BBOBS - jamstec

Seismic observation on Polynesian seafloor
with portable broadband OBS
and its result on deep mantle structure
Daisuke Suetsugu
(Institute for Research on Earth Evolution/JAMSTEC)
Japan (ERI, Univ. Tokyo; IFREE/JAMSTEC)-France cooperative project
Co-workers:
„T. Isse, S. Tanaka, M. Obayashi, H. Sugioka, C. Adam, J. Yoshimitsu,
A. Ito, Y. Fukao (IFREE/JAMSTEC)
„H. Shiobara, T. Kanazawa (ERI, Univ. Tokyo)
„G. Barruol (U. Montpellier II, France) , Alain Bonneville (IPGP,
France), Dominique Reymond (CEA, Tahiti, France)
Contents
Scientific significance of mantle plumes beneath
South Pacific –Superplume hypothesis
‡ Lack of resolution in tomographic image beneath
oceanic regions.
‡ Broadband Ocean Bottom Seismograph (BBOBS)
‡ Polynesia BBOBS (broadband ocean bottom
seismograph) array
‡ Seismic method
‡ Preliminary Results
on South Pacific mantle plumes:
‡
Current seismic networks
- Sparse in oceanic regions
Oceanic regions, 2/3 of the earth’s surface, have not been
seismologically explored well.
Why Polynesia?
-Superswell and hot spots
Superswell ≡
Broadly elevated seafloor
Adam & Bonneville (2005)
Mantle
structure
見えてきた
見えてきた
見えてきた
at present time
Downwelling
beneath IBM
African
superplume
Polynesia
Fukao et al. (2001)
South Pacific
Superplumes?
Principle of Seismic Tomography
Source
CT Scanner
Seismic tomography
X-ray transmitter
Earthquake
At arbitrary position
Biasedlocation,
only nearthe surface
Receiver
X-ray detector
Seismograph
At arbitrary position
Mostly
on
land
1/3ofthe
(only
Earth(s
surface)
CT Scanner
waves
X-ray
Seismic wave
Can scan target uniformly
Scan
target
non-uniformway
Surface wave tomography
Body wave tomography
in
a
Poor resolution beneath South Pacific
in previous tomographic models
-Checkerboard resolution test
Reconstructed
pattern
by tomography
‡
‡
(Inoue et al., 1990)
Clear black&white
pattern
→ good resolution
mantle structure
obtained by
tomography
Grey pattern
→ bad resolution
(Obayashi & Fukao, 2001)
Geometry and size of the mantle plumes have not been constrained well
because of sparse seismic stations and earthquakes in the superswell region.
Polynesia Broadband OBS
array (2003-2005)
Marquesas
Society
Pitcairn
MacDonald
Yellow: BBOBS stations (seafloor)
Red: French PLUME stations (island)
White: Permanent stations (island)
BBOBS (Broadband Ocean Bottom Seismograph)
Developed by Earthquake Research Institute, Univ. Tokyo
Inside view of BBOBS
BBOBS
-Self pop-up type
Install by free fall from a
ship
・is easy and quick
・needs flat seafloor
(bathymetry survey
required)
Vessels and submersible used
YOKOSUKA (JAMSTEC)
SHINKAI 6500 (JAMSTEC)!!
Fetu TeaII（Tahitian fishing boat)
Deployment
January, 2003
BBOBS at 4400 m depth
Recovery
August-Novemver, 2004
BBOBS on
sea surface
Sending release command
with transponder on ship
Catch a BBOBS
Examples of
teleseismic records
BBOBS station FP2 in
Polynesia
UD-component
M>5.7
Filtered at15-100 sec
Data quality
Horizontal
Vertical
Seismic methods used in the present study
‡Upper mantle → surface wave tomography
‡Transition zone (TZ) → receiver function
for depths of 410-km,660-km discontinuities
(Tomography with our data set does not provide
sufficient resolution for TZ)
‡Lower mantle → P-wave tomography
Upper mantle
- Surface wave tomography with two-station methodsuperswell
‡Surface-wave velocity is mapped to 2D surface-wave velocity
distribution at each frequency.
‡Frequency-dependence of surface-wave velocity map is inverted
for depth variation of S-velocity. → ３D S-velocity model
Upper mantle
-Surface wave tomography (Isse et al., 2006)
Checkerboard
With BBOBS
Without BBOBS
Most of the hot spots have
slow velocity roots in the
upper mantle with variable
depth extents.
Lower mantle
- P-wave tomography
Ray paths
Data
•P-wave differential times among regional stations
including the BBOBS stations
•P-wave onset times at regional stations including
the BBOBS stations
•P-wave times in the ISC catalogue
Good
resolution
Tanaka, Obayashi, Yoshimitsu et al. (2007)
faster
Large-scale variation
deeper
shallower
Small-scale variation
Lower mantle
-P-velocity model
1000km depth
slower
Cross-sectional view
Upper mantle: S-velocity by surface wave tomography using BBOBS data (Isse et al., 2006)
Transition zone: S-velocity by a previous study (Ritsema and Heijst, 2000)
Lower mantle: P-velocity by Tanaka et al. (2007) using BBOBS data
S-velocity scale: Upper mantle & Transition zone
Society
Pitcairn
S-wave image (our study)
Previous S-model (Ritsema &Heijst, 2000)
P-wave image
(our study)
Marquesas
Society
Pitcairn
MacDonald
P-velocity scale: Lower mantle
Schematic view of mantle plumes
beneath South Pacific Superswell
?
Data availability
‡
•
‡
The Polynesia BBOBS data will be made
available via the JAMSTEC web page this
year, two years after the recovery.
They were exchanged with French data of temporary PLUME network
last year.
Other BBOBS data by past experiments in
the Philippine Sea and northwestern Pacific
will be also made available via the web page
this year.
On-going BBOBS observation in the Philippine Sea
conducted by: ERI/Univ. Tokyo and IFREE/JAMSTEC
Period: 2005-2008
Target: Stagnant Pacific slab
BBOBS
+ Ocean Bottom Electro-magnetometer
Summary
z
z
The BBOBS has become reliable tool for
seafloor seismic observation and provided
waveform data of good quality equivalent
to land-based data (particularly verticalcomponent).
The BBOBS data enabled seismic imaging
of deep mantle plumes with
unprecedentedly high resolution.
Checkerboard resolution test
for P-wave tomography
Real-time OBS observation
with a buoy & satellite-based method by WHOI
Frye et al. (2006), Eos