Seismologists: Mexico earthquake's shaking worse because of soil conditions

Wednesday, September 20, 2017

PASADENA, California -- The severity of Tuesday's 7.1 earthquake in Mexico City was amplified by the quality of the soil in the region, leading to more shaking than might be seen in other quake zones like Los Angeles, according to seismologists.

The epicenter was about 80 miles east of Mexico City with a relatively deep depth, about 30 miles below the surface, according to seismologist Lucy Jones with Caltech and formerly with USGS in Pasadena. On a quake of that size, the fault would be about 30 to 40 miles long, she said.

"Mexico City is the worst possible location for earthquakes," Jones told Eyewitness News. "The soils there amplify shaking by a factor of 100 or more. By comparison the worst we get in LA is about a factor of 5."

"That whole lake bed on which it's built makes things worse every time. I wouldn't be surprised to see a lot of damage coming out of this earthquake."

Ken Hudnut, also with the U.S. Geological Survey in Pasadena, noted the quake came on a historic day - the anniversary of an 8.0 earthquake in Mexico City that killed thousands of people in 1985. Earthquake drills were being held in Mexico before Tuesday's quake struck to mark the anniversary.

"Today is a very significant day in Mexico because it's the anniversary of the big 1985 earthquake," Hudnut said. "Which actually happened on the coast way up in Michoacan. But the shaking came all the way across to Mexico City."

"From that magnitude-8 earthquake, it amplified the shaking because of the old lake beds the city is built on. There were some old-type construction buildings that collapsed and there were a lot of fatalities."

The Mexico City quake also came just 12 days after a major earthquake struck farther to the south, in the Chiapas state, killing more than 90 people. The 8.1 magnitude quake on Sept. 7 has continued to produce hundreds of aftershocks in the area.

Jones said it is difficult to determine if Tuesday's Mexico City quake would be considered an aftershock of the Sept. 7 Chiapas temblor.

"An 8.0 or above has the ability to set off quakes for hundreds of miles around and that scales with the size of the earthquake," Jones said. "So this one is at the edge of the distance that we'd expect to be a triggered earthquake but it's tectonically very similar. Both happened in the downgoing slab."

"Maybe it's related? We'll probably be writing papers on it for a while."