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Transcript
On the cutting edge: probing the internal dynamics of the Earth. Distribution of earthquakes (M>5.5, 1973-2009) The Earth’s surface is divided into tens of tectonic plates. Tectonic plates consist of the crust and the uppermost sliver of mantle. Earthquakes mostly occur along the boundaries between these plates. There are three types of plate boundaries: divergent, where new plates are born; transform, where two plates shear past one another; and convergent, where one plate subducts beneath the other. The Japanese islands spread over four plates, the boundaries between which are capable of generating frequent large earthquakes. Why do earthquakes and volcanic eruptions occur? Many Japanese people, who live in an earthquake and volcanic zone, ask this question. The crust and the uppermost mantle are divided into tens of separate tectonic plates. The vast majority of large, damaging earthquakes and active volcanoes occur along the boundaries between these plates. By understanding processes occurring at plate boundaries the mechanisms that generate magma and large earthquakes can be understood step by step. VSP (Vertical Seismic Profiling) is conducted to characterize the structure below the borehole by utilizing an array of seismic sensors temporarily clamped inside the borehole. Air guns towed by Kairei generate seismic waves, and seismic waves reflected from the fault system can be clearly observed by the borehole sensors. BBOBS (Broad Band Ocean Bottom Seismometer) BBOBS observes seismic signals across a wide range of periods from 0.1 seconds to 5 minutes. The Ring of Fire OBEM measures magnetic and electric fields beneath the ocean floor to observe their structure and activity. IFREE The Earth formed 4.6 billion years ago. The newborn Earth did not have sea and land, but was covered by an ocean of magma. The Earth evolved gradually into the arrangement of sea and land we see today. We were all born and live on the Earth, but we still have much to learn about it. Institute for Research on Earth Evolution ROV (Remotely Operated Vehicle), manned research submersible OBEM (Ocean Bottom ElectroMagnetometer) The location of volcanoes around the Pacific Ocean overlaps the regions where earthquakes occur. Volcanic activity is closely linked to the birth or subduction of tectonic plates. In some cases, volcanic activity may not be associated with plate boundaries; for example, volcanoes in Hawaii and French Polynesia form where hot material upwells from the mantle. T h e M ys t e r i o u s P l a n e t E a r t h D/V (Drilling Vessel) Chikyu and R/V (Research Vessel) Kairei HyperDolphin, Kaiko 7000-II and Shinkai 6500 are operated in order to observe the deep ocean floor. H ow m u c h d o yo u k n ow ab o u t o u r E ar t h ? The horizon from Chikyu, the Deep Sea Drilling Vessel What causes periods of extreme environmental conditions on the Earth’s surface? Diamond anvil cell Pressurizes and heats a small sample between the tips of two diamond crystals. Researchers at IFREE were the first in the world to successfully generate the pressures and temperatures experienced at the center of the Earth. Micro-milling facility Allows micro-sampling of rocks and minerals at a resolution of 1 micrometer. There has been life on Earth for more than 3 billion years, but periodically much of the life has died out - times of mass extinction. Some mass extinctions, called oceanic anoxic events (OAE), can be attributed to a lack of oxygen near the sea floor, but we are still unsure what exactly causes this lack of oxygen. The most recent oceanic anoxic event, which occurred 100 million years ago, did not last long enough to cause a mass extinction, but by investigating events associated with it we may be able to understand more about what causes oceanic anoxic events that do result in mass extinctions. We have found that this oceanic anoxic event coincided with more active plate subduction, a high number of mantle plume upwellings, a long period where the Earth’s magnetic field did not reverse, and a time when the surface of the Earth was a lot warmer than usual. Are periods when the environment on the Earth’s surface becomes extreme a manifestation of anomalous activity inside the Earth? Does activity in the Earth’s interior change the surface environment? The oceanic anoxic event from 100 million years ago provides our best opportunity to resolve this puzzle. Earth Simulator Earth Simulator is the world’s largest vector-type super computer. It is used for the vertical imaging of the Earth’s interior in mantle and core dynamics, and to model seismogenic mechanisms. 0.704105±75 0.704058±65 0.703907±20 High sensitivity mass spectrometer 0.704056±69 Analysis of element and isotope compositions in rock and mineral samples. Combining micro-milling with high sensitivity mass spectrometry allows precise determinations of strontium isotope ratios in small areas of crystals (in this example, a plagioclase feldspar crystal). 