Two devastating earthquakes struck El Salvador within a month. The first quake of January 13, 2001, which was centered off El Salvador's southern coast, damaged or destroyed nearly 108,000 houses and killed at least 944 people. A considerable amount of soil (∼200,000 m³) was fluidized on a mountain ridge rising south behind the Las Colinas area of Nueva San Salvador (Santa Tecla). The average slope was at most ∼13°, and yet the fluidized soil flowed ∼400 m across the residential area, destroying many houses and killing more than 700 people. This report outlines the findings obtained through reconnaissance by a mission dispatched by the Japan Society of Civil Engineers and the laboratory tests that followed it.