2019年10月29日 星期二

以慢動作發生的地震


以慢動作發生的地震
研究慢速滑移事件可以更加瞭解破壞性的地震
By Robert Perkins
加州理工學院進行的新研究發現稱作慢速滑移(slow slip)無聲地震(silent earthquake)的事件,其行為比以往認為的還要更接近一般地震。對於地質學家來說這項成果開啟了一條新的研究途徑:這些頻繁發生且不會造成破壞的事件,可以當作研究起來更加容易的類比,藉此來找出地震發生的原因究竟是什麼。

大約20年前,地質學家運用GPS技術追蹤以此才能查覺到的地表位移量時,首次注意到慢速滑移事件。它們的形成原因為斷層兩側以極為緩慢的速度擦過彼此,就像是慢動作播放的地震。舉例來說,一場為時數周的慢速滑移事件釋放出的能量,可能相當於一場僅有一分鐘的規模7.0地震。但由於慢速滑移事件發生在地底深處且釋放能量的速度十分緩慢,導致它們的影響範圍雖然可以到達數千平方公里,在地表造成的變形量卻極為微小。因為如此,只有在GPS技術改良到能追蹤相當微小的位移之後,科學家才注意到它們的存在。並非所有的斷層都會發生慢速滑移事件,目前為止,只有在少數幾個地點發現它們,像是西北太平洋、日本、墨西哥和紐西蘭。
加州理工學院地質學和機械與土木工程學的教授Jean-Philippe Avouac表示,由於他們才剛開始發現這些事件並記錄下來,因此仍有很多地方還不甚瞭解。「我們還有很多地方無法確定。由於這些事件產生的訊號太過微弱,而且人類活動與自然作用,像是海浪、河川與風吹的訊號都會蓋過它們,使得傳統的地震儀器無法用來研究這些事件。」Avouac說在他們的團隊開始這項研究之前,文獻中的慢速滑移事件還不夠多,造成科學家缺乏依據來確立它們的比例性質。
在華盛頓州的卡斯卡迪亞隱沒帶,太平洋板塊往東北方滑到北美板塊之下。Avouac的團隊利用352GPS站組成的觀測網得到的資料,並且運用他們設計出的全新訊號處理技術,偵測並描繪出發生在此的慢速滑移事件。研究人員分析2007年至2018年的數據,建立起一份清單記載了40多起大小不一的慢速滑移事件。他們的發現發表在1023日的《自然》(Nature)
研究人員彙整這些事件的數據之後,可以更加精準地找出慢速滑移事件的特徵。他們的重要發現之一是慢速滑移事件和一般地震都遵守同樣的比例定律。
前述的比例定律描述了發生在斷層的滑移事件,其力矩――也就是斷層滑移時釋放的彈性能多寡――和滑移時間會呈現特定關係。具體來說,這代表在面積較大的地區上發生較長距離的滑動,產生的地震持續時間也較長。科學家許久以前就已經知道地震的力矩跟持續時間的立方成正比。而在2007年,東京大學和史丹佛大學組成的研究團隊提出慢速滑移事件並非如此,其力矩似乎跟時間直接成正比。
Avouac的團隊以這份最新清單為證據,主張慢速滑移事件就跟一般的地震一樣,規模會和持續時間的立方成正比。
Avouac表示既然這些事件和一般地震有相似的行為,研究它們或許能讓我們深入了解它們更具破壞力的表親,特別是因為慢速滑移事件的發生頻率較高。在同一斷層上,規模7.0的一般地震可能每過200年才會發生一次,但是同等規模的慢速滑移事件卻可能每一或兩年就會發生一次。
「如果我們持續研究一條斷層十幾年,可能會看到這類事件發生十次。」Avouac表示。「這可以讓我們測試地震循環的模型,並得知斷層的不同段落之間如何互相影響。結果可以清楚顯示出隨著時間經過,一條大型斷層上各處的能量是如何累積起來並且釋放出去。」他說這些資訊能讓我們深入解析地震的發生機制,並且了解哪些物理過程控制了地震的發生時間與規模大小。

