INTRODUCTION

Turkey is one of the most seismically active regions in the world. As a result, it has a long history of large earthquakes that have killed many thousands of people and caused economic devastation, including the Kocaeli (M = 7.4) and Duzce (M = 7.2) events of 17 August and 12 November 1999, near Istanbul. Turkey is thus an excellent natural laboratory to study active strike-slip faulting, and active normal faulting. The chemical composition and transport of soil gases within fault zones has been the subject of extensive investigation due to the potential for geochemical anomalies to serve as precursors to seismo-tectonic activity. Volatile species such as Rn, H2, CO2, and He are observed to seep through active faults during quiescent periods when only micro-earthquakes occur intermittently at depth. Experimentally, these seeping species can be detected by chemical analyses of soil gases or soil gas efflux measurements along active faults. In this study, I have carried out geochemical studies of volatile species releasing from active faults in Turkey and Japan.

METHODS

Sampling : Soil gases were collected at 30-cm depth using a metallic probe (MU type gas collection tube; GL Sciences Inc., Japan) for chemical and carbon isotope analyses. Bubbling gases associated with hot spring or mineral spring discharges, or those emitting from river bottoms, were collected for chemical and C, He, Ne and Ar isotopic analyses. When bubbling gases were unavailable, water samples from hot springs or mineral springs were collected to measure noble gas elemental and He, Ne and Ar isotopic compositions of their dissolved gases.

CO2 efflux measurement: Diffuse CO2 efflux measurements were performed using a Portable CO2 flux meter (WEST System Srl., Italy) which is based on accumulation chamber method.

Gas Chromatography: Concentrations of CO2, He, H2, O2, N2 and CH4 in soil gases were determined using a micro-gas chromatograph (Micro-GC CP2002; Varian Inc.).

(13)C mass spectroscopy: Carbon isotope measurements for CO2 in soil gases were carried out using a MAT delta-S GC/C/MS system (Finigan MAT GmbH). Correction for mass discrimination in analyses was done with the measurement of a standard gas (CO2>99.95%, δ(13)C=-30.92‰), intermittently injected into the mass spectrometer during sample analyses.

Noble gas mass spectroscopy: The isotopic ratios of He, Ne and Ar and elemental abundances of five noble gases, He, Ne, Ar, Kr and Xe, were measured using a mass spectrometer system (MS-III) installed in the Laboratory for Earthquake Chemistry, University of Tokyo.

RESULTS AND DISCUSSION

Soil H2 and CO2 at several active faults in Japan

Seven active faults in western and central Japan are surveyed in this work. We selected the Atera, Atotsugawa, Neodani, and Yamasaki faults, because high H2 concentrations in soil gases had been reported. The other faults chosen are the Nojima fault fractured at the 1995 Hyogoken-nanbu earthquake and two faults without historical displacement in Omaezaki. The maximum H2 concentration in each active fault up to 500ppm seems to correlate with fault activity as exemplified by the time to the latest big earthquakes. Observed H2 concentrations in four faults were markedly lower than those collected in the latter half of the 1970's. These differences in H2 concentrations may attribute to different sampling depth or rock type in the active fault system. Even though we did not measure the same maximum values, we could observe the higher H2 concentrations due to the addition of the fault gases close to the fault strikes based on CO2/H2 discussion.

Release of mantle helium from forearc region of the Southwest Japan arc

Noble gas abundances and He, Ne and Ar isotopic compositions were determined for 30 samples of bubbling gases and spring waters from Shikoku, which corresponds to the forearc region of the SW Japan arc. Using those composition data, this study investigates the behaviors of mantle volatiles in the subduction system. In addition, six gas samples were analyzed for chemical and carbon isotope compositions. Observed 3He/4He ratios for the 30 samples ranged from 0.17 R/RA to 3.56 R/RA, indicating that several samples contained mantle helium, and that others contained crustal helium. The geographical distribution of 3He/4He ratios shows that two areas are releasing mantle helium with 3He/4He ratios greater than 1.2 R/RA. One area is along the Median Tectonic Line (MTL), a major active fault in Japan, which traverses northern Shikoku in an east-west direction. The other area is southwestern Shikoku, which coincides with a region where non-volcanic long-period tremors occur with epicenter depth of about 30 km (Fig.1). These two areas seem to overlap in northern Shikoku. Mantle helium, which is released in a similar manner to that from active faults and from deep tremor regions in Shikoku has also been observed in the Kii Peninsula region (Matsumoto et al., 2003). Non-volcanic deep tremors are thought to be caused by fluids that are librated from a subducting slab by dehydration in the forearc region (Obara et al., 2004). In Shikoku, the slab-derived fluids cause fracturing within the crust and ease transfer of fluids, mixed with mantle helium, to the surface. Our results also confirm that an active fault system can be an efficient path to transfer mantle helium, as Kennedy et al. (1997) suggested for the San Andreas faults.

