Eastward rupture directivity and source parameters variations in the Marmara region: implications for a future M > 7 earthquake

Earthquake rupture directivity and source parameters provide key information to understand earthquake physics and constrain seismic hazard and risk, which is particularly important for faults near urban areas. We calculate rupture directivity and source parameters for earthquakes in the Marmara region, NW Türkiye, where a M > 7 earthquake is overdue. First, we analyze directivity patterns for 31 well-constrained ML > 3.5 earthquakes along the Main Marmara Fault, in close proximity to Istanbul. We calculate source mechanisms with a waveform modeling approach and analyze directivity from apparent source-time functions using empirical Green’s functions. Most of the strike-slip earthquakes to the west of the Princess Islands segment display a predominantly asymmetric rupture towards the east with a median directivity trending 85°, consistent with the fault strike. Consequently, earthquake ground shaking may be more pronounced towards Istanbul. This may hold potentially for a future large earthquake on the Main Marmara Fault. Second, we estimate source parameters for >1.500 earthquakes with ML [1.0, 5.7] over the last 15 years. Using a spectral fitting approach, we constrain the corner frequency, seismic moment and quality factor, and calculate the static stress drop. Statistically significant spatial variations of stress drops are observed along some segments, with locally lower values in partially creeping fault zones surrounding earthquake repeaters representing a proxy for aseismic slip. The recent occurrence of M > 5 earthquakes along the overdue Main Marmara Fault did not lead to significant stress drop variations, implying that those moderate events did not significantly modify the stress level in this region which is relevant given that a M > 7 event is pending. Combined, our results underline the importance of including rupture directivity effects and source parameters when estimating seismic hazard and risk near urban areas.