Journal of Volcanology and Geothermal Research

:Journal of Volcanology and Geothermal Research?,

The background of this study is the rapid population growth in Latin America and Asia, which indicates that an increasingly number of people will be living in areas where may be seriously affected by eruption of volcanoes. As volcanic eruptions and knock-on effects have potential of causing not only casualties but also economic loss, it is both significant and necessary for scientists to decipher the nature and interpret the episodes of unrest. Therefore, this study has four major objectives: (1) to identify and classify repeated patterns of unrest to establish; (2) whether particular types of volcanoes display preferred patterns of unrest; (3) whether the length of repose affects preferred patterns of unrest; (4) whether pre-eruptive patterns can be distinguished from non-eruptive patterns of unrest.

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The statistics used in this study are primarily collected from the Smithsonian Institution Global Volcanism Program (GVP), and also selected from other available information in the published literature for some activities reported in the GVP. Meanwhile, the study also created a database which includes 228 volcanoes. By analyzing the database, this study mainly focuses on four primary sub-aerial types: large caldera, complex, shield, and stratovolcano, and finds out five primary unrest indicators: ground deformation, degassing, changes at a crater lake, thermal anomaly and seismicity. Meanwhile, the definitions for response variables are divided into three types: unrest duration, unrest indicators and inter-eruptive period. Though this study is to identify possible temporal patterns in unrest and analysis the differences between various types of volcanoes, employing standard procedures to calculate the statistics, during the process of statistics analyzing, there is no possibility to avoid biases, such as reporting bias and statistical bias.

The result of the study bases on the categories of definitions for response variables. In the unrest duration, nearly half of the data on reported unrests was missing, almost 50% of stratovolcanoes erupted after about one month of reported unrest. In the median average duration of large calderas, unrest prior to eruption was about twice as long, and shield volcanoes have a significantly longer unrest period before the onset of eruption, compared to both large calderas and stratovolcanoes. In inter-eruptive period, about 13% of statistics is missing, which covers different volcano types. In the inter-eruptive period, the study finds that the mean length of inter-eruptive period is 18,326 with a large standard deviation of 42,710. It is also indicated that outliers are mainly associated with stratovolcanoes for both pre- and non-eruptive unrests, as well as large calderas and complex volcanoes for pre-eruptive unrest. Consequently, the classes of unrest are classifies into four types: re-weakening unrest (key feature: deformation and seismic), prolonged unrest (key feature: long-term ground deformation), pulsatory unrest (key feature: seismic activity) and sporadic unrest (key feature: short-lived, intermittent activity and no apparent).

The conclusion of this study hypothesize that volcanoes with long periods of quiescence between eruptions undergo prolonged periods of unrest before eruption is not supported by the former analysis. Most eruptions during the investigation period occurred within a relatively modest amount of time after the first documented unrest, with a median average unrest duration of 79 days across all volcano types considered, regardless of the length of the inter-eruptive period. Although the findings of this study may not be representative of unrest behavior during centuries, they could have an strong impact on hazard assessment, risk mitigation and scenario planning when the unrest of volcanoes threaten human beings in the future. -w

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