In early March 2021, three shallow earthquakes, two mainshocks with M6.3 and M6.0 and one major aftershock with M5.6 impacted both the mountainous Damasi-Tyrnavos region (northern Thessaly, Greece) and the adjacent Plio-Quaternary basin. Each major event was followed by rich aftershock activity recorded by local and regional seismographs and accelerographs. Herein, we present a comprehensive analysis of the seismic sequence, from its foreshock activity starting on 28 February, 2021 and for a period of two months using new high-resolution catalogues of relocated earthquakes and hundreds of focal mechanisms. The results indicate that the aftershocks form a zone that spans ~50 km NW-SE, while focal depths range between 5 and 15 km. More than 400 focal mechanisms, computed for events with M≥ 2.5, mainly exhibit normal faulting in a NW-SE direction, while WNW-ESE to E-W normal faulting is also evidenced, in particular after the occurrence of the last major event on 12 March. The stress-field was reconstructed on a local and broader scale by inverting focal mechanism data, revealing a rotation of the σ3 axis trend from NNE-SSW, in the Damasi-broader region, to NW-SE northwards, to the region of Kozani-Grevena that hosted an Mw = 6.6 shallow mainshock in 1995. Subcrustal seismicity, present beneath those areas, implies that large-scale tectonics and plate dynamics are likely involved in the deformation of the upper crust. Coulomb stress transfer after the 3 major events of the 2021 Damasi-Tyrnavos sequence reveals that stress-loaded areas include those where most aftershocks were triggered. The analysis provides implications to the seismic hazard of the activated area, as a major NW-SE active normal fault close to Larissa city became stress-loaded, constituting a possible candidate source for significant future earthquakes.

We investigate an earthquake sequence involving an Mw = 4.6 mainshock on 2 December 2020, followed by a seismic swarm in July–October 2021 near Thiva, Central Greece, to identify the activated structures and understand its triggering mechanisms. For this purpose, we employ double-difference relocation to construct a high-resolution earthquake catalogue and examine in detail the distribution of hypocenters and the spatiotemporal evolution of the sequence. Furthermore, we apply instrumental and imaging geodesy to map the local deformation and identify long-term trends or anomalies that could have contributed to stress loading. The 2021 seismic swarm was hosted on a system of conjugate normal faults, including the eastward extension of the Yliki fault, with the main activated structures trending WNW–ESE and dipping south. No pre- or coseismic deformation could be associated with the 2021 swarm, while Coulomb stress transfer due to the Mw = 4.6 mainshock of December 2020 was found to be insufficient to trigger its nucleation. However, the evolution of the swarm is related to stress triggering by its major events and facilitated by pore-fluid pressure diffusion. The re-evaluated seismic history of the area reveals its potential to generate destructive Mw = 6.0 earthquakes; therefore, the continued monitoring of its microseismicity is considered important.

This paper presents a joint analysis of instrumental and macroseismic data regarding the 19 July 2019, Greece Mw5.1 earthquake occurred west of Athens. This earthquake ruptured a blind, south-dipping normal fault, 23 km WNW of the center of Athens, while its relocated epicentre lies in close vicinity to the one of the 1999 Mw6.0 earthquake. The maximum macroseismic intensity of the 2019 mainshock reached IEMS98 = 7.5. Scarce damage and intensities up to 5–6 were reported in the epicentral area. Higher intensities were observed at larger distances, 12–15 km east and ESE of the epicentre, alongside the banks of Kifissos River, likely related to ground motion amplification due to soft alluvial formations. Similar selectivity of increased ground motions to the east of the epicentre with respect to other azimuths, also observed during the 1981 and 1999 earthquakes, supports eastward rupture directivity of the 2019 mainshock, an effect that is possibly common for the region’s fault system. Damping of seismic effects was observed east of Aegaleo Mountain, a structure suggested to impose a stopping phase in the time histories of the 1999 and 2019 earthquakes (Fig. A1).

Sources containing descriptive information on earthquake effects in Greece from the period 1800–1899 are analysed and EMS-98 macroseismic intensities are assigned, aiming at the enrichment of the number of macroseismic datapoints contained in the Hellenic Macroseismic Database (HMDB.UoA). Based on the information provided by the analysis, intensities were re-assigned for all events reported in the sources. Compared to the MDPs presented in the Hellenic Macroseismic database, the total number of MDPs from the present study is more than double. Similar procedure was applied to the 19th century Samos and surrounding areas earthquakes including all macroseismic intensity degrees. For the case study of Samos, two macroseismic intensity data sets are presented: the damaging (IEMS98 > 6) and the non-damaging ones. The macroseismic parameters of one new earthquake were determined and the seismic history of the island showed that two earthquakes, probably similar to the 30 October 2020 Samos event, occurred in the 19th century. New earthquakes improve the seismic history and increased MDPs number allow for more accurate parameters assessment.

Knowledge and visualization of the present-day relationship between earthquakes, active tectonics and crustal deformation is a key to understanding geodynamic processes, and is also essential for risk mitigation and the management of geo-reservoirs for energy and waste. The study of the complexity of the Greek tectonics has been the subject of intense efforts of our working group, employing multidisciplinary methodologies that include detailed geological mapping, geophysical and seismological data processing using innovative methods and geodetic data processing, involved in surveying at various scales. The data and results from these studies are merged with existing or updated datasets to compose the new Seismotectonic Atlas of Greece. The main objective of the Atlas is to harmonize and integrate the most recent seismological, geological, tectonic, geophysical and geodetic data in an interactive, online GIS environment. To demonstrate the wealth of information available in the end product, herein, we present thematic layers of important seismotectonic and geophysical content, which facilitates the comprehensive visualization and first order insight into seismic and other risks of the Greek territories. The future prospect of the Atlas is the incorporation of tools and algorithms for joint analysis and appraisal of these datasets, so as to enable rapid seismotectonic analysis and scenario-based seismic risk assessment.

