Here’s Why Earthquake “Four Leaf Clover” Shock Waves Are Dangerous, Not Lucky

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Geologists have measured the devastating ‘quadruple clover’ pattern of seismic shock waves in greater detail than ever before — and the resulting results may be crucial to making our buildings and cities more resistant to major earthquakes in the future.

This four-pronged pattern has been analyzed before, but not in such depth. The team behind the new study hopes it may clear up some of the mystery surrounding how earthquake waves propagate across different frequencies.

Crucially, the alfalfa-leaf shock waves propagated at frequencies as low as 10 Hz, a level of vibration to which many buildings and structures are particularly exposed.

The four-leaf clover pattern can be seen at lower frequencies. (Trugman et al., Geophysical Research Letters, 2021)

“We find that at low frequencies a simplified and widely used four-lobe model of earthquake ground motions does a good job of describing the observed seismic wave field,” the researchers wrote in their published paper.

“However, at higher frequencies, the tetragonal radiation pattern becomes less pronounced, deteriorating due to complexity in seismic source processes and fault zone structure.”

The researchers looked at data from one of the planet’s densest seismic arrays: the LArge-n Oklahoma Seismic Survey (LASSO), which consists of 1,829 seismic sensors within an area of ​​just 15 by 20 miles (25 by 32 kilometres).

LASSO was used to measure P-wave data from 24 small earthquakes over a 28-day period in 2016, and that’s the data the new study delves into. Having sensors so close to the epicenter meant that patterns could be spotted before they fade and balance at greater distances.

Using algorithms to filter shock waves by frequency, a four-leaf clover pattern appeared, but only at lower frequencies. This may be because lower-frequency seismic waves can bypass the mixture of fractured rock found in earthquake faults, rather than reflecting and scattering in many different directions.

“What happens when you have an earthquake is that pieces of broken rock within the fault zone start moving like pin balls,” says geophysicist Victor Tsai, of Brown University in Rhode Island.

The researchers expected that the earthquakes recorded by the Lasso Array were relatively small – barely perceptible by sensors – but that the same patterns should be repeated across stronger earthquakes. The next step is to put that to the test.

Ultimately, new data like this could make earthquake assessments and modeling more accurate. It shows that while people on the ground may experience a constant level of shock waves (higher frequency waves), the buildings around them may be under more or less stress levels (lower frequency shock waves), depending on where they are in the four-leaf clover pattern.

While earthquake faults vary in terms of age, geologic composition, and other factors, the basic physics should be the same. Scientists hope to compile a catalog of earthquake zones, showing faults with the greatest potential for dangerous seismic waves and resulting damage.

“What’s important about these results is that we’re seeing variation in the motion of the Earth near the source, and that’s not taken into account in any kind of hazard models,” says the study’s first author, earthquake geophysicist Daniel Trogman of the University of Texas at Austin.

The research was published in Geophysical Research Letters.

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