Monday 05 March, 2001
Earthquake Investigations
When an earthquake struck Gujarat, India, most people's immediate concern was the human tragedy that followed. But for geologists this was an unusual opportunity to learn about the forces that cause earthquakes. Science View follows two geologists in their research.
On 26 January, 2001, Dr Becky Bendick was woken in the US by the news of an earthquake in Gujarat. The rupture in the earth's surface had created vast seismic waves that devastated towns and villages in the surrounding region of Kuchchh but it had also disturbed the earth's underlying mantle, the soft flowing zone of warm rock that is engine to the earth's overlying motions. The setting was the best opportunity in recent history for geologists to collect evidence of the processes occurring within the earth's crust.
Dr Bendick, a researcher in seismotectonics at the University of Colorado, soon found herself on a plane to India. She was accompanied by her colleague, Grant Kier, a structural geologist. Their mission was to study the resulting earthquake features and collect data that would help geologists to learn about the interaction between mantle and crust that drives earthquakes all around the world.
Getting to the Centre of the Action
Before arriving in India it was difficult for Bendick or other geologists to get information. They didn't know if the rupture caused by the earthquake had broken the earth's surface, where it was, or what it looked like. On arrival in Gujurat, a local guide took Kier and Bendick to the site of the rupture.
Bendick describes the extent of the rupture:
“We are really close to standing on the plain that actually ruptured. What we see on this bridge is that actually one side of this bridge, to the east is higher, and it's actually broken the bridge in two pieces. There's a whole series of cracks in the creek bed that show there is some stretching of the sediment.”
Mole Tracks
Cracks in the creek are not the only earthquake features to be revealed. Bendick and her colleague were excited at the sight of sand blows or mole tracks, ridges of ruptured earth with a crack in the middle of them.She explains:
“We call them mole tracks for a reason because it looks like the mother of all moles came through here. They are a compressional feature of the very top softest sediment. When you push things together it just buckles up and in a lot of these sand has just poured out of the middle. What happens is in an earthquake it shakes the sand and packs it in so water shoots up carrying the sand grains with it. The lines are the surface expression of something kilometres below."
Good Vibrations
The sandblows and rupture were the immediate visual effects of the earthquake however what is of interest to seismologists like Bendick are the seismic waves. These are tiny vibrations of the earth's crust that are produced as an earthquake occurs. To measure the waves Bendick needed to establish a network of sighting points over the area. These could be precisely mapped by satellite positioning systems.
Bendick explains what they hope to learn from the measurements that they are collecting:
“The idea is that the earthquake ruptured all the way through the crust and so it changes the geometry of the crust in relation to the upper mantle. What we're looking at is what is 50km deep and that is almost fluid. We don't know the viscosity of the whole couplet system so the rate at which it adjusts will tell us that number hopefully”
Competing Resources
In the face of the human tragedy which resulted from the earthquake these scientific investigations may appear inappropriate. Bendick justifies the research.
"Throughout this trip we have struggled with whether or not what we are doing is appropriate given the circumstances. I mean no matter how hard we try, we are using resources here that could be used otherwise to help people. We have vehicles, we are drinking water and using petrol. I think that although the immediate needs of people are so urgent here and now, what we want to do is understand the earth better and understand how earthquakes work fundamentally. Any progress we can make in that direction can only help in the big picture."
|
 |
 |
|
| Geological X-rays |
 |
|
|
"Seismic waves are to geophysists what x-rays are to the medical practitioner. They permit investigation of an inaccessible interior. Without seismic waves we would have virtually no information about the interior"
(MJ Selby, 1986)
|
|
 |
|
|
