Environment & Science

Like Nepal, LA sits on sediment easily shaken during a quake

KATHMANDU, NEPAL - APRIL 25 : Local people help with rescue work at the site of a building that collapsed after an earthquake in Kathmandu, April 25, 2015. Hundreds of people have been killed in Nepal after the Himalayan nation was hit by a powerful earthquake Saturday. At least 970 people have been killed in different parts of the country according to figures released by the Nepali Police. (Photo by Sunil Pradhan/Anadolu Agency/Getty Images)
KATHMANDU, NEPAL - APRIL 25 : Local people help with rescue work at the site of a building that collapsed after an earthquake in Kathmandu, April 25, 2015. Hundreds of people have been killed in Nepal after the Himalayan nation was hit by a powerful earthquake Saturday. At least 970 people have been killed in different parts of the country according to figures released by the Nepali Police. (Photo by Sunil Pradhan/Anadolu Agency/Getty Images)
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Like Nepal, the city of Los Angeles was built on layers of sedimentary rock that will likely amplify shaking caused by a quake, according to seismologists.

Scientists believe that the damage from the magnitude 7.8 quake in Kathmandu was made worse by these easily shaken layers.

Both cities sit near monster faults.

In Los Angeles, it’s the San Andreas, a strike slip fault where two massive plates of rock slide past each other creating a vertical rift in the earth’s crust.

(A graphic showing a strike slip fault. Image from USGS.)

The fault is believed to be capable of roughly a magnitude 8 earthquake.

In Nepal, the Himalayan collision zone occurs where one tectonic plate is thrust under another creating a horizontal fault zone deep underground.

(A graphic showing a continental collision. Image from USGS.)

The fault is under a much wider area of the country, spanning most of Nepal, according to Susan Hough of the US Geological Survey. It is able to producing a quake closer to magnitude 9.0, she said.

Both cities sit on layers of sedimentary rock left when the regions were covered in water. For L.A., those layers were deposited when the area was under an ancient sea.

Nepal picked up its sediment during a period when it was covered in a large lake. 

The details are different, said Hough, but in both cases this sediment acts like a bowl of jelly when rocked by seismic waves.

"We know that situation is going to amplify earthquake shaking considerably."

Quake expert David Oglesby at UC Riverside said the layer of sediments in Kathmandu are much shallower than in Los Angeles.

"In L.A. they go down multiple kilometers, whereas in Kathmandu they only go down a few hundred meters."

Still, he thinks in both cases the sediment can result in heightened damage during a quake.

Abhijit Ghosh, also with UC Riverside, called the Nepal quake a "great tragedy" but added that there is much to be learned from it in terms of seismic safety.

"This is a reminder that it is absolutely essential to be ready for the Big One ... and help friends and families to be prepared for the inevitable."