Papandayan is a hydrothermally active volcano in Indonesia. We revealed the subsurface structure around the Mas Crater area of Papandayan based on the magnetotelluric (MT) and geomagnetic (GM) method. For the MT method, 14 sounding stations were deployed and two of them are located close to the active fumaroles. We estimated the MT response functions using a remote reference and then modeled the data with the aid of a 1-D robust inversion. The resistivity structure can generally be divided into three layers, namely a thin resistive surface layer, a middle conductive layer, and a more resistive basement. We interpreted the middle layer to be the hydrothermal zone or clay mineral. For the GM method, we measured the total intensity at 19 data points. The IGRF and diurnal variation were subtracted from the raw data. We then obtained the 2-D magnetic susceptibility model of magnetic field anomaly using an Occam inversion. The model shows a significantly low susceptibility structure beneath the fumaroles which might correlate with thermally demagnetized rocks.
The southern part of Tohoku, Northeast Japan, is an area with significant in-land activities owing to the ongoing subduction mechanism. Among these are active volcanoes distributed on the volcanic front and back-arc, active faults throughout the area, and a recently observed swarm of shallow earthquakes on the fore-arc side. As fluids play an essential role in arc magmatism and the associated seismicity, this study aims to understand the deep fluid distribution beneath southern Tohoku to clarify the origin of the activities. A magnetotelluric survey delineating the subsurface electrical resistivity structure was used as the bulk resistivity is sensitive to the composition and connectivity of fluids. Using a newly developed joint inversion code, we estimated the resistivity structure using the inter-station horizontal magnetic field transfer function (HMTF) in addition to the conventional magnetotelluric response functions. Joint inversion with HMTF improved the recovery of low-resistivity anomalies owing to the sensitivity of the HMTF to electrical current concentration, resulting in a model with smaller data misfits. The main feature of the resulting resistivity structure is that, instead of under the volcanic front, a deep conductive body is found under the back-arc side in a position closer to a back-arc volcano (Mt. Numazawa) and a swarm of low-frequency earthquakes. Petrological studies indicate that the deep source of fluids supplying to Mt. Numazawa may be the same as that of Mt. Azuma and Mt. Adatara on the volcanic front. Magmatic fluids ascend from the upper mantle to the upper crust via different branches, resulting in multiple eruption centers. Thus, we inferred that the conductor reflects the fluid path to Mt. Numazawa. The high conductivity, especially in the uppermost mantle, may be caused by flux melting, where water or other volatiles released from the subducting slab reduce the solidus of high-temperature basaltic rocks.
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