I am wondering whether the span falling is related to lateral spreading of the river bank (on the far right side of the photo). You can see that the truss has moved to the north, pushing the next span off of it's support. However, it could just as well have been caused by ground shaking. Also, it's hard to tell, but the pier just north of the dropped truss looks like the legs have moved apart. Since I don't see a cap at the top of the columns, I think it's possible. One thing is for sure, the depth of the truss stopped the span from falling to the ground. It looks like it got wedged against the adjacent protruding truss. If this had been a girder bridge, it would have fallen as soon as it was pushed off the pier.
Chilean Bridges: El Puente Ferroviario de Longaví by Mark Yashinsky is licensed under a Creative Commons Attribution-No Derivative Works 3.0 United States License.