Seismic shockwave pattern may be redirecting earthquake damage

Date:
November 16, 2021
Source:
University of Texas at Austin
Summary:
According to physics, seismic waves from earthquakes should
travel in a four-leaf clover pattern, but in the real world they
behave more like ripples in a pond. New research has found the
pattern hidden among low- frequency seismic waves, the kind that
can topple buildings. The study looked at small earthquakes in
northern Oklahoma and could change the way scientists think about
potential damage from earthquakes.



FULL STORY ==========================================================================
New research from The University of Texas at Austin could change the
way scientists think about potential damage from earthquakes.


==========================================================================
The study examined data from one of the densest seismic arrays ever
deployed and found that earthquakes emit their strongest seismic
shockwaves in four opposing directions. The effect, which leaves a
pattern resembling a four-leaf clover, has been known for decades but
never measured in such vivid detail.

Daniel Trugman, an earthquake geophysicist at the Department for
Geological Sciences in the UT Jackson School of Geosciences, said that
the study looked at only one type of seismic shaking caused by very
small earthquakes in northern Oklahoma.

"What's important in these results is that close to the source we're
seeing a variation in ground motion, and that's not accounted for in
any sort of hazard model," Trugman said. He added that efforts were
already underway to see how the phenomena plays out in California's big
fault systems.

The analysis was published in the September issue of Geophysical Research Letters and is based on measurements of two-dozen small earthquakes
recorded by the LArge-n Seismic Survey in Oklahoma (LASSO), an array of
1,829 seismic sensors deployed for 28 days in 2016 to monitor a remote
corner of the state measuring 15 by 20 miles.

When earthquakes strike, they release a thunderclap of seismic energy at
many frequencies, but the actual ground shaking people feel ranges from
about 1 hertz to 20 hertz. The study found that low frequency energy --
about 1 to 10 hertz -- shot from the fault in four directions, but barely registered outside of the four-leaf clover pattern. This is important
because buildings are more vulnerable to low frequency waves. The
four-leaf clover pattern wasn't found for higher frequency waves, which travelled at equal strength in all directions, like ripples in a pond.



========================================================================== Co-author Victor Tsai, a geophysicist at Brown University, said that
the reason the Earth shook unevenly at different frequencies might have something to do with the complex geometry of earthquake faults and the broken-up material packed between them.

"What happens when you have an earthquake is that pieces of broken rock
inside the fault zone start to move around like pinballs," he said. The jostling pieces redirect the energy randomly but at lower frequencies,
seismic waves simply bypass the rough geologic mess near the fault,
travelling in a nice four-leaf clover pattern just as physics predicts.

This means that on the surface, a person might feel the same shaking
regardless of where they stood, but buildings -- which are sensitive
to low frequency waves -- would feel the earthquake much more intensely
within the lines of the four-leaf clover pattern.

Geophysicists have long known about this pattern; it's taught in
seismology 101. But, until now, evidence of its effect has been
sparse. That's because over large distances seismic waves are refracted regardless of frequency, smoothing out their differences and making
earthquakes seem the same in all directions.

Near an earthquake's source, however, the pattern should be
distinct. That's where the LASSO array came in. Its closely packed sensors recorded earthquakes while they were unfolding, gathering measurements
from hundreds of locations in northern Oklahoma that the U.S. Geological Survey, which funded and deployed the array, made freely available online.

To test their idea about uneven shaking near faults, Trugman developed algorithms to filter the LASSO data. At low frequencies, each earthquake
showed a four-leaf clover pattern of shaking; at higher frequencies
there was no clear pattern, just as Tsai had predicted.

Although the tremors recorded by the LASSO array were barely perceptible,
the physics that drive them should be the same for stronger quakes. The scientists have already begun examining larger faults to see whether
their age or shape can change the intensity of ground motion. Their goal
is to build a catalogue of earthquake zones, showing which faults can
generate the strongest and most dangerous types of seismic waves.

The research was funded by the U.S. Geological Survey and the National
Science Foundation. Trugman is also a researcher at the University
of Texas Institute for Geophysics, a unit of the Jackson School of
Geosciences.

========================================================================== Story Source: Materials provided by University_of_Texas_at_Austin. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Daniel T. Trugman, Shanna X. Chu, Victor C. Tsai. Earthquake Source
Complexity Controls the Frequency Dependence of Near‐Source
Radiation Patterns. Geophysical Research Letters, 2021; 48 (17)
DOI: 10.1029/2021GL095022 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/11/211116144816.htm

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