900 buildings were destroyed. The problem was ignorance. The geology of the locality was ignored. The dam was built directly over the San Francisquito Fault. Fault gouge is broken rocks and debris along the fault.
Water can easily seep through the gouge, as it did under the dam.
After two years of water slowly eroding away the gouge, a gap large enough for water to flow through formed. Once this gap formed, water could instantly rush through, as it did at midnight on March 12, 1928. The failure was actually two-fold. First, the water flowing through gap formed by the fault gouge broke off the west side of the dam. Second, a huge whirlpool formed in front of the east side of the dam.