1mm Super warm interval Volume (1million km3) 0.2 Circum-Pacific subduction zone magmatism (mantle downgoing current) 0.1 6 At IFREE we are taking up the challenge of solving the mysteries of the Earth. 2 Headquarters 2-15 Natsushima-cho, Yokosuka Kanagawa 237-0061 Japan TEL046-866-3811 FAX046-867-9025 0 Yokohama Institute for Earth Sciences 3173-25 Shouwa-machi, Kanazawa-ku Yokohama, Kanagawa 236-0001 Japan TEL045-778-3811 FAX045-778-5498 2011.05 Jurassic period IFREE, which is part of the Japan Agency for Marine-Earth Science and Technology, has divided solving the mysteries into two themes: 1) Research on the Dynamics along Plate Subduction Zones 2) Research on the Evolution of the Oceanic Crust and Mantle and conducts research and observations in oceans across the world. Now, let’s set sail on a voyage to find out more about the mysteries of the Earth. South Pacific Large Igneous Province (mantle upwelling current) 4 Institute for Research on Earth Evolution http://www.jamstec.go.jp/ifree/ Relative volume IFREE 0 150 Ma Geomagnetic field anomaly without polarity reversal Black shale (Oceanic Anoxic Events) Cretaceous period 100 Ma Tertiary Era 50 Ma 0 What makes sea and land? 70 % of the Earth’s surface is covered by water, only 30 % is land. Almost all of this water forms seas. What controls where the seas and land form? Clearly the sea floor is topographically lower than land, but it is not only the topography that is different. The rocks that make up the sea floor are different from the rocks that form the land. We still don’t know how these differences between the rocks of the sea and land are generated. In particular, the origin of the rocks that lie beneath a lot of the land, including much of Japan, known as continental crust, has long been an enigma. The Earth is the only planet in the solar system, which has a continental crust. The evolution of Earth and life on it today would have been completely different without the formation of continents; without continents and continental crust humans would not have walked the Earth. The birth of continental crust is the key to understanding the Earth and ourselves. How can we survey inside the Earth? The Earth can be compared to a boiled egg. The shell represents the Earth’s crust, the egg white the mantle, and the yolk the core. The depth from the Earth’s surface to its center is 6,400km. What is it like inside the Earth? At present, we only have the technology to drill down to about 10 km, which is not even deep enough to get through the crust and reach the mantle. Therefore how can we “see” several thousands of kilometers into the deep interior of the Earth? Research on Dynamics along Plate Subduction Zones R e s e a r c h o n t h e E vo l u t i o n o f t h e O c e a n i c C r u s t a n d M a n t l e Great earthquakes, with magnitudes higher than 8, have repeatedly occurred along the Nankai Trough every 100–150 years, resulting in devastating effects from both ground shaking and tsunamis. The last great earthquakes in this region were more than 60 years ago in 1944 (Tonankai) and 1946 (Nankai). The structure around the earthquake-generating fault system in this area has been investigated and seismic activity has been monitored continuously. In 2007, a scientific drilling project, called the Nankai Trough Seismogenic Zone Experiments, started to reveal the past and future of earthquake and tsunami generation in this area by drilling into the fault system and conducting downhole measurements, laboratory experiments and computer simulations. Oceanic crust forms at oceanic ridges where plates diverge. Oceanic crust subducts back down into the mantle at convergent margins, forming trenches, and may reach as deep as the coremantle boundary. Billions of years later some of this material may return to the surface in mantle upwellings. This is the mantle cycle; the Earth’s interior is not in a static state, it has been evolving continuously through its 4.6 billion year history. We investigate the evolutionary history of the mantle and the crust and their interactions with the surface environment of the Earth. Real-time monitoring is set in place for more accurate assessment of the next earthquake. Nankai Trough and the Drill Sites In December 2010 we installed the first borehole observatory in a well in the Nankai Trough, which can measure the strain, tilt, seismicity, pressure and temperature closer to the plate boundary. It will be connected by submarine cables to a real-time seafloor observatory network (DONET). DONET aims to establish the technology of large scale real-time seafloor research and surveillance infrastructure for earthquake, geodetic and tsunami observation and analysis. The Philippine Sea Plate subducts beneath the Japan Island at 5 cm/year. Drilling using the Japanese drilling vessel Chikyu is under way off the coast of the Kii Peninsula, above the source region of the Tonankai earthquake. 