• North Korea’s nuclear test site has collapsed ... and that may be why Kim Jong-un suspended tests | South China Morning Post
    http://www.scmp.com/news/china/diplomacy-defence/article/2143171/north-koreas-nuclear-test-site-has-collapsed-and-may-be-why-kim-jong-un
    https://cdn4.i-scmp.com/sites/default/files/styles/620x356/public/images/methode/2018/04/25/7ed3c416-47d3-11e8-85b3-af25d27017e0_image_hires_191717.JPG

    North Korea’s mountain nuclear test site has collapsed, putting China and other nearby nations at unprecedented risk of radioactive exposure, two separate groups of Chinese scientists studying the issue have confirmed. 

    The collapse after five nuclear blasts may be why North Korean leader Kim Jong-un declared last Friday that he would freeze the hermit state’s nuclear and missile tests and shut down the site, one researcher said. 

    The last five of Pyongyang’s six nuclear tests have all been carried out under Mount Mantap at the Punggye-ri nuclear test site in North Korea’s northwest.

    A research team led by Wen Lianxing, a geologist with the University of Science and Technology of China in Hefei, concluded the collapse occurred following the detonation last autumn of North Korea’s most powerful thermal nuclear warhead in a tunnel about 700 metres (2,296 feet) below the mountain’s peak. 

    The test turned the mountain into fragile fragments, the researchers found.

    Ah ! souvenirs de Beryl…

    • La page web de l’Université de science et technologie de Chine vue dans la vidéo est celle-ci

      温联星研究组
      http://seis.ustc.edu.cn/research/north-korea-nuclear-test-mountain-has-collapsed


      (a)朝鲜2017年9月3日核爆、核爆8分半钟后的塌陷(标记为20170903CL)以及2017年9月23日之后的天然地震群(9月23日双事件标记为20170923EQ1、20170923EQ2,10月12日事件标记为20171012EQ)的位置(红色圆圈)及误差范围(绿色椭圆);(b)天然地震群中三个地震的震源深度及震源机制;(c)塌陷事件过程:塌陷自核爆所致的岩体破碎区(浅蓝色区域)沿黑色箭头方向近垂直塌陷至核爆产生的中心空腔(浅红色区域)

      中国科学技术大学地震与地球内部物理实验室温联星研究组通过分析地震记录,确认朝鲜自2009年以来一直用于核试验的丰溪里万塔山已塌陷。该研究成果于2018年4月23日被国际地球物理权威学术期刊《地球物理研究快报》(Geophysical Research Letters) 接收。该研究组博士生田冬冬、姚家园为共同第一作者。研究还确认,2017年9月23日和10月12日在丰溪里试验场发生的三个小事件为核试验触发的、发生在万塔山之外的一个天然地震群。

      2017年9月3日,朝鲜在其丰溪里核试验场实施了一次地下核试验,其当量为108.3±48.1千吨,为朝鲜历次核试验中最大的一次。朝鲜2017年的核爆与其自2009年以来的历次核爆均在核试验场的万塔山下进行。与历次核爆不同,丰溪里核试验场在2017年核试验后发生了几次小事件,其中包括一个发生于核爆8分半钟后震级为4.1级的事件,两个发生于9月23日和一个发生于10月12日的小事件。虽然这些小事件引起了国际社会的极度关注,但是科学界一直不清楚这些事件的性质特征。中国科大研究利用1972个地震台数据,确定了核试验后四个小事件的震源属性特征以及它们与2017年核爆中心的相对位置。研究结果表明,核爆后8分半钟的事件为万塔山自核爆中心西北方向440米处近垂直塌陷至核爆产生的中心空腔所致;而9月23日以后发生的3个小事件则是一个位于核爆中心北侧8.4千米处的另一座山体下方的天然地震群,其震源深度至少为2.4千米。

      https://doi.org/10.1002/2018GL077095

    • La page web mentionne un article publié dans le numéro du 16 avril 2018 de Geophysical Research Letters, Vol. 45, Issue 7

      Article publié en ligne le 14 mars 2018

      North Korea’s 2017 Test and its Nontectonic Aftershock - Liu - 2018 - Geophysical Research Letters - Wiley Online Library
      https://agupubs.onlinelibrary.wiley.com/doi/10.1002/2018GL077095

      Abstract
      Seismology illuminates physical processes occurring during underground explosions, not all yet fully understood. The thus‐far strongest North Korean test of 3 September 2017 was followed by a moderate seismic event (mL 4.1) after 8.5 min. Here we provide evidence that this aftershock was a nontectonic event which radiated seismic waves as a buried horizontal closing crack. This vigorous crack closure, occurring shortly after the blast, is studied in the North Korea test site for the first time. The event can be qualitatively explained as rapid destruction of an explosion‐generated cracked rock chimney due to cavity collapse, although other compaction processes cannot be ruled out.

