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Deformation Partitioning, Fault Branching and Kinematics and Seismic-Aseismic Slip Interaction Along the Offshore Istanbul-Marmara Section of the North Anatolian Fault, NW Turkey

Session: Crustal Stress and Strain and Implications for Fault Interaction and Slip I

Type: Oral

Date: 4/22/2021

Presentation Time: 09:45 AM Pacific

Description: 

The Marmara section of the North Anatolian Fault zone (NAFZ) is late in its seismic cycle and expected to produce a large (M7+) earthquake in direct vicinity to the Istanbul metropolitan region. The efforts to image and monitor the individual offshore branches of the NAFZ segment were substantially intensified following the 1999 Izmit M7.4 earthquake. However, near-fault monitoring along large portions of the offshore fault section is challenging. Distinct seismicity features now allow to identify seismically active and inactive fault branches. The observation of seismic repeaters suggest that the western portion of the Marmara fault may be creeping to a substantial extent. In contrast, the central and eastern fault segments are largely locked.

The Sea of Marmara represents a pull-apart basin. In line with regional tectonics and the GPS-derived velocity field, the crustal stress field from inversion of earthquake focal mechanisms shows overall transtension. Local fault kinematics allow identifying distinct local normal or strike-slip segments, likely reflecting the close proximity of the vertical and maximum horizontal stress magnitudes.

Recently implemented borehole-based multi-sensor deployments focus on the eastern Marmara section immediately south of Istanbul. We observe distinct slow-slip transients detected by borehole strainmeters that occur in conjunction with local moderate (M>4) earthquakes. Furthermore, lower magnitude-detection thresholds now allow identifying systematic preparation processes leading to M>4 mainshocks as well as extended aftershock sequences complementing postseismic slow slip.

These observations show that further reducing the observational gap and broadening the signal frequency bandwidth allows to decipher deformation processes that previously remained undetected, now providing means for improved seismic hazard and risk models for the Istanbul metropolitan region.

Presenting Author: Marco Bohnhoff

Student Presenter: No

Authors