35˚ N 0 km 50 40˚N 30˚ Kii Peninsula 130˚ 140˚ 150˚E 34˚ Geological structure along the line This is the earthquake-generating plate boundary fault. Most great earthquakes occur along convergent plate boundaries. Understanding the earthquake processes requires knowledge of the location, geometry and character of the active faults. Our efforts to reveal the geological structure below the Nankai Trough use artificial acoustic waves reflected from the faults and boundaries between different types of rock. North Forearc basin Drill Sites DONET was established in March 2010 to monitor seismicity and tsunamis from the seafloor. C0002 C0001 C0003/C0004/C0008 33˚ N i anka 135˚ E Trou Subduction of 5 cm/year the Philippine Sea Plate 136˚ 137˚ 138˚ Ocean drilling reveals the relationship between the dynamics of the Earth’s interior and the environment of the Earth’s surface 139˚ 0 South Outer ridge 1 10km 2 Fluid seepage Shikoku Basin Nankai Trough 3 Depth (mbsl, km) 6 7 Oceanic crust 8 Step down Matured oceanic plate slab: Drilling to Moho discontinuity 10 Geological structure across the Nankai Trough. Large earthquakes 12 13 Fractionation: heavy materials sink and light materials float Upwelling of basalt magma Fractionation proceeds Lower crust Extremely heavy rocks “Anti-continent” Crustal structure and proposed drill sites in the Izu-Bonin-Mariana (IBM) arc Four drill sites are proposed in the Izu-Bonin-Mariana arc, each one targeting a different evolutionary stage of the arc. At the IBM-4 drill site (picture) we plan to drill 4 km below the sea floor and collect andesite and granite samples. Carbon exchange Drilling into the arc crust Sediment Mantle diamond Moho discontinuity strain NW 5320 2500 Simulated Strain across the Nankai Trough 2600 SE Cross-line 5310 5300 5290 5280 5270 5260 5250 Hole C0004D Hole C0004C Hole C0004B 2700 5240 5230 5220 Continental crust 5210 5200 Dehydration and water transport Deep carbon and water cycle 100 m VE = 1.0 “Anti-continent” High pressure carbon phase (diamond etc.) Subducted plate slab Micro-fracture zone Water transport Deep element cycle Hole C0008A Hole C0008C Depth (mbsl, m) Fault zone 2800 2900 Micro-fracture zone 3000 5cm Carbon release 3100 3200 3400 3500 The Fault zone This core sample was cut from 271 m below the sea floor across a branching fault. We recognize a narrow fault zone that runs obliquely across the core and is surrounded by a INLINE2675 fracture layer. Red dots in the rock around the fault suggest that in the past the rock experienced frictional heating due to seismic slips. Outer core 0 3300 Oceanic volcano Carbon release Upper mantle Transition zone Inner core Lower mantle D” layer 6,370km 5,200km 2,890km 660km 420km 30km 0km Imaging the mantle using seismic waves, known as seismic tomography, has revealed that the mantle convects. Subducted plates stagnate at mid-mantle depths, collapse and then sink down to the bottom of the mantle. Upwelling of hot mantle material from the bottom to the top of the mantle acts as a counter-flow. The Earth’s outer core is filled with molten iron and is also convecting, giving rise to the Earth’s magnetic field. At JAMSTEC we investigate such deep Earth dynamics using the supercomputer Earth Simulator. Cold subducted material at the core–mantle boundary may affect convection in the outer core and reverse the polarity of the Earth’s magnetic field. A reversal last occurred 100 million years ago – prior to that, they had occurred more frequently. One possibility is that a thick pile of dense material has accumulated at the boundary between the mantle and core, thermally insulating the core, preventing it from cooling. This dense material may be “anticontinent” material that has accumulated at the bottom of the mantle throughout the Earth’s history. This hypothesis still needs to be tested in detail using mantle–core dynamics. Numerical simulation of mantle convection High and low temperatures are indicated by red and blue, respectively. Subducted slabs stagnate in the mid-mantle, before collapsing and accumulating at the bottom of the mantle. Mantle upwelling takes place as a counter flow in adjacent regions. Numerical simulation successfully reproduces the mantle structure observed by seismic tomography. Core 3 4 5 Moho 6 7 Mantle 8 What’s the Moho ? This image shows the mantle-crust section, including the Moho, in the Northwestern Pacific ocean. Numerical simulation of convection in the outer core Cross-sections of outer core convections are shown on the equatorial and meridian planes, demonstrating sheetlike flows that develop parallel to the Earth’s rotational axis. Africa Mantle structure revealed by seismic wave tomography Slow (hot) and fast (cold) velocities of seismic waves are indicated by red and blue colors, respectively. Areas where cold subducted slabs have stagnated in the mantle are evident as blue patches at mid-mantle depths. These then collapse and accumulate at the bottom of the mantle, as can be seen beneath eastern Asia. In contrast, the counter-flow of hot mantle Polynesia material (red) is observed as upwellings beneath Polynesia, in the Southern Pacific, and Africa. 