Characterizing the seismic response and basin structure of Cusco (Peru): implications for the seismic hazard assessment of a World Heritage Site

Known worldwide for its rich and well-preserved pre-Columbian and Spanish architecture, the city of Cusco (Peru) is listed as a World Heritage Site since 1983. However, less well known is the seismic hazard, which represents a major threat to the city’s 400,000 inhabitants and its cultural outreach. Despite the moderate magnitudes recorded in the area, macroseismic data inferred from historical earthquakes (1650, 1950) argues for strong amplification effects of the unconsolidated sediments of the Cusco Basin during ground motion. In order to address this aggravating factor for the first time, we conducted a large-scale passive geophysical survey in the historical city center of Cusco, combining Microtremor Horizontal-to-Vertical Spectral Ratio (MHVSR) measurements and Microtremor Array Measurements (MAM). Through joint data inversion, we proposed a subsurface wave velocity model and estimated the depth of the engineering bedrock. The site response analysis not only provides an insight into the thickness of the soft sediment, but also suggests the existence of a strong geological discontinuity beneath the city center of Cusco, consistent with the trace of the Cusco fault. Moreover, the results highlight the complexity of earthquake site amplification assessment in dense urban areas. Our work paves the way for a comprehensive seismic microzonation of the entire Cusco Basin and opens up new perspectives on the potential of the MHVSR method for fault detection.