Eruption Histories of Zao and Azuma Volcanoes and Their Magma Feeding Systems for Recent Activities

摘要

2011年3月11日に発生した東北地方太平洋沖地震の後，東北地方の幾つかの火山，特に粟源に近い蔵王山と吾妻山では火山活動の活性化が見られる.本論では，両火山の噴火史と現況をレビューし，また最近の噴出物の岩石学的なデータに基づき，地下のマグマ供給系について検討した.両火山共に約100万年の噴火の歴史がある.蔵王山の活動は,約100万年前の水中噴火で特徴づけられる活動，約50-4万年前の安山岩質中小規模火山の形成で特徴づけられる活動，約3.5万年前以降の玄武岩質安山岩～安山岩質マグマの爆発的噴火で特徴づけられる活動に大きく分けられる.約3.5万年前以降の活動はさらに，約3.5-1.3万年前，9-4千年前，約2千年前以降のフェーズに分けられ，このうち最後のフェーズで現在の火口湖御釜を胚胎する五色岳が形成された.噴火記録も多数あり，御釜からの最新の噴火活動は1894～97年である.吾妻山の活動は約120-80，80-60，60-40，40-30万年前と30万年前以降の5つに大きく分けられる.各時期に，多数の安山岩質中小規模成層火山が吾妻山全域に亘って形成された.30万年前以降の活動期の中でも現在に続く活觔は.約7千年前に始まった東部の活動である.この間，約6.7，6.3，5.9～4.8千年前，4.3千年前と西暦1331年にマグマ噴火が発生した.水蒸気爆発も多数発生しており，最新のものは1977年に起こっている.蔵王山では2013年1月に火山性微動が初めて観測された.一方吾妻山ではそれまでにも火山性微動は観測されていたが，2010年の1月から振幅の大きなものが観測され始めた.火山性地震の回数が，蔵王山では2014年の4月に，吾妻山では12月に増加し，同時に火山性微動や山頂方面の膨張も観測されたため，噴火警報(火口周辺)の発令に至った.蔵王山では6月に解除されたが吾妻山では継続している.両火山共に，噴気活発化など他の予兆も認められている.両火山の最近の主な噴出物は，中間カリウム·カルクアルカリ玄武岩質安山岩～安山岩で，汚濁帯，逆累带構造を持つ斑晶や非平衡的斑晶組み合わせが認められる混合岩である.岩石学的な検討を行った結果，珪長質端成分は，蔵王山ではSiO_2量が約60％の安山岩で，温度·圧力·含水量条件は955-970℃，1.3-1.5kb，1.7-1.8％，吾妻山ではSiO_2量が約65.5％のデイサイトで，温度·圧力·含水量条件は880-890℃，0.8-1.2kb，2.75-3.25％と推定された.苦鉄質端成分は両火山共に玄武岩である.蔵王の方が吾妻よりもやや分化した組成である.浅部(地下約4～6km程度)に位置する珪長質端成分マグマに深部由来の苦鉄質端成分マグマが注入·混合して噴出したと考えられる.両火山共に，珪長質マグマは浅部火山性地庚の震源位置よりもやや深所に存在していると考えられる.浅部火山性地簾を引き起こしていると考えられる熱水は，この珪長質マグマ由来の可能性が高い.また，深部低周波地震は，その庚源域が地庚トモグラフィーによる高Vp/Vs域にあることから，メルトの移動によるものと考えられており，そのメルトが苦鉄質端成分マグマの起源である可能性が考えられる.%Since the 2011 Tohoku-Oki earthquake on March 11,2011, several dormant volcanoes have shown signs of activity. In particular, many phenomena that are precursors to eruptions have been detected at Zao and Azuma volcanoes, which are located near the hypocenter of this earthquake. Eruption histories and current conditions of these volcanoes are reviewed, and the states of present sub-volcanic systems are considered on the basis of petrologic features of recent magmatic eruption products coupled with geophysical data. Both volcanoes have eruption histories of approximately 1 million years. At Zao, the first stage,～1Ma, is characterized by sub-aqueous eruptions of basaltic magma followed in stages by small to medium-sized andesitic stratovolcanoes building at ～0.5-0.04Ma. The youngest stage,～35ka to the present, is characterized by explosive eruptions of basaltic andesite to andesitic magma. This stage is further divided into three phases;～35 to 13ka,～9 to 4ka, and ～2ka to the present During the youngest phase, the Goshikidake cone-building phase, many historical eruptions were recorded; the youngest eruption from the present crater occurred in AD 1895. At Azuma, volcanic activity is divided into five stages:1.2-0.8Ma, 0.8-0.6 Ma, 0.6-0.4Ma, 0.4-0.3Ma, and less than 0.3Ma. During each stage, small to medium-sized andesitic stratovolcanoes formed over the entire area of Azuma. The youngest activity began at about 7ka in the eastern area. Five magmatic eruptions at ～6.7ka, ～6.3ka, ～5.9 to 4.8ka,～4.3ka, and ～AD 1331 and seven phreatic eruption episodes have been recognized by tephrostratigraphic studies. Many historical eruptions have also been recorded; the youngest eruption from the present crater occurred in AD 1977. Volcanic tremors were first detected at Zao in January 2013. Although such tremors were also recorded at Azuma, those with larger amplitudes have been detected since January 2010. The number of volcanic earthquakes increased beneath Zao in April 2015 and beneath Azuma in December 2014. At those times, volcanic tremors and deformation of the summit area were observed in both cases, which led to volcanic activity warnings. At both volcanoes, other precursory phenomena have been observed, including an increase in underground temperature revealed by total magnetic force surveys and an increase of geothermal activity. At Zao, the number of low-frequency earthquakes increased about one year before the first detection of volcanic tremors was reported. Magmatic eruptions of <～35ka of Zao and <～7ka of Azuma volcanoes produced basaltic andesites to andesites, which were formed by the mixing of mafic and felsic magmas. The felsic magmas are andesitic at Zao and dacitic at Azuma, containing about 60% and 65.5% Si0_2, respectively. The temperature, pressure, and water content of these magmas are 955-970℃, 1.3-1.5kb, and 1.7-1.8% H_2O; and 880-890℃, 0.8-1.2kb, and 2.75-3.25% H_2O, respectively. The mafic magmas are basaltic in both cases, but that of Zao is slightly more differentiated than that of Azuma. In both cases, deeper mafic magmas infiltrated the shallower felsic magma chamber, which triggered the eruption. The felsic chamber locations, at ～6km at Zao and ～4km at Azuma, are deeper than the present shallow earthquakes. The shallow earthquakes are related to hydrothermal fluid released from shallow felsic chambers that have been reactivated. The deep earthquakes occurred in areas with a high P-wave to S-wave velocity ratio (Vp/Vs) at depths of about 20-37km below both volcanoes, where tomographic studies indicate melt Therefore, the mafic magma probably originated from the melt.