地震發生期間摩擦力如何變化

原文網址：http://www.caltech.edu/news/how-friction-evolves-during-earthquake-79371

地震發生期間摩擦力如何變化

Robert Perkins

加州理工學院的工程師經由在實驗室模擬地震，來記錄地震發生期間摩擦力的演變過程。他們藉此測量出過去只能靠推算得到的性質，同時也闡明了地震模型中最重要的未知參數之一。

在地震之前，靜摩擦力可以幫助固定斷層的兩側並讓它們抵住對方。在地震破裂延伸期間，斷層兩側彼此錯動時靜摩擦力會轉變成動摩擦力。動摩擦力在整場地震期間會不斷演變，因而影響地面晃動的速度和大小，使其對地震的破壞性來說至關重要。

加州理工學院工程與應用科學部(EAS)的研究學者Vito Rubino表示：「在破裂釋放地殼內部斷層的過程中，摩擦力具有關鍵地位。動摩擦力的推估值會影響地震預警科學中的許多層面，包括破裂會以多快的速率發生、地面晃動的性質以及斷層面上殘留應力的多寡。然而，動摩擦力的確切性質仍然是地震科學中最大的未知數之一。」

科學家之前一般認為動摩擦力的演變過程主要受控於破裂經過時，斷層上每一個點滑動了多遠，也就是在動態滑動過程中斷層其中一側相對於另一測的滑動距離。團隊分析了在實驗室中模擬的地震之後，發現滑動的歷程固然重要，但長期來看滑動速度實際上才是關鍵因素――不只斷層滑動距離有多長，而是滑動速度有多快。

描述研究團隊發現的論文刊登於6月29日的《自然通訊》(Nature Communications)，主要作者為Rubino。他的合作對象包括加州理工學院EAS的航空與機械工程學教授Ares Rosakis，以及EAS和加州理工學院地質與行星部合聘的機械工程與地球物理教授Nadia Lapusta。

團隊在加州理工學院由Rosakis主持，非正式名稱為「地震風洞」(seismological wind tunnel)的設施中進行這項研究。在此設施中，研究人員利用最新的高速光學量測技術和其他技術來研究地震破裂如何發生。

Rosakis表示：「我們特有的設備讓我們可以即時追蹤每一條迅速形成的剪切破裂，並且記錄沿著滑動面產生的摩擦力，藉此來研究動摩擦力定律。」Rosakis接著表示：「這讓我們首度能逐步探討每一點的摩擦力，而不需要跟傳統對摩擦力進行的研究一樣，假設滑動發生過程是各處一致的。」

為了在實驗室中模擬地震，研究人員先把一種稱為homalite，擁有跟岩石類似力學性質的透明塑膠材料做成的方塊切成兩半。然後研究人員施予壓力使兩個斷塊合在一起，來模擬沿著斷層線逐漸累積的靜摩擦力。接著，他們在想要讓震央發生的地點安裝一條小型鎳―鉻導火線，將其引爆後就能釋放該處的壓力，使得摩擦力減小而讓破裂在這條微型斷層上快速延伸。

在此研究中，研究人員運用新的量測技術來記錄模擬地震的發生過程。該技術結合了高速攝影(每秒200萬張)和一種稱為數位影像相關(digital image correlation)的技術，其原理是比較個別影像彼此之間的異同以及它們的變化過程，而能夠以次像素的精準度來追蹤運動歷程。

Lapusta表示：「包括我在加州理工學院的團隊所發展的模型在內，某些地震破裂的數值模型根據一些岩石力學實驗和理論，採取的摩擦定律是以滑動速度為變量。看到我們實驗中微型地震產生的自發性破裂可以應證這些理論讓我感到相當滿意。」

在未來工作中，團隊計畫利用他們的觀察結果來改進現今對於動摩擦力性質做出的數學模型，並且建立能更精準呈現實驗觀測結果的新模型，它們或許能改良電腦對於地震的模擬。

此研究標題為「Understanding dynamic friction through spontaneously evolving laboratory earthquakes」。支持此研究的單位為美國國家科學基金會、美國地質調查所和南加州地震中心。

How friction evolves during an earthquake

By simulating earthquakes in a lab, engineers at Caltech have documented the evolution of friction during an earthquake—measuring what could once only be inferred, and shedding light on one of the biggest unknowns in earthquake modeling.

