Unveiling the Seismic Shadows: Which District of Istanbul Will Shake the Most?

Istanbul, straddling two continents, is not just a city of unparalleled historical and cultural wealth but also one that lives in the shadow of a significant seismic threat. The North Anatolian Fault (NAF), a major source of earthquakes in the region, skirts to the south and east of this vibrant metropolis, placing it at a heightened risk of seismic activity. But in a city so vast, the question arises: which district of Istanbul will bear the brunt of such an inevitable shake?

The sequence of westerly propagating ten large (M>6.7) earthquakes on the North Anatolian Fault Zone and the seismic gap in the Sea of Marmara close to Istanbul is an indication of a large earthquake.

The Epicenter of Concern: The Riskiest Districts

Historical data and seismic studies indicate that the districts closest to the North Anatolian Fault zone beneath the Marmara Sea, especially those on the city’s European side, are more vulnerable. Our most recent study (Kalkan and Gulkan, 2024) indicates that Istanbul’s western shoreline faces heightened risk, with median spectral accelerations at 0.3 s approaching 1 g, signifying intense shaking potential. Among these, Adalar, Bakırköy, Avcılar, and Zeytinburnu stand out as areas that might experience the most intense shaking. These districts, with their dense populations and structures, many of which were built before modern seismic standards were implemented, could face significant impacts in the event of a major earthquake.

The figure above shows close-up to peak ground acceleration (PGA) estimates for the Istanbul metropolitan area considering six earthquake scenarios. The median computed PGA­­ is 0.65 g along the shoreline to the west of Istanbul (Bakırkoy district) and at Marmara Islands (Adalar district) as a result of multiple rupturing of Off-Tekirdağ, Mid-Marmara, and Islands faults.

The Science of Shaking: Understanding the Risk

Seismic risk is not only about proximity to the fault line but also about the ground beneath. Areas built on softer, sedimentary layers, such as parts of Avcilar, Atakoy, and Bakırköy amplify seismic waves, leading to more intense shaking compared to those on more solid rock. This geological variability across Istanbul means that the impact of an earthquake can differ dramatically, even within short distances.

Preparedness: The Key to Resilience

While the threat is significant, the focus now is on resilience and preparedness. Istanbul’s government and various organizations are working tirelessly to retrofit vulnerable buildings, improve emergency response systems, and educate the public about earthquake preparedness. Efforts are particularly concentrated in the high-risk districts, aiming to minimize the impact when the inevitable occurs.

Conclusion: A City Bracing for Its Future

As Istanbul faces its seismic future, understanding the areas at greatest risk is crucial for safeguarding its residents and heritage. While many districts along its shoreline may be more vulnerable, city-wide efforts to enhance resilience are a testament to Istanbul’s determination to protect its people and preserve its legacy against the forces of nature.

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Understanding the Earthquake Shaking: The Modified Mercalli Intensity Scale (MMI)

When the earth trembles, the world takes notice. But how do we measure the narrative of the ground’s fierce rumbling? Enter the Modified Mercalli Intensity Scale (MMI), a storyteller of seismic experience that narrates the drama from the ground up.

Intensity vs. Magnitude: Feeling the Difference

While magnitude scales like Richter or moment magnitude measure the energy released at the earthquake’s source, the MMI scale offers a human-centered narrative. It tells us what people felt, what damage occurred, and how the landscape changed. This scale isn’t just about numbers; it’s about experiences.

The Scale of Stories

From I, where the shaking is not felt except by a select few under favorable conditions, to XII, where damage is total, structures are uprooted, and the earth’s surface is wrenched, the MMI scale plots the plot points of an earthquake’s impact. Each level up the scale marks a significant increase in the effects felt and damage inflicted.

Click HERE to download the MMI scale in PDF.

Local Tales of a Global Phenomenon

What makes the MMI scale particularly useful is its adaptability to various settings. The same earthquake can be gentle in one location and destructive in another. By cataloging responses from different areas, seismologists can map out an earthquake’s impact in a way that resonates with the local narrative.

A Chronicle of Resilience

Beyond its scientific value, the MMI scale is a record of resilience. It highlights how communities withstand the shaking, adapt to their transformed landscape, and rebuild in the aftermath. It’s a human scale for a natural event.

In the end, the Modified Mercalli Intensity Scale does more than tell us how the Earth moved. It connects us through shared experiences and mutual understanding. It’s a reminder that while we may be separated by geography, we are united in our encounter with the natural world.

When the earth shakes again, as it inevitably will, we will turn to the MMI scale not just for data, but for the stories of survival, strength, and solidarity. It is a scale that does not just measure shakes, but also stirs the human spirit.

Stay Grounded with Knowledge

Understanding the MMI scale can help us better prepare for future seismic events. By learning from past earthquakes, we can build structures and communities that are not only earthquake-resistant but also resilient in the face of whatever the MMI scale may tell us next.

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Why Have Seismologists Moved from Richter to Moment Magnitude for Measuring Earthquake Intensity?

The Richter scale, developed in 1935 by Charles F. Richter, was the first scale to measure the size of earthquakes. The scale is logarithmic, meaning that each whole number increase on the scale represents a tenfold increase in measured amplitude and approximately 31.6 times more energy release. The scale was specifically calibrated for Southern California and used a particular type of seismograph, so it was most accurate for medium-sized earthquakes (M3 to M7) within a certain distance from the seismograph.

However, as our understanding of earthquakes has grown and technology has improved, seismologists have identified limitations with the Richter scale:

  1. Regional Limitations: The Richter scale was based on California’s geology and the specific seismographs used at the time. It does not scale well for extremely large or small earthquakes, nor does it account for variations in the Earth’s crust in different regions of the world.
  2. Energy Release: The Richter scale does not accurately estimate the energy released by very large earthquakes. The scale saturates around M7, meaning that it does not distinguish well between the energy released by the largest earthquakes, which can differ significantly.
  3. Seismograph Limitations: The original scale was based on the recordings from a particular type of seismograph that is not used as widely today. Modern seismographs provide more detailed data, and the Richter scale does not take full advantage of this.

To address these limitations, the Moment Magnitude Scale (Mw) was introduced by Hank and Kanamori (1979). It is based on the seismic moment of an earthquake, which is a measure of the total energy released by the earthquake. The moment magnitude scale is now the most common scale for measuring the size of earthquakes for several reasons:

  1. Global Applicability: Moment magnitude is calculated based on the physical properties of the earthquake (such as the rigidity of the Earth’s crust, the area of the fault that slipped, and the amount of slip) and can be used globally without regional corrections.
  2. Accuracy for Large Earthquakes: The moment magnitude scale does not saturate like the Richter scale. It provides an accurate measure of the energy release for very large earthquakes (greater than M7), which is essential for understanding their potential impact.
  3. Consistency: The scale provides a more uniform and consistent measure of an earthquake’s size, which is useful for both historical comparisons and scientific research.
  4. Detailed Data Use: Modern seismographs record a full seismic wavefield. Moment magnitude takes advantage of this data to provide a more complete picture of an earthquake’s characteristics.

Because of these advantages, the moment magnitude scale has largely replaced the Richter scale for most seismological applications, especially for earthquakes that are recorded at long distances from the epicenter or that are very large.

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