Why Scientists Dig Trenches To Find Hidden Fault Lines in Cities

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Thanks to our asphalt-giddy behavior, we've all but paved over the fault lines that zigzag through some of our riskiest seismic zones. A new video shows how a team of geologists and engineers in LA have been digging a hole to find the exact location of a fault which could prove to be especially destructive.

"Trenching" is the practice of digging a deep trench in an area which is thought to contain a fault so the soils can be examined. What scientists are looking for is evidence that the sediment bands have shifted or broken, which would demonstrate that seismic activity has occurred, and will allow them to map and investigate the fault itself in a way that's no longer possible on the surface. In this particular case, they're also using radio-carbon dating of geological material to look for an "active fault," which means the seismic activity was fairly recent, happening less than 11,000 years ago.

In the case of this particular development, which is adjacent to the Capitol Records building, neighborhood groups are fighting a pair of towers planned for the site with the argument that the area is not seismically safe (they're fighting it for a whole bunch of other reasons, too.) But I first visited the site six months ago as part of my tour of Hollywood with L.A. city seismologist Dr. Lucy Jones and she mentioned something that stuck with me: The engineers simply have to demonstrate that the proposed buildings are at least 50 feet away from any known fault. So even if they do discover a fault here, it doesn't rule out future development on the property, it just might determine where they actually locate the structure.


A graphic showing what geologists and engineers are looking for: evidence of shifted sediments and rocks


Keep in mind that this particular video was produced by the developers who want to build their skyscrapers there, so they want to put forth a rosy outlook for the property. But the general consensus from the engineers as well as the developers is that the land does not contain any active faults. A state geologist is reviewing the findings and will release a report within about 90 days. Of course, don't tell any of this to the makers of the 1974 film Earthquake. [Millennium Partners via Curbed LA]