Seismic microzonation using the Nakamura method for the El Palmar neighbourhood in the department of Santa Ana

Abstract

Introduction: The subsurface consists of a wide variety of geological materials and structures, each with distinct physical and mechanical characteristics-such as color, texture, density, and porosity. The mechanical properties are critically important in engineering because they determine how a given location (i.e., a geographically defined section of soil) will behave during an earthquake-whether seismic waves will be amplified or attenuated. Ground motion isn’t uniform across all sites; it varies depending on the properties mentioned above. The seismic amplification of a site is a phenomenon that can arise from variations in the materials that make up the subsurface. Objective: The study was conducted at each of the measurement points to identify the predominant periods at all 24 sites, and then to create a map displaying these dominant periods—enabling a better characterization of soil behavior at the different locations. Method: The spectral ratio H/V method, also known as Nakamura’s method, is one of the most widely used techniques for detecting changes in subsurface period and amplitude. This approach analyzes ambient seismic noise—small, naturally occurring vibrations generated by wind, ocean waves, and human activity—using seismometers or accelerometers. Results: In Figure 9, periods shorter than 4.5 seconds are shown in blue, while periods between 4.5 and 1 second are shown in green to red. The color red represents the longest periods, which are therefore the areas with the longest frequencies, meaning greater amplitude.