The February 6th, 2023, Mw 7.8 Pazarcık earthquake in the Turkey-Syria border region raises the question of whether such a large earthquake could have been foreseen, as well as what is the maximum possible magnitude (Mmax) of earthquakes on the East Anatolian fault system and on continental transform faults in general. To answer such questions, knowledge of past earthquakes and of their causative faults is necessary. Here, we integrate data from historical seismology, paleoseismology, archeoseismology, and remote sensing to identify the likely source faults of fourteen Mw ≥ 7 earthquakes between AD 1000 and the present in the region. We find that the 2023 Pazarcık earthquake could have been foreseen in terms of location (the East Anatolian Fault) and timing (an earthquake along this fault was if anything overdue), but not magnitude. We hypothesize that the maximum earthquake magnitude for the East Anatolian Fault is in fact 8.2, i.e. a single end-to-end rupture of the entire fault, and that the 2023 Pazarcık earthquake did not reach Mmax by a fortuitous combination of circumstances. We conclude that such unusually large events are hard to model in terms of recurrence intervals, and that seismic hazard assessment along continental transforms cannot be done on individual fault systems but must include neighboring systems as well, because they are not kinematically independent at any time scale.

The 2016 Amatrice-Norcia seismic sequence in central Italy activated a system of normal faults in the central Apennines and ruptured the surface along the Monte Vettore normal fault. Due to the complex rupture behavior, including antithetic faults and the proposed reactivation of an old thrust front, the Amatrice-Norcia seismic sequence offers a unique opportunity to study the relationship between fault complexity, surface ruptures, and earthquake source properties. Here, we focus on the first two months of the Amatrice-Norcia seismic sequence, including the 30 October 2016 Mw 6.5 mainshock near Norcia and more than 25000 aftershocks. Using continuous waveform data from 94 seismic stations with epicentral distances of up to ~100 km, we estimate source parameters of all cataloged earthquakes that exceed specific quality control criteria in a time period ranging from 24 October – 29 November 2016. Displacement spectral corner frequency and seismic moment values are fit using individual earthquake spectra, and corner frequency estimates are refined using spectral ratios. Constrained spectral parameters then provide input for static stress drop estimates based on a circular crack model. Preliminary results suggest the majority of earthquakes have static stress drop values between 1 and 10 MPa and self-similar scaling. Due to the high quality and quantity of available data, including precise earthquake locations, manually reviewed phase arrivals, and detailed mapping of surface ruptures, the Amatrice-Norcia earthquake sequence represents an opportunity to link earthquake source parameters to geological structures and surface rupture complexity. Preliminary results show correlations between high stress drop values and areas with increasing fault complexity, such as fault intersections at depth (inferred from precise earthquake hypocenters) or the mapped tip of the Monte Vettore normal fault, relative to other fault patches with fewer intersections or mapped surface trace terminations. Future work will examine whether the correlation of stress drop and fault complexity holds using refined stress drop estimates obtained using spectral ratio approaches.

Intraplate tectonic stress fields are complex due to the imprint of a long geological history. Here we use a new dataset of earthquake focal mechanism solutions and relocated events to investigate the relationship between regional stress, crustal strength, and seismicity in the Charlevoix Seismic Zone (CSZ), the most active seismic zone in eastern Canada. Our stress inversion shows that SHmax gradually rotates clockwise from approximately St. Lawrence River-parallel near the surface to river-perpendicular in the lower crust, as postglacial rebound stress becomes increasingly dominant at greater depth. The stress rotation occurs primarily between ~13 and ~26 km depth, where glacial rebound induced stress perturbation is further amplified by a “weaker” middle crust of an estimated apparent friction coefficient of ~0.5. Finally, depth-dependent b-values confirm the rheological difference between upper and middle crust in the CSZ.