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

Abstract

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 400,000 Cusco’s inhabitants and city’s cultural outreach. Despite the moderate magnitudes recorded in the area, macroseismic data inferred from historical earthquakes (1650, 1950) argue 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 historic city center of Cusco combining Microtremor Horizontal-to-Vertical Spectral Ratio (MHVSR) measurements and Microtremor Array Measurements (MAM). A subsurface wave velocity model and an evaluation of the depth of the engineering bedrock are proposed through joint data inversion. In addition to the characterization the soft sediment thickness, the site response analysis 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 the 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 blind fault detection.