The travel velocity of these two wave types is not the same (P-waves are faster than S-waves). Thus, if there is an earthquake somewhere, the first waves that arrive are P-waves. In essence, the gap in P-wave and S-wave arrival gives a first estimate of the distance to the earthquake.
With the mysterious U.S. Senator Eddie Morra (Bradley Cooper) providing him with a second drug to counteract NZT's deadly side effects, Brian uses his enhanced abilities to help FBI agent Rebecca Harris (Jennifer Carpenter).
Looking for more NZT, Brian tracks down one of Eli's coworkers and gets shot in the leg by the real murderer. He passes out, and awakes to find himself with Senator Eddie Morra, who offers him access to both NZT and a drug to counter its dangerous side effects, on the condition that he not tell the FBI.