In Lima, as in many cities, most residential buildings grow vertically over their lifespan due to incremental construction. Multiple stories are added to buildings, often without structural retrofits or even without design. As a result, the seismic vulnerability of these buildings increases, requiring the government to create resilience policies that can keep up and bend the seismic risk growth. Unfortunately, little is known about the rate of vertical expansions and risk growth to develop regional risk policies for risk reduction. To address this gap, we surveyed 1311 buildings across four districts of Lima, comparing Google Street View imagery from 2013 to 2014 with field surveys from 2023. Using these data, we calibrated transition probability matrices within a Markov chain model to represent vertical growth dynamics. Results show that vertical expansion is faster in areas adjacent to primary roads compared to secondary ones. Applying the model to three recently established settlements, we projected future vertical growth and assessed its seismic implications. Under a simulated Mw 8.8 earthquake scenario, the probability of building collapse increases from less than 10% to over 30% within three decades. These findings show that vertical growth amplifies seismic risk in informal urban areas. Thus, our study highlights the importance of integrating vertical expansion into seismic risk assessments and policy discussions.
Espinoza et al. (2026) studied this question.