Ecole Normale Superieure

This paper presents the main recent results obtained by the seismological and geophysical monitoring arrays in operation in the rift of Corinth, Greece. The Corinth Rift Laboratory (CRL) is set up near the western end of the rift, where instrumental seismicity and strain rate is highest. The seismicity is clustered between 5 and 10 km, defining an active layer, gently dipping north, on which the main normal faults, mostly dipping north, are rooting. It may be interpreted as a detachment zone, possibly related to the Phyllade thrust nappe. Young, active normal faults connecting the Aigion to the Psathopyrgos faults seem to control the spatial distribution of the microseismicity. This seismic activity is interpreted as a seismic creep from GPS measurements, which shows evidence for fast continuous slip on the deepest part on the detachment zone. Offshore, either the shallowest part of the faults is creeping, or the strain is relaxed in the shallow sediments, as inferred from the large NS strain gradient reported by GPS. The predicted subsidence of the central part of the rift is well fitted by the new continuous GPS measurements. The location of shallow earthquakes (between 5 and 3.5 km in depth) recorded on the on-shore Helike and Aigion faults are compatible with 50° and 60° mean dip angles, respectively. The offshore faults also show indirect evidence for high dip angles. This strongly differs from the low dip values reported for active faults more to the east of the rift, suggesting a significant structural or rheological change, possibly related to the hypothetical presence of the Phyllade nappe. Large seismic swarms, lasting weeks to months, seem to activate recent synrift as well as pre-rift faults. Most of the faults of the investigated area are in their latest part of cycle, so that the probability of at least one moderate to large earthquake (M = 6 to 6.7) is very high within a few decades. Furthermore, the region west to Aigion is likely to be in an accelerated state of extension, possibly 2 to 3 times its mean interseismic value. High resolution strain measurement, with a borehole dilatometer and long base hydrostatic tiltmeters, started end of 2002. A transient strain has been recorded by the dilatometer, lasting one hour, coincident with a local magnitude 3.7 earthquake. It is most probably associated with a slow slip event of magnitude around 5 ± 0.5. The pore pressure data from the 1 km deep AIG10 borehole, crossing the Aigion fault at depth, shows a 1 MPa overpressure and a large sensitivity to crustal strain changes.

The Corinthian Gulf in Greece, is the most active of a series of extending grabens which accomodate the deformation in the highly seismic Aegean region. The geodetic network established in the region has about 200 points: 50 1st order points and ~150 2nd order points. The network covers an area of about 100 x 80 km2, which correspond to an average density of 1 point every 5 km2. This dense network allows to study the main active faults in the region. Eleven field surveys were organized in 1990, 1991, 1992, 1993, 1994, June 1995, October 1995, 1997, and 2001. Two earthquakes occurred in the vicinity during the ten years period: the 1992, 18 November Ms=5.9 Galaxidi earthquake and the 1995, 15 June 1995 Ms=6.2 Aigion one. With respect to the stable Europe, we find for Peloponnessos an average displacement rate of 30 mm/yr in the N215° direction, similar to that found in previous studies. Our results show that most of the deformation in the Corinthian Gulf is localizes off-shore, in a narrow band, in the central part of the Gulf. The extension rate measured over 10 years is 11 mm/yr in the N185° direction in the middle of the Gulf (Xiloxastro) and 16 mm/yr in the N185° direction in its western part (Aigion). The southern block appears un-deformed, except the region of Aigion event. Using CNES DIAPASON software, we derived 85 interferograms of the Corinthian Gulf from 38 raw ERS SAR images acquired between 1992 and 1999. The interferograms sampling the 1995 earthquake show a clear coseismic signal reaching 250 +/- 15 mm at Psaromita cape, a value consistent with the GPS measurements. No post-seismic motion, within the error bars of SAR interferometry (+/- 15 mm), is observed during the 1995-1999 period.

The area of the western Gulf of Corinth around the city of Aigio (Achaea, northwest Peloponnese, Greece) represents an international pilot site for continuous monitoring and multidisciplinary research on earthquake processes. In the framework of the ANR-SISCOR Corinth Rift Laboratory project (2011–2014), a thorough reappraisal of the five largest (Mw >6) eighteenth–nineteenth century earthquakes was performed, namely those of 14 May 1748, 23 August 1817, 26 December 1861, 9 September 1888, and 25 August 1889. Written observations of earthquake effects were looked into in their original version and language and were placed in the context from which they originated, to avoid the translations and digests on which previous seismological studies had relied. Earthquake records were traced for 108 different localities, and 143 macroseismic intensities in European Macroseismic Scale 1998 (EMS-98) have been assigned. Earthquake-related geological phenomena have been identified and carefully mapped, to be used as a further constraint of the location and magnitude of the associated earthquakes. Finally, new parameters for the studied earthquakes have been assessed with two separate and independent strategies to quantify epistemic uncertainties. In conclusion, the 1748, 1817, and 1888 earthquakes were located in the area of Aigio; the 1861 earthquake is reckoned to be the largest in the area, with an epicentral location at sea; whereas the 1889 earthquake has been relocated to the northwest in mainland Greece, well outside the Gulf of Corinth.