WHY DO WE NEED SUBMARINE SEISMOMETERS ? - ESONET NoE

Philippe Charvis, Guust Nolet, Anne Deschamps and Yann Hello
Géoazur, Université de Nice, Observatoire de la Côte d’Azur - philippe.charvis@oca.eu
WHY DO WE NEED SUBMARINE
SEISMOMETERS?

Global seismicity map
Most earthquakes are located at plate boundaries
85 % of the total seismic moment is released
during large subduction earthquakes at active
margins
Cause of major hazard over densely populated
costal areas

Ocean bottom seismometers exists since the 30’s
One of the first OBS was deployed as early as 1937
Many different types of OBSs exist but all of them are
Free-fall portable instruments
6 – 12 month autonomy
HF to 120 sec. period sensors
No control on coupling

Global network of permanent broadband seismic stations
Lack of seismic stations in the oceans
This lack is emphasized in the southern hemisphere

Global and local seismic tomography
Mantle velocity at 2700 km 1300 km
Equatorial cross-section Polarcross-section
Traveltimes and
waveforms of recorded
seismograms are used to
reconstruct 3D wave
speed distribution in the
earth
Provides information on
the composition, thermal
structure and origin of our
planet
Red for low velocities
(compare to an average
model) and blue for high
velocities
Under-sampled regions in
white
The poor data coverage
in southern hemisphere
limits the quality of
tomographic
reconstruction

RESIF-EPOS an integrated seismic antenna
Antares
It is very unlikely that we will deploy tens of permanent sea bottom
seismometers but this need could be achieved by temporary and long-term
OBSs (several years of autonomy) with data transfer capabilities

RESIF-EPOS an integrated seismic antenna
Antares
It is very unlikely that we will deploy tens of permanent sea bottom
seismometers but this need could be achieved by temporary and long-term
OBSs (several years of autonomy) with data transfer capabilities

MERMAID drifting hydrophone buoys for global tomography
A possible and cost effective
solution to collect seismic data
in the ocean
Drifting hydrophone buoys
that will serve as floating
seismometers on the same
principle as the sounding
oceanographic Lagrangian
buoys
Detection of major
earthquake and transmission
of traveltimes
ERC advanced grant
Development, building and
deployment of 8 drifting buoys
equipped with an acoustic
hydrophone (2009-2013)

MERMAID drifting hydrophone buoys for global tomography
A possible and cost effective
solution to collect seismic data
in the ocean
Drifting hydrophone buoys
that will serve as floating
seismometers on the same
principle as the sounding
oceanographic Lagrangian
buoys
Detection of major
earthquake and transmission
of traveltimes
ERC advanced grant
Development, building and
deployment of 8 drifting buoys
equipped with an acoustic
hydrophone (2009-2013)

Earthquake Early Warning (EEW) systems
Continually process real-time seismic data to determine when a
potentially damaging earthquake is underway
Utilise the first arriving, low-amplitude P-waves to predict the
impending arrival of the higher energy later arriving (e.g. Allen and
Kanamori, 2003)
Waves which actually cause damage typically occurs 10-500 s after a
rupture starts, and even more for subduction earthquakes that
typically start 50-150 km from the nearest (onshore) building
The most advanced algorithms can differentiate between a relatively
minor M6 earthquake and a catastrophic M7-9 earthquake using only
the first few seconds’ worth of data
Seafloor real-time seismic data would greatly improve our ability to
differentiate between earthquakes that generate damaging tsunamis
and earthquakes that do not generate tsunami
Several groups in the US are starting to work on this… UC Berkeley,
Woods Hole Oceanographic Institution

The French Riviera is an active
area with a few large historical
earthquakes of magnitude > 6.0
The Antares neutrino telescope
is connected to land through
an opto-electrical cable
providing
Power
Real-time data transmission
In the deep basin (2400 m)
The Antares neutrino telescope
Submarine cable
ANTARES

