Seismic hazard across Bulgaria and neighbouring areas: Regional and site-specific maximum credible magnitudes and earthquake perceptibility

A probabilistic seismic hazard assessment is developed here using maximum credible earthquake magnitude statistics and earthquake perceptibility hazard. Earthquake perceptibility hazard is defined as the probability a site perceives ground shaking equal to or greater than a selected ground motion level X, resulting from an earthquake of magnitude M, and develops estimates for the most perceptible earthquake magnitude, M _P(max). Realistic and usable maximum magnitude statistics are obtained from both whole process and part process statistical recurrence models. These approaches are extended to develop relationships between perceptible earthquake magnitude hazard and maximum magnitude recurrence models that are governed by asymptotic and finite return period properties, respectively. Integrated perceptibility curves illustrating the probability of a specific level of perceptible ground motion due to all earthquakes over the magnitude range extending from -∞ to a magnitude M _i are then developed from reviewing site-specific magnitude perceptibility. These lead on to achieving site-specific annual probability of exceedance hazard curves for the example cities of Sofia and Thessaloniki for both horizontal ground acceleration and ground velocity. Both the maximum credible earthquake magnitude M ₃ and the most perceptible earthquake magnitude M _P(max) are of importance to the earthquake engineer when approaching anti-seismic building design. Both forms of hazard are illustrated using contoured hazard maps for the region bounded by 39°-45°N, 19°-29°E. Patterns are observed for these magnitude hazard estimates-especially M _P(max) specific to horizontal ground acceleration and horizontal ground velocity-and compared to inferred patterns of crustal deformation across the region. The full geographic region considered is estimated to be subject to a maximum credible earthquake magnitude M ₃-estimated using cumulative seismic moment release statistics-of 7.53 M _w, calculated from the full content of the adopted earthquake catalogue, while Bulgaria's capital, Sofia, is estimated a comparable value of 7.36 M _w. Sofia is also forecast most perceptible earthquake magnitudes for the lowest levels considered for horizontal ground acceleration of M _PA(50) = 7.20 M _w and horizontal ground velocity of M _PV(5) = 7.23 M _w for a specimen focal depth of 15 km.