Preferred Direction of Damage to Concrete Block Fences in the M6.7 Northridge Earthquake of 17 January 1994

The Northridge earthquake toppled three times more concrete block fences that were oriented east-west than those oriented north-south. Toppled fences were twice as likely to fall to the north as to the south. This was discovered during damage causation studies for insurance companies in our survey of more than 200 single-family residences within 29 km of the epicenter. Two hundred nineteen fences were built within 11 degrees of either north (55%) or east (45%), providing an opportunity to study the effect of orientation. Of those fences that were completely toppled, 19 were oriented east-west, while only 7 were oriented north-south. This preferred direction of damage for toppled fences was observed in all four quadrants about the epicenter and at distances of up to 17 km. In the NW quadrant, immediately above the aftershock zone, all toppled fences were oriented E-W, none were oriented N-S. The 58 fences oriented within 11 degrees of either NE or SE had no preferred direction of damage. In general, the failure rate for all 297 fences correlated with Modified Mercalli Intensity, location on alluvium as opposed to bedrock, and lack of structural reinforcement.