Ground Motion Models Considering Diverse Ground Motion Intensity Measure Parameters for the Peruvian Subduction Zone

Abstract

Se desarrolla una nueva ecuación de predicción del movimiento del suelo (GMPE) para eventos interface para componentes horizontales y verticales para la aceleración máxima del suelo (PGA), las aceleraciones espectrales (𝑆ₐ), el período medio (𝑇ₘ), la relación 𝑃𝐺𝐴⁄𝑃𝐺𝑉, y la relación 𝑃𝐺𝐴⁄(𝑃𝐺𝑉𝑓ₘ). Para ello, se seleccionaron 453 eventos interface con 3750 registros de movimiento del suelo de 533 estaciones de seis redes acelerométricas diferentes de Perú, Chile y Ecuador. Los parámetros de intensidad (IMs) antes mencionados se sometieron a regresión utilizando el método de máxima verosimilitud de dos pasos aplicable a un rango de 4.0 a 8.5 𝑀w, y hasta distancias de ruptura (𝑅ᵣᵤₚ) de 1000 km. El nuevo GMPE para PGA y 𝑆ₐ provee un mejor ajuste con la data que comunes GMPE extranjeros, que tienden a subestimar los movimientos del suelo en 𝑀w superiores a 7.0 en un rango específico de 𝑅ᵣᵤₚ. Además, se sugiere una nueva clasificación de suelos y coeficiente de sitio que son bastante similares al código sísmico chileno (NCh 433). Además, el ratio 𝑃𝐺𝐴⁄𝑃𝐺𝑉𝑓ₘ es un parámetro de relación innovador propuesto en este estudio que tiene menor incertidumbre en comparación con los otros IMs. Además, este nuevo IM puede contribuir a generar aceleraciones sintéticas. Finalmente, la relación PGA vertical a horizontal de 2/3 sugerida en el código sísmico peruano (E030) puede sobreestimar la PGA para movimientos verticales del suelo en escenarios críticos.

This dissertation is written for the purpose of developing a Ground-Motion Prediction Equation (GMPE) for the Peruvian subduction zone, which relies on foreign GMPEs for Probabilistic Seismic Hazard Assessment (PSHA) from which is base the current Peruvian seismic code (E030). The proposed GMPE model not only predict common Intensity Measures (IM) for both horizontal and vertical components like peak ground acceleration (PGA) and Spectral accelerations (𝑆ₐ) between 0.2 and 5.0s, but also covers other IMs such as the mean period (𝑇ₘ), the 𝑃𝐺𝐴⁄𝑃𝐺𝑉 ratio, the 𝑃𝐺𝐴⁄𝑃𝐺𝑉𝑇ₘ ratio and the 𝑃𝐺𝐴⁄(𝑃𝐺𝑉𝑓ₘ) ratio. The latter two parameter relates PGA, peak ground velocity (PGV) and mean frequency (𝑓ₘ), which is the inverse of the 𝑇ₘ. To do so, a complied dataset of 2803 events were collected mainly from Peru, Chile and Ecuador between 4.0 and 8.5 𝑀w from 1951 to 2020. The current Slab2 three-dimensional geometry of the South America subduction zone as well as other 18 sections derived from different studies were used to define the Peru, Ecuador and Chile subduction zone. This enabled to identify interface, inslab and crustal earthquakes, which represent the 45%, 33% and 22% considering the entire dataset, respectively. So, only 453 interface events were selected with 3750 ground motion records from 533 stations, which comprise six different networks from Peru, Chile and Ecuador. The aforementioned IMs were used as regressor parameters for both horizontal and vertical components to derive GMPEs applicable to a range of 4.0 to 8.5 𝑀w, and a range up to rupture distances (𝑅ᵣᵤₚ) of 1000km by assessing the two-step maximum likelihood. The new GMPE for PGA and 𝑆ₐ performs better than commonly used foreign GMPEs, which tend to underestimate ground motions at 𝑀w higher than 7.0 at a specific 𝑅ᵣᵤₚ range. This implies the E030 is currently providing lower PGA at lower 𝑅ᵣᵤₚ distances for interface events which exclusively harbours the strongest events higher than 8.0 𝑀w for the entire dataset used in this study. Also, a new soil classification and site coefficient are suggested which are quite similar to the Chilean Seismic code (NCh 433). These proposed site coefficients do not underestimate PGA values for soil classes 𝑆₀, 𝑆₂ and 𝑆₃ as the Peruvian Seismic code (E030) does for soils under inelastic behavior caused by strong earthquakes. Besides, the 𝑇ₘ, the 𝑃𝐺𝐴⁄𝑃𝐺𝑉 ratio, and the 𝑃𝐺𝐴⁄𝑃𝐺𝑉𝑓ₘ provide less residuals compared to PGA, 𝑆ₐ, and the 𝑃𝐺𝐴⁄𝑃𝐺𝑉𝑇ₘ ratio. Meanwhile, the 𝑃𝐺𝐴⁄𝑃𝐺𝑉𝑓ₘ is an innovate ratio parameter proposed in this study that has the lowest uncertainty compared to the other IMs. Also, this new IM may contribute to generate reliable synthetic motions with the same proxy applicability range as previous stated when a strong dataset is not available. In addition, the 𝑃𝐺𝐴⁄𝑃𝐺𝑉 ratio indicates that ground motions (GMs) can be classified as high and intermediate range for the South America subduction zone. Besides, the 𝑇ₘ variations indicate that both structures and soils with fundamental periods between 0.1 and 0.70s are more vulnerable to resonate to critical scenarios for both horizontal and vertical motions. Finally, the vertical to horizontal PGA ratio of 2/3 suggested in the Peruvian Seismic code (E030) may overestimate the PGA for vertical ground motions. This study found that this ratio depends strongly on moment magnitude, rupture distances and shear shave velocity (𝑉ₛ₃₀) and for critical scenarios this ratio may be up to 0.55.