Local Site Effects Evaluation Using Geophysical Methods

Introduction

SDG 11 (Sustainable Cities and Communities) emphasizes considering natural events to create safe and sustainable cities (SDG, 2023). To build sustainable, safe, and durable cities against the effects of earthquakes, which are the most important natural events, the structural and physical properties of the underground should be determined with macro and micro-zoning. This book chapter emphasizes the importance and necessity of macro and micro-zoning studies. Different damages are observed in areas affected by earthquakes in many parts of the world (Oluwafemi et al., 2018; Wada et al., 2018; Koren and Rus, 2021; Zarski et al., 2022). The most critical factors in earthquakes that cause damage to structures are the factors such as the magnitude, mechanism, location, and duration of the earthquake, as well as the physical and structural properties of the underground where quake waves radiate and the regional behavior of that environment. Some situations, such as the transmission characteristics of these environments, their dynamic elastic properties, the absorption and expansion of earthquake vibrations, the focusing effect of earthquake waves originating from the base topography, the channeling of earthquake waves into a region as a result of successive reflections in low-velocity layers constitute the regional characteristics of the environment. These features significantly affect the ground structure near the surface and its engineering structures. Therefore, it is essential first to reveal the deep underground system. The structural condition of deep underground structures can be determined by applying many geophysical methods (Aki and Richard, 1980; Alverez, 1990; Ulugergerli et al., 2007; Pamukçu et al., 2014; Chaljub et al., 2021).

The duration and intensity of elastic wave vibration at a particular location depend on many factors, such as the space from the quake epicenter, the earthquake’s magnitude, and the soil properties of that location (Bullen and Bolt, 1985; Cilsalar and Temiz, 2020). Although seismic waves emanating from the source travel many kilometers within the crust, forming bedrock until they reach the soil layers, their distance within the soil layers is usually less than 100 m. However, soil layers are indispensable in deciding the features of the observed motion on the earth. Soil layers are like a filter for seismic waves. While seismic waves at some frequencies are damped, others are amplified. This situation is revealed by the geophysical methods realized in the field and the determination of the characteristics of the artificially produced seismic wave frequencies in the ground.

While earthquake waves propagate, a seismic impedance difference occurs between the rock and soil. This status causes the amplitude of earthquake waves to increase in the soil near the surface. Aki and Richards (1980) also stated that seismic impedance refers to the medium's resistance against particle movement. From this point of view, the importance of the seismic velocity values, the dominant periods of the bedrock, and the upper layers in soil amplification are revealed. Loose, porous ground conditions amplify the seismic wave amplitude. Therefore, loose and abundantly porous units are shaken more than tight and hard rocks. Geophysical engineers and international organizations (Regulations such as NEHRP, EuroCode, TBDY) effectively use shear wave velocities (Vs) to define the soil’s tightness or looseness. Soils with small Vs magnify the earthquake shaking and cause greater damage.

Existing settled cities cannot show sufficient resistance against the effects of natural events such as earthquakes. This situation is seen especially in undeveloped and developing countries. Large earthquakes such as the 6-7 February 2023 Kahramanmaraş (Türkiye) with Mw=7.8 and 7.5 resulted in the death of tens of thousands of people in collapsed cities, injuries of tens of thousands of people, and economic losses of billions of dollars. These results arise from poor construction and the wrong selection of areas where cities are settled. Even today, when choosing residential areas, micro zoning is often considered while macro zoning is ignored. This book chapter emphasizes the need for macro zoning before micro zoning.