The moderate size 2019 September Mw 5.8 Silivri earthquake unveils the complexity of the Main Marmara Fault shear zone

Abstract

Summary The unbroken section of the North Anatolian Fault beneath the Sea of Marmara is a major source of seismic hazard for the city of İstanbul. The northern and currently the most active branch, the Main Marmara Fault (MMF), is segmented within a shear zone and exhibits both partially creeping and locked behavior along its 150 km length. In September 2019, a seismic activity initiated near MMF, off-coast the town of Silivri, generating 14 earthquakes ≥ Mw3.5 in a week. The Mw5.8 Silivri earthquake, is the largest in the Marmara Sea since the 1963 Mw6.3 Çınarcık earthquake. Our analyses reveal that the activity started in a narrow zone (∼100 m) and spread to ∼7 km following an Mw4.7 foreshock within ∼2 days. The distribution of relocated aftershocks and the focal mechanisms computed from regional waveforms reveal that the Mw5.8 earthquake did not occur on the MMF, but it ruptured ∼60° north-dipping oblique strike-slip fault with significant thrust component located on the north of the MMF. Finite-fault slip model of the mainshock shows 8 km long rupture with directivity toward east, where the ruptured fault merges to the MMF. The narrow depth range of the slip distribution (10–13 km) and the aftershock zone imply that the causative fault is below the deep sedimentary cover of the Marmara Basin. The distribution of aftershocks of the Mw5.8 event is consistent with Coulomb stress increase. The stress changes along MMF include zones of both stress decrease due to clamping and right-lateral slip, and stress increase due to loading.

Seismological and Engineering Characteristics of Strong Motion Data from 24 and 26 September 2019 Marmara Sea Earthquakes

“…Our estimation of 0.75 m as the average source dislocation of the MW5.7 event is larger and in good agreement with the slip range considered in Figure 7 of Karabulut et al (2021). Average source dislocations calculated through Thingbaijam et al (2017) are shown in Table 3 as well.…”

“…In the case of 26 th September earthquake, moment magnitudes stated by both organizations are identical with those of KOERI (KOERI-RETMC 2019a& 2019b. The focal mechanism solution of the 24 th September earthquake is compatible with the fault mechanism of NAF's central Marmara segment and exhibits a strike-slip dominancy (KOERI-RETMC 2019a, Karabulut et al 2021). However, there are different views about the faulting mechanism of the 26 September 2019 event.…”

“…For the MW4.5 event, due to lack of a fault plane solution, we have assumed that RJB equals to epicentral distance (Repi). As for the MW5.7 event, Karabulut et al (2021) suggested an elliptic finite-fault model, whose rupture zone has a length of 5 km on the east and 3 km on west of the epicentre. There is about 8% and 3% decrement in distance when RJB with respect to its surface projection is considered instead of Repi for stations 3408 (closest) and YLVH (farthest) respectively.…”

“… Fault type (SoF): Style of faulting is accepted as strike-slip for both MW4.5 and MW5.7 events based on Karabulut et al (2021).…”

“…However, there are different views about the faulting mechanism of the 26 September 2019 event. While the solutions by KOERI-RETMC (2019b), AFAD (2020), USGS (2021) and Eyidogan and Sevilgen (2019) indicate dominantly thrust faulting with dipping planes ranging from 35 to 55 degree, according to Karabulut et al (2021) the event is dominantly strike-slip and took place on a 60-degree north-dipping fault. A total of 70 and 76 stations located in and around Istanbul were selected to analyse the MW4.5 and MW5.7 earthquakes, respectively.…”