b-value variations preceding the mainshock of the 2023 Kahramanmaraş earthquakes

The analysis of the precursory behavior has been growing in the last decades around the paradigm of earthquake prediction. Amongst the plethora of precursors, the b-value has been deemed as a proxy for the state of stress on a seismogenic source. On 6th February 2023 a mainshock of Mw 7.8 followed by another event of Mw 7.5 within a few hours of difference hit Kahramanmaraş region. Both events occurred on different fault zones of the East Anatolian Fracture Zone (EAFZ). While the first one struck the Pazarcik segment on the main segment of the EAFZ, the second event did it on the Sürgü-Misis fault zone at the west of the EAFZ. In a context of complex rupture behavior, a stress redistribution and transient stress caused by the first event are two of the likely triggering mechanisms of the second event. Our b-value analysis relies on the earthquake catalog from AFAD in the period from early 2015 till the previous moments of the first event on February 2023. To homogenize magnitude reported we excluded Md (few events) and convert Ml to Mw using an empirical relation. As the seismicity along the EAFZ is high, and to avoid statistical bias caused by earthquake clusters, we declustered the earthquake catalog. After assessing the magnitude of completeness to guarantee that the catalog gathers the minimal quality requirements, we examined the temporal evolution and the spatial distribution of b-value. Concerning temporal evolution, we detected that 40 days before the mainshock a sudden decrease of the b-value from 0.85 until its minimum 0.5 on the event’s day. With regard to the spatial distribution of b-value, mainshock’s epicenter is on an elongated region with general strike NE-SW and minimum b-value of 0.76.  

Work supported by the Portuguese Fundação para a Ciência e a Tecnologia (FCT) I.P./MCTES through national funds, UIDB/04683/2020 (ICT) and UIDP/04683/2020 (ICT)