On the landslide event in 2010 in the Monarch Butterfly Biosphere Reserve, Angangueo, Michoacan, Mexico

In February 2010, 19 fatalities and economic damage were caused by a regional landslide episode in the state of Michoacan, M,xico. The municipalities of Angangueo, Ocampo, Tiquicheo de Nicolas Romero, Tuxpan, and Tuzantla were severely damaged, with Angangueo being the most affected. The event involved a series of debris flows, of which four were the most significant; these four caused 16 deaths in addition to considerable damage to roads, electricity, and the water supply system, with indirect consequences in crop production, cattle farming, and tourism. The area affected by these four flows was calculated as 282 km(2), with an estimated 697,346 m(3) of mobilized material. General observations indicated that the initiation sources of the debris flows were on deforested zones. The present research is concentrated on the Angangueo basin, an area situated within the Monarch Butterfly Biosphere Reserve. Given the lack of rain gauges in the area of interest, records from neighboring points were used to build a comprehensive overview of the extreme precipitation event that triggered the devastating debris flows. The nearest rain gauge, Laguna del Fresno, situated 21 km to the south, recorded 204 mm of rainfall from 1 to 5 February, equivalent to 30% of the mean annual rainfall. Moreover, during a 24-h period the El Bosque rain gauge recorded 144.5 mm of precipitation, the equivalent of 2,270% of the mean rainfall for the same month (6.36 mm). The occurrence of a hailstorm preceding the rainfall event is notable; conditions in the superficial soil layer would have included an increased pore water pressure. Presumably, before the 2,000-year return period extreme rainfall event, thawing of hail and consequent moisture and/or pore-pressure increase result in decreased frictional strength. This paper presents a spatial analysis of the distribution of these landslides, mainly debris flows, as well as general observations on the triggering mechanism, the strength properties of the materials involved, and the societal impact.