Chivas USA struck first when second-half substitute Zemanski fired a shot from 30 yards in the 64th minute that proved too much for Earthquakes goalkeeper Jon Busch.
LaBrocca scored when he found himself alone on net and easily beat Busch in the 84th minute for his fifth goal of the season.
San Jose has yet to win on Chivas USA’s home field since returning to MLS in 2005.
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