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!