11:00 AM - 1:00 PM
[SVC30-P02] Construction of a database for thickness and grain size data of pyroclastic fall in Japan
Keywords:pyroclastic fall deposits, fall thickness, grain size, volcanic ash, database study
Modes of pyroclastic fall hazards are categorized to deposit loading, ash inhalation, and ash adhesion. Loading hazards such as roof collapse occur with large amounts of pyroclastic fall. In contrast, ash inhalation and adhesion hazards to the human body, plants, animals, equipment and facility occur even if smaller amounts of fine ash fall. Therefore, the grain size is important from the viewpoint of modes of pyroclastic fall hazards.
This study extracts thickness and grain size data of pyroclastic fall deposits in Japan from previous geological studies by GIS analysis, and constructs a database. The distance and azimuth between an eruptive center and an outcrop are combined with the data of thickness (Th), maximum pumice (MP), maximum lithic (ML), and median size (Md).
The cases in the database include eruptions with a Volcanic Explosion Index (VEI) of 2 to 8. Although there are more than 500 eruptions with isopach data (Uesawa et al., Submitted), only about 60 eruptions have some grain size data. For Md, only eight eruptions have been found. We clarified that the minimum layer thickness is mostly up to 10 cm, and there are very little data for layer thicknesses below that.
We focus on the Md, which is expected to be the most representative of the grain size characteristics of deposits and less dependent on the analyst. The ratio of the thickness to the median grain size (Th⁄Md) varies with the azimuth between the eruptive center and the outcrop and has a maximum value near the principal axis of pyroclastic fall. This means that for the same eruption, the grain size is larger off the principal axis for the same thickness. The relationship between Th⁄Md and eruptive volume (dense rock equivalent) shows that Th⁄Md near the principal axis increases as the eruptive volume increases. The Th⁄Md is about 101.0-101.5 for an eruptive volume of 0.1 km3, while it becomes 103.0-104.0 for a volume of 10 km3 or more. This means that the larger the eruption volume is, the smaller is the Md of the deposits near the principal axis of the pyroclastic fall for the same thickness. One of the possible controlling factors for the relationship is the relationship between eruptive volume and duration.
In the future, we also add data of maximum grain size of pumice and lithic to a database to geologically estimate the plume height and eruption rate. We will compare the above estimates with the database of magma properties (Takeuchi et al., 2021, JVGR), and aim to understand the relationship between magma properties, plume dynamics and pyroclastic fall dispersion.
Takeuchi et al. (2021) Relationships between magmatic properties and eruption magnitude of explosive eruptions at Japanese arc volcanoes during the last one hundred thousand years, JVGR 419, 107345.
Uesawa et al. (Submitted) Creating a digital database of tephra fall distribution and frequency in Japan. Submitted to JAV.
This study extracts thickness and grain size data of pyroclastic fall deposits in Japan from previous geological studies by GIS analysis, and constructs a database. The distance and azimuth between an eruptive center and an outcrop are combined with the data of thickness (Th), maximum pumice (MP), maximum lithic (ML), and median size (Md).
The cases in the database include eruptions with a Volcanic Explosion Index (VEI) of 2 to 8. Although there are more than 500 eruptions with isopach data (Uesawa et al., Submitted), only about 60 eruptions have some grain size data. For Md, only eight eruptions have been found. We clarified that the minimum layer thickness is mostly up to 10 cm, and there are very little data for layer thicknesses below that.
We focus on the Md, which is expected to be the most representative of the grain size characteristics of deposits and less dependent on the analyst. The ratio of the thickness to the median grain size (Th⁄Md) varies with the azimuth between the eruptive center and the outcrop and has a maximum value near the principal axis of pyroclastic fall. This means that for the same eruption, the grain size is larger off the principal axis for the same thickness. The relationship between Th⁄Md and eruptive volume (dense rock equivalent) shows that Th⁄Md near the principal axis increases as the eruptive volume increases. The Th⁄Md is about 101.0-101.5 for an eruptive volume of 0.1 km3, while it becomes 103.0-104.0 for a volume of 10 km3 or more. This means that the larger the eruption volume is, the smaller is the Md of the deposits near the principal axis of the pyroclastic fall for the same thickness. One of the possible controlling factors for the relationship is the relationship between eruptive volume and duration.
In the future, we also add data of maximum grain size of pumice and lithic to a database to geologically estimate the plume height and eruption rate. We will compare the above estimates with the database of magma properties (Takeuchi et al., 2021, JVGR), and aim to understand the relationship between magma properties, plume dynamics and pyroclastic fall dispersion.
Takeuchi et al. (2021) Relationships between magmatic properties and eruption magnitude of explosive eruptions at Japanese arc volcanoes during the last one hundred thousand years, JVGR 419, 107345.
Uesawa et al. (Submitted) Creating a digital database of tephra fall distribution and frequency in Japan. Submitted to JAV.