Periodic upward migration model for intermediate-depth earthquakes in Vrancea, Romania

Vrancea intermediate-depth earthquakes with a depth range of 60-160 km occur frequently. By using data from ROMPLUS catalog, we analyze earthquakes with M >= 7.0 that occurred during 1500-2000. We propose a periodic upward migration (PUM) model for these earthquakes. According to this model, (1) active zones at the bottom of seismic regions migrate upward with the velocity V, with the activity starting from the year T(S) and, (2) this migration repeats every T(P) years, and at a given depth, seismic active period continues for T(1) years. We test the model using the likelihood function and compare the model with a uniform Poisson model. A simulation using the bootstrap method indicates that the PUM model for M >= 7.5 and M >= 7.3 earthquakes is statistically significant at the 5% level and is better than the Poisson model. We propose the following regularity for Vrancea M 7 intermediate-depth earthquakes. Three earthquakes occur at a depth of 140-160 km, 110-140 km, and 80-110 km at the beginning, at the middle, and at the end of each century, respectively. This phenomenon repeats every century. The next M 7 earthquake is expected to occur at a depth of 140-160 km soon.