Seasonal variations of seismic velocities in Greece measured from seismic noise cross-correlations

Greece is the most earthquake-prone country in Europe, as it is located at the intersection of the Eurasian and African plates, as well as at the western end of the North Anatolian fault zone.

The long term goal of this study is to monitor the spatio-temporal evolution of the mechanical properties of the crust around the Gulf of Corinth in Greece that is associated with seismic swarms and large magnitude (Mw>5) earthquakes. We found that the seismic velocity changes induced by tectonic processes in the upper crust in Greece are masked by the velocity variations associated with environmental factors such as seasonal changes of temperature and hydrological parameters. The aim of the present study is to quantify the seismic velocity changes in the upper crust that are due to these environmental parameters. To that end, we use 6 years (2015-2020) of continuous vertical noise recording in 142 stations and calculate daily auto-correlations. We use the stretching method to measure seismic wave velocity variations (dv/v) with a sliding window of 2 months in the [1-3]s period band corresponding to the shallow crust.  We find that in several regions, the seismic velocities exhibit strong seasonal variations in particular in karstic areas. We use data from 495 meteorological stations in order to assess if it is possible to model the observed seasonal variations of dv/v from temperature changes and rainfall. Preliminary results indicate that 1- the seasonal changes of temperature are unlikely to explain the seasonal changes of seismic velocities and 2- in several regions, the variations of the groundwater level induced by rainfalls can at least partially explain the observed velocity variations.

To complete this study, we turned to an analysis of GPS traces as an independent means of assessing modeled velocity variation and to analyze hydrologic processes within aquifers.