[U-13] February 2023 Earthquake Sequence of the East Anatolian fault zone in Turkey

[U13-09] Seismic motions and structural damage in various locations during the 2023 Kahramanmaraş, Turkey, earthquakes.

*Shinichiro Mori¹, Shigeto Osato², Tatsuro Chiba³, Nurcan Meral Ozel⁴, Gulten Polat⁵ (1.Ehime University, 2.Soil Research, 3.Asia Air Survey, 4.Bogazici University, 5.Yeditepe University)

Keywords:seismic ground motions, structural damage, field survey, pulse wave, directivity effect, basin edge effect

On February 6, 2023, at 4:17 and 13:24 Turkey local time, two earthquakes of magnitudes 7.8 and 7.5 occurred successively in southeastern Turkey, which were called the Kahramanmaraş Earthquake. The two earthquakes, the largest ever recorded in Turkey, caused strong seismic motions that caused extensive damage in areas along the East Anatolian Fault and in northern Syria, and the number of victims reported at the end of February, over 50,000, was the largest in modern and contemporary Turkey. We attempted to analyze the earthquake ground motions and assess the damage based on information available on the Internet immediately after the earthquakes. The inverse analysis of long-period ground motions immediately after the earthquakes showed that the fault movement from the epicenter of the Kahramanmaras earthquake to the northwest along the East Anatolian Fault Zone was noteworthy, but the shaking from the epicenter to the south-southwest through the Hatay province was reflected in the seismic intensities based on the questionnaire to residents. AFAD in Turkey conducted strong-motion seismic observations and released the results immediately after the earthquake. The distribution of the maximum acceleration also showed large acceleration in the direction extending south-southwest from the epicenter, and there was concern that there would be more damage in the south-southwest direction from the epicenter. The results of the AFAD seismic motion analysis showed that some of the observation points had strong seismic pulse waves that were characteristic of the vicinity of the epicenter fault and have predominant periods from 1 to 3 seconds. It was necessary to observe the structural damage from the viewpoints of ground motion amplification and near-source pulse wave effects. The topography, geology, and ground conditions, which differ greatly from those in Japan, are also important perspectives when investigating and discussing structural damage. In Turkey, among the various types of structures, the main types are 1- and 2-story houses of masonry construction and mid- to high-rise reinforced concrete apartments (reinforced concrete (RC) frame structures with masonry infill walls). This RC construction type constitutes the bulk of the building stock in Europe. RC buildings in Turkey are designed according to the seismic design code, and this perspective is also essential in the damage analysis, as the building designed according to the code (AC) and not-according to the code (NAC) are the main indicators in past earthquake damage analysis studies. In the field survey, it is essential to observe the quality of the concrete and the details of the rebar system, such as the main bar and shear reinforcement bars. Satellite images and the results of damage detection based on these images were made public by many organizations immediately after the earthquakes. The field damage survey can be conducted efficiently and accurately by effectively referring to these images in advance. The authors prepared for the field survey by conducting various analyses immediately after the earthquake on February 6. For the field survey, Mori and Polat conducted the field survey from March 3 to March 7 in cooperation with JICA and in liaison with AFAD. Microtremor measurements and observations of building damage were conducted at AFAD seismograph sites in Gaziantep, Kahramanmaras, Hatay, and Adiyaman provinces and in areas of concentrated damage in those cities and towns. At the intersections of highways and roads, and surface fault ruptures (those previously detected by the USGS and those that could be determined in the field), the presence and status of highway damage and structural damage were confirmed. A lightweight small drone for structural inspection was used to determine the distribution of building damage around the seismographs and for detailed observation of damaged buildings that escaped collapse. In Antakya, the city with the highest concentration of collapsed buildings, we observed the damage concentration zone, which can be called the earthquake zone, and observed the damage around the seismographs from the sky. We infer a possible basin edge effect expected from the interaction of the basin structure of Antakya city.