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Deformation Partitioning, Directivity Effects, and Stress-Drop of Seismicity Along the Main Marmara Fault Offshore Istanbul/Türkiye in the Light of an Overdue M7+ Earthquake

Description: 

The Marmara section of the North Anatolian Fault zone (NAFZ) in northwestern Türkiye is late in its seismic cycle and expected to produce a large M>7 earthquake in direct vicinity to the Istanbul metropolitan region. Monitoring the NAFZ below the Sea of Marmara is a challenge due to absence of near-fault islands except for its eastern section. High resolution seismic monitoring has been achieved through setting up the borehole-based GONAF (Geophysical Observatory at the North Anatolian Fault) infrastructure monitoring seismicity as well as slow deformation transients.

Analyzing the occurrence of repeating earthquakes as indicator for seismic creep we identify several repeater families below the eastern and –to a lesser extent- also below the central Sea of Marmara. We conclude on a systematic transition of deformation along the Main Marmara Fault from dominantly creeping in the west , an intermittent state of locked-creeping deformation at its central part, and the fully locked Princess Islands segment immediately south of Istanbul. Small, but statistically significant, spatial stress-drop variations are observed among some segments of the fault. Lower average stress-drop values are observed in zones surrounding earthquake repeaters, which may imply a weaker fault along the creeping region. Recent M>5 earthquakes along the Main Marmara Fault did not exhibit significant changes in the mean stress-drop, suggesting no reduction of fault stress level or fault strength due to the occurrence of these events. To improve rupture scenarios for the pending large Marmara earthquake we estimate the rupture directivities for 30 M>3.5 earthquakes along the Main Marmara Fault. We constrained earthquake moment tensors for these events providing evidence for their potential fault planes and observe a predominant eastward rupture direction for half of the events, most of them located along the western and central parts of the Main Marmara Fault. This suggest that about 50% of the analyzed events displayed a preferential orientation of energy release towards Istanbul, hence probably increasing the ground shaking in a major event.

Session: Characteristics and Mechanics of Fault Zone Rupture Processes, from Micro to Macro Scales - II

Type: Oral

Date: 5/2/2024

Presentation Time: 10:45 AM (local time)

Presenting Author: Marco

Student Presenter: No

Invited Presentation: 

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