Continental scientific drilling projects were promoted actively around the world in the latter part of the 20th century. The super deep drilling projects developed drilling technologies, and highlighted a lack of understanding of the Earth's interior. The International Continental Scientific Drilling Program was initiated to promote continental scientific drilling projects globally and international cooperation. Geology needs to be taken into consideration more and technology for preventing bore-hole break-outs needs to be developed in continental drilling projects. More work is required to take a fresh look at the Earth based on the facts obtained from the surface. Continental scientific drilling is a potentially important tool for undertaking this study of the Earth in areas such as the subsiding atoll concept and ocean floor spreading. Because accretion and arc magmatism accompanying subduction of the ocean floor are important in the formation of continental crust, continental scientific drill projects on the Japanese Islands will play an important role in geology and human society.
The history and performance of the International Continental Scientific Drilling Program (ICDP) is reviewed in conjunction with the contributions of Japanese scientists. ICDP is a land-based equivalent of the Integrated Ocean Drilling Program (IODP), but has a much smaller budget. Consequently, the combined fund is used solely to cover direct costs, that is, drilling, logging, ICDP workshops, and a small number of committee meetings. The organizational structure of ICDP is simple; the Assembly of Governors (AOG), the Executive Committee (EC), and the Science Advisory Group (AOG), each meet once a year to decide all details promptly. All indirect costs such as salaries are covered by the host country (Germany). On the other hand, its performance is excellent: nineteen scientific drilling projects have been conducted since its inauguration in 1996 including Unzen volcano drilling and other five projects proposed by Japanese lead proponents. Besides, the COREF project, coral reef drilling at the Ryukyu islands, which was proposed by Japanese principal investigators (PIs), was approved recently and drilling will start in the near future. Continental drilling will become increasingly important when using scientific results to establish a sustainable future for human society.
Drilling into volcanoes is important to obtain an understanding of subterranean three-dimensional geometrical and thermal structures. Even shallow drilling in volcanic areas provides various information including the eruption histories of the volcanoes. The magmatic system and conditions at eruption sites together with eruption and development histories are important to understand magma processes in the upper crust, forecast future eruptions, and mitigate volcanic disasters. Using the holes drilled as observation wells is an additional advantage for monitoring the volcanic activity in-situ. Volcano and magma development histories have been revealed by deep drilling at Mauna Kea volcano (Hawaii), Unzen volcano, Fuji volcano, etc. Drillings was carried out into a hot basalt lava lake and a volcanic conduit of a recently erupted Kilauea volcano (Hawaii) and Unzen volcano, respectively. During geothermal drilling at Kakkonda and in Iceland, a solidifying magma chamber and rhyolite magma, respectively, were accidentally drilled. Three-D subsurface structures under caldera volcanoes have been understood by carrying out geothermal drillings at Long Valley, Aso, and Nigorikawa calderas. In addition, some projects to drill at caldera volcanoes have been undertaken or are being prepared as ICDP projects. Recently, national economic problems have made it difficult for researchers to propose deep drilling projects at volcanoes and calderas as an ICDP project. Under this situation, close international cooperation among researchers related to these projects or participation in geothermal exploration drilling projects may be better solutions.
The author reviews scientific drilling projects investigating lacustrine sediments in Japan. The lake sediments are excellent archives for paleoenvironments and tectonic movements, and are especially useful when sediments have sequences of annual bands. The earliest deep drilling projects, namely 200 m and 1,400 m drillings, were performed at Lake Biwa in 1971 and 1982-1983, respectively. The results revealed detailed climate changes and tectonic movements of the Omi basin over the past 1.6 Ma. Sediment cores from Lake Suigetsu provide an excellent chronological key based on annual bands for the past 50 ka or more. Furthermore, studies on sediment cores from Lake Nojiri and the lacustrine Takano Formation provided high time resolution data on paleoclimate for the past 160 ka: these data are based on the total organic carbon content and pollen composition of the sediment cores. The impregnation method to make a thin section of lake sediment is essential for counting annual layers to avoid miscounting varves. Lacustrine sediments exposed on land may also be useful archives of the Pleistocene as shown by case studies on Takano and Yoshino Formations. Sample storage systems and sub-sampling method such as double-L channel are useful for systematic research on sediments.
There have been fewer studies on strata formation in tectonically subsiding areas than in tectonically stable and uplifting areas. In this study, (1) 1000 year scale sediment stacking patterns, (2) seismic subsidence and meltwater pulse (MWP) 1B, and (3) event stratigraphy derived from seismic subsidence are revealed on the basis of sedimentary facies and 162 radiocarbon dating of three sediment cores obtained from the latest Pleistocene to Holocene Shinano River incised-valley fill (Alluvium) in the Echigo Plain. Aggradation is dominant in sediment stacking patterns of the incised-valley fill due to tectonic subsidence from the Nagaoka Plain Western Margin Fault Zone. Two continuous layers with abundant marine diatoms and rapid sediment accumulation rates are identified in transgressive salt marsh sediments. These layers may be due to seismic subsidence, but the latter layer (11.2-11.6 cal kyr BP) coincides with MWP 1B. If MWP 1B exists, the thickness of the latter layer indicates that the magnitude of the relative sea-level jump is less than 4 m. The event stratigraphy derived from seismic subsidence in salt marsh sediments is characterized by grading, dominant marine diatoms, and intense sediment accumulation rates. The presence of this event stratigraphy in the Echigo Plain suggests that seismic events strongly affect strata formation in tectonically subsiding areas in Japan.
