Brief communication: Radar images for monitoring informal urban settlements in vulnerable zones in Lima, Peru

Lima city, Peru’s capital, has about 9.6 million inhabitants and keeps attracting more residents searching for a better life. Many citizens, without access to housing subsidies, live in informal housing and shack settlements. A typical social phenomenon in Lima is the sudden illegal occupation of areas for urban settlements. When such areas are unsafe against natural hazards, it is important to relocate such a population to avoid significant future losses. In this communication, we present an application of Sentinel-1 SAR images to map the extension of a recent occupation of an area with unfavorable soil conditions 5 against earthquakes.

Furthermore, Lomo de Corvina is also classified as Zone IV due to the existence of deep eolian sand deposits and the largest fundamental vibration period found in Lima city, which is slightly larger than 1 s (CISMID, 2011). Recent studies have shown 55 that Lomo de Corvina might evidence important values of amplification factors due to the generation and interference of surface waves along the slope (Gonzales et al., 2019). It is worth mentioning that the collapse of the light makeshifts built during the recent invasions may not have represented an effective danger condition to the inhabitants. However, non-engineering masonry houses could have been constructed in the short term if the inhabitants were not removed.

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This section shows how the informal settlements were observed from satellite imagery. Unfortunately, the areas of interest were cloud-covered in most of the days after the invasion. From the beginning of April 2021 until May 5, only one product from the  Sentinel-2 constellation recorded on April 20 was cloud-free. On the other hand, synthetic aperture radar (SAR) images are designed to pass through the clouds; thus, we focus on SAR images from the Sentinel-1 constellation. Figure 1d shows a color composite of backscattered intensity recorded on different dates. The red band denotes an image recorded on December 10, 65 2020, and the green and blue band denote the image recorded on April 14, 2021. Red tones are observed in the new informal settlements, which express an increment of the backscattering in time. Such increment is originated from the double bounce scattering mechanism produced by the shelters . Figures 2a and 2b depict a scheme of the backscattering mechanism under two scenarios. When the radar pulses emitted by the satellite reach a flat area, only a small fraction of the energy is backscattered to the sensor (Figure 2a). In the presence of an object (Figure 2b), the radar pulses bounces-off the ground towards the object, and then it is reflected from the object to the sensor. Therefore, a larger fraction of the energy is backscattered to the sensor.
In the case of the new informal settlements, such objects are indeed the makeshift shelters. We exploit this pattern to map the extent of the occupied area. Figure 2c shows the time series backscattered intensity at some representative points of the recently occupied areas (top) and existing built-up areas (bottom). As expected, a clear increment is observed in the invaded area from April 14. 2021. Therefore, to map the recently occupied areas, a threshold for each pixel coordinate is computed.

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The threshold is set as the average plus two times the standard deviation of the time series backscattering intensity until April 2, 2021. Then, if the backscatter intensity recorded on April 14, 2021, is greater than the threshold, it is assumed that the area is occupied. Figure 3 depicts the extent of the invaded areas in Morro Solar and Lomo de Corvina.
The results show that a recent squatter settlement could be identified from radar images. Considering the open access to Sentinel-1 imagery, it represents an opportunity to implement a sustainable, low-cost system to monitor informal urban growth 80 over unsafe areas and perform hazard mitigation actions, such as relocation. Prompt measures can prevent significant losses to society. For instance, the mapped settlements are 438,000 m 2 and 265,300 m 2 in Morro Solar and Lomo de Corvina, respectively. Note from Figure 3b that the computed area in Lomo de Corvina is underestimated because of the layover effect of SAR images on steep slopes. From a visual inspection of the urban settlements nearby the recently occupied areas, we estimated a total of 3051 houses in Morro Solar and 1595 in Lomo de Corvina that would have been exposed to strong ground 85 motion amplification and potential landslides during earthquakes.

Conclusions
Informal urban growth is a recurrent problem in Peru that has increased in recent years. When the settlement is located in a hazardous area, an assessment of the extent of the occupation can be valuable to perform a proper relocation. In this study, we analyzed a recent informal settlement in two areas in Lima, the capital of Peru, through SAR images of 10 m pixel resolu-90 tion. A time-series analysis was performed to identify increments of the backscattering intensity at the areas occupied by the settlements. An increment in the SAR intensity is observed from April 14, 2021, which is consistent with local media information regarding the dates of the invasion. We identified 438,000 m 2 occupied in the Morro Solar, and 265,300 m 2 at Lomo de Corvina. One limitation is that the area identified at Lomo de Corvina is underestimated because of the significant layover effect in steep slopes. A future extension of this work will implement an automated monitoring system of informal settlements 95 in unsafe areas against natural hazards. The monitoring system, constrained by the acquisition dates of Sentinel-1 images, can identify settlements within few days. However, through a potential integration of other satellite constellations, a near-real time monitoring system can be achieved.
Author contributions. LM, FG, CG, and MD conceived and designed the study. All authors contributed in the analysis of the data, discussion of the results, and preparation of the manuscript.