Dynamics of microseismicity and its relationship with the active structures in the western Corinth Rift (Greece)

Abstract

We analyse the complete earthquake archive of the western Corinth Rift using both crosscorrelations between pairs of event waveforms and accurate differential traveltimes observed at common stations, in order to identify small-scale fault structures at depth. The waveform database was generated by the dense Corinth Rift Laboratory network and includes about 205 000 events between 2000 and 2015. Half of them are accurately relocated using doubledifference techniques. The novelty of this relocated catalogue is the integration of the recent westernmost earthquakes due to the extension of the network in 2010 to the western extremity of the Corinth Rift and the consideration of the whole database over more than 15 yr. The total relocated seismicity exhibits well-defined clusters at the root of the main normal faults mainly between 5 and 10 km depth in the middle of the gulf and illuminates thin active structure planes dipping north about 20 • under the northern coast. Some seismicity is observed in the footwall of the main active faults, along the West and East Helike faults. We also built a multiplet database based on waveform similarity taking into account cross-correlation coefficients weighted by signal-to-noise ratios. Short-term multiplets are concentrated in the middle of the gulf along the Kamarai fault system, in a 1-2 km thick layer at 6-8 km depth, interpreted as a highly fractured geological layer. They are often associated to slow seismic migration velocities occurring in this zone during strong swarm episodes and are thus likely to be triggered by pore pressure variations. On the other hand, most long-term and regular multiplets are located deeper (7-10 km), under the northern coast, within a layer less than 0.3 km thick. They occur at the border of nearly planar structures with low seismicity rate, which we identify as fault planes, and they may be explained by aseismic slip on the fault surface around them. This supports the existence of an immature structure growing downdip towards the north at the base of the active geological layer, which possibly connects to the ductile middle crust around 15 km depth, as suggested by the occurrence of deeper events in the continuity of the 1995-fault plane. The different migration velocities (from 0.05 km d −1 to several km d −1) highlighted during the western 2014-swarms indicate that both pore pressure and creep diffusion are operating in the fault zone. The fast migrations observed in the Psathopyrgos fault zone, where a slow slip event was detected by dilatometers in 2002, compare with that for creeping faults. To the west, from spatial distribution of events, we show that the Rion-Patras fault connecting the western extremity of the Corinth Rift fault system to the Patras Rift, is dipping around 60 • northwest with a rake angle of −115 •. Finally, we identified two new areas within the central active zone which may correspond to large scale, locked asperities on active fault surfaces, similar in size to the main asperity broken during the 1995, M W 6.3, Aigio...

Imbricated Aseismic Slip and Fluid Diffusion Drive a Seismic Swarm in the Corinth Gulf, Greece

“…This elongated structure may be a plane with a low dip angle (10–30°) toward the north. It is consistent with the preferential alignment of seismicity observed by Lambotte et al (2014) and Duverger et al (2018) and interpreted as the detachment plane of the Phyllade nappes. While the structure of the main plane is well described by the seismicity locations, the secondary plane is not clear because the seismicity mainly gathers at the intersection between both planes.…”

“…The high extensional deformation rate of the Corinth Gulf is mainly accommodated by seismic swarms and occasionally by mainshocks (Duverger et al, 2018). Focusing on a small and short prolific swarm, we observe that fluid diffusion at slow migration velocity of the order of m/day dominantly controls the overall migration of the seismicity.…”

“…The seismicity expansion is stopped on its bottom end below the intersection with a low dipping plane at 6.3‐km depth. The latter structure may correspond to the low‐angle detachment plane highlighted by seismic activity in which normal faults are rooting (Duverger et al, 2018; Lambotte et al, 2014). Below this layer, there is no seismicity, but anomalies in V P /V S ratio show evidences of fluid (Gautier et al, 2006).…”

A new crustal fault formed the modern Corinth Rift

“…Changes in elevation of the longitudinal river profile major inflexions and the reduced magnitude of related river channel steepening are also consistent with this inference . Further supporting evidence are the recent hanging-wall uplift in the footwall of the Aigio F and its age (50-60 ka; Cornet et al, 2004), as well as the large seismicity in the Aigio, Fassouleika and Psathopyrgos faults (Bernard et al, 2006;Boiselet et al, 2014;Duverger et al, 2018). Eastwards propagation is likely to have occurred at younger times than those suggested by Leeder et al (2012).…”

Ground Deformations in the Corinth Rift, Greece, Investigated Through the Means of SAR Multitemporal Interferometry

“…The large gradient of strike‐slip velocities between the south and the north coast at both ends of the Rio‐Antirio bridge support a model of deformation accumulation in a very narrow locked layer in the crust, with the rest being unlocked, and therefore not associated with strong seismicity (at least in the last decades until present). There might well be alternative models, for example, models invoking a low angle decollement at some depth in the crust, presumably 8–10 km if it is of the same nature of the one assumed at base of the Psathopyrgos fault (Duverger et al, ). Here we have not reviewed the other possible models nor investigated any of them, focusing our approach on the questions/answers that simple crack models in elastic media can answer.…”