雷達顯示北韓最近一次核武試爆之後山體坍塌的詳細情形
By Robert Sanders
在北韓總統承諾朝鮮半島「去核化」的同時，一組國際科學家團隊也公布了目前為止最為詳細的圖像，顯示北韓最近一次地下核武試爆的整體情形。該次試爆在2017年9月3日進行，也是北韓史上規模最大的一次核武試爆。
這幅新影像呈現出引爆點上方的山丘因為爆炸而產生的變化。其凸顯出當我們想要更加精準地監控北韓和世界其他地方的核武試爆地點和爆炸當量時，除了地震儀紀錄之外，稱作SAR(合成孔徑雷達)的衛星雷達產生的影像也具有相當重要的價值。
研究人員包括了新加坡南洋理工大學地球觀測站的Teng Wang、Qibin Shi、Shengji Wei和Sylvain Barbot；美國加州大學柏克萊分校的Douglas Dreger和Roland Bürgmann；德國波茨坦地質科學研究中心的Mehdi Nikkhoo；德國漢諾威大學的Mahdi Motagh；以及北京中國科學院的Qi-Fu Chen。他們的成果在正式發表於期刊《科學》之前，於本周先行發布線上版。
這場爆炸發生於北韓北部豐溪里的萬塔山核試驗場，它對當地造成的震動相當於規模5.2的地震。根據全球和區域地震觀測網的紀錄，以及德國TerraSAR-X和日本ALOS-2的雷達成像衛星在事件前後對地表的測量結果，團隊顯示地下核武爆炸把萬塔山最多往外推了3.5公尺，並讓高度下降了大約50公分。
透過在電腦上模擬這起事件，他們可以精確指出爆炸發生的地點與深度，其位於山頂正下方400至600公尺處。
他們也更加精準地定位出在核彈爆炸8.5分鐘後發生的另一起地震事件(或許也能稱為「餘震」)，大約位在核彈爆炸地點南方700公尺處。這個位置差不多是在核彈引爆處和一條通行隧道入口的中間，因此其成因可能是這條隧道部分發生坍方，或者是過往的核彈爆炸產生的坑洞崩塌所致。
「這是首次完整地把地下核武試爆對地表造成的水平垂直位移以影像呈現出來讓大眾看見。」主要作者，新加坡南洋理工大學地球觀測站的Teng Wang表示。
比廣島核彈還大的爆炸
這次核試驗是北韓的第六次核武試驗，也是在萬塔山進行的第五次。研究人員結合所有數據，預估這次核武試爆的爆炸當量介於12至30萬噸，威力大約是二次大戰期間美國發射至廣島的原子彈的10倍。這種等級的核彈可能是是小型的氫彈(核融合)，也有可能是大型的原子彈(核分裂)。
他們描繪出來的新情況跟上禮拜另外兩篇報告有所差異。其中一篇被期刊《地球物理研究通訊》(Geophysical
Research Letters)接受出版的論文定位出來的爆炸地點，大約位於這篇新研究認定地點的西北方一公里處；且其結論認為這次爆炸使得整座山再也不適合讓未來的核武試爆使用。
「SAR在監測爆炸時確實具有相當獨特的能力，因為它可以直接得到當地地表的影像；不像運用地震學方法得知來源的性質時，是分析事件往外散布出去的震波被遠方地震站接收到的訊號。」加州大學柏克萊分校的地球和行星科學教授Dreger表示，他也是柏克萊地震學實驗室的成員之一。「SAR可以對事發當地進行某種程度上的實地測量，這是相當難以取得的數據。此研究是首度有人結合衛星和地震數據，來實際模擬地下爆炸的機制。」
「跟傳統光學成像衛星的成像技術不同，SAR在白天或夜晚，以及任何天氣狀況下都能測量地表變形。」共同作者，Dreger的同事 Roland Bürgmann表示。他也是加州大學柏克萊分校的地球和行星科學教授。「藉由精確追蹤影像的像素在不同方向的位移量，我們可以測出萬塔山在立體空間中的完整變形量」
據Dreger所說，新的資訊顯示的情況如下：爆炸發生在萬塔山山頂下方超過半公里處，高熱使得花崗岩蒸發而形成一個寬約50公尺的坑洞――相當於一個足球場的大小――受到破壞的岩石總體積則有大約300公尺寬。爆破可能在短短數分鐘之內就把整座山推高2公尺並往外推最多達3至4公尺；在數個小時到數天之後，坑洞上方的岩石坍方而造成山體塌陷。
在核彈爆炸8.5分鐘之後，附近有一個地下坑洞崩塌而形成了規模4.5，呈現出內爆特性的餘震。
之後，寬度可能達1至2公里的大量被震碎的岩石逐漸壓密，使得萬塔山相較於爆炸之前往下降了大約0.5公尺。
Dreger 表示：「爆炸過後的萬塔山可能會持續壓縮。這些不會產生震波的作用會以較慢的速度進行。」
Dreger 表示雖然利用地震波可以區別爆炸和自然產生的地震，但是不確定性卻很大。爆炸通常也會使得附近的斷層產生地震，或是引發其他會讓岩石移動的作用而產生跟地震很相似的震動訊號，因此會混淆分析結果。SAR的數據可以顯示當地從靜止狀態下產生的移動，因此能提供額外的條件幫助科學家縮小可能的來源。
Dreger 表示：「我們希望結合大地測量的數據和地震數據的分析結果，之後可以更準確地分辨地震和爆炸事件。毫無疑問的，這也可以讓我們在估計爆炸的當量以及來源深度時得到更好的結果。」
「這項研究顯示當未來有任何大型的地下核武試爆發生時，太空遙測可以幫助我們瞭解該起事件的特性。」Wang 表示，「目前監控私自進行的核武試爆是依賴全球地震網，不過衛星搭載監控系統的潛力卻一直沒有被好好開發。」
本研究經費來自於新加坡南洋理工大學地球觀測站，以及美國空軍研究實驗室。
Radar reveals details of mountain
collapse after North Korea’s most recent nuclear test
