A Two-Scale Preparation Phase Preceded an Mw 5.8 Earthquake in the Sea of Marmara Offshore Istanbul, Turkey
Virginie Durand, Stephan Bentz, Grzegorz Kwiatek, Georg Dresen, Christopher Wollin, Oliver Heidbach, Patricia Martínez‐Garzón, Fabrice Cotton, Murat Nurlu, Marco Bohnhoff
Abstract
Abstract We analyze the spatiotemporal evolution of seismicity during a sequence of moderate (an Mw 4.7 foreshock and Mw 5.8 mainshock) earthquakes occurring in September 2019 at the transition between a creeping and a locked segment of the North Anatolian fault in the central Sea of Marmara, northwest Turkey. To investigate in detail the seismicity evolution, we apply a matched-filter technique to continuous waveforms, thus reducing the magnitude threshold for detection. Sequences of foreshocks preceding the two largest events are clearly seen, exhibiting two different behaviors: a long-term activation of the seismicity along the entire fault segment and a short-term concentration around the epicenters of the large events. We suggest a two-scale preparation phase, with aseismic slip preparing the mainshock final rupture a few days before, and a cascade mechanism leading to the nucleation of the mainshock. Thus, our study shows a combination of seismic and aseismic slip during the foreshock sequence changing the strength of the fault, bringing it closer to failure.