How do you block the car chase in Los Angeles?

You have to make an earthquake, go to the bridge, try to get on it, follow the broken bridge and you should see a car in the side. Grind on the side, over the car and the car will fall off the bridge onto the road where the police cars and the van come along and skid to a halt with the car in the way. The person in the van will then shoot his gun at the police car.

To make the earthquake, you need to slide on four rails around the starting building (they'll say something north, south, etc., so grind all four and the earthquake starts).

Well first you must complete the objective of creating an earthquake

Near where you begin, you'll find some rails that go down stairs. Grind some of these and you'll eventually hit an "Earthquake Rail". You'll get confirmation with something like, "Southern Earthquake Rail." Anyway, there's four total, and they're all in that same block of the city. Just keep hitting rails and you'll get it.