WHY DO WE NEED SUBMARINE

SEISMOMETERS ?

Philippe Charvis, Guust Nolet, Anne Deschamps and Yann Hello

Géoazur, Université de Nice, Observatoire de la Côte d’Azur - philippe.charvis@oca.eu

Global seismicity map

Most earthquakes are located at plate boundaries85 % of the total seismic moment is released during large subduction earthquakes at active marginsCause 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 oceansThis lack is emphasized in the southern hemisphere

Global and local seismic tomography

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

Mantle velocity at 2700 km 1300 km

Equatorial cross-section Polarcross-section

RESIF-EPOS an integrated seismic antenna

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

Antares

RESIF-EPOS an integrated seismic antenna

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

Antares

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

Submarine cable

The Antares neutrino telescope

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)

ANTARES

Submarine cable

The Antares neutrino telescope

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)

ANTARES

23-2-1887 M~6.2

Broad band seismometer Guralp CMG 3T in specific titanium casing

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

Seismic noise at the sea bottom

Relation between NS and EW motions

The linearity indicates the tilt of seismometer is constant and allows correction

of the seismic signal (Crawford et al.)

Bef

ore

bu

ryin

gA

fter

bu

ryin

gStrong current Weak current

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

Whydoweneedsubmarineseismometers?PhilippeCharvis,GuustNolet,AnneDeschampsandYannHello

Géoazur,ObservatoiredelaCôted’Azur,UniversitédeNiceSophia­Antipolis,IRD,INSU­CNRSBât.4,250rueAlbertEinstein–LesLucioles1,SophiaAntipolis–06560Valbonne–FranceTél:+33492942692–Email:philippe.charvis@oca.eu

Theseismicactivityontheearthsurfaceislocatednearthetectonicplateboundaries,mostofthembeinginthedeepocean(expansioncenters)orneartheirmargins(subductionzones).Furthermore,85%ofthetotalamountofseismicmomentisreleasedduringlargeearthquakes(M>7.5km/s)locatedatsubductionzones.Theselargeearthquakescausemajorhazardsoverdenselypopulatedcoastalareas.Very early in the history of seismology the need for sea‐bottom sensors was identified to improvelocalization of earthquakes. One of the first ocean bottom seismograph was deployed as early as 1937(EwingandEwing,1961).Suttonetal. (1965)emphasizedthe interest toconductobservationsofseismicmotionandothergeophysicalparameterson theoceanbottomoverextendedperiodsof timeandoverawiderangeoffrequencies.

SeismicimagesofthedeepearthEarthquakes generate seismic waves propagating through the earth that can be recorded by permanentseismic networks installed on continents and on some oceanic islands (e.g. the Global SeismographicNetwork consisting of 150 very broadband stations, distributed worldwide and capable of recording allseismicvibrationsfromlocaltolargeteleseismicevents).Traveltimes and waveforms of recorded seismograms can be used to reconstruct the three‐dimensionalwave speed distribution in the earth by a procedure known as seismic tomography or to image specificboundaries in the deep earth (core‐mantle boundary,…). This provides information on the composition,thermal structure andoriginof ourplanet.Nevertheless, theunequal geographical repartitionof stations,locatedonlyoncontinentsandmostly inthenorthernhemisphere, leadstoanunequaldatacoveragethatlimitsthequalityoftomographicreconstructionsandimagesoftheinterioroftheEarth(Fig.1).

Figure1.ApolarcrosssectionthroughaPwavespeedanomalymodel(vanderHilstetal.,1997)showsundersampledregionsinwhite.ThishighlightsthepoorresolutionofmantlestructureintheSouthernHemisphereandbeneathmajoroceansduetothescarcityofseismicstationsintheoceans.

