PRELIMINARY REPORT OF THE GEOLOGICAL EFFECTS AND DAMAGE DISTRIBUTION OF THE APRIL 4th, 2010 CUCAPAH-EL MAYOR EARTHQUAKE, MEXICALI VALLEY, BC, MEXICO

Twenty four hours after the Cucapah-El Mayor Earthquake occurs, the Mexicali Valley was inspected. The aim of the reconnaissance work was to obtain direct evidences of ground ruptures in the area of the epicenter as well as to document any other phenomena related to the seismic event. We found a lot of damage in the small villages (e.g., Town Cucapah Mestizo, Ejido Nayarit, Ejido Zacamoto, Ejido Oaxaca) and in the agricultural areas that lie closer to the epicentral zone; this damage is associated primarily to the extensive soil liquefaction, and in second term to the fracturing of the ground. The great amount of water and sand extruded from underground due to liquefaction, inundate several of the already cited small towns. Although this geologic phenomenon took place in the entire region, it is important to note that, its effects decrease to the east of the Nuevo León-Oaxaca-Ledón boundary. On the other hand, in relation to the ground cracking and deformation because of the earthquake shaking, at the field level and in a low-fly, high-resolution aerial photographs taken by INEGI few days after the earthquake, the largest fractures in the area were observed in the agricultural fields, near the drains or irrigation channels, and some of them cutting the pave roads and dirt roads connecting the ejidos. These fractures seem to be in most of the cases purely extensional features, but it is not uncommon to find ground cracks with vertical displacements in the order of tens of centimeters. In general, the fracture network has a NNW-to-NNE orientation. In synthesis, we can say that the damage inventory derived from the April 4^th earthquake includes: zones of inundation, buried zones by liquefied sand ejected, ruptures on the pave and dirt roads, cracking of the hydraulic infrastructure, tilting of power line towers, and partial or total collapse of many houses. Between 25,000 and 35,000 persons were evacuated and are waiting to see where they are going to be relocated.