The French Riviera is an active
area with a few large historical
earthquakes of magnitude > 6.0
The Antares neutrino telescope
is connected to land through
an opto-electrical cable
providing
Power
Real-time data transmission
In the deep basin (2400 m)
The Antares neutrino telescope
Submarine cable
ANTARES
23-2-1887 M~6.2

Broad band seismometer
Guralp CMG 3T in specific
titanium casing

Seismic noise at the sea bottom
D
N
O
S
A
J
J
M
A
M
F
J

Seismic noise at the sea bottom
D
N
O
S
A
J
J
M
A
M
F
J

Before burying
After burying
Relation between NS and EW motions
Strong current Weak current
The linearity indicates the tilt of seismometer is constant and allows correction
of the seismic signal (Crawford et al.)

The Ligurian Sea submarine observatory

Geophysicists need permanent sea bottom observatories
Real-time monitoring of earthquakes (landslides and tsunamis)
Multi-sensors
Broad band seismometers, accelerometers (strong motion), pressure gauge,
tiltmeters,…
Real-time data transmission for earthquake early warning
Located at active zones (subduction,,…)
Monitoring fluids and relation with seismic events and seismic activity
Geodetic milestone for future underwater geodetic measurements
(quantification of coupled fault segment)
Ligurian submarine platform
Test zone for the development of new technologies
Local and global seismic imaging of the earth
Fleet of drifting hydrophone buoys
Long-term deployment of wide-band OBSs with increased autonomy (3
years) and possibility of regular data recovering and instrument check

Why
do
we
need
submarine
seismometers
?
Philippe
Charvis,
Guust
Nolet,
Anne
Deschamps
and
Yann
Hello
Géoazur,
Observatoire
de
la
Côte
d’Azur,
Université
de
Nice
Sophia­Antipolis,
IRD,
INSU­CNRS
Bât.
4,
250
rue
Albert
Einstein
–
Les
Lucioles
1,
Sophia
Antipolis
–
06560
Valbonne
–
France
Tél
:
+33
492
94
26
92
–
Email:
philippe.charvis@oca.eu
The
seismic
activity
on
the
earth
surface
is
located
near
the
tectonic
plate
boundaries,
most
of
them
being
in
the
deep
ocean
(expansion
centers)
or
near
their
margins
(subduction
zones).
Furthermore,
85%
of
the
total
amount
of
seismic
moment
is
released
during
large
earthquakes
(M>
7.5
km/s)
located
at
subduction
zones.
These
large
earthquakes
cause
major
hazards
over
densely
populated
coastal
areas.
Very
 early
 in
 the
 history
 of
 seismology
 the
 need
 for
 sea‐bottom
 sensors
 was
 identified
 to
 improve
localization
 of
 earthquakes.
 One
 of
 the
 first
 ocean
 bottom
 seismograph
 was
 deployed
 as
 early
 as
 1937
(Ewing
and
Ewing,
1961).
Sutton
et
al.
(1965)
emphasized
the
interest
to
conduct
observations
of
seismic
motion
and
other
geophysical
parameters
on
the
ocean
bottom
over
extended
periods
of
time
and
over
a
wide
range
of
frequencies.
Seismic
