On 3 March 2021, the Mw6.3 Tyrnavos earthquake shook much of the Thessalia region, leading to extensive damage in many small towns and villages in the activated area. The first main shock was followed in the next day, on 4th of March 2021, by an “equivalent” main shock with Mw6.0 in the adjacent fault segment. These are the largest earthquakes to strike the northeastern part of Thessalia since the M6.3, 1941 Larissa earthquake. The main shocks triggered extensive liquefaction mainly along the banks of the Titarisios tributary where alluvial flood deposits most probably amplified the ground motions. Our seismic monitoring efforts, with the use of recordings of the regional seismological network along with a dense local network that was installed three days after the seismic excitation initiation, led to the improved understanding the geometry and kinematics of the activated faults. The aftershocks form a north–northwest–trending, east–northeast–dipping, ~40 km long distribution, encompassing the two main ruptures along with minor activated structures, consistent with the rupture length estimated from analysis of regional waveform data and InSAR modeling. The first rupture was expanded bilaterally, the second main shock nucleated at its northern tip, where from this second rupture propagated unilaterally to the north–northwest. The focal mechanisms of the two main shocks support an almost pure normal faulting, similar to the aftershocks fault plane solution determined in this study. The strong ground motion of the March 3 main shock was computed with a stochastic simulation of finite fault model. Coseismic displacements that were detected using a dense GPS / GNSS network of five permanent stations located the Thessaly region, have shown an NNE–SSW extension as expected from the nature and location of the causative fault. Coulomb stress changes due to the coseismic slip of the first main shock, revealed that the hypocentral region of the second main shock was brought closer to failure by more than 10 bars.

Summary The reliability of the ground motion simulation is highly dependent on the quality of the site response (or site effects) evaluation. There are several methods to estimate the site Amplification Factor (SAF) corresponding to Fourier Amplitude Spectra, either by using numerical simulation tools or empirical approaches. A widely used empirical method is the “Standard Spectral Ratio” (SSR) technique (Borcherdt, 1970) based on the Fourier Amplitude Spectra (FAS) ratio of the seismic record at a target site to the corresponding one at a nearby amplification-free “rock” site (reference site). The main limitation of this method lies in the availability of a reference site relatively close to the target one. In this study a SAF estimation technique at a target site in relation to a distant reference site is presented and evaluated. This technique is based on the retrieval of the minimum phase Source Time Function (mpSTF) at a pair of examined sites (target-reference), with the Spectral Factorization analysis of Coda waves (SFC) proposed by Sèbe et al., (2018), (2005). The so derived mpSTF are considered as a convolution of the actual source function, and of the SAF, so that the FAS ratio of the mpSTF, derived at one site (target) and at a distant reference site, should be an estimate of the target SAF. The latter is confirmed in this is study. Under the conditions of a common STF at the examined sites and of similar coda waves excitation factor, the ratio of the FAS of the mpSTFs (target over reference site) can safely approach the actual SAF, at least when target-reference distance is up to ∼60 km and provides encouraging results at longer distances. This technique was applied at 24 sites in western Greece in relation to 4 reference sites located at varying distances from the target ones (from 0.4 km to 110 km). More than 700 STFs were calculated for 89 moderate magnitude earthquakes (3.9 ≤ M ≤ 5.1) located in this high seismicity area and SAFs were determined from each pair of target-reference stations using common seismic sources. Finally, the average SAFs were computed and compared to the ones computed by other methods (e.g. SSR, GIT, HVSR) demonstrating the reliability and robustness of the proposed technique in site effect estimation.

Generalized inversion techniques (GITs) have become popular for determining seismological parameters (e.g., source, attenuation, and site response), particularly in low-to-moderate seismicity regions. Indeed, GITs can potentially provide reliable site-response estimates when a minimum number of recordings is available, as well as valuable information about source parameters and regional attenuation characteristics. Significant advances have been made on GITs in which different approaches and hypotheses were investigated, such as the application of “nonparametric” and “parametric” inversion schemes. In this context, several scientific questions have arisen that depend on the final scope of the GITs: What is the optimal inversion strategy for a given dataset configuration? What is the impact of the different choices, assumptions, and implementations on the reliability of the results? Is it possible to quantify the associated epistemic uncertainties? Here, we have considered and compared the different approaches of GITs to improve the understanding of each for use in different applications. A methodological benchmark that includes different GIT methods and dataset configurations is set up to fulfill the objective, using a simple synthetic dataset, a French regional sparse dataset, and an Italian national dense dataset. The benchmark is developed in two phases: (1) phase I: a free phase with no common constraints; and (2) phase II: a constrained phase with unified reference conditions. Despite unifying the reference conditions in the different inversions, the variability was not reduced. Discrepancies are observed between different terms of GITs. Site responses appear to be the most robust estimates, compared to source and attenuation terms. The way that stress drops of earthquakes and quality factors for crustal attenuation are parameterized appears to lead to significant variability between different approaches. Finally, uncertainties are addressed by quantification of the inter-method variability for the different terms and parameters.

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