## 2 Materials and Methods
### Classification of Seismic Activity  
In seismic countries, there is daily seismic activity, where energy is released over a short or long period. Scientific works are mainly focused on the study of earthquakes that release more energy over a long period [9; 18; 26; 27]. However, this work focused on the study of seismic activity in the range of low to moderate intensity in a short period in the coastal zone of the Biobio region since this energy may be directly related to the risk of deformation  
Figure 1: (**a**) Chile located in South America, (**b**) Biobio region, and (**c**) zoom of the study area and their municipalities. Source: Adapted from Spatial Data Infrastructure Chile ([[https://www.ide.cl/](https://www.ide.cl/)]([https://www.ide.cl/](https://www.ide.cl/)), accessed on 26 December 2021). in the study area. Earthquakes are classified according to their magnitude and intensity. According to the Seismological Service of the University of Chile [50] ([[https://www.csn.uchile.cl/](https://www.csn.uchile.cl/)]([https://www.csn.uchile.cl/](https://www.csn.uchile.cl/)), accessed on 17 January 2022), the magnitude measures the energy released in the hypocenter of the earthquake, which is the place where the friction of the plates or their rupture occurs. This is measured with specialized instruments that, where they are located, will register the same magnitude range. Meanwhile, the intensity is not even one; there are several, and we speak of intensities, which objectively measure the violence with which an earthquake is felt at various points in the affected area. Measuring the intensity indicates objectively the violence with which an earthquake is felt at various points in the affected area. The measurement is made by observing the effects or damage produced by the tremor on buildings, objects, the terrain, and the impact it has on people. Its value depends on the distance from the epicenter, type of construction, quality of soil or rock in the locality, and the place where people live [50]. In this work, the United States Geological Survey (USGS) magnitude classification (Richter) was used, as shown in Table 1. For example, a magnitude 5.3 might be computed for a moderate earthquake, and a minor earthquake might be rated as magnitude 3.9. Because of the logarithmic basis of the scale, each whole-number increase in magnitude represents a ten-fold increase in measured amplitude; as an estimate of energy, each whole-number step in the magnitude scale corresponds to the release of about 31 times more energy than the amount associated with the preceding whole-number value. The seismological records around the study area were downloaded from the Centro Sismologico Nacional website (CSN; [[http://www.sismologia.cl/](http://www.sismologia.cl/)]([http://www.sismologia.cl/](http://www.sismologia.cl/)), accessed on 17 January 2022). The historical recompilation of the daily earthquakes was recorded in the range of 2.0 \(\leq\) M \(\leq\) 5.9 of different magnitude scales covering the time interval from 2006 to 2017. In addition, geological fault data available from SERNAGEOMIN were analyzed ([[https://www.sernageomin.cl/](https://www.sernageomin.cl/)]([https://www.sernageomin.cl/](https://www.sernageomin.cl/)), accessed on 17 January 2022). The density of low- and medium-intensity earthquakes was estimated using a kernel function in geographical information system (GIS) to fit a smoothly conical surface to the location of each earthquake, using magnitude as the weight [51]. In this study, the local magnitude (MI) was also used to analyze the intensity of earthquakes. This is determined using the internal waves (primary P waves and secondary S waves) captured by the seismographs of the CSN stations near the place where the earthquake was generated. The W phase (Mww) is also reported, which is a measure of the size of the earthquake and is related to the amount of energy released in the form of elastic waves (internal or surface waves), as well as long-period waves with periods between 200 and 1000 s [50].