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Abstract

Two earthquakes struck eastern Turkey and caused severe damage on 6^th February 2023. The magnitudes are Mw7.8 and Mw7.7, respectively. The rupture processes of the two earthquakes are significantly different. The Mw7.8 earthquake has a gradual initial rupture, resulting in a longer duration time. In addition, the epicenter is located at a small branch.We focus on the initial rupture constrained by teleseismic data. We set up fault planes based on preliminary online geodetic and seismology results by other researchers. After a series of tests, we could not simulate the initial rupture well based on the uniform sub-fault size (6x4km). We carry out a two-step inversion strategy. Firstly, we used a rectangle fault with a length of 26km and a width of 30km to simulate the first 12s teleseismic P waveforms. The sub-fault size is 2x2km. The strike and dip angles are 220° and 70°, respectively. The residual teleseismic waveforms between synthetic and the whole observed teleseismic waveforms are calculated.Secondly, Two rectangle segments with larger sub-fault sizes and different strike angles (220° and 235°) are applied to simulate the residual teleseismic waveforms. The sub-fault size is 6x4km.The result indicates that the initial rupture propagated downward along the dip-slip direction which is a nearly pure strike-slip event. The moment is 1.3x10^19 Nm, and the corresponding moment magnitude is Mw6.7, about 2% of the total moment.A significant normal component could be observed at the shallow depth between the initial pure strike-slip patch and the body patch, which is consistent with the tectonic background.