Analysing the earthquakes that hit Turkiye and Syria on February 6, 2023, researchers say that statistically there was about a 7 per cent chance of an earthquake triggering a "doublet", indicating this behaviour was not anomalous.
They said that while the occurrence of two large earthquakes taking place so close in time as a "doublet" was uncommon, the first mainshock may have created stress changes in the area of the second earthquake's epicentre that caused failure at the Surgu-Misis Fault Zone (SMFZ), about 90 kilometres to the north of the first earthquake.
Offering an initial look at the earthquakes in south-central Turkiye and northwestern Syria, three studies have been published in the journal The Seismic Record, including details on how, where, and how fast the earthquakes ruptured and how they combined as a "devastating doublet" to produce damaging ground shaking.
An analysis led by Gesa M. Petersen of Helmholtz Centre Potsdam GFZ German Research Centre for Geosciences, Germany, suggested that the first mainshock ruptured up to 560 kilometres in multiple phases over a total of 117 seconds, while the second earthquake ruptured about 115 kilometres over 32 seconds, with aftershocks being distributed along roughly 160 kilometres of the SMFZ. In this study, the researchers traced the rupture direction of each earthquake, showing that the first involved multiple directional phases and segmented rupture.
Their analysis of mainshocks and aftershocks further provided new details about how the East Anatolian Fault (EAF) and SMFZ ruptured and illuminated a previously unmapped fault segment close to the Turkish city of Malatya. EAF is located at the tectonically active and complex junction between the Anatolian, Arabian, and African plates.
In another paper, Dara E. Goldberg of the U.S. Geological Survey and colleagues used optical and radar imagery and mapped more than 340 kilometres of rupture associated with the mainshock and roughly 175 kilometers of rupture associated with the subsequent event.
Aftershocks from the two earthquakes were found to demonstrate a variety of mechanisms, including strike-slip, normal and thrust faulting, said Petersen and colleagues. Strike-slip mechanisms similar to the two mainshocks were observed on the northeastern EAF segments and along most of the SMFZ, while the team observed normal faulting aftershocks in the southwestern EAFZ segments, as well as clustered at the western termination of the SMFZ.
Combined with the way the rupture process appeared to jump between different fault segments, Petersen and colleagues suggested the evolution of the Turkiye earthquake rupture processes to be similar in their degree of complexity to the 2022 Denali, Alaska of 2016 Kaikoura, New Zealand earthquakes.
In a third paper, the researchers from the King Abdullah University of Science and Technology, Saudi Arabia, write that strong motion recordings made during the first mainshock found that peak ground acceleration (the maximum ground acceleration that occurs during earthquake shaking at a particular location) reached up to 2g locally. This measure corresponds to extreme perceived shaking and very heavy damage.
Rupture in both earthquakes stopped abruptly, which could have contributed to radiation of strong seismic shaking, these researchers note. Ground motions from the second mainshock would have then hit buildings weakened by the first mainshock, possibly increasing damage and destruction as a result.