      Plain Language Summary
      North Korea detonated its strongest underground nuclear test in September 2017. It attracted the public interest worldwide not only due to its significant magnitude (6.3 mb) but also because it was followed 8.5 min later by a weaker event. Was the delayed shock a secondary explosion, an earthquake provoked by the shot, or something else? We answer these questions, thanks to unique data from near‐regional broadband stations. We basically solve a simple problem—fitting observed seismograms by synthetics. The good fit means that we understand why and how the seismic waves are radiated. According to our model, the explosion created a cavity and a damaged “chimney” of rocks above it. The aftershock was neither a secondary explosion nor a triggered tectonic earthquake. It occurred due to a process comparable to a “mirror image” of the explosion, that is, a rock collapse, or compaction, for the first time documented in North Korea’s test site. Interestingly, shear fault motions, typical for natural earthquakes, were extremely small both in the explosion and in the aftershock. Small natural earthquakes also occur at the test site, and geotechnical works might trigger them. Thus, all studies related to rock stability of the site, and prevention of radioactive leakage, are important.


    • Figure 4
      Inferred interpretation of (a–c) mainshock and (d–f) nontectonic aftershock. Dominant body forces equivalent to seismic radiation are shown for an assumed depth of 1.5 km. The force couples are annotated with their relative size. Scaling factors for mainshock and aftershock are 5.33e17 and 3.40e16 Nm, respectively. The events radiated as an opening and closing horizontal crack, with a significant compensated linear vector dipole contribution. Schematic sketch (g) shows the structural elements and processes, discussed in the text. (h) Vertical components of normalized full‐band raw data of Event 1 (red) and Event 2 (black). Traces of Event 2 are plotted with opposite sign; thus, the surface waves match with Event 1. It illustrates the “mirror‐image” character of the two sources. Note also the absence of high‐frequency body phases in the records of Event 2, similar to “collapse” events (Engdahl, 1972; Ryall & Savage, 1969; Willis, 1963). Origin time is at t = 0.

    • La fermeture du site et l’effondrement de la cheminée ne convainquent pas tout le monde…

      Optimism About Korea Will Kill Us All – Foreign Policy
      http://foreignpolicy.com/2018/04/30/optimism-about-korea-will-kill-us-all

      Last week’s inter-Korean summit, and North Korean leader Kim Jong Un’s declaration that he would “close” his nuclear test site by May, were greeted widely with celebration. But contrary to the hoopla, we have now arrived at an especially dangerous moment in Washington’s relationship with Pyongyang. We are on the verge of letting our hopes get in the way of our survival.

      Consider the now widespread view that North Korea’s test site is unusable or that the mountain that contains it has collapsed. This was always garbage reporting. You can download the two academic papers that are said to have originally made these claims — they say nothing of the kind. What the papers do is prove that, after North Korea’s big nuclear test in September 2017, the cavity created by the explosion collapsed in on itself. We already knew that probably happened (although it is cool to see it demonstrated through seismology).

      But the collapsing of the cavity and shrinking of the mountain do not mean the tunnels leading to it collapsed, let alone that the mountain itself had done so. And, of course, there are two other nuclear test complexes underneath entirely different mountains at the site. Kim was quoted as making this point himself: “Some said we will dismantle unusable facilities, but there are two more larger tunnels [in addition to] the original one and these are very in good condition as you will get to know that when coming and seeing them.” But commentators in the West, hoping for a diplomatic breakthrough (whether for political or more idealistic reasons), still heard what they wanted to hear about the condition of North Korea’s program.

      Les articles signalés sont d’une part celui pointé ci-dessus et aussi celui-ci (27/04/2018)

      Collapse and Earthquake Swarm after North Korea’s 3 September 2017 Nuclear Test - Tian - - Geophysical Research Letters - Wiley Online Library
      https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2018GL077649

      Abstract
      North Korea’s 3 September 2017 nuclear test was followed by several small seismic events, with one eight‐and‐a‐half minutes after the test and three on and after 23 September 2017. Seismic analysis reveals that the first event is a near vertical on‐site collapse toward the nuclear test center from 440±260 m northwest of the test site, with its seismic source best represented by a single force with a dip angle of 70°‐75° and an azimuth of ~150°, and the later events are an earthquake swarm located 8.4±1.7 km north of the test site within a region of 520 m, with a focal depth of at least 2.4 km and a focal mechanism of nearly pure strike‐slip along the north‐south direction with a high dip angle of 50°‐90°. The occurrence of the on‐site collapse calls for continued monitoring of any leaks of radioactive materials from the test site.

      (pdf téléchargeable : que de la technique…)