The scientists found that tremors emit their strongest seismic shockwaves in four opposing directions. They came to the conclusion by examining data from one of the densest seismic arrays ever deployed. The scientists said that the effect of earthquakes leaves a pattern resembling a four-leaf clover. Daniel Trugman, a geophysicist in the UT Jackson School of Geosciences, said the study looked at a type of seismic shaking caused by very small earthquakes in northern Oklahoma.
Trugman noted that one of the most important aspect seen in the study was that variation in ground motion was seen close to the source, which has not been accounted for in any sort of hazard model. Geophysical Research Letters published the findings of the study, which was funded by The US National Science Foundation.
The study is based on measurements of two-dozen small earthquakes recorded by the Large-n Seismic Survey in Oklahom. When earthquakes strike, it releases a thunderclap of seismic energy at several frequencies. However, the actual ground shaking people feel ranges from about one 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. They said that this qwas important as buildings are more vulnerable to low frequency waves. They noted that the four-leaf clover pattern was not found in higher frequency waves, which travelled at equal strength in all directions, like ripples in a pond.
FOUR-LEAF CLOVER PATTERN.
Co-author Victor Tsai, a geophysicist at Brown University, said the reason the Earth shook unevenly at different frequencies may relate to the complex geometry of earthquake faults and the broken-up material packed between them. Tsai said that when an earthquake happens, pieces of broken rock inside the fault zone start to move around like pinballs. 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, the researcher said. This means that on the surface, a person might feel the same shaking regardless of where he or she stood, but buildings — which are sensitive to low frequency waves — would be affected by the earthquake much more intensely within the lines of the four-leaf clover pattern.