Earthquakes produce various kinds of frequencies, from the long, rolling ground motions that make buildings sway, to the jerky, high-frequency vibrations that cause colossal damage to structures. Two geophysicists from Brown University propose a new theory that could explain the occurrence of these high-frequency earthquakes while challenging the old notions of seismology.

Destructive Earthquake frequencies

Geophysicists from Brown University recently published a research article in which they proposed that the main source of high-frequency vibration during destructive earthquakes is the collision of rocks inside a fault zone. This hypothesis is quite different from the traditional view in the field of Seismology. The researchers postulate that this new explanation might help explain puzzling seismic patterns made by some destructive earthquakes. This theory could also aid in predicting which faults are likely to generate more destructive earthquakes.

Seismologists have used an overly simplistic model for studying earthquakes called Brune-Haskell (BH) model for the past 5 decades. New models have built upon the BH model which has added complexity but these new models still rely on the old physics of the BH model. Therefore it was necessary to come up with a new theory that could define seismic observations that do not reconcile with the BH model. This is why the researchers of the present study – Victor Tsai and Greg Hirth propose an elastic collision of structures within a rupturing fault zone.

Revising old notions of Seismology

“The way we usually think of earthquakes is that stress builds on a fault until it eventually ruptures, the two sides of the fault slide against each other, and that slip alone is what causes all the seismic activity we observe,” said Victor Tsai, an associate professor at Brown University. “The idea of this paper is to determine if there is something other than just slip. The fundamental question is: If you have objects colliding inside the fault zone as it slips, what physics could result from that?”

Drawing from mathematical models that depict the collisions of rocks during mass wasting processes (landslides), the researchers constructed a model that predicts the potential effects of rock collisions in fault zones. The model proposes that collisions could indeed be the principal driver of high-frequency vibrations.

Destructive earthquake in Guatemale 2017
Earthquake damage in San Marcos, Guatemala (2017)
Image Credit: ElBorisOMG (Wikimedia Commons) / CC 4.0 BY-SA

According to the researchers, “Combining the collision model with more traditional frictional slip models offers plausible explanations for seismological observations that don’t quite fit the traditional model alone.” This combined model helps in illustrating repetitive quakes — earthquakes that happen at the same place in a fault and have almost similar seismic waveforms. The odd thing about these types of destructive earthquakes is that they often have contrasting magnitudes, yet still generates ground motions that are almost similar. That’s difficult to explain by traditional models alone but makes more sense with the collision model added. 

“If you have two earthquakes in the same fault zone, it’s the same rocks that are striking together — or at least rocks of essentially the same size,” Tsai said. “So if collisions are producing these high-frequency vibrations, it’s not surprising that you’d get the same ground motions at those frequencies regardless of the amount of slip that occurs.”

The collision model can also aid in answering, why earthquakes at more mature fault zones tend to cause less devastation when compared to quakes of the same magnitude at more immature faults. Over time, seismic events tend to grind down the rocks in a fault, making the faults smoother. The collision model predicts that smoother faults with less jagged rocks colliding would produce weaker high-frequency vibrations.

The researchers said that further investigation into the matter is required by the scientific community to validate the model. If proved valid, the collision model could be very helpful in describing which faults are likely to produce destructive earthquakes as compared to less damaging quakes.

“In some sense, it might mean that we know less about certain aspects of destructive earthquakes than we thought,” Tsai said. “If fault slip isn’t the whole story, then we need a better understanding of fault zone structure.”


STORY

Researchers develop new explanation for destructive earthquake vibrations
Kevin Stacey | March 3, 2020 | Brown University, USA.

RESEARCH ARTICLE

Elastic Impact Consequences for High‐Frequency Earthquake Ground Motion
Geophysical Research Letters

CITE THIS ARTICLE: "Geophysicists develop theory to decipher destructive earthquakes," in Geoscience Daily, March 8, 2020, https://geosciencedaily.com/earth/new-theory-destructive-earthquake/.