研究人員建構新的理論來解釋造成破壞的地震波如何產生
高頻震波是地震產生的震動中最具破壞力的一種類型。布朗大學的研究人員得出了一套新的理論可以解釋它們如何產生。
圖中是位在緬因州的一條斷層帶。研究人員提出地震時斷層帶內部的岩石互相碰撞，可能是造成損害的高頻震波成因。圖片來源：Julia
Carr
地震會產生許多不同頻率的地震波，像是滾動一般的低頻震波會讓摩天大樓搖晃，而像抽搐一般的高頻震波則會讓房屋和其他小型建築受到嚴重損害。布朗大學的兩位地球物理學家對於高頻震波的可能形成方式有了新的解釋。
在發表於《地球物理研究通訊》(Geophysical Research Letters)的論文中，布朗大學的教師Victor
Tsai和
Greg Hirth提出地震發生時，斷層帶內的岩石互相碰撞是高頻震波的主要來源。研究人員表示這和傳統的解釋十分不同，但或許有助於解開某些地震奇特的震波模式。此外也可以讓科學家預測哪些斷層可能發生破壞力較強的地震。
「一般我們對地震的理解是因為應力持續累積在斷層上，直到斷層終於承受不住使得兩邊互相錯動。而我們所觀察到的地面震動全都是由這股滑動所造成。」布朗大學地球、環境與行星科學系的副教授Tsai表示。「這篇論文的構想是評估有沒有滑動以外的作用參與其中。最根本的問題是：如果斷層帶滑動時裡面的物體互相撞擊，那會產生什麼樣的物理現象？」
Tsai和Hirth的基本概念來自於描述山崩或者其他類型的碎屑流發生時，裡面的岩石會如何撞擊的數學模型，接著他們建構出新的模型來預測斷層帶的岩石碰撞時可能會產生什麼效應。模型顯示出碰撞確實有可能是高頻振動的主要來源。研究人員表示不太符合傳統摩擦滑動模型的地震觀察結果，將碰撞模型和傳統模型結合之後可以提供合理的解釋。
舉例來說，模型結合起來可以解釋重複地震——一種發生在斷層內同一地點，地震波形幾乎相同的地震序列。這些地震奇特的地方在於它們的規模雖然常常差距很大，但是造成的地表運動模式卻幾乎相同。研究人員表示這種現象如果只用滑移很難去解釋，但是加入撞擊模型之後就能得到較為合理的結果。
「如果兩個地震都是發生在斷層帶的相同地方，那麼撞在一起的岩石也會是一樣的，至少它們的大小基本上會相同。」Tsai表示。「因此，如果高頻振動是由碰撞產生，那麼不管滑動了多少，它們的高頻地表運動會有相同的行為也就沒什麼好驚訝的。」
碰撞模型也有助於解釋為什麼發育成熟的斷層(長時間下來發生過許多地震的斷層)和發育不成熟的斷層發生相同規模的地震時，前者造成的災害通常較少。這是因為隨著時間流逝，地震一再發生會把斷層內的岩石磨碎。碰撞模型預測較為平滑的斷層中，內部的岩石稜角較少，因此互相碰撞時產生的高頻振動也會較弱。
Tsai表示還需要進行更多研究才能完全證明他們的模型，但初步成果顯示此理論是有潛力的。如果之後可以確實證明模型的效力，或許能幫助科學家分類哪些斷層可能產生破壞程度較高的地震。
「人類已經觀察到某些類型的斷層似乎比其他斷層產生更多的高頻振動或者更少，但還不清楚為什麼這些斷層屬於某個類別或是另一類。」他說。「我們提供了一個可行的基礎框架來瞭解這項問題，甚至可以歸廣至全世界的所有斷層。一般來說，內部構造較為圓滑的平滑斷層產生的地震高頻震波可能較少，而粗糙的斷層產生的地震則會有較多的高頻震波。」
研究也提出對於地震，我們可能需要重新審視某些抱持已久的看法。
「某種層面上這或許代表了對於地震的某些部份，我們的認知其實比我們以為的還少。」Tsai表示。「如果斷層滑移並非故事的全貌，那麼我們就需要去更加了解斷層帶的構造。」
Researchers develop new explanation
for destructive earthquake vibrations
High-frequency vibrations are some of the
most damaging ground movements produced by earthquakes, and Brown University
researchers have a new theory about how they’re produced.
Earthquakes produce seismic waves with a range of
frequencies, from the long, rolling motions that make skyscrapers sway, to the
jerky, high-frequency vibrations that cause tremendous damage to houses and
other smaller structures. A pair of Brown University geophysicists has a new
explanation for how those high-frequency vibrations may be produced.
In a paper published in Geophysical Research Letters, Brown faculty members Victor Tsai and
Greg Hirth propose that rocks colliding inside a fault zone as an earthquake
happens are the main generators of high-frequency vibrations. That’s a very
different explanation than the traditional one, the researchers say, and it
could help explain puzzling seismic patterns made by some earthquakes. It could
also help scientists predict which faults are likely to produce the more
damaging quakes.
“The way we normally think of earthquakes is that
stress builds up on a fault until it eventually fails, the two sides slip
against each other, and that slip alone is what causes all the ground motions
we observe,” said Tsai, an associate professor in Brown’s Department of Earth,
Environmental and Planetary Sciences. “The idea of this paper is to evaluate whether
there’s something other than just slip. The basic 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 describe the
collisions of rocks during landslides and other debris flows, Tsai and Hirth
developed a model that predicts the potential effects of rock collisions in
fault zones. The model suggested the collisions could indeed be the principal
driver of high-frequency vibrations. And combining the collision model with
more traditional frictional slip models offers reasonable explanations for earthquake
observations that don’t quite fit the traditional model alone, the researchers
say.
For example, the combined model helps explain
repeating earthquakes — quakes that happen at the same place in a fault and
have nearly identical seismic wave forms. The odd thing about these quakes is
that they often have very different magnitudes, yet still produce ground
motions that are nearly identical. That’s difficult to explain by slip alone,
but makes more sense with the collision model added, the researchers say.
“If you have two earthquakes in the same fault zone,
it’s the same rocks that are banging together — or at least rocks of basically
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 also may help explain why quakes
at more mature fault zones — ones that have had lots of quakes over a long
period of time — tend to produce less damage compared to quakes of the same
magnitude at more immature faults. Over time, repeated quakes 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.
Tsai says that more work needs to be done to fully
validate the model, but this initial work suggests the idea is promising. If
the model does indeed prove valid, it could be helpful in classifying which
faults are likely to produce more or less damaging quakes.
“People have made some observations that particular
types of faults seem to generate more or less high-frequency motion than
others, but it has not been clear why faults fall into one category or the
other,” he said. “What we’re providing is a potential framework for
understanding that, and we could potentially generalize this to all faults
around the world. Smoother faults with rounded internal structures may
generally produce less high-frequency motions, while rougher faults would tend
to produce more.”
The research also suggests that some long-held ideas
about how earthquakes work might need revising.
“In some sense it might mean that we know less about
certain aspects of earthquakes than we thought,” Tsai said. “If fault slip
isn’t the whole story, then we need a better understanding of fault zone
structure.”
原始論文：Victor C.
Tsai, Greg Hirth. Elastic Impact Consequences for High‐Frequency
Earthquake Ground Motion. Geophysical Research Letters,
2020; DOI: 10.1029/2019GL086302
引用自：Brown University. "Researchers develop new
explanation for destructive earthquake vibrations."
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