Moment magnitude estimates in the Marmara Sea region (NW Turkey) inferred from modeling of coda waves

Abstract

Subduction of the African plate beneath the Anatolian plate in the Hellenic and Cyprus trenches, as well as collisional tectonics between the Eurasian and Arabian plates has led a westward motion of the Anatolian plate in the counter-clockwise rotation. Under these forces, Anatolia has been actively deforming along with two major shear deformation zone characterized by the North Anatolian Fault Zone (NAFZ) and East Anatolian Fault Zone (EAFZ). Western branch of the NAFZ is divided into two strands within the Marmara Sea located in the Marmara region. Even though the northern strand is linear southern strand is located at the transition zone from the shear to western Anatolian extensional deformation regime and this may result in a more complex faulting network at this part of the region. In general Marmara region hosting these two seismically active branches of the NAFZ poses the most industrial and populated settlement areas of Turkey such as Istanbul with more than 15 million habitants. Thus, a precise earthquake magnitude estimation is of utmost important for making a better seismic hazard assessment in Marmara region. In the present study, coda derived moment magnitude 𝑀𝑤−𝑐𝑜𝑑𝑎, was estimated for small-to-moderate earthquakes (2.5 ≤ 𝑀𝐿 ≤ 5.7 within a radius of 200 km) occurred between 2018 and 2020 in this region. We have used 6557 digital three-component waveforms recordings that were recorded at 121 seismic stations from these 303 local earthquakes. We established source displacement spectra obtained from a coda wave modeling approach in which the unknown source, site, and attenuation terms are simultaneously inverted. The forward prediction of coda waves was achieved considering acoustic Radiative Transfer Theory (RTT). The inversion procedure performed for several different frequency bands to obtain seismic moment 𝑀0, and thus the coda-derived moment magnitude 𝑀𝑤−𝑐𝑜𝑑𝑎 for each event. The use of coda waves in quantifying source properties of local earthquakes is advantageous mainly these scattered wavefields is less sensitive to source radiation pattern and directivity effects. Besides coda wave part can be more useful even if the seismogram is clipped in cases recording site within a close proximity to the earthquake. Resultant 𝑀𝑤−𝑐𝑜𝑑𝑎 values show significant similarities with previous studies and the local magnitude values reported by the KOERI. Eventually, this study presents an empirical relation between 𝑀𝐿 and 𝑀𝑤−𝑐𝑜𝑑𝑎 for Marmara region.