Soil gas survey in Marmara region, NW Turkey

The effluxes, concentrations, and carbon isotopic compositions of soil CO2 were measured along the north western North Anatolian Fault (NAF) in the Izmit-Sapanca Fault, Iznik-Mekece fault and Gazikoy-Saros fault, in Marmara Region. CO2 efflux was measured at 180 points along these faults with portable instrumentation in both faulted and unfaulted areas. Spatial and temporal variability of surface CO2 effluxes was observed to be higher at faulted NAF, relative to comparable background areas (Fig.2). Carbon isotopic compositions of 42 soil gas samples measured show δ(13)C (-23.3to -15.6‰) values of soil CO2 in both faulted and unfaulted areas, which are indicative of biogenic CO2, even though CO2 effluxes in faulted areas reached values as high as 309 g m(-2) d(-1). The CO2 flux anomalies are therefore consistent with fault-related biogenic gas flow and do not yield evidence for degassing of deeply derived CO2.

3He/4He in Marmara region NW Turkey

Measured 3He/4He ratios for 36 samples in this study range from 0.26 R/RA (M5) to 4.22 R/RA (M67). Figure 3 shows the 3He/4He ratio plotted as a function of distance from a strike of the NAF. The 3He/4He ratios along the NAF are higher in two locations than those of other areas in Marmara. In the 70 km zone to the north and 90 km south of the strike of the NAF, four sites provide 3He/4He ratios above 1 R/RA, with the highest ratio of 4.22 R/RA at Sarkoy. This result is attributable to the effective transfer of mantle helium through this fault system with fluids, as Kennedy et al. (1997) showed for the San Andreas Fault and Dogan et al. (2006) in Shikoku MTL.

The bubbling gas samples showed a wide range of δ(13)C(CO2) of -0.6 〜 -28 ‰, apparently in the range of mantle carbon (-6.5±2.5‰), marine carbonate (-1 〜 +2‰), and organic sediment (-20 〜 -40‰).

CONCLUSION

(1) Helium isotope surveys in SW Japan and NW Turkey imply that helium with higher 3He/4He is transferred from mantle source through the pathways of active faults; MTL and NAF. Such a transfer of mantle helium is also observed at the San Andreas fault, USA (Kennedy et al., 1997). In addition, mantle helium appears to the surface in the region where non-volcanic long-period tremors occur in the fore-arc region of SW Japan (Obara, 2002)

(2) CO2 effluxes are higher at faulted areas of the NAF than those in background area. However, δ(13)C(CO2) values of soil CO2 are -23.3 to -15.6‰ both in faulted and unfaulted areas, indicative of biogenic origin. Therefore, CO2 efflux anomaly may be caused by fault-related biogenic gas flow.

(3) Soil H2 anomaly in the fracture zone is observed at active faults in Japan, which displaced recently. However, no H2 anomaly is observed at NAF. This may attribute to the difference in basement rock and moisture environment around each fault.

(4) It is important to develop the continuous measuring system of volatile species released from the active fault, to monitor the fault activity or future seismic events.

Fig.1 Areas of mantle helium release with 3He/4He ratios above 1.2 R/RA and the epicenter distribution of the deep long-period tremors (Obara, 2002).

Fig.2 Location of soil gas survey transects and plot of each transect as a function of distance (km) from strike of fault and flux Flux (gm(-2)d(-1)). 0km presents fault strike.

Fig.3 Plot of 3He/4He ratios as a function of distance from the main strike of the North Anatolian Fault (NAF).

