To monitor a segment of the North Anatolian
seismic fault near Istanbul, an international team of researchers, in
particular from CNRS and Université de Bretagne Occidentale, has
installed a network of transponders on the floor of the Sea of Marmara.
The aim is to measure motion of the sea floor on either side of this
segment. The data collected during the first six months reveals that the
fault is probably locked in the region of this segment, suggesting that
there is a progressive build-up of energy that could be released
suddenly. This could cause a major earthquake in the Istanbul area.
The study, carried out by a collaboration of researchers from France, Germany and Turkey, is published in Geophysical Research Letters.
The North Anatolian fault, which caused destructive earthquakes in
Turkey in 1999, is comparable to the San Andreas fault in California. It
marks the boundary between the Eurasian and Anatolian tectonic plates,
which move relative to each other at a speed of around 2 cm per year.
The behavior of one underwater segment of the fault, located a few tens
of kilometers from Istanbul in the Sea of Marmara, particularly
intrigues researchers, since there has apparently been no seismic
activity there since the eighteenth century. How does this segment
behave? Does it continuously creep? Does it regularly give way,
occasionally causing small, low-magnitude quakes? Or is it locked,
making it likely that it will one day rupture and cause a major
earthquake?
Observing the motion of a submarine fault in situ over a period of
several years is no easy matter. To meet this challenge, the researchers
are testing an innovative underwater remote sensing method, using
active, autonomous acoustic transponders remotely accessible from the
sea surface. Placed on the sea floor on either side of the fault at a
depth of 800 meters, the transponders take it in turns to interrogate
each other in pairs, and measure the round-trip time of an acoustic
signal between them. These time lapses are then converted into distances
between the transponders. The variation in these distances over time is
used to detect motion of the sea floor and any deformation of the
network of transponders, and thus infer the displacement of the fault.
Specifically, a network of ten French and German transponders was set up
during an initial sea cruise1 in October 2014. The first six months of
data (travel time, temperature, pressure and stability)2 have confirmed
that the system is performing well. Following calculations, the data
show no significant motion of the monitored fault, within the network's
resolution limits. The distances between the transponders, which are
between 350 and 1700 meters apart, are measured with a resolution of 1.5
to 2.5 mm. The segment is therefore probably locked or nearly locked,
and is accumulating stress that could trigger an earthquake. However, it
will be necessary to acquire data for several years in order to confirm
this observation or show that this part of the fault has a more complex
behavior.
Going beyond this specific demonstration, if this approach, known as
acoustic seafloor geodesy, proves to be robust in the long term (in this
case, three to five years are planned, within the limits of the
autonomy of the batteries), it could be included within a permanent
underwater observatory as an addition to other observations (seismology,
gas bubble emission, etc) for in situ real-time monitoring of the
activity of this particular fault, or of other active submarine faults
elsewhere in the world.
The work was carried out by the Laboratoire Domaines Océaniques3
(LDO, CNRS/Université de Bretagne Occidentale), in collaboration with
the Laboratoire Littoral Environnement et Sociétés (CNRS/Université de
La Rochelle), GEOMAR (Kiel, Germany), Centre Européen de Recherche et
d'Enseignement de Géosciences de l'Environnement (CNRS/Collège de
France/AMU/IRD), the IFREMER's Laboratoire Géosciences Marines, the
Eurasian Institute of Earth Sciences at the Istanbul Technical
University (Turkey), and the Kandilli Observatory and Earthquake
Research Institute at Bogazici University, Istanbul. This paper is
dedicated to the memory of the Principal Investigator of the project,
Anne Deschamps, CNRS researcher at LDO, who passed away shortly after
leading the successful deployment of the acoustic transponders.
Notes:
1. By the oceanographic vessel Pourquoi pas?, with the help of IFREMER's Laboratoire Géosciences Marines
2. The data was collected from the surface during the campaign of the German oceanographic vessel Poseidon in April 2015.
3. This laboratory is attached to the Institut Universitaire Européen de la Mer -- IUEM (CNRS/UBO/IRD)
Story Source:
The above post is reprinted from
materials provided by
Le Centre national de la recherche scientifique (CNRS).
Note: Materials may be edited for content and length.
Journal Reference:
- P. Sakic, H. Piété, V. Ballu, J.-Y. Royer, H. Kopp, D. Lange, F.
Petersen, M. S. Özeren, S. Ergintav, L. Geli, P. Henry, A. Deschamps. No
significant steady state surface creep along the North Anatolian Fault
offshore Istanbul: Results of 6 months of seafloor acoustic ranging. Geophysical Research Letters, 2016; DOI: 10.1002/2016GL069600
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