Broadband stochastic simulation of earthquake ground motions with multiple strong phases with an application to the 2023 Kahramanmaraş, Turkey (Türkiye), earthquake

Stochastic ground motion simulation models are often less accurate at lower frequencies than at higher frequencies when fitting recorded data unless supplemented by a deterministic forward directivity velocity pulse model. Moreover, time‐modulated stochastic models, which adjust ground motion amplitudes over time, typically use functions that fail to capture multiple strong‐motion phases. The February 2023 Turkey (Türkiye) earthquake exhibited diverse recordings, including near‐fault and far‐field motions with pulse‐like and non‐pulse‐like characteristics, along with single and multiple strong‐motion phases. To better represent such a diverse set of recordings, this study enhances a fully non‐stationary site‐based stochastic model without combining it with a deterministic model. Improvements include a new band‐pass filter with upper‐ and lower‐frequency limits, which refines the representation of the low‐frequency content. Moreover, a time‐modulating function that can represent energy arrival in multiple strong phases is introduced. The reference model's parameters are identified by fitting to the energy content, zero‐level crossings, and cumulative counts of positive‐minima and negative‐maxima of a target accelerogram. This fitting procedure is modified to address the increased number of parameters. These improvements broaden the reference model's applicability while preserving its simplicity, a key aspect appealing to engineering practitioners. The improved model's applicability is demonstrated by simulating a dataset from the February 2023 Türkiye earthquake, and the accuracy is tested using a pulse‐like Next Generation Attenuation Relationships for Western United States dataset. Validations are performed based on total energy, zero‐level crossings, Fourier amplitude spectrum, elastic response spectra, and peak ground motion parameters. Validations are performed schematically in the time and frequency domains and quantitatively using goodness‐of‐fit scores, various validation‐metrics errors, and inter‐period correlations. Overall, the improved stochastic model can effectively simulate a set of diverse ground motion recordings, including near‐fault pulse‐like records, records with multiple strong phases, and far‐field motions across a broad frequency range.