Probing Earth's Deep Structure
Using earthquakes as a ground penetrating sonar illuminator.
By digging a hole around half a metre deep, Dr Nick Rawlinson can peer down and figure out the nature of the earth below to a depth of around 300 km! Well, maybe that’s oversimplifying things. He first has to place a sensor at the bottom of the hole that picks up vibrations from the earth. He then has to dig a number of other holes, each with its own sensor, forming an array hundreds of kilometres wide. Over several months this array picks up vibrations, seismic waves, travelling through the planet from distant earthquakes. The signal at each hole is compared with the signals picked up at the other holes, and the resulting data set is then mathematically transformed to yield a picture of the planet below.
Dr Rawlinson is a seismologist based in the Research School of Earth Sciences, Australia’s leading institution when it comes to the seismic monitoring of our region’s deep geology. For most of the last ten years he’s been deploying seismic arrays across south east Australia in order to better understand what lies underneath.
“We’re currently establishing an array of 36 seismometers around south western NSW,” says Dr Rawlinson. “They’ll be positioned across the Hay Plains and beyond.
“Each station is about 40 to 50 km apart, and they’re going to sit out there for around seven months. They’ll record the seismic waves from large earthquakes occurring at the margins of distant tectonic plates in places like Japan, Taiwan, Fiji and New Guinea.”
“The idea is that you use these big distant earthquakes to illuminate the structure underneath the array,” explains Dr Rawlinson. “It’s similar to going to the hospital and having a CAT scan where you have all these X-rays going through your body at different angles.
“In seismology we use a lot of seismic recorders spread out in an array. Energy from big distant earthquakes penetrates quite deeply into the planet and comes up underneath the array. As these seismic waves approach the surface, their paths cross in many directions, which is similar to the X-rays going through the body, and we can use that to image the variations in structure beneath the array.”
The recording stations that Dr Rawlinson and colleagues set up consist of a seismometer, buried about half a metre in the ground, and a recording box. The box contains a signal amplifier, a Global Positioning System module that allows each station to compare timing between stations down to thousandths of a second, a memory card for the storage of several months of data, and a battery.
According to Dr Rawlinson, people skills are an important aspect of setting up these arrays. “In the process of gaining permission to set up recording stations on private land I must have knocked on 300 doors,” he says. “Most people we meet are only too happy to let us onto their land once we’ve explained our purpose. Some are initially suspicious - one lady even answered the door with a knife in her hand, but they all come around when we explain we’re monitoring earthquakes.
“When we actually get onto a private property we then make a decision as to where to best place the recorder. We have to be careful with cattle and sheep as they often destroy the equipment. Other problems include the risk of bushfires sweeping through an area and taking out a station. “While we believe that much of south eastern Australia was created by subduction accretion, the process of one tectonic plate colliding with and descending beneath another plate, there are still many unanswered questions about the deeper structure of our region. Our work sheds light on this. It’s fundamental research that’s explaining how south east Australia has evolved over time.
“While establishing these arrays can be a very laborious task, it’s also one of the interesting aspects of this work – you meet different people in parts of Australia that most Australian’s never get to see.”