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New earthquake calculations predict their probable destructiveness

The Pinball Fault Slide Model Takes on Techniques from Avalanche Mathematics




Repeated tremors shook the center of Taiwan for many days and weeks after the earthquake of magnitude 7.7, which occurred there in 1999. The new earthquake model was able to explain the differing power of these shocks.

When a fault slides , it generates a whole sequence of different seismic waves. Long waves of low frequency can spread over a large distance from the source and sway high buildings such as skyscrapers. High-frequency waves perfectly shake houses and bridges, and sometimes completely destroy them. For most of the past fifty years, seismologists have assumed that the entire set of these waves generates friction that occurs when the fault slides.

Now, a couple of geologists from Brown University have given their own history of the origin of the waves. Using mathematical models inspired by the counting of landslides and avalanches, the researchers argue that these destructive high-frequency waves are generated not by the slip itself, but by the geological processes occurring inside the fault, reminiscent of a pinball game .

“They get pretty pretty,” said Elizabeth Cochran , a seismologist at the US Geological Survey. “It would certainly not have occurred to me to describe the fault as they did.”

The new model, published last month in the journal Geophysical Research Letters, will still need to be tested in future earthquakes to see how accurately it predicts their properties. However, if confirmed, it will reverse our understanding of the destructive potential of earthquakes, and perhaps help save lives.

Geological pinball


According to traditional models of earthquakes, when a block of the earth's crust begins to slip and rub against another, friction between them generates seismic waves. Seismologists recognize the simplicity of these models compared to real processes occurring in the area of ​​the fault line. However, they accurately describe the low-frequency component of an earthquake wave set - a critical early indicator of earthquake magnitude and vital information.

However, traditional models are not able to explain the large number of high-frequency waves generated by the earthquake, said Lucille Bruhat., an expert in earthquake physics from the Higher Normal School in Paris, who did not participate in this study. This becomes a problem when you try to figure out why certain cracks are more damaging.

Cochran argues that traditional models associate these high-frequency waves with fault oscillations — unpredictable crack movements, sometimes arising, then decaying. However, since the physics of cracking is very difficult to study, such assumptions are not easy to confirm. “You can't make an earthquake in a laboratory,” said Robert Graves , a geophysicist at the US Geological Survey who did not participate in this study.


In traditional models, high-frequency waves are associated with fault oscillations - unpredictable crack movements, sometimes arising, then damping.
The new “pinball” model speaks of the collision of different stones with each other, generating high-frequency waves. The size of the stones varies from a few meters in diameter to the football field.
Long waves of low frequency can travel a long distance from the source and sway high buildings like skyscrapers.
High-frequency waves perfectly shake houses and bridges, and sometimes completely destroy them.

To better understand these waves, Victor Tsai and Greg Hirth, two geologists from Brown University, studied the mathematics of debris flows - when stones of various sizes periodically collide with each other. Then they applied it to the emerging faults. There is not much free space inside the fault, so what is happening is reminiscent of a “ball-filled pinball machine,” Tsai said. Balls are stones of various sizes, from several meters in diameter to a football field.

When Tsai and Hirt added this crowd to traditional models, the resulting combination described both low-frequency waves and their high-frequency counterparts.

To some extent, the pinball mechanism can be considered an extension of traditional ideas that protrusions and lumps located on the fault walls are responsible for high-frequency waves. However, Tsai and Hirth developed this idea, developed a special pinball mechanism, and studied the exact mathematics that describe it. They turned the assumption into something tangible and verifiable. They are not just “trying to do abstract science,” said Bruhat. “They are really trying to test a physical idea and see how it works.”

Stones and hard surfaces


This new model could help solve long-standing seismological puzzles. For example, in 1999, Taiwan suffered from a deadly earthquake of magnitude 7.7. During repeated shocks, some parts of the fault cracked again, and each time the earth moved in the same direction. However, the magnitude of these shocks was constantly changing for some reason.

Traditional models do not provide satisfactory explanations for this fact. But according to the new “pinball” model, all these repetitive shocks are related to the fact that “balls” of the same size hit the same fault point, causing the earth to move in one direction. However, some of the repetitive shocks probably occurred as a result of a greater number of simultaneous hits, which is why their magnitude was higher.

The new model can also explain why earthquakes on mature faults — old ones that have slipped many times — usually result in less damage than earthquakes of the same magnitude on fresh faults. Earthquakes of the first category with a long history of tremors constantly ground their large fragments, due to which fewer collisions occur in them, and the generated high-frequency waves become weaker.

Graves says that if the model is confirmed, then scientists can carefully study the fault zones and use their geometry to predict the destructive high-frequency waves of future earthquakes. Also, this idea can work in the other direction: if the model gives a more accurate description of the high-frequency component of the earthquake, scientists are likely to be able to more accurately determine the geometric properties responsible for the shocks, said Bruhat.

It will take many more such cascades of earthquakes that were observed in Taiwan - as well as shocks from both mature and immature faults - to assess the relative pros and cons of the old and new models. Seismologists will want to see which one better describes the observations that they will receive on the surface.

However, the new model looks “definitely intriguing,” Graves said. “I think it is believable and worthy of additional tests.”

“I myself immediately admit that there is no evidence that this model is definitely correct, and the old one is definitely wrong,” Tsai said. However, if new ideas prove their superiority, it will force seismologists to "rethink their understanding of earthquakes."

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