Magnetotelluric and temperature monitoring after the 2011 sub-Plinian eruptions of Shinmoe-dake volcano

Abstract

Three sub-Plinian eruptions took place on 26-27 January 2011 at Shinmoe-dake volcano in the Kirishima volcanic group, Japan. During this event, GPS and tiltmeters detected syn-eruptive ground subsidence approximately 7 km to the WNW of the volcano. Starting in March 2011, we conducted broad-band magnetotelluric (MT) measurements at a site located 5 km NNW of the volcano, beneath which the Shinmoe-dake magma plumbing system may exist. In addition, temperature monitoring of fumaroles and hot-springs near the MT site was initiated in July 2011. Our MT data record changes in apparent resistivity of approximately ±5%, along with a ±1• phase change in the off-diagonal component of the impedance tensor (Z xy and Z yx ). Using 1-D inversion, we infer that these slight changes in resistivity took place at relatively shallow depths of only a few hundred meters, at the transition between a near-surface resistive layer and an underlying conductive layer. Resistivity changes observed since March 2012 are correlated with the observed temperature increases around the MT monitoring site. These observations suggest the existence beneath the MT site of pathways which enable volatile escape.

Three‐dimensional resistivity structure and magma plumbing system of the Kirishima Volcanoes as inferred from broadband magnetotelluric data

“…Taking into account that the erupted volume of magma is on the order of 0.01 km 3 , we assume that the resistivity structure has not significantly changed since those observations were made. Although the measurement sites in 2010 and 2011 do not overlap with the previous sites, the continuous MT observation at Iwo‐Yama since May 2011 suggested that a resistivity change occurred only at a shallow level (less than several hundred meters) and its amplitude was subtle less than ±10% [ Aizawa et al ., ].…”

Imaging the hydrothermal system beneath the Jigokudani valley, Tateyama volcano, Japan: implications for structures controlling repeated phreatic eruptions from an audio-frequency magnetotelluric survey

“…The accumulation of upwelling volcanic gas beneath this low permeable layer can lead to an increase in temperature and pressure that can potentially cause hydrothermal eruptions. This low permeable clayey layer is conductive and can be imaged using magnetotelluric (MT) methods (e.g., Ussher et al 2000;Aizawa et al 2013). The fact that gases are not as conductive as fluids or clays means that these low permeable clayey caps are often underlain by slightly resistive regions.…”

Magnetotelluric transect of Unzen graben, Japan: conductors associated with normal faults

“…After data collection, the binary time series data were converted to text format time series data with physical units (mV/km and nT) using the frequency responses of induction coils and data loggers, and the dipole length of the electric field. Because the time series data were affected by cultural noise from a 60 Hz power line, we performed notch filtering to eliminate 60 Hz noise and its associated odd-order overtones (Aizawa et al 2013). Next, we calculated the MT response function over a 0.003-3276.8-s period range using a bounded influence remote reference code (Chave and Thomson 2004).…”

“…To obtain unbiased impedance, remote reference processing was performed by canceling incoherent noise between an observation site and a reference site (Gamble et al 1979). As a reference magnetic site, we used data from the MT monitoring station at Iwo-Yama, located in the Kirishima Volcanic Complex (Aizawa et al 2013). Although data quality over shorter periods was successfully improved, the data show significant scatter at periods greater than 10 s. By visual inspection, we excluded outliers from the smoothed sounding curve.…”