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

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 (Zxy and Zyx ). 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

Constraining the subducting slab in the 2-D inversion of MT data in Southern Tohoku, NE Japan

令和4年度 Conductivity Anomaly研究会日時：令和4年12月26日（月）09:25-18:30, 12月27日（火）09:00-16:30場所：京都大学防災研究所連携研究棟3階301号室およびZoo

Seismicity controlled by resistivity structure : the 2016 Kumamoto earthquakes, Kyushu Island, Japan

The M JMA 7.3 Kumamoto earthquake that occurred at 1:25 JST on April 16, 2016, not only triggered aftershocks in the vicinity of the epicenter, but also triggered earthquakes that were 50–100 km away from the epicenter of the main shock. The active seismicity can be divided into three regions: (1) the vicinity of the main faults, (2) the northern region of Aso volcano (50 km northeast of the mainshock epicenter), and (3) the regions around three volcanoes, Yufu, Tsurumi, and Garan (100 km northeast of the mainshock epicenter). Notably, the zones between these regions are distinctively seismically inactive. The electric resistivity structure estimated from one-dimensional analysis of the 247 broadband (0.005–3000 s) magnetotelluric and telluric observation sites clearly shows that the earthquakes occurred in resistive regions adjacent to conductive zones or resistive-conductive transition zones. In contrast, seismicity is quite low in electrically conductive zones, which are interpreted as regions of connected fluids. We suggest that the series of the earthquakes was induced by a local accumulated stress and/or fluid supply from conductive zones. Because the relationship between the earthquakes and the resistivity structure is consistent with previous studies, seismic hazard assessment generally can be improved by taking into account the resistivity structure. Following on from the 2016 Kumamoto earthquake series, we suggest that there are two zones that have a relatively high potential of earthquake generation along the western extension of the MTL

Earthquake Prediction Research by Means of Telluric Potential Monitoring : Progress Report No. 1 : Installation of Monitoring Network

In order to test the applicability of the VAN-method of short term earthquake prediction which has reportedly been extremely successful in Greece, a telluric potential monitoring network has been set up in Japan under a cooperative program with the Nippon Telegraph an Telephone Corporation (NTT). So far, 22 stations have been tested. Mainly due to logistic and financial constrainst, at all except two temporary stations existing NTT electrodes are used. At each station, telluric potential difference is sampled every 20 seconds on six dipoles set in different lengths and azimuths. Data stored at each station are transmitted to the central station at the Earthquake Research Institute (ERI) by telephone once a day. At several stations artificial noise is too high even to detect the potential changes due to geomagnetic variations but there are some stations yielding reasonably noise-free data.地電位差の局地的ないし地域的な時間的変化の観測による地震予知方法の妥当性を検討している.この方法はギリシアで1980年頃以来非常な成功をおさめていると報告されているVAN法と呼ばれるものである.本報告はその第1報として,わが国での観測網の設置及び,それから得られる記録の概況について述べる.この研究は日本電信電話会社(NTT)との協同研究であり,各観測点では同社の通信用アースを電極として使用して,6本の測線(基線長数km)が設けられた.データは10秒のローパスフィルタをかけた後,12ビットにA/D変換され,20秒サソプリングでメモリに蓄えられる(容量は2日分).地震研究所からは毎日1回各観測点に電話がかかり,データは同所に転送される(1200bps,1観測点あたり約15分を要する).従って,データは準リアルタイムで見ることができる.1989年4月現在までに,全国22ケ所に観測点が設置されたが,日光・長野・和歌山など8地点では人エノイズ(主として直流電車による)が大きすぎるため観測は中止された.その他の14地点では程度の差はあるが,おおむね良好な記録が得られており,各地の地磁気変動による電位差変化を比較すると,海岸効果や,地下電気伝導度分布などについて有用な情報がもたらされている.また弟子屈,土佐中村,会津若松などでは,地震の前兆シグナルかも知れない地電位差変化もみられている.それらについては次報以下で報告する

Magmatic–hydrothermal system of Aso Volcano, Japan, inferred from electrical resistivity structures

We investigated the magmatic–hydrothermal system of Aso Volcano, Japan, using broadband magnetotelluric (MT) data. To establish the nature of the shallow crust, a previous resistivity model based on data from 100 measurement sites in and around Aso volcano was revised using data from 9 additional sites near Naka-dake crater, which is located in the central part of the volcano. The components of MT impedance and the tipper vector were used to obtain the resistivity structure by three-dimensional inversion. The resistivity structure shows a subvertical low-resistivity (< 1 Ωm) column-shaped body beneath Naka-dake crater that extends from − 600 m to 10 km below sea level (BSL) and dips steeply to the north-northeast. The position of the upper part of the column is displaced eastward compared with the previous model and does not overlap the position of the presumed magma reservoir inferred previously from seismic and geodetic observations underneath the western side of Naka-dake crater at a depth of 5 km. We interpret this low-resistivity column to be a magmatic–hydrothermal system composed of brine and magma that were transported to Naka-dake crater from the main deep-seated magma reservoir. A horizontal low-resistivity (< 10 Ωm) layer occurs beneath post-caldera cones at the depths of 0–2 km BSL, and this layer extends laterally from the upper part of the low-resistivity column. We interpret this low-resistivity layer as representing a shallow hydrothermal system that has developed around the central column-shaped magmatic–hydrothermal system

Significance of Electromagnetic Surveys at Active Volcanoes : Toward Evaluating the Imminence of Wet Eruptions

The detection capability of various anomalous phenomena preceding volcanic eruptions has considerably progressed as the geophysical monitoring networks have become denser and multi-disciplinary. However, current eruption forecasting techniques, from a practical perspective, still have much scope for improvement because they largely depend on empirical techniques. In the past decade, three-dimensional modeling based on the electromagnetic sounding methods such as magnetotellurics (MT) have become a practical choice, and its recent applications to active volcanic fields has revealed certain common features among volcanoes. Information about the resistivity structure, especially in ‘wet’ volcanic fields, is useful for the provisional screening of the eruption potential from the viewpoint of the subsurface structure, and, thus, may contribute to the evaluation of eruption imminence in a broad sense. In this study, for evaluation purposes, we present the roles and possible further applications of the subsurface resistivity structure studies by demonstrating the preliminary results and interpretations of an MT survey that we performed in the Kuttara Volcanic Group, northern Japan

Report on DELP 1985 Cruises in the Japan Sea : Part V : Heat Flow Measurements

Heat flow measurements were carried out in the Japan Sea at three stations on the KH82-4 cruise of R/V Hakuho-Maru and twelve stations on the DELP-85 WAKASHIO cruise. In the Yamato Basin, heat flow is very uniform and consistent with the existing data. The ages of the Yamato Basin and Tsushima Basin are estimated to be about 25 m.y. if based on the heat flow data. It was proved that the heat flow is variable in the Mogami Trough which was previously thought to be a low heat flow zone. A high heat flow value possibly associated with an anomalous reflector was obtained in the hypocentral region of the 1983 Nihonkai-Chubu Earthquake.自鳳丸KH82-4航海では3地点,若潮丸DELP-85航海では12地点において,地殻熱流量測定を実施した.大和海盆においては,熱流量はきわめて一様であり,従来報告されている値とよく一致している.熱流量データから推算される大和海盆及び対馬海盆の年令は,約25m.y.である.最上トラフはこれまで低熱流量帯であると考えられていたが,今回の測定ではばらつきの大きい値が得られた.1983年日本海中部地震の震源域において高熱流量が測定されたが,これは同じ地点での3.5kHz音波探査で観測された特異な反射記録に関連している可能性がある