Download The Mysterious Planet Earth - Japan Agency for Marine

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