Magma transport along structural boundaries in the upper crust: insights from broad-band magnetotelluric constraints on the structure beneath Unzen volcano, Japan

Abstract

Summary Unzen volcano, located on Shimabara Peninsula, Nagasaki, Japan, is an active volcano that has been intensively monitored since 1989, one year before the most recent eruption in 1990–1995. Previous earthquake and surface deformation studies have revealed that magma is transported obliquely from a magma reservoir beneath Tachibana Bay, to the west of Shimabara Peninsula. Here we conduct broadband magnetotelluric (MT) surveys at 99 sites around Shimabara Peninsula to investigate the crustal structure beneath Unzen volcano that is related to magma migration. A three-dimensional resistivity model that is constructed from 25 broadband MT sites and 45 telluric sites shows a broad high-resistivity zone beneath Shimabara Peninsula and low-resistivity zones to the west and east of the peninsula. An unexpected observation is the spatial alignment of the high-resistivity zone with a seismic low-velocity zone at 3–15 km depth. Quantitative analysis indicates this high-resistivity zone contains <0.7% melt under the assumption that the melt is stored in a good porosity network, while <11% melt in relatively poor pore network. We infer this high-resistivity, low-velocity zone to be a highly crystallized mush zone with low permeability. The hypocenters and pressure sources of the 1990–1995 eruption are distributed along the boundary between the high- and low-resistivity zones beneath the western part of the peninsula. We therefore conclude that the magma migrated along a structural boundary that possessed a relatively high permeability. Previous studies have suggested that eruptible magma is usually transported vertically upward through the center of the mush zone, whereas the present results reveal that magma can be transported along the upper boundary of a highly crystallized mush zone.

Does a Shallow Magma Reservoir Exist in the Wudalianchi Volcanic Field? Constraints From Magnetotelluric Imaging

“…In some volcanic areas, magma reservoirs have been observed to have lateral offsets when extending to depth, such that their surface projection is not coincident with the location of the volcano (Bedrosian et al., 2018; Cordell et al., 2018; Hill et al., 2009, 2022; Lerner et al., 2020; Triahadini et al., 2023); however, most of these magma reservoirs are found in the lower crust or mantle (Kelbert et al., 2012; H. Zhang et al., 2016), or they coexist with another shallow magma reservoir (or multiple reservoirs) that directly supplies magma to an overlying volcano (Díaz et al., 2020). Indeed, magma reservoirs in the upper crust are typically located beneath the volcano (Comeau et al., 2015; Cordell et al., 2019; Hata et al., 2016; Hill et al., 2015, 2020).…”