The study of oceanic lithosphere, of the ocean‐continent boundary, and of subduction zones is of majorscientific,societalandeconomicinterest.Becauseofthelackofpermanentsea‐bottomseismometersthesestudiesareconductedovershortperiodoftime(afewweekstoafewmonthsatmost)usingportableoceanbottomseismometers.Thisapproachisveryrestrictingbecauseofthelimitedperiodofrecording,thepoorcouplingoftheinstrumentswiththesea‐bottomandthelimitedband‐widthofsensors.Local and global seismic imaging of the earth needs long‐term and permanent deployment ofwide‐bandseismic sensors that will provide denser and more homogeneous data coverage. Ocean bottomseismometersandmooredhydrophonesarecapableofaddressingthecoveragegap,buttheyareexpensiveto manufacture, deploy and maintain and cannot communicate their recordings without prohibitivelyexpensivecabling.A possible solution to increase geographic data coverage for global tomography is the deployment of anumberofdriftinghydrophonebuoysthatwillserveasfloatingseismometersonthesameprincipleasthesoundingoceanographicLagrangianbuoy.Thistypeofinstrument,providinganeasy,cost‐effectivewaytocollectseismicdataintheocean,wasprototypedbySimonsetal.(2006).

Real‐timemonitoringofearthquakesMajorearthquakescausehumanandeconomiclossesdirectlyrelatedtothestrongmotionofthegroundorbyinducedphenomenaliketsunamisandlandslides.Earlywarningsystems for tsunamisandearthquakeshavebeendeveloped in therecentyears tomitigateassociateddamages. For earthquakesearlywarning (EEW), systems continuallyprocess real‐time seismicdatatodeterminewhenapotentiallydamagingearthquakeisunderway.Theyutilizethefirstarrivinglow‐amplitudeP‐wavestopredicttheimpendingarrivalofthehigherenergylaterarrivingwaves,whichactuallycausedamage.Subductionzonemega‐thrustslike2004SumatraaregreatcandidatesforEEWbecausetheytypically start 50‐150 km from the nearest inhabited area,meaning there is several tens or hundreds ofseconds to proceed with precautions, including shutting off gas lines and stopping trains. This can beachievedonlywithdedicatedcabledsea‐bottomobservatoriesthatcantransmittheseismicsignalreal‐timetoprocessingcenters.Neverthelessforacademicpurposestheaccesstothedatainalmostreal‐timeisalsoimportanttocheckifthe instrument is operating properly, to adapt themulti‐sensors acquisition scheme to the variation of aparameter.Forexample,anearrealtimeconnectiontoshore,allowingtransmissionofat leastasubsetofthedatawillallowthepossibilitytomodifyacquisitionparametersforothersensors(avalanchesensors,…).

TheLigurianunderwaterscientificplatform

Figure2:viewoftheAntaresCMG3TseismometerduringitsinstallationbyROVVictorofIfremer.

The Antares neutrino telescope, installed in the Ligurian Sea, isconnected to land through an opto‐electrical cable that providespower and data transmission from the coast to the deep basin(Aguilaretal.,2007).Usingthisopportunity,weinstalledin2005abroadband CMG3T seismological sensor specifically designed forthis experiment that was used to test the technology and theinstallationofthesensor(Deschampsetal.2003).In the next years, a more ambitious project is to install several

sensors forearthquakes,slope instabilitiesandsubmarineavalanchesoffshoreNice, interconnectedtotheAntarestelescopewithanew,light,opticalmicro‐wire(Valdyetal.,2007).

ConclusionsThere isamajorneed for submarineandsea‐bottomobservation in seismology,butalso tomonitor slowdeformationoftheseafloorusinggeodetic(acoustic)measurementandtiltmeters.Theneedsvaryfromreal‐timeacquisitionallowingearlywarning forearthquakesor tsunamis, tomuchmoredensersetof sensors(driftingsonobuoy,autonomousoceanbottominstruments)fromwhichthedatacanberetrievefromtimetotime.Thelatterareimportantbecausetheywillbemuchmorecheapertodevelop,deployandmaintainandwillallowdenseenoughnetwork.

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