Emergent failure process of a M4.2 earthquake offshore Istanbul observed from GONAF downhole recordings

Marco Bohnhoff, Peter E. Malin, Murat Nurlu, & Felix Bluemle

Published July 19, 2018, SCEC Contribution #8160, 2018 SCEC Annual Meeting Poster #039

The main branch of the North Anatolian Fault Zone below the Sea of Marmara is facing a high probability for a M>7 earthquake. Recently implemented monitoring efforts such as the downhole GONAF observatory nor allow for detecting low-magnitude seismicity along the eastern Marmara section offshore of Istanbul. The June25, 2016, M4.2 earthquake off the Armutlu peninsula was the largest local earthquake in years. Low-noise recordings from the closest vertical seismic GONAF array allowed to detect a series of earthquakes preceding and following the mainshock down to M~0. In the 64 hours before the M4.2 event, a total of 18 M>0 earthquakes with S-P times within 0.1 sec of the M4.2’s S-P time took place. They reflect a type of self-organization and preparatory phase of the mainshock. In the subsequent 48 hours at least 3 to 4 times as many similar size-and-S-P time events were also detected. The relative magnitudes of the detected events span from the detection limit of the array at M~0 to M~3.5 as measured on the same scale as the M4.2. Based on their close hypocenter locations the entire series of detected events appear to fill a local seismic gap, whose dimensions are on the same order as the M4.2 rupture. A large fraction of the preceding and following events have similar waveforms. Average cross-correlation coefficients of foreshocks show a clear increase during the ten hours before that mainshock possibly reflecting a preparation process in direct vicinity to the mainshock hypocenter. If processed in near-real time this might support approaches in earthquake forecasting.

Key Words
earthquake nucleation, forecasting, downhole seismology

Citation
Bohnhoff, M., Malin, P. E., Nurlu, M., & Bluemle, F. (2018, 07). Emergent failure process of a M4.2 earthquake offshore Istanbul observed from GONAF downhole recordings. Poster Presentation at 2018 SCEC Annual Meeting.

Related Projects & Working Groups
Earthquake Forecasting and Predictability (EFP)