Untersuchung der Deformation, Seismizität und vulkanischen Gefährdung des Columbo Seamount (Santorini, Ägäisches Meer)
Zusammenfassung der Projektergebnisse
The occurrence of seismic swarms is often assumed to be caused by fluid- or magma intrusions. However, since most intrusions do not reach the surface, direct evidence for this relation is usually missing, and the interpretation of swarm earthquakes remains speculative. Joining local seismicity with simultaneous submarine tilt and deformation measurements may provide such an evidence, and is therefore a promising novel approach to better investigating and characterizing the unrest of a submarine volcano. This is what the Columbo project contributed, with the special and demanding situation that seismicity and tilt/deformation was measured at the seafloor using low-cost, in-house developed, free-fall ocean-bottom stations. The Columbo project was successful, and revealed new and interesting insights into the relation between fluid-induced seismicity and crustal deformation beneath an active, submarine volcano. These results were derived out of large interdisciplinary data including new structural and geophysical data from Columbo seamount. Our free fall, self leveling Ocean Bottom Tiltmeter (OBT) observed short baseline tilt at a resolution of about 15 nrad. The tiltmeter is mounted on the preexisting Hamburg Ocean Bottom Seismometer (OBS) carrier system, with a baseline of about 1.5m. It was additionally equipped with a hydrophone to record seismic data, and with an absolute pressure sensor to observe uplift and subsidence. Columbo volcano, a seamount about 15 km NE of Santorini island has attracted increased attention since island based monitoring on Santorini indicates a high seismicity rate clustering around the seamount and possible crustal deformation which both might represent fluid migration in the subsurface. The passive experiment lasted 10 month, between June 2006 and March 2007. Four OBT systems were deployed along a profile perpendicular to the main known fracture zones stricking NE-SW across the summit region of the Columbo Submarine Volcano. The network was completed by four OBSs in the vicinity of the seamount and additional land seismometers on the surrounding islands. The azimuthal gaps of the Greek monitoring network between the islands were closed and the magnitude threshold of the permanent network was significantly decreased through our experiment. The installation of zero offset seismic stations on top of the volcano helped to better constrain the depth of the local earthquakes beneath Columbo. In general, the location accuracy was high. About 4000 local earthquakes have been manually picked and located during the time of the project and six earthquake swarms beneath Columbo have been analyzed for their seismicity pattern and migration velocities of seismic fronts. Four of these swarms were classified as possibly magma-dike-induced. Moment tensor solutions of earthquakes stronger than MW > 3 helped to better understand the stress field and its perturbations. Significant and reliable tilt signals were measured during to the occurrence of the strongest earthquake swarms. We propose an upward migrating, localized vertical intrusion (magma or gas) to explain the simultaneous observation of seismicity and tilt. As a zero order model we employ an analytical volume-source model which explains the major features and characteristics of seismicity and deformation. It is possibly one of the first consistent observations of small-scale sub-seafloor deformation and seismicity underneath a volcanic seamount using free fall stations. The marine magnetic and gravity data were analyzed and interpreted with respect to the possible location, depth and extend of intrusion zones beneath Columbo and the Columbo-Santorini complex. The results support the findings obtained and models derived from seismicity and tilt with respect to the magmatic system beneath Columbo. The analysis of marine seismic profiles is currently subject of a Diploma thesis, and preliminary results further confirm our current understanding of the interaction between intrusions and local tectonics.
Projektbezogene Publikationen (Auswahl)
- 2006: Towards a risk assessment for the Santorini–Columbo volcanic complex. EOS, vol. 87, No. 39, pp. 401 and 407
C. Hübscher, M. Hensch, A. Dehghani, T. Dahm, M. Hort, I. Dimitriadis, T. Taymaz
- EGELADOS workshop 2006: Report on the Poseidon Research Cruise 338 in the area of Santorini
Dehghani, Hensch, Dahm, Hübscher
- EGELADOS workshop Sep. 3-4, 2009: On the interelaton of fluid induced seismicity and crustal deformation
Hensch & Dahm
- PhD thesis (2009): On the interrelation of fluid-induced seismicity and crustal deformation at the Columbo Submarine Volcano (Aegean Sea, Greece). University of Hamburg, Jan. 2010
Martin Hensch
