Exploring Why the February 6, 2023, Kahramanmaras Earthquakes Were So Destructive: Further Analysis of the SGM Data and Bidirectional Effects

The sequential earthquakes that occurred on February 6, 2023, particularly the Pazarcik (MW 7.8) and Elbistan/Ekinozu (MW 7.6) events, caused significant structural damage and loss of human lives in the city of Kahramanmaras and nearby areas. This study exemplifies the structural damage patterns observed in RC buildings based on multiple field surveys made by the authors. Particular attention is given to the effects of simultaneous bidirectional seismic loading, characteristics of the recorded strong ground motion (SGM), and the interrelationships among various SGM parameters derived from the available data. Based on the peak ground acceleration (PGA) values, several strong-motion stations were selected, and corresponding pseudoacceleration spectra were generated. These spectra were subsequently compared with the target design spectra outlined in both the current and previous Turkish seismic codes, emphasizing the discrepancies between the observed seismic demands and the expectations set forth by the codes. The analyses show that high Arias and Housner intensity values, along with surpassing PGA design thresholds, indicate a significant and sustained input of energy. Following this, the bidirectional combination coefficients, commonly taken to be 30% or 40% of the orthogonal component added to 100% of the principal direction in many design codes, are revisited. For the selected strong-motion records, pseudoelastic velocity spectra were generated for various damping ratios (from undamped to significantly damped systems), and the corresponding Housner’s spectral intensity values were computed. Intensities derived from both the bidirectional and unidirectional velocity spectra were combined, and the corresponding percentage values for the contributions from the other direction were numerically calculated. The analyses revealed that more than half of the examined stations had computed combination coefficients that exceeded the commonly assumed 30% value. These findings suggest that the current bidirectional combination approach used in the current seismic design codes, particularly for RC buildings, may underestimate actual multidirectional demands. This emphasizes the need to reconsider and potentially revise the treatment of bidirectional effects in seismic design practices. It is shown that bidirectional earthquake effects could have contributed to the widespread damage and collapses observed during the February 6, 2023, Kahramanmaras earthquake sequence.