Monitoring submarine volcanic eruptions
and drifting icebergs in the South Pacific Ocean
Olivier Hyvernaud
CEA/DASE, Laboratoire de Géophysique
BP 640 — Tahiti, French Polynesia ([email protected])
COMMISSARIAT A L’ENERGIE ATOMIQUE
Monowai volcano, Kermadec arc, was discovered in 1977 : a patch of brown and turbulent
water was observed by Royal New Zealand Air Force. The summit of the seamount is now
130m deep. Since 1988, the Laboratoire de Géophysique, CEA/DASE, uses its ‘T Phase’ sta-
tions to monitor closely the submarine hydroacoustic activity of the seamount.
Hydroacoustic paths from source TO RSP stations and IMS hydrophones.
Many T waves marked out the drift and the progressive crumbling of B15A. Finally, on December 27, at 03:22, an
unusual swarm of T waves indicated that knife-shaped B15A was breaking up into 4 pieces. We found that the
most of the hydroacoustic events where generated by B15A, but some of them came from small pieces accompa-
nying the giant. In February and March 2008, we detected the last events from small melting pieces. The source
mechanism of these hydroacoustic events was not clearly known, but the locations and some anomalies of ampli-
tude suggested that the sources were due to cracks at the periphery of B15A.
Since May 24, 2002, Monowai volcano is in constant activity. Several Sea Beam surveys have shown at least two
dramatic changes of the volcano : in May 24, 2002, a very strong T wave was associated with a SE sector col-
lapse. A second slide was identified on the SW slope between 2004 and 2007 but could not be associated with a
strong T wave because the RSP network was shadowed by the edifice. Since 2002, cyclic collapses and regrowth
of the volcano generates episodic swarms of T waves of different shapes and origin: hour-long tremors, subma-
rine explosive activity, very shallow volcano-seismic earthquakes.
The continuous real-time seismic recordings of RSP Network enable to follow all the stages of Monowai erup-
tion. IMS H03N hydrophones are occasionally very useful to constrain the localization of particular Monowai
sources and to reduce the size of error ellipse.
At present, Monowai is still continuing sending out T waves. LDG has frequent contacts with the Institute of
Geological and Nuclear Sciences of New Zealand to keep them informed.
RSP Network records numerous T waves from Antarctic icebergs. During summer 2008-2009, we particularly stu-
died the giant iceberg B15A. The parent iceberg of B15A calved off the Ross Ice Shelf in 2000, and broke up into
several pieces. B15A, 27 x 122km, the size of Long Island, NY, slowly moved in the Ross Sea, broke Drigalsky Ice
Tongue in 2005 and began to drift in the Pacific towards north in mid 2008. In October 2008, it was near 56°S,
167°W. Between November an February, we detected 798 hydroacoustic events coming from Pacific-Antarctic
Ocean, and 412 events were located with RSP and H03N data. The T phases were very short, with a weak to me-
dium amplitude. They were different from T phases of colliding icebergs. K. Stuart, from MERS Lab, BYU., pro-
vided us with accurate Qscat tracking positions of B15A, and LAADS, NASA, made available high resolution
images of Terra and Aqua MODIS instrument. These data gave us the opportunity to track closely the drift and
final breaking-up of this icy giant.
Hydroacoustic source locations (orange circle) during December 2008
with error ellipses, track of B15A in red, and known QScat positions
(MERS Lab.) in December (green stars). In yellow, the QScat position of a
son of B15A.
B15A on Dec 25, 2008 with many located hydroacoustic
sources. Image Terra MODIS, LAADS, NASA.
H03N Hydrophones waveforms from Monowai.
RSP ‘T Phase’ stations waveforms from Monowai.
H03N
RAR
PMO&VAH
TAOE
TBI RKT
MEH PPT
MONOWAI
Monowai seamount in 2004(Chadwick et al, 2008).
Hydroacoustic activity of Monowai since 2002 :
cumulated number of events.
Landslide on the SE slope.
RSP Network
Tonga Trench
Louisville Ridge
South America
Volcanic Activity :
Drifting Icebergs :
Chadwick, W. W., Jr., I. C. Wright, U. Schwarz-Schampera, O. Hyvernaud, D. Reymond, and C. E. J. De Ronde (2008), Cyclic eruptions and sector collapses at Monowai Submarine Volcano, Ker-
madec Arc: 1998-2007, Geochem. Geophys. Geosyst., Q10014, doi:10.1029/2008GC002113.
May 5, 2009
May 5, 2009
1.5Pa
1.5Pa
1.5Pa
2µ/s
.8µ/s
.2µ/s
3µ/s
1µ/s
.4µ/s
.4µ/s
.15µ/s
B15A on Dec 4, 2008 with a located hydroacoustic source at
01:29 UTC. Image Terra MODIS, LAADS, NASA.
B15A on Dec 29, 2008 with located hydroacoustic sources at
18:42 and 15:55 UTC. Image Terra MODIS, LAADS, NASA.