When the Earth changes face

By James Dacey

When an earthquake strikes, it can change a landscape dramatically within just a few seconds. Tectonic plates that usually creep past each other suddenly speed up to several thousand kilometres per hour, which can result in large displacements of the land.

An international team of scientists has managed to build the most detailed before-and-after picture to date of a landscape altered by an earthquake – a region near Mexicali in northern Mexico that was struck by a magnitude-7.2 quake in April 2010.

The geoscientists, working with the National Center for Airborne Laser Mapping, used LiDAR to survey the region, which had already been mapped by the Mexican government in 2006 using the same technique. This colourful animation shows the extent to which some areas of land were thrust upwards while other areas were forced down during the 2010 earthquake.

LiDAR – an abbreviation of “light detection and ranging” – involves flying over a region and firing a laser beam at the ground then detecting the reflected beams. By measuring the time delay between the transmission of the pulse and the detection of the reflected signal, scientists can estimate the elevation of the land surface.

In the study, researchers used LiDAR to measure surface features to within a few inches over an area of 360 square kilometres, within a few days. Publishing their findings in the journal Science, the scientists have identified features such as warping of the ground surface adjacent to faults, which did not show up in earlier ground surveys. These features, they say, could help geophysicists to better understand how earthquakes occur within multiple-fault systems like the one in northern Mexico.

“The 2010 Mexicali earthquake did not occur on a major fault, like the San Andreas, but ran through a series of smaller faults in the Earth’s crust. These minor faults are common around major faults but are under-appreciated,” says Michael Oskin of the University of California, lead author of the Science paper.

For an insightful introduction to earthquake science and the efforts to predict the occurrence of earthquakes, you can watch this short film just released on physicsworld.com. Keep an eye on this website throughout March for further content relating to the physics of the Earth.