Turns out that this effect owes everything to the sick bass line of '70s pop supergroup Earth, Wind and Fire. The story goes like this: Spielberg was driving along in his car one day, grooving to a tasty jam, when he noticed that his rear view mirror was vibrating with the bass. Because Spielberg is Spielberg, he thought, "Hey, this would be a great way to let people know a big-ass monster is coming." Blasting "Shining Star" at full volume next to a glass of water wouldn't cut it, though -- they tried that. In fact, hey tried various methods and talked to different sound experts, but nothing could get the perfectly circular ripples Spielberg was looking for. Even Kubrick would have said, "Bro, it's just a water cup. Let it go."
Or, alternatively, terrorized the water until it reacted accordingly.
Michael Lantieri, head of special dinosaur effects, eventually noodled around on his guitar until he figured out the precise note necessary to make the water ripple. That's right: While actors were doing their best to look scared beyond all rational thought in the heart-pounding T-Rex scene, there was a guy lying under the jeep, strumming a guitar string over and over. That's acting.