Monitoring the Variations in Crustal Seismic Velocity and Anisotropy Associated with the 2023 Kahramanmaraş Earthquakes, Türkiye, using Ambient Noise Cross Correlation

On the 6^th February 2023 two devastating earthquakes of Mw 7.8 and Mw 7.7 with left-lateral strike-slip mechanisms occurred along the East Anatolian Fault (EAF) and the Sürgü-Çardak Fault (SÇF) in southeastern Türkiye. The doublet nucleated and instantaneously ruptured for ~350 km and ~160 km during the complex network of multi-fault segments reaching a maximum slip of ~8 m and >10 m, respectively. The consecutive large earthquakes are likely to have caused permanent changes in the shallow crustal properties, especially in the vicinity of the fault zone. The variations in crustal velocity and anisotropy during the pre-, co-, and post-seismic periods could be efficiently monitored using the ambient noise data. The primary objective of this work is to monitor isotropic velocity changes for the pre-, co-, and post-seismic periods, as well as rapid changes in seismic anisotropy potentially caused by coseismic stress field rotation beneath the EAFZ. To achieve this, we analyze continuous three-component digital recordings from 52 broadband seismic stations located along the EAF, the SÇF and surroundings that are operated by AFAD (Turkish National Seismic Network) and KOERI (Kandilli Observatory and Earthquake Research Institute). First, we analyze the daily correlation functions of all rotated components (ZZ, TT, RR, ZT, TZ, TR, RT, RZ, and ZR) in order to obtain the isotropic seismic velocity change. Second, we rotate the nine-component cross-correlation tensors (CCTs) to minimize tangential components (TZ, ZT, TR, RT), as expected to be zero for an isotropic medium with randomly distributed noise. This approach enables us to monitor the temporal variations of crustal anisotropy before, after, and during these two devastating earthquakes, effectively. Here we present our preliminary results on the spatiotemporal variations of crustal anisotropy derived from ambient seismic noise cross-correlations between station pairs during the 2023 Kahramanmaraş doublet.