Jennifer Lopez Shakes Her Ass at Benefit Concert for Disaster Relief

Jennifer Lopez is going all out to help raise money for recent hurricane and earthquake victims ... by shaking her booty for a whopping 13 seconds (we counted)!!!

J Lo just kicked off "SOMOS LIVE!" -- a benefit concert airing on VH1 to aid those affected by the hurricanes in Puerto Rico, the Caribbean and South Florida, as well as the earthquakes in Mexico -- and she kicked it off with a bang.

Lopez also took an apparent shot at Trump over his treatment of Puerto Rico, by saying ... "There are some messes you can't clean up with paper towels."

Live performances in L.A. by J Lo, Mary J. Blige, Jamie Foxx and others, and in Miami by Marc Anthony, DJ Khaled and more are being broadcast while celebs like Kim Kardashian, Debra Messing and Ted Danson work the phone for donations.

Earlier in the night, Bruno Mars wowed the crowd by singing -- beautifully, of course -- in Spanish