Walter Salazar

By Walter Salazar

This book chapter explains the fundamental concepts of the probabilistic seismic hazard and site effect evaluation. It is divided into four parts: firstly, the theoretical background of the probabilistic seismic hazard methods is explained to compute the earthquake loads used in structural analysis of buildings, namely, the rigid-zone method, the free-zone methods, and the characteristic models. We emphasize the physical meaning of the seismic coefficient prescribed in the seismic code regulations and its association with the return period of ground motion and spectral ordinates. The interconnection of the return period, the recurrence interval, and the lifetime concepts are explained to clarify misconceptions among these terms in connection with the probability of exceedance of motion. Secondly, the seismic hazard methods are applied employing volcanic chain seismicity data, and preliminary seismic hazard maps for rock site are presented for flat topography conditions along El Salvador. Thirdly, the site effects in terms of the amplification of ground motion are studied using soil profiles characterized by the interbedding of lava flows and volcanic ashes. Finally, we present a summary that highlights the most important concepts explained in this book chapter.

Part of the book: Earthquakes

By Walter Salazar, Garth Mannette, Kafele Reddock and Clevon Ash

This book chapter explains the methodology to determine the shear wave velocity VS profile employing microtremors array data at Port of Spain, Trinidad, and its implication in the seismic amplification and liquefaction hazard in the city. We divide this study into five sections; firstly, we introduce a description of the spectral autocorrelation method and the genetic algorithm schemes to retrieve the Vs and thickness of soil layers. Secondly, we validate the soil profiles via inspection of the ellipticity pattern at such sites; we also compared the observed horizontal-to-vertical spectral ratios (H/V) with the synthetic ones derived by the Diffuse Field Approach and 1D theoretical SH wave amplification functions. Thirdly, we compute the shear wave velocity in the first 30 m obtained from our genetic inversion and compared with the ones estimated by the empirical formulas based on geomorphological conditions. Fourthly, we present a preliminary liquefaction hazard map based on the level of H/V microtremor ratios and the fundamental period of vibration. Finally, we conclude with further recommendations for planning purposes in the city of Port of Spain.

Part of the book: Natural Hazards

By Walter Salazar

This chapter aims to provide a methodology to test the probability distributions of earthquakes in terms of the interoccurrence times (ITs), namely, the time between consecutive earthquakes of a specific magnitude. First, we compile a new earthquake catalog for the El Salvador subduction zone within moment magnitude M 5.0–8.12 comprising historical and instrumental data for 1609–2019. Secondly, we explain the fundamentals of the Weibull and Poisson distributions and verify the IT probability fits when considering the clustered catalog. We find that the Weibull distribution fits all ITs, while the Poisson distribution fails to explain the natural seismicity patterns for small magnitude bins. Besides, we test the assumption that the declustering process leads to a Poisson probability distribution when removing foreshocks and aftershocks in the earthquake catalog. Finally, the classical Gutenberg–Richter relationship and conditional magnitude probabilities are calculated as an essential input in any seismic hazard assessment.

Part of the book: Earthquakes