Measuring multifault rupture with differential lidar

A California resident might argue that a large earthquake is not a subtle event. But it can have subtle aspects. Michael Oskin, a geologist at the University of California, Davis, and his colleagues have quite literally shed light on the subtleties surrounding the 2010 El Mayor-Cucapah earthquake in Mexico.

A fault is a crack in Earth's crust. Earthquakes occur when faults rupture. At the San Andreas Fault and other large faults where earthquakes occur repeatedly and have come to be expected, it's easy to see how much slip occurred at the surface. Slip refers to the relative displacement of formerly adjacent points on opposite sides of the fault. The amount of slip at the surface is similar to the amount of slip at depth and thus a measure of the amount of energy released.

But large earthquakes, such as El Mayor-Cucapah, can occur far from any major faults. If there is no discrete break at the surface, it's much more difficult to identify slip and strain rates and analyze the ruptures and deformation caused by an earthquake. Such analysis is necessary for hazard mitigation, however. That's where Oskin's study comes in.

Oskin’s team used light detection and ranging (lidar) to illuminate the physical features of the northern Baja California region and measure the distance the light travels to each point on the targeted area. With that data, they produced a very high resolution map. Lidar is exceptional at measuring vertical resolution.

Use of lidar to map an area is not new. Indeed, a lidar map, albeit at lower resolution, of the same area had already been completed in 2006. By comparing the older map with the more recent one, Oskin was able to differentiate pre- and post-earthquake surface features and show exactly what happened in 2010.

It turns out that the El Mayor-Cucapah earthquake actually warped Earth’s surface. That's very subtle.

“[Having this data] is a matter of luck!” says Oskin. When the 2006 lidar map was made, nobody knew that an earthquake would later occur at that site. “It's an experiment that wanted to be done, and you have to get lucky to get the data set.” The study was published recently in Science.

El Mayor-Cucapah comprises several faults located broadly in the plate boundary zone. Earthquakes that link disparate faults can be disparate themselves in terms of their geographical location.

One part of the mapped region with particularly low slope allowed subtraction of the 2010 and 2006 data sets to focus almost entirely on vertical distancing without any lateral motion to reduce resolution. That area, which had multiple isolated faults, provided insight into the process of how the faults link together.

“By linking together lots of small disconnected faults in one single rupture, during that one energetic event that unzips them all, it produces a larger earthquake than from a single fault,” explains Oskin.

Other lidar mappings are now also being made. The San Andreas Fault has been imaged as an example of a big “before” experiment. After the next earthquake it will be imaged again to provide a before-and-after study of an earthquake-prone area.

Although changes to surface features revealed by lidar differentiation may be subtle, such data can allow people to anticipate earthquake damage and the potential problems that can occur.