Seismicity at the convergent plate boundary offshore Crete, Greece, observed by an amphibian network

Journal of Seismology (Impact Factor: 1.39). 04/2009; 14(2):369-392. DOI: 10.1007/s10950-009-9170-2


We investigate microseismic activity at the convergent plate boundary of the Hellenic subduction zone on- and offshore south-eastern
Crete with unprecedented precision using recordings from an amphibian seismic network. The network configuration consisted
of up to eight ocean bottom seismometers as well as five temporary short-period and six permanent broadband stations on Crete
and surrounding islands. More than 2,500 local and regional events with magnitudes up to M
L = 4.5 were recorded during the time period July 2003–June 2004. The magnitude of completeness varies between 1.5 on Crete
and adjacent areas and increases to 2.5 in the vicinity of the Strabo trench 100km south of Crete. Tests with different localization
schemes and velocity models showed that the best results were obtained from a probabilistic earthquake localization using
a 1-D velocity model and corresponding station corrections obtained by simultaneous inversion. Most of the seismic activity
is located offshore of central and eastern Crete and interpreted to be associated with the intracrustal graben system (Ptolemy
and Pliny trenches). Furthermore, a significant portion of events represents interplate seismicity along the NNE-ward dipping
plate interface. The concentration of seismicity along the Ptolemy and Pliny trenches extends from shallow depths down to
the plate interface and indicates active movement. We propose that both trenches form transtensional structures within the
Aegean plate. The Aegean continental crust between these two trenches is interpreted as a forearc sliver as it exhibits only
low microseismic activity during the observation period and little or no internal deformation. Interplate seismicity between
the Aegean and African plates forms a 100-km wide zone along dip from the Strabo trench in the south to the southern shore-line
of Crete in the north. The seismicity at the plate contact is randomly distributed and no indications for locked zones were
observed. The plate contact below and north of Crete shows no microseismic activity and seems to be decoupled. The crustal
seismicity of the Aegean plate in this area is generally confined to the upper 20km in agreement with the idea of a ductile
deformation of the lower crust caused by a rapid return flow of metamorphic rocks that spread out below the forearc. In the
region of the Messara half-graben at the south coast of central Crete, a southward dipping seismogenic structure is found
that coalesces with the seismicity of the Ptolemy trench at a depth of about 20km. The accretionary prism south of Crete
indicated by the Mediterranean Ridge showed no seismic activity during the observation period and seems to be deforming aseismically.

KeywordsHellenic subduction zone-Microseismicity-Forearc sliver-Crete-Amphibian seismic network-Return flow

