Newswise — Three recent publications in The Seismic Record, an open-access journal, provide preliminary insights into the earthquakes that occurred on February 6, 2023, in south-central Türkiye and northwestern Syria. These studies examine the rupture characteristics, spatial distribution, and rupture velocity of the earthquakes, as well as their combined impact as a "destructive pair" that resulted in significant ground shaking.
The pair of earthquakes, measuring 7.8 and 7.6 in magnitude respectively, occurred at the intricate and seismically active intersection of the Anatolian, Arabian, and African plates along the East Anatolian Fault (EAF) lines. Historically, major earthquakes in Türkiye have predominantly transpired along the North Anatolian Fault, with only three moderately sized earthquakes, the largest being magnitude 6.8, happening on the EAF over the last five decades.
First Analyses of Rupture and Aftershocks
The epicenter of the initial mainshock, measuring 7.8 in magnitude, is situated approximately 15 kilometers east of the East Anatolian Fault (EAF). In contrast, the epicenter of the subsequent earthquake, magnitude 7.6, is located around 90 kilometers north of the first earthquake on the Sürgü-Misis Fault Zone (SMFZ).
According to a study conducted by P. Martin Mai and colleagues from King Abdullah University of Science and Technology, the initial earthquake caused bilateral ruptures along the East Anatolian Fault (EAF) spanning approximately 350 kilometers (including aftershocks) in a duration of 80 seconds. This rupture resulted in surface fault displacements exceeding six meters. Similarly, the second earthquake also exhibited bilateral ruptures, covering approximately 170 kilometers (including aftershocks) in a timeframe of 35 seconds, with surface fault offsets surpassing seven meters.
A research team led by Gesa M. Petersen from Helmholtz Centre Potsdam GFZ German Research Centre for Geosciences conducted an analysis revealing that the initial mainshock exhibited a multi-phase rupture, extending up to 560 kilometers over a total duration of 117 seconds. On the other hand, the second earthquake ruptured approximately 115 kilometers in a span of 32 seconds, with subsequent aftershocks distributed along roughly 160 kilometers of the Sürgü-Misis Fault Zone (SMFZ). The study also provided insights into the rupture directions of each earthquake, demonstrating that the first earthquake involved multiple phases and segmented ruptures. Additionally, their analysis of both mainshocks and aftershocks shed light on a previously unknown fault segment near the city of Malatya in Turkey.
Dara E. Goldberg and a team of researchers from the U.S. Geological Survey utilized optical and radar imagery, which was accessible in the initial days following the earthquake sequence, to analyze and determine the rupture trace. As additional imagery became accessible, the National Earthquake Information Center (NEIC) of the U.S. Geological Survey continuously updated their source characterization and impact analyses to incorporate the newly acquired knowledge about the extent of the ruptures. In total, the research group mapped over 340 kilometers of rupture linked to the mainshock and approximately 175 kilometers of rupture associated with the subsequent earthquake.
According to Mai and colleagues, the occurrence of two large earthquakes happening in such close proximity in time, known as a "doublet," is not common. However, they suggested that the initial mainshock might have induced stress changes in the region surrounding the epicenter of the second earthquake. These stress changes potentially contributed to the failure along the Sürgü-Misis Fault Zone (SMFZ), leading to the occurrence of the second earthquake.
Goldberg and colleagues present a different perspective, arguing that statistically, there is approximately a 7% probability of an earthquake triggering a doublet. They suggest that this finding indicates that the occurrence of such a doublet is not anomalous or unexpected based on existing statistical analysis and earthquake behavior.
Petersen and colleagues observed a range of faulting mechanisms in the aftershocks following the two earthquakes. These mechanisms included strike-slip, normal, and thrust faulting. Strike-slip mechanisms, similar to those observed in the mainshocks, were identified along the northeastern segments of the East Anatolian Fault (EAF) and throughout most of the Sürgü-Misis Fault Zone (SMFZ). In contrast, the team noted occurrences of normal faulting aftershocks in the southwestern segments of the EAFZ, as well as clusters of such aftershocks at the western end of the SMFZ.
Petersen and colleagues propose that the complexity of the earthquake rupture processes observed in Türkiye, including the manner in which the rupture appeared to propagate across different fault segments, bears similarities to the complexities observed in the 2022 Denali earthquake in Alaska and the 2016 Kaikoura earthquake in New Zealand. This suggests that the evolution of the earthquake ruptures in Türkiye shares a similar degree of complexity with these notable earthquake events.
Rapid Response
Goldberg and colleagues highlight the important role of the U.S. Geological Survey's National Earthquake Information Center (NEIC) in swiftly characterizing earthquakes worldwide. They mention that researchers often rely on NEIC's products, such as ShakeMap and PAGER (Prompt Assessment of Global Earthquakes for Response), especially in the immediate aftermath of significant earthquakes. Regarding the Türkiye earthquakes, they report that within the first 24 hours of the sequence, the USGS public earthquake event web pages received an impressive number of visits, totaling 1,035,364. This indicates the high demand for information and data related to the earthquakes and the value of the NEIC's resources in such situations.
As a key source of information for government agencies to gauge their response in the aftermath of an earthquake, the National Earthquake Information Center (NEIC) swiftly released its initial ShakeMap for the magnitude 7.8 earthquake just 15.7 minutes after the sequence began. Additionally, NEIC provided its PAGER assessment 21.2 minutes after the origin time of the earthquake, reflecting a similar timely response for the subsequent event. This highlights the NEIC's efficiency in delivering crucial information and assessments to assist in post-earthquake decision-making processes.
Ground Motion Details
According to Mai and colleagues, the analysis of strong motion recordings captured during the initial mainshock revealed that peak ground acceleration (PGA) reached locally up to 2g. The value of 2g signifies an exceptionally high level of ground acceleration, indicating extreme perceived shaking and the potential for significant structural damage. The recorded PGA highlights the severity and intensity of the ground shaking experienced during the earthquake.
According to the researchers, the rupture in both earthquakes abruptly ceased, which may have played a role in the radiation of intense seismic shaking. As a consequence, the ground motions generated by the second mainshock would have impacted buildings that were already weakened by the first mainshock. This interaction between the two events could have potentially intensified the damage and destruction, amplifying the overall impact on structures and infrastructure.
Goldberg and colleagues point out that the PAGER (Prompt Assessment of Global Earthquakes for Response) system, which is the U.S. Geological Survey's loss estimation tool, potentially underestimated the overall impact of the earthquake sequence. The reason for this underestimation is that PAGER does not account for repeated shaking caused by aftershocks. To provide a more accurate estimation of losses for this damaging earthquake sequence, the researchers suggest using a composite ShakeMap that incorporates the maximum shaking intensity at each location throughout the entire earthquake sequence. This approach would take into consideration the cumulative effects of both the mainshocks and aftershocks, providing a more comprehensive assessment of the seismic impact and potential losses.
Goldberg and colleagues propose that if a composite ShakeMap, which considers the maximum shaking intensity throughout the entire earthquake sequence, had been utilized as input for PAGER, the final assessment of the sequence would have resulted in an estimation of approximately 30,000 total deaths and $51 billion in economic losses. By incorporating the cumulative effects of the mainshocks and aftershocks, a composite ShakeMap would provide a more comprehensive and accurate representation of the seismic impact, resulting in a higher estimation of casualties and economic losses compared to the original PAGER assessment.
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