Magnitude dependence from the parameters of the source, the Earth’s structure and the receiver: A case study

Synthetic seismograms are used to investigate the dependence of the estimates for four magnitude types (Md, Ml, MS_BB, Mb_BB) on various parameters of the seismic sources, the Earth’s structure and the receiver. Relations between the magnitudes and various parameters are well known and described by analytical equations, but it is hard to apply them directly in practice for a certain event or network configuration. We tested a model of an earthquake in the middle of the Balkan Peninsula region with a station configuration, similar to the stations from the Virtual Seismological Network of Sofia University (VSNSU). Syngine software package, available from the IRIS web page, was used to calculate synthetic seismograms. A number of tests were performed to investigate: the magnitude’s dependence on the azimuth epicenter-station; the behavior of magnitude estimates for epicentral distances ranging from 100 to 1000 km; the changes in magnitude for earthquakes with a varying depth between 0 and 300 km; the magnitude variations changing focal mechanism parameters (strike, dip and rake); the impact of different velocity models on the magnitude estimates; the influence of errors in the coordinates; and the magnitude behavior changing the seismic moment of the event. All tests demonstrate variations in different magnitude estimates up to 2 magnitude units. In the last test, we compared the magnitudes measured by synthetic seismograms and several real seismograms from VSNSU. Tested earthquake has magnitude Ml=4.5, origin time 2016-05-22, 08:58:31 UTC, location 41.62° N, 23.29° E, and depth 1 km. Seismograms from stations RDO, TIR, DJES, DIVS, TIRR and APE were used in the calculations. Observed differences are mainly due to the usage of a global model in the generation of synthetics instead of a regional one as well as the fact that the periods of the synthetics are in the range 2 -100 s while smaller periods are typical for local and regional events.