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    • "During the last ∼40 yr many studies have been conducted on coastal areas of Crete mainly to constrain its vertical tectonic motion (Flemming, 1978; Pirazzoli et al., 1982, 1996; Stiros, 2001; Shaw et al., 2008; Gallen et al., 2014; Strobl et al., 2014; Tiberti et al., 2014). The resulting tectonic models require uplift and, in some cases, subsidence associated with a range of inferred mechanisms including upper-plate faulting (both normal and reverse), slip on the subduction thrust, sediment underplating or isostatic adjustments to mass deficit beneath Crete (e.g., Pirazzoli et al., 1982, 1996; Stiros, 2001; Meier et al., 2007; Snopek et al., 2007; Shaw et al., 2008; Ganas and Parsons, 2009; Becker et al., 2010; Gallen et al., 2014; Strobl et al., 2014; Tiberti et al., 2014). However, these models are typically conditioned by data from a limited section of the island (either in the west or east) and this article is the first that we are aware of to examine paleoshorelines along the entire Cretan coastline (Fig. 1). "
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    ABSTRACT: Keywords: paleoshorelines beachrock eustatic sea-level rise rock uplift Eastern Mediterranean Crete Paleoshorelines of Late Quaternary age in western Crete do not exclusively increase in age with rising altitude as is generally observed worldwide. At numerous sites, for example, Late-Holocene paleoshorelines decrease in age with increasing altitude while in other cases paleoshorelines at similar altitude vary in age by tens of thousands of years. We propose that the observed paleoshoreline altitude–age relationships can be accounted for by eustatic sea-level changes and tectonic rock uplift without requiring substantial errors on radiocarbon ages or tectonic subsidence, as has been previously proposed. To test this model we use a dataset consisting of altitude and age data for 71 individual paleoshorelines sampled from 21 sites distributed along the entire Cretan coastline. These data include radiocarbon ages of marine biota (40 new dates) within beachrock resting on paleoshorelines ranging up to 48 kyr BP in age and ≤20 m above present sea-level. We find that paleoshoreline formation reflects Late Holocene tectonic rock uplift in western Crete, preceded by eustatic sea-level rise and by >10 kyr BP rock uplift along the entire island. Our observations contravene existing models as they suggest that some paleoshorelines, and their associated lithified beachrock, survived passage through the wave-zone multiple times and formed throughout the sea-level cycle (i.e., preservation is not restricted to highstand deposits). These results may have application globally in regions where erosion-resistant carbonate beachrock mantles paleoshorelines.
    Earth and Planetary Science Letters 12/2015; 431:294-307. DOI:10.1016/j.epsl.2015.09.007 · 4.73 Impact Factor
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    • "The N–S striking extensional faults are difficult to discern at the scale of the map and are thus not highlighted. The rectangle outlines the study area shown in Fig. 3. al., 2001; Ring et al., 2001, 2003; ten Veen and Kleinspehn, 2003; Kreemer and Chamot-Rooke, 2004; Peterek and Schwarze, 2004; Rahl et al., 2005; Meier et al., 2007; Becker et al., 2010; Shaw and Jackson, 2010; Özbakır et al., 2013). The correctness of one model versus another has direct implications for the processes driving orogenesis above the Hellenic subduction zone as well as the potential seismic hazard posed by these faults. "
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    ABSTRACT: The island of Crete occupies a forearc high in the central Hellenic subduction zone and is characterized by sustained exhumation, surface uplift and extension. The processes governing orogenesis and topographic development here remain poorly understood. Dramatic topographic relief (2–6 km) astride the southern coastline of Crete is associated with large margin-parallel faults responsible for deep bathymetric depressions known as the Hellenic troughs. These structures have been interpreted as both active and inactive with either contractional, strike-slip, or extensional movement histories. Distinguishing between these different structural styles and kinematic histories here allows us to explore more general models for improving our global understanding of the tectonic and geodynamic processes of syn-convergent extension. We present new observations from the south–central coastline of Crete that clarifies the role of these faults in the late Cenozoic evolution of the central Hellenic margin and the processes controlling Quaternary surface uplift. Pleistocene marine terraces are used in conjunction with optically stimulated luminesce dating and correlation to the Quaternary eustatic curve to document coastal uplift and identify active faults. Two south-dipping normal faults are observed, which extend offshore, offset these marine terrace deposits and indicate active N–S (margin-normal) extension. Further, marine terraces preserved in the footwall and hanging wall of both faults demonstrate that regional net uplift of Crete is occurring despite active extension. Field mapping and geometric reconstructions of an active onshore normal fault reveal that the subaqueous range-front fault of south–central Crete is synthetic to the south-dipping normal faults on shore. These findings are inconsistent with models of active horizontal shortening in the upper crust of the Hellenic forearc. Rather, they are consistent with topographic growth of the forearc in a viscous orogenic wedge, where crustal thickening and uplift are a result of basal underplating of material that is accompanied by extension in the upper portions of the wedge. Within this framework a new conceptual model is presented for the late Cenozoic vertical tectonics of the Hellenic forearc.
    Earth and Planetary Science Letters 07/2014; 398:11–24. DOI:10.1016/j.epsl.2014.04.038 · 4.73 Impact Factor
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    • "Normal faulting on the island of Crete forms in response to the southward slab roll-back of the Hellenic margin in conjunction with the southwestward expulsion of the Aegean microplate (Le Pichon and Angelier, 1979; Meulenkamp et al., 1988). It manifests itself through impressive limestone fault scarps that traverse the island over two prevailing orientations (N–S and NW–SE) and considerable historic seismicity (Becker et al., 2010; Fassoulas, 2001; Meier et al., 2004). The Spili Fault is a normal fault that traverses the central part of Crete with a NW–SE orientation (300–320°) (Fig. 1a). "
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    ABSTRACT: Editor: Y. Ricard Keywords: rare earth elements carbonate scarp soil paleoearthquake normal fault Crete Recent work (Carcaillet et al., 2008; Manighetti et al., 2010) has utilised a well-established earthquake record on a normal fault in Italy (the Magnola Fault) to successfully test a new method for identifying paleoearthquakes on carbonate rocks: that of chemical analysis of their exhumed fault planes. Here we take the next natural step, applying this novel method on a notionally active normal fault in Greece, the Spili Fault, for which no paleoearthquake record exists. Despite the 'blind' sampling, data reveal an outstanding record of systematic fluctuations in the concentrations of Rare Earth Elements (REE) and Yttrium (Y) upscarp, which closely resemble those recorded on the Magnola Fault. Chemical analysis of 35 core-samples extracted from a 10 m high section of the exhumed Spili Fault plane records upscarp depletion in the REE-Y concentrations at an average rate of ca. 9.3%/m. Depletion is overprinted by locally increased REE-Y concentrations upscarp. A minimum of four such concentration fluctuations, with wavelengths ranging from 0.5 to 3 m, are recorded. Each fluctuation is interpreted to be generated by at least one paleoearthquake that episodically exhumed a zone of the fault plane. Each zone consists of an upper domain that is enriched in REE-Y and a lower un-enriched domain. REE-Y enrichment is due to the prolonged (at least few 100's of years) contact of the limestone with the soil, whereas the un-enriched domain reflects instantaneous uplift from depths greater than the base of the soil, during the same earthquake. The REE-Y analytical method cannot resolve individual small-sized earthquakes (with slip less than the thickness of the soil-cover) and/or individual large-and small-sized earthquakes which are clustered in time (repeat timeb 100's yr). It may therefore yield better results when applied on large (≥20 km) carbonate faults that rupture the earth's crust at most once every ca. 0.5 kyr; nevertheless the number of identified earthquakes should always be treated as a minimum.
    Earth and Planetary Sciences Letters 08/2011; 309(1). DOI:10.1016/j.epsl.2011.06.015 · 4.73 Impact Factor
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