Using LiDAR to create maps of Earthquakes
LiDAR stands for Light Detection And Ranging. It is an extremely precise method of measuring the distance to an object, by bouncing a stream of laser pulses off the object to be detected as the emitter moves around. When used on a large object such as the ground,. It is quite easy to use it to create a terrain map which is extremely accurate.
In this case, the researchers mounted their LIDAR equipment on a small plane. Even from a height of a thousand feet up, it still managed to scan the ground accurately to within a few inches. They covered 140 square miles this way, and they did so, twice, each trip taking three days.
The first trip was in 2006, so technology was not as accurate as modern LIDAR, but it was accurate enough. Not everything had moved in the recent quake that prompted the second scan, so many, many land features in the 140 square mile area could be used to perform a baseline check on to marry the data sets up.
The second scan took place in 2011, after a devastating earthquake had shook the region in question - Mexicali, northern Mexico. The earthquake that hit the area in April that year had come completely out of the blue, and no major fault lines had been involved.
By comparing the two separate scans, it was possible to see exactly which parts of the terrain had shifted the most, and so precisely identify which fault lines were responsible. From the ground, features like the five-foot escarpment created when part of a hillside abruptly moved up and sideways are readily visible.
But the LiDAR survey further reveals warping of the ground surface adjacent to faults that previously could not easily be detected. For example, it revealed the folding above the Indiviso fault running beneath agricultural fields in the floodplain of the Colorado River, something impossible to see with the naked eye.
Ken Hudnut, a geophysicist with the U.S. Geological Survey and co-author on the paper, made the first use of airborne LiDAR about 10 years ago to document surface faulting from the Hector Mine earthquake. But pre-earthquake data were lacking. Since then, NCALM has carried out LiDAR scans of the San Andreas system (the B4 Project) and other active faults in the western U.S. (a component of the EarthScope Project), thereby setting a trap for future earthquakes, he said.
In this case, fortunately, our CICESE colleagues had set such a trap, and this earthquake fell right into it and became the first ever to be imaged by before and after LiDAR. It is a thrill for me to be on the team that reached this important milestone," Hudnut said.
To view the data, it was so huge that there was of course only one option. A high-grade virtual reality system was used. Specifically the CAVE VR system at UC Davis W.M. Keck Center for Active Visualization in Earth Sciences. This much like any CAVE system, surrounds the users on six sides (four walls, floor and ceiling) with the data, and allows full immersive manipulation, using natural body gestures as opposed to avatars.
The data revealed that seven minor faults had moved at once to trigger the 7.2 earthquake, proving beyond all doubt that major faults do not have to be involved in major earthquakes, and something just as deadly can come easily from smaller faults as well. So, in order to protect against major earthquakes, all smaller fault lines really need to be monitored as avidly as the major ones, and VR has delivered the data to prove it.