　本論文は7章からなる。第1章はイントロダクションであり、活断層からの揮発性物質放出研究の研究例と、研究対象のトルコ、アナトリア断層についてまとめ、本研究の目的を述べた。この種の研究はトルコではこれまで行われておらず、近い将来に大地震が起きることが懸念されるアナトリア断層西端域での揮発性物質放出の特徴を把握し、日本の活断層地域の結果と比較しつつ、地震活動の予測に役立てるのが本研究の目的である。第2章は、活断層周辺の現地でおこなうCO2拡散放出量測定法，土壌ガスや温泉ガス，温泉水などの試料採取方法、実験室での化学分析法、同位体分析法についてまとめた。

　第3章から第6章では、日本やトルコで行った調査研究の結果が述べられている。第3章では日本の主要な7カ所の活断層で行った土壌ガスの観測研究結果が述べられている。多くの活断層では高濃度のH2が検出され、最後に断層が動いた年代が新しい程H2濃度が高い傾向が見えたが、1970年代の結果に比べて低いことも示され、断層破砕帯内部に蓄積した気体そのものを採取していないためであることが分かった。

　第4章では、日本で最大規模の活断層である中央構造線に沿ってマントル起源のヘリウムの放出を調べた結果が述べられている。中央構造線は九州に発し四国を東西に縦断し近畿東海へとのびているが、本研究では四国全域にわたって温泉や鉱泉湧出に伴って放出するガスや水を密に採取し、その中に含まれるヘリウム同位体比を測定した。四国はまたフィリップ海プレート沈み込みの島弧の前弧域にもあたる。その結果，マントル起源を示す3He/4He比の高いヘリウムは、中央構造線に沿って放出していることが認められたほか，それより南側に分布する非火山性低周波微動が起きている領域でも認められた。この種の低周波微動は沈み込んでいるプレートからその上を覆うマントルに脱水供給された流体の移動に原因が求められており、この結果はプレート沈み込みの前弧域での物質循環をあたらたに見つけたことになり，活断層からの放出が世界的には2例目であることを考えると，学術的にもっと価値ある発見と言える。

　第5章では、トルコ，アナトリア断層の西端域でマルマラ海を挟む地域での土壌ガス調査研究結果が述べられている。断層を横切る4本の断面でCO2拡散放出量測定をおこなった結果、マルマラ海西側の2断面では、断層面上で放出量が高く，断層から離れると低いと言う明瞭なパターンがえられた。世界的にもこれほど明瞭なパターンははじめてである。しかしながら，放出しているCO2の炭素同位体比は生物起源を示す値で一定であり、放出量のパターンとは符合しなかった。このことは地表近くの土壌中の生物活動で生産されたCO2が断層面に沿って地表へ放出しやすくなっていることを示しており，活断層に沿っての物質放出に新たな知見を加えることができた。また、日本の活断層で特徴的なH2の放出は全く見られず、H2の生成が微小地震活動による岩石破砕面と水との反応であるとして、トルコと日本の基盤岩の違いによることを指摘している。

　第6章は第5章と同じ地域のヘリウム同位体比の結果を示している。この地域は大陸地殻起源の低い3He/4He比が混入しているガス試料や水試料が多くを占めていたが，数少ないマントル起源の高い3He/4He比はアナトリア断層上で見られ，活断層に沿ってマントル起源のヘリウムが放出することはこの第3例目で普遍的な現象と考えられるに至った。なお、マルマラ海西側で最高値が得られたことは、断層面でCO2拡散放出が顕著であることと考え合わせると，この領域は揮発性物質が通りやすい性質を持つことを示したことになる。

　第7章では、結果のまとめと今後の見通しを述べている。研究全体を通じて、活断層からは普遍的にマントル起源のヘリウムが放出していることが明らかになり，沈み込み帯では前弧域の沈み込むプレートからの流体の移動もマントルヘリウム放出に寄与することをはじめて示した。また、トルコ，日本ともに断層面からは生物活動起源のCO2が出ているが、水素は日本の断層でしか検知できず，基盤岩の違いを表している。最後に今後トルコで地震予知を目ざした地球化学観測を行うなら、揮発性物質が通りやすい性質を持つマルマラ海西側が最適であることを指摘している。

　なお、本論文の第1，2章の一部と第3章は森俊哉，角森史昭、野津憲治と、第1，2章の一部と第4章は角野浩史、長尾敬介、野津憲治と、第5，6章は野津憲治との共同研究であるが、論文提出者が主体となって分析及び検証を行ったもので、論文提出者の寄与が十分であると判断する。

　したがって、博士(理学)の学位を授与できると認める。