As North
Korea’s president pledges to “denuclearize” the Korean peninsula, an
international team of scientists is publishing the most detailed view yet of
the site of the country’s latest and largest underground nuclear test on Sept.
3, 2017.
The new picture of how the explosion
altered the mountain above the detonation highlights the importance of using
satellite radar imaging, called SAR (synthetic aperture radar), in addition to
seismic recordings to more precisely monitor the location and yield of nuclear
tests in North Korea and around the world.
The researchers – Teng
Wang, Qibin Shi, Shengji Wei and Sylvain Barbot from the Earth Observatory at
Nanyang Technological University in Singapore, Douglas Dreger and Roland
Bürgmann from the University of California, Berkeley, Mehdi Nikkhoo from the
German Research Centre for Geosciences in Potsdam, Mahdi Motagh from the
Leibniz Universität Hannover, and Qi-Fu Chen from the Chinese Academy of
Sciences in Beijing – will report their results online this week in advance of
publication in the journal Science.
That explosion took place
under Mt. Mantap at the Punggye-ri nuclear test site in the country’s north,
rocking the area like a 5.2-magnitude earthquake. Based on seismic recordings
from global and regional networks, and before-and-after radar measurements of
the ground surface from Germany’s TerraSAR-X and Japan’s ALOS-2 radar imaging
satellites, the team showed that the underground nuclear blast pushed the surface
of Mt. Mantap outward by as much as 11 feet (3.5 meters) and left the mountain
about 20 inches (0.5 meters) shorter.
By modelling the event on a
computer, they were able to pinpoint the location of the explosion, directly
under the mile-high summit, and its depth, between a quarter and a third of a
mile (400-600 meters) below the peak.
They also located more
precisely another seismic event, or aftershock, that occurred 8.5 minutes after
the nuclear explosion, putting it some 2,300 feet (700 meters) south of the
bomb blast. This is about halfway between the site of the nuclear detonation
and an access tunnel entrance and may have been caused by the collapse of part
of the tunnel or of a cavity remaining from a previous nuclear explosion.
