A microseismic study in the western part of the Gulf of Corinth (Greece): implications for large-scale normal faulting mechanisms

Abstract

We present the results of a dense seismological experiment in the western part of the Gulf of Corinth (Psathopyrgos-Aigion area), one of the most active rifts in the Aegean region for which we have precise tectonic information. The network included 51 digital stations that operated during July and August 1991, covering a surface of 40 x 40 km2.Among the 5000 recorded events with M L ranging between 1.0 and 3.0, we precisely located 774 events. We obtained 148 well-constrained focal mechanisms using P-wave first motions. Of these, 60 also have mechanisms obtained by combining the P-wave first motions with the S-wave polarization directions. The observed seismicity is mainly located between 6 and 11 km depth. Most of the fault-plane solutions correspond to E-W-striking normal faulting, in agreement with the geological evidence. Most of the well-determined mechanisms indicate a nodal plane dipping 10-25" due north and a steep south-dipping plane. A similar asymmetry is also seen in the seismicity distribution and in the overall geological structure of the Corinth Rift. We discuss this evidence and the inference of a deep detachment zone, a structure where the major faults seen at the surface appear to root. A large part of the microseismic activity appears to cluster in regions near the junctions of the main faults with the proposed detachment zone. This feature of the microseismicity is interpreted in terms of stress transfer and stress concentration in regions of probable nucleation of future large earthquakes.

The role of pre-existing thrust faults and topography on the styles of extension in the Gran Sasso range (central Italy)

“…Below this depth, it is generally considered that the high-angle normal faults (rupturing during seismic events) assume shallower dips rooting into the ductile lower crust (Eydogan and Jackson, 1985;Rigo et al, 1996). This work shows that a major shallowing in normal fault geometry is possible in the upper crust as a result of the reactivation of pre-existing thrust faults.…”

“…It is possibly the depth at which normal faults assume a shallower dip which controls the extent of antithetic faulting. In the case of a high-angle normal fault detaching at the base of the upper crust (as in the Corinth rift; Rigo et al, 1996) hanging-wall deformation could be partly accommodated at the base of the brittle layer, thus reducing deformation by antithetic faulting. For a normal fault branching on an upper crust low-angle discontinuity all the hanging-wall deformation is accommodated mostly through the formation of antithetic faults.…”

“…6, 8 and 11) in well-studied major normal fault systems (e.g. Gulf of Corinth; King et al, 1985;Armijo et al, 1996;Rigo et al, 1996), antithetic faulting generally accommodates less of the overall extension. It is possibly the depth at which normal faults assume a shallower dip which controls the extent of antithetic faulting.…”

High-angle, not low-angle, normal faults dominate early rift extension in the Corinth Rift, central Greece

“…If a mature, seismically active LANF or detachment does exist beneath the western Corinth Rift, slip on such a structure must be able to account for the long-term (tens of thousands to hundreds of thousands of years) geometry and pattern of vertical displacement across the rift, which is well constrained from seismic reflection imaging and geomorphology data. The southern margin is uplifting with late Quaternary uplift rates of 0.8-2.0 mm/yr determined from uplifted marine terraces and wave-cut notches (e.g., Armijo et al, 1996). Long-term uplift-to-subsidence ratios across the largest faults bordering the southern margin are estimated at 1:1.2 -1:2.2, measured from the elevation of features of comparable age in the footwall and hanging wall (McNeill et al, 2005(McNeill et al, , 2007.…”

Strain localisation in mechanically layered rocks beneath detachment zones: insights from numerical modelling

“…Crustal-scale thermo-mechanical models of this detachment have shown that kinematics of high angle normal faults in its hanging wall as well as the observed formation of out-of-sequence faults can only be reproduced for low effective viscosity of the order of 10 19 to 10 20 Pa s within the shear zone (Le Pourhiet et al, 2004) . However, with such a large strain rate, brittle-ductile interactions are still expected to occur at small scale, as attested by the repeated occurrence of small-magnitude earthquakes (Rigo et al, 1996). Lecomte et al (2012) have shown that these events can be interpreted as slip on neoforming shear bands within a kilometer-scale shear zone.…”