The seismic activity rate is one of the most significant factors in seismic hazard modeling. Although it is usually estimated from observed seismicity, a complete picture of the possible earthquakes is not always available since catalogs of the observed earthquakes are short and incomplete. Long-term physics-based numerical simulations, providing a comprehensive range of earthquakes, are a decent way to overcome such deficiency. With this contribution, we built a seismic hazard model for the Alborz region, Iran, using a long-term physics-based synthetic earthquake catalog, enriched with the additional consideration of background seismicity derived from a deformation model. 200,000 yr synthetic catalogs for the Alborz region, Iran, are used and validated by considering the recurrence time of large-magnitude events estimated from the paleoseismological investigation on individual faults. The magnitude-frequency distribution (MFD) from the synthetic earthquake catalog is then compared with the MFD based on observation, which overall indicates good compatibility, although there are discrepancies for some faults. The estimated peak ground acceleration (PGA) for the Alborz region varies in the ranges of 0.16-0.52g and 0.27-1.0g for 10% and 2% probability of exceedance in 50 yr, respectively. The absolute natural logarithm differences averaged across the region are similar to 0.21, corresponding to an average of 23% difference in PGA values in comparison with the most up-to-date observed-based hazard model. Hazard curves for several populated cities are also presented and compared with the other independent estimates. The proposed procedure could be an alternative approach to evaluate seismic hazard for a seismically active region, in particular for those without a complete catalog of observed earthquakes.