images
of
the
deep
earth
Earthquakes
 generate
 seismic
 waves
 propagating
 through
 the
 earth
 that
 can
 be
 recorded
 by
 permanent
seismic
 networks
 installed
 on
 continents
 and
 on
 some
 oceanic
 islands
 (e.g.
 the
 Global
 Seismographic
Network
 consisting
 of
 150
 very
 broadband
 stations,
 distributed
 worldwide
 and
 capable
 of
 recording
 all
seismic
vibrations
from
local
to
large
teleseismic
events).
Traveltimes
 and
 waveforms
 of
 recorded
 seismograms
 can
 be
 used
 to
 reconstruct
 the
 three‐dimensional
wave
 speed
 distribution
 in
 the
 earth
 by
 a
 procedure
 known
 as
 seismic
 tomography
 or
 to
 image
 specific
boundaries
 in
 the
 deep
 earth
 (core‐mantle
 boundary,…).
 This
 provides
 information
 on
 the
 composition,
thermal
structure
and
origin
of
our
planet.
Nevertheless,
 the
 unequal
 geographical
 repartition
 of
 stations,
located
only
on
continents
and
mostly
in
the
northern
hemisphere,
leads
to
an
unequal
data
coverage
that
limits
the
quality
of
tomographic
reconstructions
and
images
of
the
interior
of
the
Earth
(Fig.
1).
Figure
1.
A
polar
cross
section
through
a
P
wave
speed
anomaly
model
(van
der
Hilst
et
al.,
1997)
shows
undersampled
regions
in
white.
This
highlights
the
poor
resolution
of
mantle
structure
in
the
Southern
Hemisphere
and
beneath
major
oceans
due
to
the
scarcity
of
seismic
stations
in
the
oceans.
The
 study
 of
 oceanic
 lithosphere,
 of
 the
 ocean‐continent
 boundary,
 and
 of
 subduction
 zones
 is
 of
 major
scientific,
societal
and
economic
interest.
Because
of
the
lack
of
permanent
sea‐bottom
seismometers
these
studies
are
conducted
over
short
period
of
time
(a
few
weeks
to
a
few
months
at
most)
using
portable
ocean
bottom
seismometers.
This
approach
is
very
restricting
because
of
the
limited
period
of
recording,
the
poor
coupling
of
the
instruments
with
the
sea‐bottom
and
the
limited
band‐width
of
sensors.
Local
 and
 global
 seismic
 imaging
 of
 the
 earth
 needs
 long‐term
 and
 permanent
 deployment
 of
 wide‐band
seismic
 sensors
 that
 will
 provide
 denser
 and
 more
 homogeneous
 data
 coverage.
 Ocean
 bottom
seismometers
and
moored
hydrophones
are
capable
of
addressing
the
coverage
gap,
but
they
are
expensive
to
 manufacture,
 deploy
 and
 maintain
 and
 cannot
 communicate
 their
 recordings
 without
 prohibitively
expensive
cabling.
A
 possible
 solution
 to
 increase
 geographic
 data
 coverage
 for
 global
 tomography
 is
 the
 deployment
 of
 a
number
of
drifting
hydrophone
buoys
that
will
serve
as
floating
seismometers
on
the
same
principle
as
the
sounding
oceanographic
Lagrangian
buoy.
This
type
of
instrument,
providing
an
easy,
cost‐effective
way
to
collect
seismic
data
in
the
ocean,
was
prototyped
by
Simons
et
al.
(2006).