1 Crust Juvenile continental crust Mantle convection 0 2 Revealing the dynamics of the Earth’s mantle and core low The past is the key to the future. We have already obtained cylindrical ‘core’ samples from landslides and an active fault zone branching from the plate boundary. In 2010 our challenge started to sample fault rock from the seismogenic plate boundary 7000 meters below the sea floor, using the drilling vessel Chikyu. Sampling fresh rocks from the plate boundary where the seismic waves are generated will help us to understand the mechanisms behind the earthquakes. Middle crust Basalt Water supply via subduction and water-fluxed mantle melting high Sampling fresh fault rock from the mega-quake zone. Andesite Oceanic ridge Oceanic plate slab Back-arc basin Geological structure across the Nankai Trough revealed by imaging using artificial acoustic waves. Since the plate is subducting continuously, strain energy will accumulate along the any portion of the plate boundary fault that is locked. When this strain exceeds the strength limit of the rocks around the boundary, ruptures, or sudden offsets along the plate boundary cause an earthquake. By simulating and monitoring how the strain accumulates we can improve the quality of earthquake prediction. Basalt Continental crust Sea floor Trench 11 Nankai Trough as seen from the Shikoku Basin looking north. Partial melting Oceanic crust The first proposal to drill to the mantle was made in 1959. It was called “Project Mohole” as the target was to reach the Mohorovičić discontinuity, or Moho. This is the boundary between the crust and the mantle, which occurs at depths ranging from 10 to 60 km below the surface. Technological limitations at the time prevented “Project Mohole” from reaching the mantle. However, today we have Chikyu, a drilling research vessel with the capabilities to drill to the Moho and beyond. By drilling to the Moho we will be able to obtain a picture of the cycles operating in the Earth’s interior by directly analyzing material recovered from the mantle. In addition, the drilling will recover an entire section of oceanic crust, the analyses of which will comprehensively reveal the processes that form the oceanic crust. Volcanic arc 9 Philippine Sea plate Immature oceanic plate slab: Drilling to Moho discontinuity Carbon exchange Subduction zone drilling Rhyolite Oceanic crust consists of basalt. The crust that underlies the continent, known as continental crust, is largely made up of rocks called andesite and granite. It has long been debated where continental crust forms. Our seismic studies in the IzuOgasawara-Mariana island areas, where two plates of oceanic crust are colliding to form an intra-oceanic arc system, have revealed that andesite and granite form beneath infant arc systems. We will drill into the crust of this arc system in order to recover for the first time rock samples where andesite and granite are forming to investigate how continental crust originates. Broad-band seismometer module being lowered into the well (photograph taken from a remotely-operated vehicle). The Earth’s mantle is slowly convecting over a timescale of a few billion years. This internal convection involves carbon released from the Earth’s core and water input from the Earth’s surface through subduction of tectonic plates. The carbon is stored at great depths as diamond and released as carbon dioxide gas at the surface by volcanoes. Massive degassing may cause climate change, such as global warming. Subducted water decreases the melting temperature of the mantle, reducing its viscosity and accelerating convection, and also enhancing melting, causing volcanic activity. 4 5 Décollement Splay fault Branching of the fault system Deployment of borehole observatory Journey to the Mantle: Our dreams come true Where two plates of oceanic crust collide the colder, older plate is forced to subduct and takes water in the rocks down into the mantle. Because mantle material melts at a lower temperature if water is present, molten rock is generated. This rises buoyantly and accretes to the oceanic crust that is not subducting. This accreted arc crust may melt due to additional heat supplied from continuously accreting molten rock. Fractionation occurs in the arc crust as heavy materials sink and lighter materials rise. The lighter materials eventually form continental crust. Heavy materials form “anti-continent” material, which eventually breaks off from the lighter continental crust and falls back into the deep Earth. Continental crust formation occurs in the arcs C0006/C0007 gh Normal faults Accretionary prism DONET (Dense Ocean-Floor Observatory Network for Earthquakes and Tsunamis). Formation process of the continental crust Inner core km