Reliable estimates of the moment magnitude of earthquakes as representative of the earthquake source energy are vital for an understanding of seismic hazard in regions prone to tectonic activity. To address this demand, our researchers Berkan Özkan, Assoc. Prof. Dr. Tuna Eken and Prof. Dr. Tuncay Taymaz have tested a method involving coda wave modelling moment magnitudes estimates of earthquakes in the Sea of Marmara, northwestern Türkiye. 

This study modeled the source displacement spectrum for 303 local earthquakes recorded at 49 regional seismic stations between 2018 and 2020. The coda wave traces, which were extracted from each event, were then analysed across 12 frequency bands ranging from 0.3 to 16 Hz. The resultant coda-based moment magnitudes showed strong agreement with traditional local magnitude estimates. However, for
smaller earthquakes (magnitude < 3.5), there was some variation between the local and coda-derived moment magnitudes. This is probably due to some incorrect assumptions about anelastic attenuation or the finite sampling intervals of seismic data.The scaling relationships between total radiated energy and seismic moment suggest that earthquakes in the Sea of Marmara behave in a non-self-similar way. What we can see is that the bigger earthquakes in the region have different ways of rupturing, which means they release more seismic energy than smaller events.Researchers also found an easy-to-use relationship that can help us understand the differences we see between local and coda-derived moment magnitude estimates.

Findings of this research were published at the Solid earth journal in this year. More details about the study can be found as follows:

Özkan, B., Eken, T., Gaebler, P., and Taymaz, T.: Coda-derived source properties estimated using local earthquakes in the Sea of Marmara, Türkiye, Solid Earth, 15, 1303–1317, https://se.copernicus.org/articles/15/1303/2024/, 2024.

Figure. Spatial distribution of 303 local events (2.5 ≤ ML ≤ 5.7) that occurred between 2018 and 2020 displayed with circles colour-coded according to focal depths reported by the KOERI catalogue. White triangles indicate stations used in the present work (Özkan et al., 2024).

Figure. Black squares indicate observed source displacement spectra and grey curves represent predicted source displacement spectra for all 303 individual local earthquakes (Ozkan et al., 2024)

Figure. Scatter plot of Mw-coda as a function of ML. The bold grey line represents the linear regression fit, and dashed lines are the standard deviation (Ozkan et al., 2024).