GPS determination of ongoing
ground deformation at Santorini Caldera, Greece
Andrew V. Newman, School of Earth and Atmospheric Sciences, Georgia Institute of Technology, USA
and
Stathis Stiros, Geodesy Lab, Dept of Civil Engineering, Patras University, Greece
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Santorini Caldera in the southern Aegean, is part of a well developed
but still very active volcanic system fueled by subduction along the
Hellenic arc. The caldera is partially submerged, with only pieces
of caldera wall, flanks and central post-caldera lavas exposed above the
sea level, comprising a grouping of five small islands. The system had
its most recent caldera-forming event around 1650 B.C. in a massive
series of Plinean eruptions that expelled some 60 km3 of volcanic
material [Sirgudsson et al., 2006], burying the previous island surface,
and possibly contributing to the demise of the Minoan civilization.
Additionally, the eruption likely caused wide-spread and locally large
tsunami waves across the Aegean and Mediterranean Seas. The system
remains active with ongoing smaller pyroclastic and phreatic eruptions,
the most recent in the 1950’s, forming the central islets atop of the
submerged caldera floor.
In late-spring 2006, with UNAVCO engineer support, we established a
network of two continuous GPS across the caldera, and will soon finish a
third site atop one of the young central islets. As well, we performed a
first GPS campaign of 18 previously established and new sites across the
5-island group. Currently, we are planning a second set of measurements
for spring-summer 2007. Through the continuous and campaign measurements
we hope to establish the rate of ongoing deformation there and determine
if there exists any significant transient deformation that would yield
valuable information about near source rheology and pressure history.
This information may additionally be useful for understanding the early
post-caldera resurgence in a mostly submerged environment. As well, this
information may be useful for early hazard awareness and mitigation during
future regional volcanic crises.
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*click on illustration for a larger view (138 kb)
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Figure 1: Station NOMI, being installed by UNAVCO Engineer and
Project Manager, Jim Normandeau. The site, which is installed atop of
the Santorini Convention Center near the volcanolgical observatory, sits
on the Eastern Caldera wall and overlooks the town of Fira.
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*click on illustration for a larger view (120 kb)
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Figure 2: Station KERA, installed on Thirasia, and is looking
east towards the young volcano islands Nea Kameni and Palea Kameni (tranlated to "new" and "old burnt islands")
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*click on illustration for a larger view (38 kb)
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Figure 3: Station map, of campaign GPS benchmarks and established continuous sites.
The campaign sites were first occupied in Summer 2006 and consist of existing surveying benchmarks
and new sites. The continuous GPS sites, KERA and NOMI (figures 1 and 2) were established in 2006,
while station PKMN is planned to be established in summer 2007.
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*click on illustration for a larger view (63 kb)
*click on illustration for a larger view (63 kb)
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Figure 4a,b:
The difference of two stations in daily determined positions
can be plotted over time. The uncertainty of this signal can also be
determined and plotted. When the daily positions are analyzed over time,
a trend for the annual motion can be determined and a velocity can be
calculated. These plots show the position changes for KERA and NOMI
with respect to station TUC2 which is located on the Aegean micro-plate
in western Crete. A linear approximation was used and a constant velocity
estimate was determined. Slope (velocity) error was corrected for
time dependent uncertainties. A repeating annual signal can also be observed
from the data. Most of the signal observed can be considered insignificant
given the calculated values and uncertainties.
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*click on illustration for a larger view (26 kb)
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Figure 5:
Map of Santorini with the two running
continuous stations marked. After processing
the GPS data, the velocity of each station can be
determined and plotted as a vector. The velocity
was plotted as it was calculated in Figures 3 and
4 with respect to TUC2. Ellipses show regions
of 1 σ uncertainty. It can be observed that the
stations appear to have different velocities, but
because of the uncertainty in the signal, conclusions
based on such differences cannot yet be made.
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Additional field photos are here.
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References:
- Sirgudsson, H., S. Carey, M. Alexandri, G. Vougioukalakis, K. Croff,
C. Roman, D. Sakellariou, C. Anagnostou, G. Rousakis, C. Ioakim,
A. Gogou, D. Ballas, T. Misaridis, and P. Nomikou,
Marine Investigations of Greece’s Santorini Volcanic Field,
Eos, Trans. Am. Geoph. Un., 87 (34), 2006.
- Farmer, G. F, A.V. Newman, P. Psimoulis, S. Stiros, Geodetic Characterization of
Santorini Caldera from Continuous GPS Measurements, EOS, Trans. Am. Geoph. Un.,
Fall, 2007.
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anewman gatech.edu | Updated:
Wed Dec 19 16:33:53 EST 2007
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