Abstract:
In this study are presented the spectra of P- and S-waves for earthquakes from the seismic cluster (foreshock – main earthquake – aftershock) of the May 24, 2009 earthquake with moment magnitude Mw5.3, occurred in Valandovo seismogenic zone (situated in FYR of Macedonia close to Bulgaria-Greece border region). Earthquake parameters (seismic moment, source radius, stress drop) are estimated applying Brune model. The model has been used extensively and it has been shown that it gives a good agreement with observations from many different tectonic regions and for a large range of magnitudes. The seismic moment (M0 ) is a direct measure of the tectonic size (product of rupture area, average static displacement and share module). Stress drop (Δσ) is the average difference between initial and final stress along a fault before and after an earthquake. It is an important physical parameter associated with the rupture along the fault. Earthquake occurrence is well described with the Poisson process if earthquake sequences or clusters are excluded. Such groups (or clusters) of earthquakes are foreshocks and aftershock sequences. Foreshocks are one of the few well-documented precursors to large earthquakes. Foreshocks are those earthquakes that occur before (months, days, hours) the main event in the same area. Aftershocks occur after the main shock and their frequency decays through time with approximately reciprocal of elapsed time since the main earthquake. The database is compiled using digital data from the Bulgarian Seismological Network – NOTSSI (National Operative Telemetric System for Seismological Information). Displacement spectra are generated for P- and S- waves to estimate some source parameters, such as seismic moment, stress drop, source radius and moment magnitude. The source parameters are estimated for 11 earthquakes: for five foreshocks with magnitude Мp in interval 2.8 ÷ 4.3, for the main event with magnitude Мw=5.3 and for five aftershocks with magnitude Мp in interval 4.1 ÷ 5.0. The estimated stress drop values for the main event is within the expected range for moderate earthquakes. The stress drop values for the foreshocks are lower than stress drop value for the main event. The strongest aftershocks are characterized with higher stress drop values than stress drop value for the main event.