Analysis of signals of a borehole strainmeter in the western rift of Corinth, Greece

Abstract

This paper presents the first analysis of the records of an elliptical 3-component Sacks-Evertson borehole strainmeter. This highresolution prototype by the Carnegie Institution of Washington, is installed since 2006 in the western rift of Corinth, Greece. We first present the calibration and the correction from external influences, in order to quantify the detection level of the instrument. We show evidence for pore pressure diffusion from the sea, mostly affecting one component. Neglecting this effect, a first order correction reduces the signal by 90% at tidal periods for 2 components and about 70% for the third one. The residual noise vary from 1 nstrain at 1-hour period to 10 nstrain at 1-day period. It allows to detect slow earthquakes lasting 1 day down to magnitude 4 at an hypocentral distance of 8 kilometers. The uncorrected records at periods smaller than semidiurnal does not reveal any slow strain transient with strong amplitude. During the closest seismic swarm to the site in 2011, the analysis of the records reveals strain steps occuring at the arrival times of seismic waves radiated by the local earthquakes, uncorrelated with the amplitudes and mostly related to dynamic pore pressure instabilities.

Keywords

• high strain deformation zones
• fractures and faults
• tides and planetary waves
• instrumental noise
• external forcing correction