the oceans. The oceans were moving too! In fact, as North America and Eurasia move apart from each other, they reasoned, the oceanic crust moves with the continents and new ocean floor is created at the seam along huge chains of volcanoes (mid-ocean ridges). This process is known as seafloor spreading. Lamont ships traversing the world’s seas had recently found vast and mysterious mountain chains circling the Earth and bisecting many oceans, but no one really knew what they were. (Hint: them’s the mid-ocean ridges.)

One of the key pieces of evidence came from the community of scientists measuring rocks’ magnetic fields. As you probably know, the Earth has a strong magnetic field (hence compasses being a thing), generated within its core. But rocks (especially volcanic ones) can host their own magnetic fields, inherited from the Earth’s field at the time that they are created.

It turns out everyone was rather keen on magnets in those days because the Cold War was afoot and WWII had just happened, and submarines were a bit of a concern… Detecting subs through their magnetic fields was a hot idea, but it required knowing the background field of the rocks pretty well so you could detect anomalies caused by an infiltrating sub, presumably captained by Sean Connery. As scientists mapped the seafloor, they noticed the magnetic field of the rocks looked like a series of stripes. First a bunch of ‘Normal’ polarity magnets, then a bunch of ‘Reversed’ polarity ones. They figured out that this pattern results from the Earth’s magnetic field periodically reversing (so the magnetic North pole randomly flips South, every few 100,000 years, and then back). As volcanic rocks get churned out along these long chains of volcanoes, they hold onto the field at the time of their eruption and you end up with magnetic ‘stripes’ of Normal or Reversed polarity.

>>>>>  Excellent video demonstrating seafloor spreading <<<<<

Now, according to the theory of seafloor spreading, you should see these stripes on either side of the chains of underwater volcanoes, and they should be symmetrical. This would have definitively proved that seafloor is created at the mid-ocean ridges and moves off on either side like symmetric conveyor belts (see animation above). But magnetised rocks are disobliging little buggers and all the random timings of flips (coupled with unknown rates of volcanic production) was making robust observations of this phenomenon elusive.

Until… Eltanin 19. The re-purposed US Navy icebreaker Eltanin was a Research Vessel used by Lamont scientists (among others) from 1962-72, completing 52 research cruises in Antarctic waters and surveying vast swaths of the southern oceans. Importantly, the good ship Eltanin carried a magnetometer. During its 19th research cruise, it took a long traverse across the Pacific-Antarctic ridge and measured the magnetic field of the rocks. When then-graduate-student Walter Pitman plotted the data, he saw something that irreversibly (c’mon) changed the face of Earth science. The ‘magic’ Eltanin-19 magnetic profile was perfect. It was so detailed, and so symmetric, it proved that seafloor spreading was real and turned ‘fixists’ into ‘mobilists’ wherever it was published. It also provided the key to all the other oceans: by stretching or squashing other magnetic profiles, you could show they, too, fit Eltanin-19 (and the amount of stretching or squashing told you how slow or fast the spreading was!)

From Pitman & Heirtzler (1966), Magnetic Anomalies over the Pacific-Antarctic Ridge, Science, 154, 1164-1171.

From Pitman & Heirtzler (1966), Magnetic Anomalies over the Pacific-Antarctic Ridge, Science, 154, 1164-1171.

 

Although it took a few more years for the legendary founding director of Lamont (Maurice ‘Doc’ Ewing) to be convinced, the then-younger generation were already sprinting into the future with a series of seminal papers that established the fundamentals of Plate Tectonics. As Neil Opdyke commented yesterday, “Only when we began to ask the correct questions did the answers begin to appear”. The marriage of the ‘correct questions’ with the reams of data collected by Lamont’s global fleet of research vessels proved potent. Graduate students were publishing in Science and Nature for fun. Contorted fixist geological narratives fell away. More and more unexplained phenomena slotted into place. And a new paradigm was born.

p.s. Sorry about all the puns.

p.p.s. Aside from sharing stories about cantankerous advisors and preposterous field exploits, the venerable alumni had a few tips for the aspiring researchers in the audience:

  • Respect the data - if the data disagree with your ideas, you are probably wrong.
  • Don’t have ‘darling’ theories.  See above. Be prepared to challenge accepted wisdom.
  • It’s better to be right than first. (No details were given regarding being left or last.)

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