Earthquakes in slow motion
Studying "slow-slip" events could shed light on destructive temblors
A new study from Caltech finds that so-called "slow slip" or "silent" earthquakes behave more like regular earthquakes than previously thought. The discovery opens the door for geoscientists to use these frequent and nondestructive events as an easy-to-study analog that will help them find out what makes earthquakes tick.
Slow-slip events were first noted about two decades ago by geoscientists tracking otherwise imperceptible shifts in the earth using GPS technology. They occur when faults grind incredibly slowly against each other, like an earthquake in slow motion. For example, a slow-slip event that occurs over the course of weeks might release the same amount of energy as a minute-long magnitude-7.0 earthquake. Because they occur deep in the earth and release energy so slowly, there is very little deformation at the surface, although the slow events might affect an area of thousands of square kilometers. As such, they were only noted when GPS technology was refined to the point that it could track those very minute shifts. Slow-slip events also do not occur along every fault; so far, they have been spotted in just a handful of locations including the Pacific Northwest, Japan, Mexico, and New Zealand.
As they have only just begun to be detected and cataloged, a lot remains unknown about them, says Jean-Philippe Avouac, Caltech's Earle C. Anthony Professor of Geology and Mechanical and Civil Engineering. "There's a lot of uncertainty. You can't study them using traditional seismological techniques because the signal they create is too faint and gets lost in the noise from human activities as well as from natural geological processes like ocean waves, rivers, and winds." Before Avouac's group began this study, there were not enough documented slow-slip events to determine their scaling properties reliably, he says.
Avouac's group designed and applied an innovative signal processing technique to detect and image the slow-slip events along Washington state's Cascadia Subduction Zone, where the North American tectonic plate is sliding southwest over the Pacific Ocean plate, using a network of 352 GPS stations. The researchers analyzed data spanning the years 2007 to 2018 and were able to build a catalog of more than 40 slow-slip events of varied sizes. Their findings appear in Nature on October 23.
Compiling data from these events, the researchers were able to characterize the features of slow-slip events more precisely than previously possible. One key finding from the study is that slow-slip events obey the same scaling laws as regular earthquakes.
In this context, the scaling law describes the "moment" of a slip event on a fault—which quantifies the elastic energy released by slip on a fault—as a function of the duration of slip. In practical terms, that means that a big slip across a broad area yields a long-lasting earthquake. It has long been known that the moment of an earthquake is proportional to the cube of the amount of time the earthquake lasts. In 2007, a team from the University of Tokyo and Stanford suggested that slow-slip events appear to be different, with the moment seemingly directly proportional to time.
Armed with their new fleshed-out catalog, Avouac's team argues that the magnitudes of slow-slip events also are proportional to the cube of their duration, just like regular earthquakes.
Since these events behave similarly to regular earthquakes, studying them could shed light on their more destructive cousins, Avouac says, particularly because slow-slip events occur more frequently. While a traditional magnitude-7.0 earthquake might only occur along a fault every couple of hundred years, a slow-slip event of that magnitude can reoccur along the same fault every year or two.
"If we study a fault for a dozen years, we might see 10 of these events," Avouac says. "That lets us test models of the seismic cycle, learning how different segments of a fault interact with one another. It gives us a clearer picture of how energy builds up and is released with time along a major fault." Such information could offer more insight into earthquake mechanics and the physics governing their timing and magnitude, he says.
原始論文:Sylvain Michel, Adriano Gualandi, Jean-Philippe Avouac. Similar scaling laws for earthquakes and Cascadia slow-slip eventsNature, 2019; 574 (7779): 522 DOI: 10.1038/s41586-019-1673-6
引用自:California Institute of Technology. "Earthquakes in slow motion.”

沒有留言:

張貼留言