The sPg wave is one of the regional depth phases useful for focal depth determination in the Regional Depth Phase Modeling Technique. We investigated whether sPg phases were observed for events in Mongolia. We analyzed data from the Central Mongolia Seismic Experiment for the 2012 Bayanbulag earthquake to investigate whether sPg were observed. There are phases that arrived after Pg on the observed records. The arrival time differences in the distance range between these phases and Pg do not change much in the epicentral distance range from 55 km to 70 km, which suggests that the observed phases are sPg waves.
We calculated synthetic seismograms using the code of the reflectivity method using a model recently obtained for South-Central Mongolia and measured arrival time differences between sPg and Pg waves from them. A comparison of the arrival times from the synthetic and observed seismograms in the epicentral distance range from 55 km to 70 km suggests that the depth of the Bayanbulag earthquake is around 10 km. It indicates that this epicentral distance range (55 km to 70 km) is good to be used to determine focal depths using the sPg-Pg pair based on the synthetic waveforms.
Keywords: Focal depth, Regional depth phase, sPg, Bayanbulag earthquake, Synthetic waveform.