• 非マグマ性の火山活動に伴う消磁及び地盤変動のソースの特徴
  • On the Source Characteristics of Demagnetization and Ground Deformation
    Associated with Non-magmatic Activity

橋本, 武志

宇津木, 充

大倉, 敬宏

神田, 径

寺田, 暁彦

三浦, 哲

井口, 正人

  • Other non-magmatic unrest
  • Other demagnetization
  • Other phreatic eruption
  • Other Mogi source
  • Other statistical evaluation
  • NDC 453
  • We collectedinformation on non-magmatic unrest events reportedmainly of Japanese volcanoes that have been monitored for extended time periods, with the aim of conducting a statistical evaluation of these phenomena, which can sometimes leadto an eruption. We assessedthe geomagnetic fieldchanges andgroundd eformation, which indicate demagnetization andinflation, respectively. We then compiledthe source depth, intensity, andthe rate (i.e., magnetic moment rate / deformation rate), based on the single magnetic dipole model or the Mogi model, since these models are the most frequently citedin the literature.We foundclear positive correlations between the source depth andits intensity andrate, exhibiting a linear trendon the log-log scatter plots. We confirmedthat all the magnetic events andsome of the deformation events were plotted markedly above the cutoff line related to the detection limit. We suspected that the geometry of the monitoring network and/or the simplistic point-source modeling could contribute to these positive correlations ; otherwise some physical requirements such as subsurface structure might be the cause. Although the number of unrest events collectedwas small, we investigatedthe validity of the evaluation of these unrest events from different viewpoints. Upwardd eviation from the linear trendon the scatter plot was foundto be a potentially useful criterion, whereas the source depth had no definitive relationship with the subsequent occurrence of eruptions. We assert that observable non-magmatic unrest events shouldbe plottedwithin a diagonal area between the lowerright andupper-left domains in a scatter plot of source intensity (or rate) andd epth. This is because weak sources at great depths cannot be detected, while overly strong sources close to the ground surface are physically impossible. In the intermediate zone, “unerupted” and “erupted” unrest events both occur. Based on this idea, we propose a statistical evaluation of the severity of unrest events, by measuring the deviation from the “unerupted unrests”, although this hypothesis must be testedwith a sufficient number of unrest events in the future. Meanwhile, care must be taken in interpreting the data collectedhere, since they are basedon the simplifiedpoint-source models that assume a uniform half-space, andthey may not accurately recreate the physical entities. Further studies that can relate the physical conditions and their time evolution in the subsurface to geophysical monitoring records at the groundsurface will be necessary to evaluate how such unrest events are relatedto eruptions.
Date Issued 2019-06-30
Languagejpn, eng
NIItypejournal article
Identifier URI
  • isIdenticalTo DOI
  • isIdenticalTo NAID 130007676213
    • ISSN 0453-4360
    • 火山
    64(2), 103-119