A new technique allows researchers to map Earth's interior by looking at seismic waves created by daily human activity.
A team of scientists looked at tiny ground tremors created by activity from vehicles and even human footsteps, allowing them to achieve a sought-after Earth-imaging technique called ambient noise body wave tomography, Stanford's School of Earth, Energy & Environmental Sciences reported.
"It's a technique that scientists have been trying to develop for more than 15 years," said Nori Nakata, who is the Thompson Postdoctoral Fellow at the School of Earth, Energy & Environmental Sciences.
The researchers created the maps by applying newly-developed signal processing technique to a type of seismic waves called "body waves." These types of waves travel through the Earth (as opposed to across its surface), allowing for better measurements of spatial resolution.
"Scientists have been performing body-wave tomography with signals from earthquakes and explosives for decades," said study coauthor Jesse Lawrence, an assistant professor of geophysics at Stanford. "But you can't control when and where an earthquake happens, and explosives are expensive and often damaging."
This is why researchers have been after a way to perform body wave tomography that does not rely on earthquakes or explosives. The feat has been difficult because body waves have lower amplitudes than surface waves, making them harder to observe. To get around this roadblock, the researchers used a new software processing technique, called a body-wave extraction filter. The filter allowed them to analyze ambient noise data picked up thousands of sensors installed across the California port city of Long Beach to monitor underground oil reserves.
"When I saw the Long Beach data, I realized I had all of the pieces in my hand to isolate body- wave energy from ambient noise," Nakata said. "I was excited, but at the same time I was skeptical my idea would work."
Using the refined filter, the researchers were able to create map that reached down to a depth of more than half a mile. One map revealed the Newport-Inglewood fault, which is visible in surface wave maps but not in as brilliant of detail. The map provided new information about the velocity of seismic waves near the fault, which could offer new insight into the surrounding rocks' composition.
"This has been something of a holy grail in Earth imaging, and Nori's work is a first-of-its-kind study," said geophysicist Greg Beroza, the Wayne Loel Professor at Stanford, who was not involved in the study. "His groundbreaking achievement is sure to be widely emulated."
The findings were published in a recent edition of the Journal of Geophysical Research: Solid Earth.
