Local perturbations in the western rift of Corinth (Greece) caused by the 2011 Mw 9 Tohoku earthquake: joint observation of strain, water-level and seismic signals

The Gulf of Corinth (Greece) is one of the most seismic regions in Europe, producing some strong earthquakes in the decades, 1 to 1.5 cm/yr of north-south extension, and frequent seismic swarms. This structure is a 110 km long, N110E oriented graben bounded by systems of very recent normal faults. The Corinth Rift Laboratory (CRL) project is concentrated in the western part of the rift, around the city of Aigion, where instrumental seismicity and strain rate is highest. In the aim to detect transient slow deformation and creep phenomenon, possibly associated with the numerous microseismic swarms, three Sacks-Evertson borehole strainmeters have been deployed since few years. They are devoted to support the GPS observations, especially at short periods. During the 2011 Mw 9 Tohoku earthquake, the passing of long-period surface waves have initiated free oscillations of the gulf (commonly called seiches) at different periods. The dominant oscillations exhibit a period of 5-7 min with related amplitude of about 5-8 cm of water. This is also observed on the records of the strainmeters installed on Trizonia island (TRZ) and in Monasteraki (MOK). We took advantage of this observation to investigate in details the principal mechanisms of resonance of the gulf of Corinth. Furthermore, the observation of the dynamic strain induced by the longest period surface waves (150-200 sec) shows an anti-correlation between the TRZ dilatometer response and the MOK strainmeter, located 15 km apart. This puzzling observation is consistent with a similar anti-correlation between the solid tidal signal at 24-hrs period recorded at TRZ and the theoretical solid tide. This could be related to the presence of a shallow fault close to the instrument, which would creep in response to seismic wave related stress. However, further observations are necessary to better understand the response of the TRZ sensor to internal forcing, as for instance the strain associated with the earth free oscillations excited by the Tohoku earthquake.