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Individual Seismogenic Sources

General information
GRIS001
Southwestern Crete
Basili R.(1), Tiberti M.M.(1)
Basili R.(1), Tiberti M.M.(1)
1) Istituto Nazionale di Geofisica e Vulcanologia; Sismologia e Tettonofisica; Via di Vigna Murata, 605, 00143 Roma, Italy
20-Jun-2006
20-Jun-2006
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Parametric information
ParameterQualityEvidence
35.42 / 23.69 OD Based on coseismic uplift modelling.
130.0 OD Based on coseismic uplift modelling.
86.0 OD Based on coseismic uplift modelling.
5.0 EJ Inferred from geodynamic considerations.
54.3 EJ Inferred from geodynamic considerations.
314 LD Based on geological and seismological data.
35 LD Based on geological and seismological data.
90 LD Based on geodetic and seismological data.
17.5 OD Based on coseismic uplift modelling.
1.0…5.3 OD Based on geological observations.
3302…17500 OD Based on geological constraints.
8.4 OD Based on coseismic uplift modelling.
LD=Literature Data; OD=Original Data; ER=Empirical Relationship; AR=Analytical Relationship;EJ=Expert Judgement;
 
Information about the associated Earthquakes
21 Jul 365 Guidoboni et al., 1994.
Unknown See "Commentary" for information.
 
 
Active Faults
Active Folds
Commentary

 

COMMENTS

The characteristics of this source are based on a good set of data on raised shorelines that were interpreted as the effect of coseismic uplift of the 365 AD earthquake (Pirazzoli et al., 1982; 1996).

We determined the location, length, width, slip per event, and magnitude of this source through elastic dislocation modeling of the coseismic uplift data, assuming uniform slip on a reverse fault plane embedded in a homogeneous half space (Okada, 1985, BSSA, vol. 75, p. 1135-1154). The fault strike, dip and rake were determined a priori based on the average geometry of the geological structure.

The age and elevation of the several shorelines show a history of about 3,300 years of continuous subsidence before the uplift episode of the 365 AD earthquake. The subsidence rate is about 0.5 mm/y and the coseismic uplift is about 9 m. This suggests a minimum recurrence interval of 3,300 y, taken as the shortest time elapsed without any uplift episode. Based on the assumption that the island of Crete exhibits net long-term uplift resulting from the repetition of 365 AD-like earthquakes and implying that interseismic subsidence does not overtake the uplift, we estimate that the maximum recurrence interval is 18,000 years. Putting these figures in a strictly-periodic recurrence model yields a slip rate of 1.0-5.3 mm/y.

OPEN QUESTIONS

The depth of the 365 AD Crete earthquake is probably the main problem in defining the fault parameters.

Was the 365 AD an interplate earthquake (sensu Bonhoff et al, 2005)? If this is the case, this earthquake must have occurred along the subduction plane. However, there are two main hypotheses on the geometry of the subduction. In the hypotheses by Le Pichon and Angelier (1979), Jolivet (1993), Giunchi et al. (1996) and Papazachos (1996) the source of the 365 AD earthquake may lay at any depth along the subduction plane. In contrast, in the hypotheses by Bohnhoff et al. (2001, 2005), Casten and Snopek (2006), and Makris and Yegorova (2006) the source of the 365 AD earthquake cannot be shallower than 15 km.

An alternative hypothesis is that the fault propagates upward as a splay in the overriding plate (Shaw and Jackson, 2010). In such a case, the fault could also be shallow.

SUMMARIES

Pirazzoli et al. (1982) These investigators identified recent movements of the Earth crust by means of a systematic survey along the coasts of the islands of Crete and Antikithira. Marks of several shorelines left by sea-level stands during the past 4,000 yrs were analyzed on a multidisciplinary basis, including 60 radiometric datings. The results indicate that between 4,000 and 1,700 yr. B.P. a block of crust about 150 km-long underwent a series of ten rapid subsidence episodes (from 10 to 25 cm each time) without noticeable tilting. These movements ended about 1,530 yr B.P., when a block of crust approximately 200 km long, including the block above, was uplifted about 10 m and tilted northeastwards in a single event, suggesting coseismic uplift.

Pirazzoli et al. (1996) The authors of this paper present uplift data for Greece and the Eastern Mediterranean related to the so-called Early Bizantine Tectonic Paroxysm (EBTP). The areas uplifted at that time include Antikythira and western Crete, where the amount of uplift reached its maximum (9 m). Lateral correlations of uplifted shorelines of the same age are shown for these two areas. The authors ascribe the observed uplift to the 365 AD earthquake.

Papazachos et al. (2000) These investigators define the plate boundaries in the Hellenic Arc area by means of locations of 961 shallow and intermediate depth earthquakes which occurred between 1956 and 1995. Reliable fault plane solutions for 77 shallow and intermediate depth earthquakes are also used to define the interaction between the different plates in the arc. They find that an ocean–continent type of interaction occurs on a curved surface, which follows the convex (outer) side of the sedimentary arc (western Peloponnese–west of Cythera–south coast of Crete–east coast of Rhodes) and dips at low angle (~30°) towards the Aegean sea. Coupling between the subducted oceanic crust and the overriding of the Aegean lithospheric plate takes place along this surface. The deep branch (100–180 km) of the Wadati–Benioff zone dips freely (without coupling) at a high angle (~45°) beneath the south Aegean trough and the volcanic arc. The large magnitude shallow seismicity (h 20 km) observed in the southwestern convex (outer) side of the arc (Ionian sea) is attributed to the fast southwestward motion of the Aegean plate.

Drakos and Stiros (2001) These investigators carried on an elastic dislocation analysis of the earthquake uplift deduced from coastal data by Pirazzoli et al. (1982). The fault parameters of the two models found to match the observed uplift constraints are as follows:

Model 1: strike 271°, dip 45°, depth 100 km, length 120 km, width 135 km, slip 20 m, Mo 9.72*1028, Mw 8.7;

Model 2: strike 292.5°, dip 40°, depth 70 km, length 105 km, width 100 km, slip 16 m, Mo 5.04*1028, Mw 8.5.

The authors chose the second set of parameters as the most reliable reporesentation of the earthquake source.

Makris and Yegorova (2006) A 3-D density model for the Cretan and Libyan Seas and Crete was developed by gravity modelling constrained by five 2-D seismic lines. Velocity values of these cross-sections were used to obtain the initial densities using the Nafe–Drake and Birch empirical functions for the sediments, the crust and the upper mantle. The crust outside the Cretan Arc turns out to be 18 to 24 km-thick, including a 10 to 14 km-thick package of sediments. The crust below central Crete at its thickest section is 32 to 34 km-thick and consists of continental crust of the Aegean microplate, which in its turn is thickened by the subducted oceanic plate below the Cretan Arc. The oceanic lithosphere appears to be decoupled from the continental lithosphere starting from central Crete eastward along an E-W oriented section.

 
Pictures

 

Past sea-level markers in western Crete (Paleochora) Details
Tectonic framework and bathymetry of the area Details
Velocity cross-sections Details
Average focal mechanisms for earthquakes along the Hellenic arc Details
Depth and focal mechanisms of interplate earthquakes along the Hellenic Arc Details
Map of coseismic uplift observations Details
Age and elevation of dated uplifted Holocene shorelines in western Crete and Antikithira Details
Map of the effects of the AD 365 earthquake Details
Contours of the inferred uplift of Crete during the AD 365 earthquake Details
References

 

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