A new analysis of earthquake rupture directivity provides essential insights for seismic hazard and risk assessments in urban areas, particularly concerning the Main Marmara Fault near Istanbul in western Türkiye.

Based on the correlation between rupture directivity and the direction of the transported seismic energy, a team of researchers led by Dr. Xiang Cheng and Prof. Patricia Martínez-Garzón from the GFZ Helmholtz Center for Geosciences in Potsdam, Germany, has shown that quakes in the Marmara region transport a particularly large amount of energy and thus a destructive force in the direction of Istanbul.

Their study has been published in the journal Geophysical Research Letters. They analyzed 31 well-constrained ML ≥ 3.5 earthquakes in this region. The unveiled critical patterns could influence preparedness for future seismic events in one of the world’s most populous cities.

Earthquakes are a natural phenomenon that can result in devastating impacts, especially in densely populated regions. In particular, understanding the behavior of these seismic events is crucial for mitigating risks and enhancing preparedness.

In recent years, it has become evident that the energy transported by seismic waves can be stronger in certain directions—commonly in the direction of the rupture—and weaker in others, which has important consequences for the damage potential in populated regions.

Analyses of the Sea of Marmara based on smaller earthquakes and modeling

A team of researchers led by Dr. Cheng and Prof. Martínez-Garzón of the GFZ Helmholtz Center for Geosciences in Potsdam, Germany, investigated such directional effects. They analyzed 31 well-constrained earthquakes of magnitude ML > 3.5 in the Sea of Marmara, west of the Istanbul megacity. Smaller earthquakes occur more often and can thus be studied in greater details, being a blueprint for “big ones” that occur more rarely but with bigger implications.

In their study, the research team compared modeled and measured waveforms to calculate source mechanisms and then measured the earthquake durations in different directions to estimate directivity effects of moderate earthquakes in the Istanbul-Marmara region.

The findings reveal that most of the studied earthquakes below the Sea of Marmara west of Istanbul exhibit a predominantly eastward rupture. This results in more energy directed toward the metropolis. The median directivity trends at 85° from the North, aligning closely with the strike of the Main Marmara Fault.

“This directional tendency suggests that ground shaking is more pronounced in Istanbul during such seismic events,” says Dr. Xiang Chen, first author of the study and post-doc scientist at GFZ during the study.

This information is particularly vital given that the Main Marmara Fault is considered to be late in its seismic cycle, meaning that a large earthquake is overdue. The present study does not reduce concerns about the implications of a large earthquake in the region.

“Depending on where a future large earthquake would nucleate, these asymmetric rupture patterns could lead to heightened ground motion towards the urban center of Istanbul,” states Prof. Patricia Martínez-Garzón, working group leader at GFZ Section 4.2 Geomechanics and Scientific Drilling and corresponding author of the study.

Considerations recommended for seismic hazard maps

When estimating seismic hazard maps for certain regions, rupture directivity, e.g. the preferential direction in which earthquakes radiate their energy, is not yet taken into account.

“We want and plan to include directivity effects in the next generation of seismic hazard maps used in earthquake engineering, and results such as these are fundamental to enabling this development,” says Prof. Fabrice Cotton, co-author of the study and Head of GFZ-Section 2.6 Seismic Hazard and Risk Dynamics.

The measured data for this study were partly delivered by the Plate Boundary Observatory (GONAF), which has been operated in the Marmara region since 2015 by the GFZ Helmholtz Center for Geosciences, in collaboration with the Turkish Disaster and Emergency Management Presidency (AFAD). It comprises various types of instrumentation, including seismometers that are installed in boreholes to precisely monitor and measure earthquake activity in the region.

“A key objective of our observatory is to better monitor the small and moderate earthquakes in the Marmara region, to prepare as good as possible for when a large earthquake ruptures near Istanbul,” adds Prof. Marco Bohnhoff, Head of Section 4.2 Geomechanics and Scientific Drilling and Leading Scientist of GONAF geophysical observatory.

Implications for urban planning and the protection of the population

The implications of this study underscore the necessity for urban planners, policymakers, and emergency response coordinators to incorporate detailed seismic risk assessments into their planning frameworks.

“Evaluating potential earthquake impacts based on improved scientific methodologies can drastically enhance the resilience of Istanbul’s infrastructure and communities,” emphasizes Patricia Martínez-Garzón.

This research not only sheds light on the seismic behavior of the Main Marmara Fault near Istanbul, but also serves as a critical reminder of the ongoing threat that earthquakes pose to urban environments worldwide. As cities become increasingly vulnerable due to population density and infrastructure challenges, understanding the nuances of seismic activity remains vital for safeguarding communities.