The purpose of this paper is to determine calibration constants of instrumental Greek earthquakes in order to calculate the basic seismic parameters of historical earthquakes (magnitude, epicenter, focal depth) via macroseismic data. Two different approaches are adopted for calibration procedure. The first implements the macroseismic estimation of earthquake parameters (MEEP procedure) and is based on macroseismic data points (MDPs). The second approach calculates macroseismic magnitude based on isoseismal areas, using both linear and multiple regression techniques. The datasets used for analysis comprise of 121 instrumental earthquakes with 7247 MDPs and 123 isoseismal maps. Validation of the results is performed using six instrumental earthquakes in order to verify the calibration parameters. Finally, calibration constants are successfully applied for parameters calculation of eight eighteenth century events. Thus, the application of the results to historical earthquakes contributes to the improvement of the seismic picture of Greece.

In this paper, we present a detailed study of a shallow seismic swarm which took place in the area of Oichalia (SW Peloponnesus), between August and December 2011. The seismic crisis started on 14/8/2011 with an Mw=4.8 earthquake and was followed by more than 1600 events, several of which having magnitude over 4.0. The activity was recorded by local temporary and regional permanent seismic stations. Thousands of records were collected and routinely analyzed. P- and S-wave arrival times were manually picked and incorporated in the HYPOINVERSE algorithm together with a new optimum local velocity model. Hypocentral solutions were improved by applying a double-difference method. Focal mechanisms show that the activated fault zone is dominated by dip-slip normal faulting, trending NNW–SSE, with the average T-axes orientation being N70°E, consistent with regional tectonics. We have investigated towards stress triggering and fluid diffusion, by employing Coulomb stress transfer, spatio-temporal and Frequency–Magnitude Distribution (FMD) analyses. The negligible Coulomb stress transfer and seismicity rate changes that were calculated imply for a stress deficit in the broader study area, hence an external triggering mechanism is required to justify the observed pattern. The b-values increase towards the SSE, compatible with the similarly directed migration of seismicity, showed that the Oichalia swarm could possibly be adapted to an Epidemic Type Aftershock Sequence model (ETAS). Fluid diffusion is reflected in the spatio-temporal hypocenter migration. Clustering analysis, combined with the temporal distribution of b-values, has shown that the swarm evolved in three major phases, the first two being initiated by major events, which were probably triggered externally due to fluid injection that brought the seismogenic volume into a critical state, likely followed by afterslip. The last phase signified a relaxation period, with dispersed seismicity throughout the area and the b-values gently diminishing towards unity.

A systematic study of historical earthquakes leading to the quantification of earthquake effects in terms of macroseismic data points (MDPs) and, consequently, earthquake parameters has been carried out in the last decade at the Laboratory of Seismology of the University of Athens. For each earthquake, the available background information was evaluated and the corresponding macroseismic intensities assessed in terms of the European Macroseismic Scale 1998. A considerable amount of these MDPs contributed to the Archive of Historical Earthquake Data inventory through European initiatives (NERIES and SHARE). Based on the structure of the European Database, the local version of the Hellenic Macroseismic Database (HMDB.UoA) was designed incorporating historical earthquakes of the period 1000–1899 from the eastern Aegean area, central Greece and Ionian Islands. In its present form, the HMDB.UoA includes 90 events with Imax ≥ 7 (868 MDPs) and 1,088 events with Imax < 7 (1,273 MDPs). The database is hosted on the website http://macroseismology.geol.uoa.gr/.

The Adriatic region was chosen as one of the test areas in the GSHAP program and, consequently, its seismic hazard was computed. The standard hazard map chosen by GSHAP represents PGA with a 475-year return period. Some other parameters, as the spectral acceleration and the uniform hazard response spectra for the main Adriatic towns, have been computed for a better representation of the regional hazard. The most hazardous area remains identified in the Cephalonia zone, where strong earthquakes frequently occur. The Southern Apennines are characterised by a slightly lower hazard, while the Adriatic Sea itself, the Po plain and the Apulian peninsula are almost aseismic.

Subcrustal seismicity recorded in the southern Aegean sea during a 7-week microearthquake study was low compared with shallow seismicity. Most intermediate depth seismicity occurred beneath the western and eastern ends of the Hellenic arc. This distribution confirms that a slab of lithosphere is being subducted at a very shallow (<15°) angle for 200 km beneath the western end (Peloponnese) but more steeply beneath the eastern end (Dodecanese). We could locate only one intermediate depth event beneath the central pan of the arc, where teleseimically located intermediate depth earthquakes also are infrequent. T axes for most of the 22 focal mechanisms of subcrustal earthquakes are roughly parallel to the local dip direction of the seismic zone. Between depths of 40 and 80 km, the mechanisms are more confused than at greater depth, perhaps because some of these earthquakes did not occur within the downgoing slab. Earthquakes deeper than 80 km, and within the subducted slab, have nearly horizontal P axes that trend NNE-SSW in the eastern part and NNW-SSE in the western part of the arc. These deeper mechanisms show horizontal P axes along strike, perhaps in response to the contortion of the slab or to the westward motion of Turkey, as well as lengthening downdip, probably in response to gravity acting on excess mass in the slab. Thus the short slab, both downdip and along strike, subducting beneath the Aegean is subjected to a more complex set of forces than the long slabs of the Pacific.