“This is the first time the
complete three-dimensional surface displacements associated with an underground
nuclear test were imaged and presented to the public,” said lead author Teng
Wang of the Earth Observatory of Singapore at Nanyang Technological University.
Blast
larger than Hiroshima bomb
Putting all of this
together, the researchers estimate that the nuclear test, North Korea’s sixth
and the fifth inside Mt. Mantap, had a yield between 120 and 300 kilotons,
about 10 times the strength of the bomb dropped by the United States on
Hiroshima during World War II. That makes it either a small hydrogen, or
fusion, bomb or a large atomic, or fission, bomb.
The new scenario differs
from two reports last week, one of which has been accepted for publication in
the journal Geophysical
Research Letters, that pinpointed the blast nearly a kilometer to
the northwest of the site identified in the new paper, and concluded that the blast
rendered the entire mountain unfit for future nuclear tests.
“SAR really has a unique
role to play in monitoring explosions because it is direct imaging of the local
ground surface, unlike seismology, where you learn the nature of the source
analyzing waves radiating outward from the event at distant stations,” said
Dreger, a UC Berkeley professor of earth and planetary science and a member of
the Berkeley Seismological Laboratory. “SAR provides some measure of ground
truthing of the location of the event, a very challenging thing to get at. This
is the first time anyone has actually modeled the mechanics of an underground
explosion using satellite and seismic data together.”
“As opposed to standard
optical imaging satellite imagery, SAR can be used to measure earth deformation
day and night and under all weather conditions,” added Dreger’s colleague and
co-author Roland Bürgmann, a UC Berkeley professor of earth and planetary science.
“By precisely tracking the image pixel offsets in multiple directions, we were
able to measure the full three-dimensional surface deformation of Mt. Mantap.”
According to Dreger, the
new information suggests the following scenario: The explosion occurred more
than a quarter mile (450 meters) below the summit of Mt. Mantap, vaporizing
granite rock within a cavity about 160 feet (50 meters) across – the size of a
football stadium – and damaging a volume of rock about 1,000 feet (300 meters)
across. The blast likely raised the mountain six feet (2 meters) and pushed it
outward up to 11 feet (3-4 meters), though within minutes, hours or days the
rock above the cavity collapsed to form a depression.
Eight and a half minutes
after the bomb blast, a nearby underground cavity collapsed, producing the
4.5-magnitude aftershock with the characteristics of an implosion.
Subsequently, a much larger
volume of fractured rock, perhaps 1 mile (1-2 kilometers) across, compacted,
causing the mountain to subside to about 1.5 feet (0.5 meters) lower than
before the blast.
“There may be continuing
post-explosion compaction at the mountain. It takes time for these aseismic
processes to occur,” Dreger said.
While it is possible to
discriminate explosions from natural earthquakes using seismic waveforms, the
uncertainty can be large, Dreger said. Explosions often trigger nearby
earthquake faults or other natural rock movements that make the seismic signals
look earthquake-like, confusing the analysis. The SAR data revealed that
additional constraints from the local static displacement can help to narrow
down the source.
“I am hoping that by
jointly analyzing the geodetic and seismic data, we will be able to improve
discrimination between earthquakes and explosions, and certainly help with
estimating the yield of an explosion and improving our estimation of source
depth,” Dreger said.
“This study demonstrates
the capability of spaceborne remote sensing to help characterize large
underground nuclear tests, if any, in the future,” Wang said. “While
surveillance of clandestine nuclear tests relies on a global seismic network,
the potential of spaceborne monitoring has been underexploited.”
The work was supported by
the Earth Observatory of Singapore, at Nanyang Technological University, as
well as the U.S. Air Force Research Laboratory.
原始論文：Teng Wang, Qibin Shi, Mehdi Nikkhoo, Shengji Wei, Sylvain
Barbot, Douglas Dreger, Roland Bürgmann, Mahdi Motagh, Qi-Fu Chen. The rise, collapse, and compaction of Mt.
Mantap from the 3 September 2017 North Korean nuclear test. Science, 2018 DOI: 10.1126/science.aar7230
引用自：University
of California - Berkeley. "Radar reveals details of mountain collapse
after North Korea's most recent nuclear test.
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