The moment magnitude (Mw)7.2 mainshock of the Sierra El Mayor-Cucapah earthquake occurred at 3:40 pm local time (22:40:42 universal time coordinated) on April 4, 2010. The epicenter was located in the southern Mexicali Valley at the foot of the Sierra El Mayor and Sierra Cucapah. The seismogenic fault rupture propagated both northwesterly into the mountains and southeasterly toward the Gulf of California for a total surface rupture length of approximately 120 kilometers. Near-field strong motion recordings, with stations located entirely east and north of the fault rupture, measured peak ground accelerations of 0.40 to 0.54 g in the lower Mexicali Valley. North of the international border, peak ground accelerations were recorded at 0.59 g southwest of El Centro, 0.38 to 0.56 g in El Centro, and 0.27 g in Calexico.