Aug 19, 2024 05:46 PM
https://www.bbc.com/future/article/20240...l-of-jelly
EXCERPT: Los Angeles is located south of a giant a bend in the San Andreas fault where the plate boundary clearly changes direction. "If you see it from the air, it's amazing," says Jones. "It's so bizarre – you can look down and see the fault valley and then it just turns."
Around the turn, the region is chock full of faults. Over millions of years, the faults churned and pushed slabs of bedrock into multiple mountain ranges and deep basins. Gravity, water and wind act like sandpaper, wearing down the mountains, and carrying the debris into the basins. Over time, the basins have been filled with sediment.
The bowl-shaped basin of rock under Los Angeles is up to five miles (8km) deep, filled with a mixture of gravel, sand and clay. The contrast between the hard rock and softer sediment are big factors that cause some seismic weirdness for cities like Los Angeles.
During an earthquake, seismic waves are modulated by geology, says John Vidale, professor of seismology at University of Southern California. "The primary factor is just how hard is the ground and how deep is the structure that has soft [material] near the surface," he says. Seismic waves will move faster in denser material like rock, versus softer and less dense sediment.
As seismic waves travel through the basin, their behaviour changes when they encounter the loose sediment. "[The wave] is now having to travel at a much slower speed, but it still has to carry the same amount of energy per unit time," said Jones. As the wave slogs through the sediment, the amplitude, or wave height, gets bigger.
Put another way, imagine the Los Angeles basin as a giant bowl of jelly – the dense rocky mountains and underlying rock make up the bowl, while the sediment fill is represented by the gelatinous mixture. "If you shake the bottom [of the bowl] a little bit, the top flops back and forth quite a bit," says Vidale. And atop this quivering mass of jelly is the megacity of Los Angeles.
This means the amplitude of the waves within a basin can be significantly bigger than those moving through rock...(MORE - missing details)
EXCERPT: Los Angeles is located south of a giant a bend in the San Andreas fault where the plate boundary clearly changes direction. "If you see it from the air, it's amazing," says Jones. "It's so bizarre – you can look down and see the fault valley and then it just turns."
Around the turn, the region is chock full of faults. Over millions of years, the faults churned and pushed slabs of bedrock into multiple mountain ranges and deep basins. Gravity, water and wind act like sandpaper, wearing down the mountains, and carrying the debris into the basins. Over time, the basins have been filled with sediment.
The bowl-shaped basin of rock under Los Angeles is up to five miles (8km) deep, filled with a mixture of gravel, sand and clay. The contrast between the hard rock and softer sediment are big factors that cause some seismic weirdness for cities like Los Angeles.
During an earthquake, seismic waves are modulated by geology, says John Vidale, professor of seismology at University of Southern California. "The primary factor is just how hard is the ground and how deep is the structure that has soft [material] near the surface," he says. Seismic waves will move faster in denser material like rock, versus softer and less dense sediment.
As seismic waves travel through the basin, their behaviour changes when they encounter the loose sediment. "[The wave] is now having to travel at a much slower speed, but it still has to carry the same amount of energy per unit time," said Jones. As the wave slogs through the sediment, the amplitude, or wave height, gets bigger.
Put another way, imagine the Los Angeles basin as a giant bowl of jelly – the dense rocky mountains and underlying rock make up the bowl, while the sediment fill is represented by the gelatinous mixture. "If you shake the bottom [of the bowl] a little bit, the top flops back and forth quite a bit," says Vidale. And atop this quivering mass of jelly is the megacity of Los Angeles.
This means the amplitude of the waves within a basin can be significantly bigger than those moving through rock...(MORE - missing details)