剛剛發生的是主震嗎？
By Michèle Marti
目前為止還沒有辦法預測強烈地震發生之後，是否還有更強的地震會接著發生。不過蘇黎世聯邦理工學院瑞士地震研究所的學者進行的新研究，讓人們未來有望做出近乎即時的預測。
2016年義大利中部一場強烈地震之後發生了規模更大的餘震，摧毀了許多村莊與城鎮。圖片來源：keystone
雖然多數大地震發生前並沒有前震，但發生之後都會有數以千計的餘震，其頻率與規模會隨著時間經過而逐漸下降。然而，在某些案例中，一場大地震過後接著發生的另一場地震強度甚至還要更強。2016年襲擊義大利中部，以及2019年7月發生在美國加州里奇雷斯特市的一連串地震就是這樣。
目前為止還沒有辦法預測大地震過後，是否有可能發生另外一場規模相近，甚至還要更大的地震。但是蘇黎世聯邦理工學院瑞士地震研究所的Laura
Gulia和Stefan
Wiemer最近發表在《自然》(Nature)的研究結果，讓我們可以冀望人類很快就能辦到這件事，而且是進行即時預測。這項科研成果可能會在許多方面產生深遠的影響，像是保障人民的安全；疏散民眾時可以做出更有根據的決定；搜救人員可以依此決定目標；以及安裝相關措施來確保發電廠之類的重要設施安全無虞。
研究作者探討的相關參數稱為b值，代表了地震規模與發生次數之間的關係。實驗室試驗顯示b值可以間接顯示地殼內部的應力狀態。在地震活躍的地區b值通常趨近於1，代表規模3的地震數目會比規模大於等於4的地震還要多出10倍左右。他們根據近期的地震資料，設計出一套方法來判斷一個地震序列是否已經結束，或者還有更為強烈的地震會接著發生。
交通號誌系統
研究人員表示b值在一套地震序列當中的變化是有系統的。為了證實這項論點，他們檢驗了58個地震序列數據，並且想出一個交通號誌系統來代表接下來的情況。當b值下降10個百分點或者更多，交通號誌就會變成紅燈，代表規模更大的地震有立即發生的危險。不過多數情況下b值會上升10個百分點以上，這時交通號誌就會轉成綠燈，代表一切安全。這是因為系統預測此序列就像典型的地震序列一樣會逐漸平息。在研究人員探討的資料庫中，80%的序列都是這種情形。而b值上升下降的幅度小於10個百分點時，交通號誌就會顯示為黃燈，意味接下來的狀況還不明朗。
結果顯示研究人員設計的交通號誌系統，可以精確預測他們檢視的95%案例。b值的變化透露出一個地震序列將會如何發展，經由觀測該數值便能指出是否有更加強烈的地震可能接著發生。不過他們的成果在變成可以實際保護民眾的系統之前，仍然需要用來檢驗其他資料庫以證實其效力。此外，要讓該系統成功運作也需要密集的地震觀測網，並且有相應的能力進行資料處理。目前來說，並非所有能從這種交通號誌系統獲益的地區都具備上述條件。
Was that the main quake?
So far it has not been possible to
predict whether a strong quake will probably be followed by a stronger one or
not. A new study by researchers from the Swiss Seismological Service at ETH
Zurich gives rise to hopes of being able to make predictions in almost real
time.
Whereas most major earthquakes are not preceded by
foreshocks, they are always followed by thousands of aftershocks, whose
frequency and magnitude fade over time. However, in some cases a major
earthquake is followed by an even more powerful one. This was what happened in
the sequences of earthquakes that hit Central Italy in 2016 or Ridgecrest,
California (USA) in July 2019.
Up to now, there was no way of predicting whether a
powerful earthquake was likely to be followed by one of even greater magnitude.
But the results of a study recently published in Nature by Laura Gulia and Stefan Wiemer from the Swiss
Seismological Service (SED) at ETH Zurich awaken hopes that we will soon be
able to do just that, in real time. Such a scientific discovery would have
far-reaching consequences for civil protection, enabling more reliable
decisions about evacuating people, allowing rescue workers to target their
efforts accordingly, and permitting the implementation of measures to secure
critical infrastructure, such as power stations.
Based on recent seismic data, the authors of the
study have devised a method that can be used to determine whether a sequence of
earthquakes is rather ending or will be followed by an even more powerful
earthquake. The relevant parameter they examined was the so-called b-value,
which characterises the relationship between the magnitude and number of
quakes. Laboratory measurements show that this value indirectly indicates the
state of stress in the Earth's crust. In seismically active regions the b-value
is usually close to 1, meaning that there are about 10 times as many magnitude
3 earthquakes than quakes with a magnitude of 4 or higher.
A traffic light
system
The researchers have now demonstrated that the
b-value changes systematically in the course of an earthquake sequence. To
prove this, they examined data from 58 sequences and came up with a
traffic-light system indicating what would happen next. When the b-value drops
by 10 percent or more, the traffic light turns red, suggesting acute danger of
an even more powerful quake. In most cases, though, the b-value rises by 10 percent
or more and the traffic light turns green, giving the all-clear by predicting a
typical sequence that will gradually fade away. This happened in 80 percent of
the sequences captured in datasets examined by the researchers. The traffic
light shows amber when the b-value rises or falls by less than 10 percent,
meaning it is unclear what will happen next.
The traffic-light system devised by the researchers
turned out to be accurate in 95 percent of the cases they examined. The
observed change in the b-value betrayed how a sequence would develop,
indicating whether or not an even more powerful earthquake would follow. That
said, their findings will have to be verified by examining other datasets
before such a system can actually be used for civil protection. The system's
successful deployment would also require a dense seismic network and
corresponding data processing capacity. By no means all regions that could
benefit from such a traffic-light system currently have these things.
原始論文：Laura
Gulia, Stefan Wiemer. Real-time
discrimination of earthquake foreshocks and aftershocks. Nature, 2019; DOI:
10.1038/s41586-019-1606-4
引用自：ETH
Zurich. “Was that the main quake?”
沒有留言:
張貼留言