这是一个要我们采取有效措施的紧急事件.
这是一个动词不定式作定语的结构,介系词to是不能省略的,因为to的宾语就是所修饰的那个名词,这种作定语的动词不定式还有：
I have parents to take care of.
I need a knife to cut it with.
也可以把介系词提前,后面用一个关系代词which或whom来引导不定式,如：
It is an urgent thing to which for us to take effective measures.
I need a knife with which to cut it.
希望我能帮助你解疑释惑.