Rayleigh wave group velocity tomography in the Aegean area

Data from a large-scale experiment which took place in Greece during the period January-July 1997 have been used to investigate the structure of the Aegean area using surface waves. During this experiment, 30 seismic broadband instruments were deployed throughout the whole Greek area. Additional data during the period 1996-2000 from other temporary networks have been included in the dataset. One hundred eighty-five events with magnitudes 4.0≤ Mw ≤5.5 recorded by these stations have been collected and processed. The individual dispersion curves of the group velocity of Rayleigh waves for each source-station path have been calculated, producing more than 700 paths covering the studied region. These curves have been used to determine Rayleigh group velocity maps using a 2D-tomography method. On the basis of a regionalization of the dispersion measurements, local averaged dispersion curves have been obtained and non-linearly inverted to obtain models of shear-wave velocity versus depth. Since the dispersion curves in the period range 5 s ≤ T ≤30 s are mostly affected by the crustal structure, the model velocities are estimated down to a depth of approximately 35-45 km. The results from the non-linear Hedhehog inversion as applied to a few local dispersion curves show a crustal thickness of approximately 32 km for the Northern Aegean Sea, and a relatively thin crust of approximately 22-24 km for the Southern Aegean Sea. © 2002 Elsevier Science B.V. All rights reserved

Corrigendum to “Seismic imaging of Santorini: Subsurface constraints on caldera collapse and present-day magma recharge” [Earth Planet. Sci. Lett. 514 (2019) 48–61](S0012821X19301360)(10.1016/j.epsl.2019.02.033)

The authors regret errors in the last two paragraphs of Section 6.3 (p. 57), these should read as follows: 6.3 Multistage, nested caldera collapse: In this scenario, the seismic and geological results from Santorini provide observational evidence for models of multistage, nested caldera formation during progressive caldera subsidence (Acocella, 2006). During stage A, the inner collapse column would be formed along outward-dipping reverse faults with breakup of the roof rock (Fig. 6f). During stage B, a new outer ring of collapse would cause subsidence of the entire topographic caldera and the opening of new vents during phase 4 (Fig. 6c). Accordingly, the geologically distinctive LBA eruptive phases form as a direct result of geological processes occurring during each stage of caldera formation (Fig. 6). It is quite possible that all three of the above scenarios (Fig. 6d-f) play a role in generating the inner cylinder of high porosities at Santorini. Thus, rock breakup by reverse faulting during inner caldera collapse may be accompanied by, or even promoted by, fracturing and reaming of the volcanic vent during violent magma-water interactions. In addition, the upper portions of the high-porosity cylinder are probably formed by the deposition of eruptive volcanic products including tuffs, pyroclasts, and ignimbrites. © 2019 Elsevier B.V

Tectonism and Its Relation to Magmatism Around Santorini Volcano From Upper Crustal P Wave Velocity

At extensional volcanic arcs, faulting often acts to localize magmatism. Santorini is located on the extended continental crust of the Aegean microplate and is one of the most active volcanoes of the Hellenic arc, but the relationship between tectonism and magmatism remains poorly constrained. As part of the Plumbing Reservoirs Of The Earth Under Santorini experiment, seismic data were acquired across the Santorini caldera and the surrounding region using a dense amphibious array of >14,300 marine sound sources and 156 short-period seismometers, covering an area 120 km by 45 km. Here a P wave velocity model of the shallow, upper-crustal structure (<3-km depth), obtained using travel time tomography, is used to delineate fault zones, sedimentary basins, and tectono-magmatic lineaments. Our interpretation of tectonic boundaries and regional faults are consistent with prior geophysical studies, including the location of basin margins and E-W oriented basement faults within the Christiana Basin west of Santorini. Reduced seismic velocities within the basement east of Santorini, near the Anydros and Anafi Basins, are coincident with a region of extensive NE-SW faulting and active seismicity. The structural differences between the eastern and western sides of Santorini are in agreement with previously proposed models of regional tectonic evolution. Additionally, we find that regional magmatism has been localized in NE-SW trending basin-like structures that connect the Christiana, Santorini, and Kolumbo volcanic centers. At Santorini itself, we find that magmatism has been localized along NE-SW trending lineaments that are subparallel to dikes, active faults, and regional volcanic chains. These results show strong interaction between magmatism and active deformation. ©2019. American Geophysical Union. All Rights Reserved