Real‐time
monitoring
of
earthquakes
Major
earthquakes
cause
human
and
economic
losses
directly
related
to
the
strong
motion
of
the
ground
or
by
induced
phenomena
like
tsunamis
and
landslides.
Early
warning
systems
for
tsunamis
and
earthquakes
have
been
developed
in
the
recent
years
to
mitigate
associated
damages.
For
earthquakes
early
warning
(EEW),
systems
continually
process
real‐time
seismic
data
to
determine
when
a
potentially
damaging
earthquake
is
underway.
They
utilize
the
first
arriving
low‐
amplitude
P‐waves
to
predict
the
impending
arrival
of
the
higher
energy
later
arriving
waves,
which
actually
cause
damage.
Subduction
zone
mega‐thrusts
like
2004
Sumatra
are
great
candidates
for
EEW
because
they
typically
 start
 50‐150
 km
 from
 the
 nearest
 inhabited
 area,
 meaning
 there
 is
 several
 tens
 or
 hundreds
 of
seconds
 to
 proceed
 with
 precautions,
 including
 shutting
 off
 gas
 lines
 and
 stopping
 trains.
This
 can
 be
achieved
only
with
dedicated
cabled
sea‐bottom
observatories
that
can
transmit
the
seismic
signal
real‐time
to
processing
centers.
Nevertheless
for
academic
purposes
the
access
to
the
data
in
almost
real‐time
is
also
important
to
check
if
the
 instrument
 is
 operating
 properly,
 to
 adapt
 the
 multi‐sensors
 acquisition
 scheme
 to
 the
 variation
 of
 a
parameter.
For
example,
a
near
real
time
connection
to
shore,
allowing
transmission
of
at
least
a
subset
of
the
data
will
allow
the
possibility
to
modify
acquisition
parameters
for
other
sensors
(avalanche
sensors,…).
The
Ligurian
underwater
scientific
platform
Figure
2:
view
of
the
Antares
CMG3T
seismometer
during
its
installation
by
ROV
Victor
of
Ifremer.
The
 Antares
 neutrino
 telescope,
 installed
 in
 the
 Ligurian
 Sea,
 is
connected
 to
 land
 through
 an
 opto‐electrical
 cable
 that
 provides
power
 and
 data
 transmission
 from
 the
 coast
 to
 the
 deep
 basin
(Aguilar
et
al.,
2007).
Using
this
opportunity,
we
installed
in
2005
a
broadband
 CMG3T
 seismological
 sensor
 specifically
 designed
 for
this
 experiment
 that
 was
 used
 to
 test
 the
 technology
 and
 the
installation
of
the
sensor
(Deschamps
et
al.
2003).
In
 the
 next
 years,
 a
 more
 ambitious
 project
 is
 to
 install
 several
sensors
for
earthquakes,
slope
instabilities
and
submarine
avalanches
offshore
Nice,
interconnected
to
the
Antares
telescope
with
a
new,
light,
optical
micro‐wire
(Valdy
et
al.,
2007).
Conclusions
There
is
a
major
need
for
submarine
and
sea‐bottom
observation
in
seismology,
but
also
to
monitor
slow
deformation
of
the
seafloor
using
geodetic
(acoustic)
measurement
and
tiltmeters.
The
needs
vary
from
real‐
time
acquisition
allowing
early
warning
for
earthquakes
or
tsunamis,
to
much
more
denser
set
of
sensors
(drifting
sonobuoy,
autonomous
ocean
bottom
instruments)
from
which
the
data
can
be
retrieve
from
time
to
time.
The
latter
are
important
because
they
will
be
much
more
cheaper
to
develop,
deploy
and
maintain
and
will
allow
dense
enough
network.
References
Aguilar
J.A.
and
the
ANTARES
Collaboration
(2007).
The
data
acquisition
system
for
the
ANTARES
neutrino
telescope.
Nucl.
Instrum.
Meth.,
A570,
107‐116
Deschamps,
A.,
Hello,
Y.,
Charvis,
P.,
Guralp,
C.,
Dugué,
M.,
and
Levansuu,
D.,
2003,
Broad‐band
seismometer
at
2500
m
depth
in
the
Mediterranean
Sea,
in
EGU‐AGU
spring
Meeting
(Nice).
Ewing,
J.
and
Ewing,
M.:
1961,
'A
Telemetering
Ocean
Bottom
Seismograph',
J.
Geophys.
Res.
66,
3863‐3878.
Simons
F.J.,
G.
Nolet,
J.
M.
Babcock,
R.
E.
Davis,
and
J.
A.
Orcutt
(2006).
A
Future
for
Drifting
Seismic
Networks.
Eos,
Vol.
87,
No.
31,
1
August
2006,
p
305,
307.
Sutton,
G.
H.,
G.
McDonald,
D.
D.
Prentiss,
and
S.
N.
Thanos,
“Ocean
bottom
seismic
observations,”
in
Proceedings
IEEE,
1965,
vol.
53,
p.
1909.
Valdy,
P.,
Ciausu,
V.,
Leon,
P.,
Moriconi,
P.,
Rigaud,
V.,
Hello,
Y.,
Charvis,
P.,
Deschamps,
A.,
and
Sillans,
C.,
2007,
Deep
sea
net:
an
affordable,
and
expandable
solution
for
deep
sea
sensor
networks.
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Symposium
on
Underwater
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International
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on
Scientific
Use
of
Submarine
Cables
and
Related
Technologies
2007:
Tokyo,
Japan,
p.
172‐5.
van
der
Hilst,
R.
D.,
S.
Widyantoro,
and
E.
R.
Engdahl
(1997),
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