Three strong earthquakes ruptured the northwest Thessaly area, Central Greece, on the 3, 4 and 12 March 2021. Since the area did not rupture by strong earthquakes in the instrumental period of seismicity, it is of great interest to understand the seismotectonics and source properties of these earthquakes. We combined relocated hypocenters, inversions of teleseismic P-waveforms and of InSAR data, and moment tensor solutions to produce three fault models. The first shock (M_w = 6.3) occurred in a fault segment of strike 314° and dip NE41°. It caused surface subsidence −40 cm and seismic slip 1.2–1.5 m at depth ~10 km. The second earthquake (M_w = 6.2) occurred to the NW on an antithetic subparallel fault segment (strike 123°, dip SW44°). Seismic slip of 1.2 m occurred at depth of ~7 km, while surface subsidence −10 cm was determined. Possibly the same fault was ruptured further to the NW on 12 March (M_w = 5.7, strike 112°, dip SSW42°) that caused ground subsidence −5 cm and seismic slip of 1.0 m at depth ~10 km. We concluded that three blind, unknown and unmapped so far normal fault segments were activated, the entire system of which forms a graben-like structure in the area of northwest Thessaly.

Significant seismicity anomalies preceded the 25 October 2018 mainshock (M_w = 6.8), NW Hellenic Arc: a transient intermediate-term (~2 yrs) swarm and a short-term (last 6 months) cluster with typical time-size-space foreshock patterns: activity increase, b-value drop, foreshocks move towards mainshock epicenter. The anomalies were identified with both a standard earthquake catalogue and a catalogue relocated with the Non-Linear Location (NLLoc) algorithm. Teleseismic P-waveforms inversion showed oblique-slip rupture with strike 10°, dip 24°, length ~70 km, faulting depth ~24 km, velocity 3.2 km/s, duration 18 s, slip 1.8 m within the asperity, seismic moment 2.0 × 10²⁶ dyne*cm. The two largest imminent foreshocks (M_w = 4.1, M_w = 4.8) occurred very close to the mainshock hypocenter. The asperity bounded up-dip by the foreshocks area and at the north by the foreshocks/swarm area. The accelerated foreshocks very likely promoted slip accumulation contributing to unlocking the asperity and breaking with the mainshock. The rupture initially propagated northwards, but after 6 s stopped at the north bound and turned southwards. Most early aftershocks concentrated in the foreshocks/swarm area. This distribution was controlled not only by stress transfer from the mainshock but also by pre-existing stress. In the frame of a program for regular monitoring and near real-time identification of seismicity anomalies, foreshock patterns would be detectable at least three months prior the mainshock, thus demonstrating the significant predictive value of foreshocks.

On 26 November 2019 an Mw6.4 earthquake ruptured along the Adriatic coast of Albania causing extensive destruction, mainly in Durrës city and Thumanë town, claiming 51 human lives. Maximum intensity in both areas reached the level of VIII-IX. On 21 September 2019 a strong foreshock (Mw5.6) occurred at nearly the same epicenter causing considerable damage. Fault-plane solutions of the main shock showed reverse faulting striking NW-SE and likely dipping to east indicated by the regional tectonics. In the long-term sense the earthquake has not been a surprise since the area experienced destructive earthquakes in the past as well. We present observations collected during a post-event field survey as well as a seismic source model of the main shock. Apart from the strong motion and amplification phenomena due to loose soil conditions and soil liquefaction, the severe building damage is attributed to the synergy of several other factors including poor workmanship and construction quality, ageing of materials, impact of the September 21st earthquake and pre-existing stress on buildings suffering differential displacements due to soft soil conditions in their foundations. A finite fault model was developed from the inversion of 30 teleseismic P-wave records following the methodology by Hartzell and Heaton (1983, 1986). The hypocenter was manually located offshore but close to Durrës at depth of ~22 km using the NLLoc procedure and the Ak135 velocity model based on 71 seismic phases at distances ≤550 km. Based on this solution a rectangular fault was assumed of 29 km in length with a depth ranging from 15 to 27 km which is large enough to describe successfully the slip for an earthquake of Mw=6.4. A kinematic parameterization of the earthquake fault was used to identify the space-time distribution of slip. Synthetics were calculated for each cell in which the fault is divided and are compared with the recorded data during an inversion producing the solution vector, i.e. the slip for each cell. Strike of 345° and dip of 22° were found to better fit the data. The rake vector was allowed to vary within the range from 65° to 155° permitting up-dip and possible left lateral strike-slip movement of the hanging wall domain relatively to the footwall. Rupture velocity was allowed to vary from 2.4 km/s to 3.6 km/s, the best fit found for 2.6 km/s. The heterogeneous spatial slip distribution obtained shows a complex rupture process with maximum slip at the source of ~1.5 m with the rake vector at that point being of 115° but an arithmetic mean of the rake for the cells with significant slip over 10 cm is 99°. This implies that the thrust component is the one that played the important role in the rupture process. Significant values of slip were also found in a second patch in the northern area of the fault at depths from 19 to 26 km. The total duration of rupture was nearly 16 s, while the total seismic moment released was Mo=5.0x1018 N*m, corresponding to Mw=6.4.