InvestigatingtheaftershockssequencefollowingtheMw7.3Iran earthquake(November12,2017)

Amajorearthquake,withmagnitudeMw=7.3,strucktheNorthwesternpartofIranonNovember122017,causing extended damage and casualties. The aftershock sequence is still ongoing, as testified by the two recent M6.4 (November 25, 2018) and M5.9 (January 6, 2019) events that occurred in the area more than one year after the mainshock. Thus, we investigate the aftershock sequence following this earthquake, with the aim to understand whether it fits the classical laws of aftershocks occurrence (e.g. the Omori Law), and to explore whether observed deviations from these laws could provide some insights about the occurrence of possible further large aftershocks. TheDelta/Sigmamethod(Caccamoetal.,2005.PhysicsoftheEarthandPlanetaryInteriors153,175–180),isused for this purpose, which is based on the observation of possible anomalies in the temporal decay of an aftershock sequence. Specifically, if the decay can be modeled as a non-stationary Poissonian process, where the intensity function is equal to n(t) = K · gt + c)−p + K1, the number of aftershocks in a short time interval ∆tis the mean value n(t)·∆t, with a standard deviation Sigma2 = n(t)·∆t. Given the difference between observed and estimatednumber ofevents Delta =|Nobs-Ncalc|,it was found that,before theoccurrence of large aftershocks, the Delta/Sigma ratio may reach rather high values, which can be considered as seismic anomalies. The Delta/Sigma method has been applied to several aftershocks sequences worldwide and different improvements of the method havebeendevelopedduringthelastyears,butitstillrequiresrigorousandextensivetestingtoassessitscapability in describing observed aftershocks occurrence. ThebasicdatausedinthisstudyhavebeenextractedfromtheregionalcatalogoftheIranianSeismologicalCenter (IGTUcatalog),whichisavailableathttp://irsc.ut.ac.ir/bulletin.php/.TheIGTUcataloglistseventswithmagnitude M≥2.5, recorded in the Iranian plateau and adjacent areas since 1 January 2006. Various experiments are carried out in order to verify if this method could identify the large aftershocks (with M≥4.8), which occurred in the region surrounding the Mw 7.3 mainshock occurred in November 2017. Different input parameters and configurations of the method are considered, in order to check the stability and statistical significanceoftheobtainedresults.ThepreliminaryresultsofthestudyindicatethattheapplicationofDelta/Sigma methodtotheNorthwesternregionofIran,surroundingtheMw7.3earthquakeoccurredinNovember2017,could highlightthepossibleoccurrenceofseveralsignificantaftershocks.Basedontheseresults,theanalysisisexpanded toANSSglobaldataprovidedbyUSGS,inordertoassessthestabilityofthedetectedpattern,whichmightprovide relevant information for seismic risk assessment.