研究揭露了大自然本身防止地震發生的機制

 原文網址：https://news.ucr.edu/articles/2021/04/19/study-reveals-workings-natures-own-earthquake-blocker

這項發現指出了可以更加精準的預測地震的方式

By Jules Bernstein

一項新研究發現了自然界具有的「地震閘道」決定了哪些地震的規模可以增加到8以上。

地震研究人員在紐西蘭阿爾卑斯斷層附近採集沉積物以進行這篇研究。(圖片來源：Jamie Howarth/Victoria University of Wellington)

有的時候這道閘門會把地震擋在規模7的範圍，但是通過的就能增加到規模8以上，釋放出比規模7的地震高出32倍以上的能量

「地震閘門就像是在施工的單線道前的指揮號誌。有的時候你停下車會看到代表『通行』的綠燈標誌，其他時候代表『停下』的紅燈標誌則是要你等到狀況有所改變，」加州大學河濱分校的地質學家Nicolas Barth表示。

研究人員在研究紐西蘭的阿爾卑斯斷層時發覺了這道閘門。他們判斷這條斷層未來50年有75%的機率產生的地震會造成破壞，模型也顯示該地震造成的破裂有82%的機率穿過這道閘門而變成規模8以上。他們的見解最近發表在期刊《自然―地球科學》(Nature Geoscience)。

Barth所屬的國際團隊成員包括了威靈頓維多利亞大學、紐西蘭地質與核子科學研究所、奧塔哥大學與美國地質調查局的科學家。

他們的研究結合了兩種研究地震的方法：地質學家蒐集過去的地震留下的痕跡，而地球物理學家則運用電腦進行模擬。研究人員聯合這兩種方法才得出新的見解，進而預估阿爾卑斯斷層未來發生的地震的行為模式。

「大地震會造成劇烈晃動，也會造成山崩而讓大量土石順流而下或是進到湖泊裡面，」主要作者，威靈頓維多利亞大學的地質學家Jamie Howarth表示。「我們可以鑽取數公尺深的湖床沉積物，從裡面辨識出地震曾經在附近造成晃動而形成的特殊模式。透過這些沉積物的定年結果，我們可以精準判斷地震發生的年代。」

阿爾卑斯斷層附近的沉積物紀錄的地震可以追溯到數千年前。(圖片來源：Jamie Howarth/Victoria University of Wellington)

他們從阿爾卑斯斷層沿線的六個地點採集沉積物紀錄，從中辨認出過去4000年20個地震事件的大小。這是世界上同樣類型的紀錄中最為詳細的之一。

這份地震紀錄的完整程度讓研究人員有難得的機會，可以和電腦產生的十萬年地震紀錄互相比較。研究團隊使用的地震模擬程式碼是由加州大學河濱分校的特聘名譽教授James Dieterich所研發。

只有幾何型態與阿爾卑斯斷層符合的斷層模型才能重現他們的地震數據。「模擬顯示發生在地震閘門附近規模6到7的小地震可以改變應力，中斷連續發生的大地震，」Barth表示。「我們知道最近三次的破裂都通過了這道閘門。在我們最符合下次地震的模型當中，也有82%的機率會通過。」

放眼紐西蘭之外，地震閘門對於密切研究加州地區的研究人員來說也是相當重要的領域。南加州地震中心是個超過100個學術單位組成的組織，核心成員之一為加州大學河濱分校。他們也把地震閘門放在研究的優先順位。研究人員特別關注聖伯納迪諾附近的卡洪山口，由於聖安德魯斯斷層和聖哈辛托斷層在此交接，因此可能會形成地震閘門的行為模式，進而調控下次大地震的強度。

「我們的數據和模型已經越來越仔細，而讓我們有機會能開始預測地震的模式。不只是地震發生的機率，我們還能預測它的大小以及影響範圍，使我們可以做出更好的事前預備。」

 

Study reveals the workings of nature's own earthquake blocker

Findings point way toward more detailed earthquake predictions

A new study finds a naturally occurring “earthquake gate” that decides which earthquakes are allowed to grow into magnitude 8 or greater.

Sometimes, the “gate” stops earthquakes in the magnitude 7 range, while ones that pass through the gate grow to magnitude 8 or greater, releasing over 32 times as much energy as a magnitude 7.

“An earthquake gate is like someone directing traffic at a one-lane construction zone. Sometimes you pull up and get a green ‘go’ sign, other times you have a red ‘stop’ sign until conditions change,” said UC Riverside geologist Nicolas Barth.

Researchers learned about this gate while studying New Zealand’s Alpine Fault, which they determined has about a 75 percent chance of producing a damaging earthquake within the next 50 years. The modeling also suggests this next earthquake has an 82 percent chance of rupturing through the gate and being magnitude 8 or greater. These insights are now published in the journal Nature Geoscience.

Barth was part of an international research team including scientists from Victoria University of Wellington, GNS Science, the University of Otago, and the US Geological Survey.

Their work combined two approaches to studying earthquakes: evidence of past earthquakes collected by geologists and computer simulations run by geophysicists. Only by using both jointly were the researchers able to get new insight into the expected behavior of future earthquakes on the Alpine Fault.

“Big earthquakes cause serious shaking and landslides that carry debris down rivers and into lakes,” said lead author Jamie Howarth, Victoria University of Wellington geologist. “We can drill several meters through the lake sediments and recognize distinct patterns that indicate an earthquake shook the region nearby. By dating the sediments, we can precisely determine when the earthquake occurred.”

Sedimentary records collected at six sites along the Alpine Fault identified the extent of the last 20 significant earthquakes over the past 4,000 years, making it one of the most detailed earthquake records of its kind in the world.

The completeness of this earthquake record offered a rare opportunity for the researchers to compare their data against a 100,000-year record of computer-generated earthquakes. The research team used an earthquake simulation code developed by James Dieterich, distinguished professor emeritus at UC Riverside.

Only the model with the fault geometry matching the Alpine Fault was able to reproduce the earthquake data. “The simulations show that a smaller magnitude 6 to 7 earthquake at the earthquake gate can change the stress and break the streak of larger earthquakes,” Barth said. “We know the last three ruptures passed through the earthquake gate. In our best-fit model the next earthquake will also pass 82% of the time.”

Looking beyond New Zealand, earthquake gates are an important area of active research in California. The Southern California Earthquake Center, a consortium of over 100 institutions of which UCR is a core member, has made earthquake gates a research priority. In particular, researchers are targeting the Cajon Pass region near San Bernardino, where the interaction of the San Andreas and San Jacinto faults may cause earthquake gate behavior that could regulate the size of the next damaging earthquake there.

“We are starting to get to the point where our data and models are detailed enough that we can begin forecasting earthquake patterns. Not just how likely an earthquake is, but how big and how widespread it may be, which will help us better prepare,” Barth said.

原始論文：Jamie D. Howarth, Nicolas C. Barth, Sean J. Fitzsimons, Keith Richards-Dinger, Kate J. Clark, Glenn P. Biasi, Ursula A. Cochran, Robert M. Langridge, Kelvin R. Berryman, Rupert Sutherland. Spatiotemporal clustering of great earthquakes on a transform fault controlled by geometry. Nature Geoscience, 2021; DOI: 10.1038/s41561-021-00721-4

引用自：University of California - Riverside. "Study reveals the workings of nature's own earthquake blocker.”