San Miguel, Baja California Norte, earthquakes of February, 1956: A field report

abstract The San Miguel, Baja California Norte, earthquakes of February 9 and 14, 1956, were felt over most of northern Baja California and parts of southern California. They originated on the San Miguel fault. This newly named fault runs from the southern end of the Sierra Juárez, near the eastern end of the Agua Blanca fault, about N 60° W across the Álamo Plain. At least twelve miles of surface rupture occurred along this line during the February shocks. Cracks were uniformly open and arranged en échelon, stepping to the left. Magnitude of displacement was variable, but the direction was always right-lateral and up to the northeast. Surface evidence indicates that the fault is nearly vertical. A zone of springs, linear hills and valleys, displaced drainage, and closed basins extends beyond the zone of surface rupture for fifty miles northwest to the vicinity of Guadalupe. The main shock of February 9 occurred close to the northwest projection of the surface trace; the two major aftershocks on February 14 were near the southeast projection. This implies progressive faulting from northwest to southeast, similar to the southwest-to-northeast faulting in the Arvin-Tehachapi earthquake. Slides occurred on the steep slopes of the Sierra Juárez. Temporary warm springs were formed a few miles from the fault, and near-by existing springs had variations in their flow. Total property loss was small because there were few objects to be damaged. The six-building community of San Miguel, astride the fault, was severely hit: three adobe houses were destroyed and a frame schoolhouse was shifted from its inadequate foundations. Two brush shacks were relatively undamaged. Elsewhere, adobe and unreinforced concrete block structures suffered varying degrees of damage, most of which has gone unrepaired. The most common form of damage was cracking upward and outward from lintels. The principal damage in and around the Álamo Plain, the area of surface rupture, was apparently from the shock of February 9. Beyond the southeast end of the surface trace, the only reported damage was from the aftershocks of February 14.