Damage and Shaking Intensity in the M5.7 Canyondam Earthquake

An M5.7 earthquake occurred southeast of Lake Almanor, CA, at 8:47 PM on May 23, 2013. Double-difference relocations of the main shock and aftershocks indicate that the earthquake nucleated at 11 km depth and ruptured up dip on a fault striking 292° and dipping 70° to the northeast. The earthquake cracked foundations, broke chimneys, and ruptured plumbing around Lake Almanor. We canvassed communities around the lake and to the south and east for earthquake damage, adding reports from our interviews to the geocoded 'Did You Feel It?' reports and to a set of damage reports collected by the Plumas County Office of Emergency Services. Three communities suffered significant damage. In Lake Almanor West, 14 km and 290° from the hypocenter, one wood-frame house was shifted on its foundation, the cripple wall of another house was racked, and water and gas pipes in five houses were ruptured. This damage indicates the shaking approached MMI 8. In Lake Almanor Country Club, 10 km and 310° from the hypocenter, more than 40 chimneys were cracked, broken, or collapsed, a coupling for the municipal water tank was ruptured, and a 200-foot long fissure opened on a slope facing the lake. This damage indicates shaking between MMI 7 and MMI 8, consistent with the accelerograph recording of PGA = 38% g and PGV = 30 cm/s at the Fire Station in Lake Almanor Country Club. This CSMIP station and a PG&E station on the crest of the Butt Valley Dam obtained the only recordings within 50 km of the epicenter. In Hamilton Branch, 10 km and 345° from the hypocenter, a foundation of a wood-frame house was damaged, and 14 chimneys and a water pipe were broken, indicative of MMI 7 shaking. All three communities are underlain by Tertiary and Quaternary basalts. The communities of Chester, Westwood, and Greenville were less damaged, suffering cracked drywall, broken windows, and objects thrown from shelves. The intensities in the three most strongly damaged communities increase as the azimuth from the source approaches the fault strike. This damage pattern could be caused by rupture directivity if the earthquake or the strongest sub-event of the earthquake ruptured up dip and to the west.