It's a passive construction. The clue is the past participle together with an actual or potential "by" clause. To recast it as active: a) Take what is in the "by" clause and make it the subject -- "severe earthquakes" b) Take the past participle and rewrite it as a finite verb (a) in the same tense as had been used by the verb "be" in the passive version and (b) agreeing with the new subject -- "affect" c) Remember your adverbs -- "never" d) Your old subject becomes the new object -- "my country" e) Put it all together -- "Severe earthquakes never affect my country"

In both sentences, "struck" and "affected" are verbs in the passive voice. "To be" before the past participle forms the passive voice. By the way, it should be "earthquakeS" in the second sentence. The sentences could be rewritten as "Severe earthquakes never affect my country" and "Earthquakes often strike California".

If it's Passive voice how to make Active?