In Rokken-gawa lowland, located near Lake Hamana in central Japan, along the Nankai Trough, a sand sheet dated at around 1400–1300 BC (∼3400 cal BP) was identified along a coring transect. The sand sheet is possibly associated with a high-energy event such as a tsunami or storm. The sand sheet, composed of cross-stratified well-sorted fine to very fine sand is characterized by a maximum thickness of ∼25 cm and extends over 600 m inland from the former coastline. It erosively covers a brackish muddy substrate and is overlaid with freshwater peat deposits. In some cores, the sand sheet is classified into lower and upper sub-layers by a mud drape between them. The lower sub-layer is characterized by current ripples indicative of a landward flow direction and shows inversely graded bedding. Based on the following sedimentological and paleoecological diagnostic criteria it is suggested that the sand sheet was deposited by a tsunami: (1) The sand sheet shows a clear fining- and thinning landward trend, suggesting an extremely long inundation distance. (2) The lower and upper sub-layers comprising the sand sheet indicate a turnover of sediment flows (possibly up flow to return flow). Mud drape separating the lower and upper sub-layers indicates a slack water condition during the transitional stage between up flow and return flow. (3) The sand sheet yields an open marine diatom assemblage suggesting the intrusion of seawater into the freshwater marsh environment. (4) The presence of the sand sheet in the sedimentary sequence can be linked to an abrupt paleoenvironmental change along the coastline, whereby the study area evolved from a semi-open brackish coastal environment into a freshwater peaty marsh environment due to the closure of a tidal inlet.
Several fault-drilling projects have been conducted with the common aim of seeking direct access to zones of active faulting and understanding the fundamental processes governing earthquakes and fault behavior, as well as the factors that control their natural variability. Here, we review recent scientific drilling projects related to the Nojima Fault which slipped during the 1995 Hyogo-ken Nanbu Earthquake, the Chelungpu Fault which slipped during the 1999 Taiwan Chi-Chi earthquake, the San Andreas Fault Observatory at Depth, and the Nankai Trough Seismogenic Zone Experiment. We also briefly introduce the ongoing drilling research by the Geophysical Observatory at the North Anatolian Fault Zone, the Deep Fault Drilling Project at the Alpine Fault, and the Japan Trench Fast Drilling Project. One of the main findings of fault-drilling research is a better understanding of the physico-chemical processes of the primary slip zone during an earthquake, which is closely related to the mechanism of dynamic fault weakening. In the case of the Chelungpu fault, integrated research with borehole experiments, core sample analyses, and numerical simulations were performed, and the results indicate that thermal pressurization occurred during the 1999 earthquake, explaining the peculiar seismic behavior during the earthquake. These fault-drilling projects related to active faults certainly improve our knowledge and understanding of earthquakes. In addition, we discuss new technical problems related to handling core samples, identifying the latest slip zone, and overprinting by ancient earthquake events.
Current geothermal power generation from engineered geothermal system (EGS) technologies has two bottle-necks in practical use: one is that the recoverability of injected water is about 50% or less than that in fracture-dominant regions such as Japan, which inevitably requires replenishing large volumes of injected water throughout the power generation operation, and the other is that the injected water raises pore fluid pressures in crustal rocks, causing induced-earthquakes. This paper proposes a new power generation method, which has the potential to resolve these two bottle-necks using EGS technologies in ductile zones. With this method, an artificial brittle fracture reservoir system is completely surrounded by ductile zones at a temperature exceeding 500°C, the presence of which has already been confirmed at the Kakkonda geothermal field, northeastern Japan. The profitability of this method is highly dependent on the depth of drilling, but this concept could dramatically expand exploitable thermal conduction geothermal resources beyond the brittle zones.
The deep subsurface biosphere has been regarded as an emerging topic in geo-bioscience and industry for the past few decades, and has been approached by terrestrial and seafloor drillings. Terrestrial sites have better proximity and greater relevance to the anthroposphere and technosphere, i.e., human habitats and societies, than do seafloor sites. Therefore, understanding the subterranean biosphere has more direct importance to issues related to a sustainable civilization, and issues such as formation/maturation of hydrocarbon reservoirs and ore deposits, disposal of radioactive wastes and carbon dioxide, and postulated association between seismogenic and microbial activities. Microbiological studies in the terrestrial deep subsurface have been prompted to respond to such human-related issues, and microbial life in sedimentary and crystalline rocks as well as pore-filling fluids has been studied to evaluate rock stability and (im) mobilization of redox-sensitive elements/nuclides, for instance. This is in contrast to subseafloor microbiology, which focuses more on microbial interactions with hydrothermal circulation, relevant biogeochemical processes including gas hydrate formation, associated diversity of life, and modern analogs of origin-of-life. Avoiding man-induced contamination of cored samples and pumped fluids has been a microbiological issue. Technical (both instrumental and operational) measures to minimize contamination were first developed in subterranean microbiology, because of easier accesses to test sites for repetition, evaluation, improvement, etc. of attempted measures on land. Then, anti-contamination expertise was introduced into subseafloor practices, and anti-contamination protocols and facilities are now better developed by the Integrated Ocean Drilling Program (IODP) than the International Continental Scientific Drilling Program (ICDP). Newly developed techniques are also applied to measure/monitor geological and geochemical parameters that are used to characterize microbial habitats and processes occurring there. Lessons from subterranean microbiology are directly applicable to subglacial microbiology that may retrieve microbial life from sub-million-year-old ice cores, although additional measures are needed for glacier drilling. Because land and icy surfaces are common in Earth-like planets or potentially life-bearing satellites, lessons (experiences and expertise) from subterranean microbiology should be applicable to astrobiological searches for extraterrestrial life.