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The M w 7.8 earthquake in Turkey on 6 February 2023 was extraordinary for various reasons. It originated in depth of only 10 km, ruptured along a fault plane around 300 km long and the surface was covered by an extensive network of high-quality seismic instruments. The strong motions resulted in a vast number of tragic casualties and huge material losses in Turkey and Syria. However, abundant and proximate seismic observations of this event and numerous aftershocks give an opportunity to deepen the understanding of earthquake processes. In this study, we carried out an assessment of coseismic changes of seismic velocity using passive image interferometry. We used data from one strong-motion and twenty-four broad-band sensors. We observed coseismic drops of seismic velocity, which reached up to −1.79 per cent at a location directly at the ruptured East Anatolian Fault Zone. Along the M w 7.8 earthquake fault, we observe frequency dependence of the velocity changes. At frequencies above 0.5 Hz, the velocity drops seem to be higher at locations close to the ruptured faults than in the more distant areas. [ABSTRACT FROM AUTHOR]

The Global Positioning System (GPS), although it has existed for only 30 years, is an important source for active tectonics, resulting in estimates of plate motions very close to geologic estimates over millions of years. GPS is also used for elastic block models to calculate slip rates for a better understanding of Earth's active crustal deformation. GPS-derived velocity fields may be used as the basis for clustering analysis to create a preliminary definition of block geometry. In this study, we used published horizontal velocity fields to evaluate the effects of data dependences on determining the optimum number of clusters with algorithms. For this purpose, we used different variations of velocity fields in Turkey and tested four different algorithms that are Davies–Bouldin index, the elbow method, GAP statistics algorithm and the silhouette method. We also clustered velocity components with the k -means technique and compared the results with previous studies. [ABSTRACT FROM AUTHOR]