[求助] 杨继难句82中的which 到底指代什么？

As rock interface are crossed, the elastic characteristics encountered （by seismic waves）generally change abruptly, which causes part of the energy to be reflected back to the surface, where it is recorded by seismic instruments.

请问，此句中的which到底指代什么？通过后面的动词causes， 我认为which一定代表一个单数名词，但characteristics and seismic waves却都是复数的，所以说which might refer to the all sentence. However, 在GMAT中，我记得which cannot be used to stand for all sentence. so I am really confusing now. 请指点！谢谢！