Before an earthquake, static friction helps hold the two sides of a fault immobile and pressed against each other. During the passage of an earthquake rupture, that friction becomes dynamic as the two sides of the fault grind past one another. Dynamic friction evolves throughout an earthquake, affecting how much and how fast the ground will shake and thus, most importantly, the destructiveness of the earthquake.

"Friction plays a key role in how ruptures unzip faults in the earth's crust," says Vito Rubino, research scientist at Caltech's Division of Engineering and Applied Science (EAS). "Assumptions about dynamic friction affect a wide range of earthquake science predictions, including how fast ruptures will occur, the nature of ground shaking, and residual stress levels on faults. Yet the precise nature of dynamic friction remains one of the biggest unknowns in earthquake science."

Previously, it commonly had been believed that the evolution of dynamic friction was mainly governed by how far the fault slipped at each point as a rupture went by—that is, by the relative distance one side of a fault slides past the other during dynamic sliding. Analyzing earthquakes that were simulated in a lab, the team instead found that sliding history is important but the key long-term factor is actually the slip velocity—not just how far the fault slips, but how fast.

Rubino is the lead author on a paper on the team's findings that was published in Nature Communications on June 29. He collaborated with Caltech's Ares Rosakis, the Theodore von Kármán Professor of Aeronautics and Mechanical Engineering at EAS, and Nadia Lapusta, professor of mechanical engineering and geophysics, who has joint appointments with EAS and the Caltech Division of Geological and Planetary Sciences.

The team conducted the research at a Caltech facility, directed by Rosakis, that has been unofficially dubbed the "seismological wind tunnel." At the facility, researchers use advanced high-speed optical diagnostics and other techniques to study how earthquake ruptures occur.

"Our unique facility allows us to study dynamic friction laws by following individual, fast-moving shear ruptures and recording friction along their sliding faces in real time," Rosakis says. "This allows us for the first time to study friction point-wise and without having to assume that sliding occurs uniformly, as is done in classical friction studies," Rosakis adds.

To simulate an earthquake in the lab, the researchers first cut in half a transparent block of a type of plastic known as homalite, which has similar mechanical properties to rock. They then put the two pieces together under pressure, simulating the static friction that builds up along a fault line. Next, they placed a small nickel-chromium wire fuse at the location where they wanted the epicenter of the quake to be. Triggering the fuse produced a local pressure release, which reduced friction at that location, and allowed a very fast rupture to propagate up the miniature fault.

In this study, the team recorded these simulated earthquakes using a new diagnostic method that combines high-speed photography (at 2 million frames per second) with a technique called digital image correlation, in which individual frames are compared and contrasted with one another and changes between those images—indicating motion—are tracked with sub-pixel accuracy.

"Some numerical models of earthquake rupture, including the ones developed in my group at Caltech, have used friction laws with slip-velocity dependence, based on a collection of rock mechanics experiments and theories. It is gratifying to see those formulations validated by the spontaneous mini-earthquake ruptures in our study, " Lapusta says.

In future work, the team plans to use its observations to improve the existing mathematical models about the nature of dynamic friction and to help create new ones that better represent the experimental observations; such new models would improve computer earthquake simulations.

The study is titled "Understanding dynamic friction through spontaneously evolving laboratory earthquakes." This research was supported by the National Science Foundation, the U.S. Geological Survey, and the Southern California Earthquake Center.

原始論文：V. Rubino, A. J. Rosakis, N. Lapusta. Understanding dynamic friction through spontaneously evolving laboratory earthquakes. Nature Communications, 2017; 8: 15991 DOI: 10.1038/ncomms15991

引用自：California Institute of Technology. "How friction evolves during an earthquake: By simulating quakes in a lab, engineers study the way that friction changes along a fault during a seismic event."