Article

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

ABSTRACT

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 100 km 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 20 km 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 20 km. 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.

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    • "We use the altitudes and radiocarbon ages of these paleoshorelines in combination with sea level curves to analyze, for the first time, uplift rates from the entire length of Crete over the last 50 kyr (Figure 2a). Our analysis highlights strong temporal transients in uplift rates that varied little spatially on Crete prior to the period of Minoan civilization (~2700–1450 years B.C.), despite an interpreted apparent eastward change from seismic to aseismic uplift [Meier et al., 2007; Zachariasse et al., 2008; Becker et al., 2010; Gallen et al., 2014; Strobl et al., 2014] "
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    ABSTRACT: The Hellenic subduction margin in the Eastern Mediterranean has generated devastating historical earthquakes and tsunamis with poorly known recurrence intervals. Here stranded paleoshorelines indicate strong uplift transients (0-7 mm/yr) along the island of Crete during the last ∼50 kyr due to earthquake clustering. We identify the highest uplift rates in western Crete since the demise of the Minoan civilization and along the entire island between ∼10 and 20 kyr B.P., with the absence of uplifted Late Holocene paleoshorelines in the east being due to seismic quiescence. Numerical models show that uplift along the Hellenic margin is primarily achieved by great earthquakes on major reverse faults in the upper plate with little contribution from plate-interface slip. These earthquakes were strongly clustered with recurrence intervals ranging from hundreds to thousands of years and primarily being achieved by fault interactions. Future great earthquakes will rupture seismically quiet areas in eastern Crete, elevating both seismic and tsunami hazards.
    Full-text · Article · Dec 2015 · Geophysical Research Letters
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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.
    Full-text · Article · Dec 2015 · Earth and Planetary Science Letters
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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.
    Full-text · Article · Jul 2014 · Earth and Planetary Science Letters
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