插入语是对主句中什么对象的修饰

But detractors maintain that the terraces could also have been created by geological activity, perhaps related to the geologic forces, in which case they have nothing whatever to do with Martian water.

请问在这个句子中，双逗号中间的这个插入语，是修饰了前一句中的terraces 还是修饰了geological activity？

perhaps related to the geologic forces 不是同位语，是形容词短语作非限制性定语修饰geological activity，等于一个非限制性定语从句：which is perhaps related to the geologic forces