Learn Swedish: Ett vulkaniskt skalv inträffade igår. - A volcanic quake occurred yesterday.

Yes, hände works and is very common. However, inträffade (from the verb inträffa) has a slightly more formal or journalistic tone, often translated as 'occurred.' It is frequently used for specific events, accidents, or natural disasters.

What is the difference between ett skalv and en jordbävning?

En jordbävning specifically means 'an earthquake.' Ett skalv translates closer to 'a quake' or 'a tremor.' While they are often used interchangeably, skalv can refer to smaller tremors or be paired with other words, as seen with ett vulkaniskt skalv (a volcanic quake) here.

Does vulkaniskt end in -t here because it is an adverb describing how the quake occurred?

No, it is an adjective modifying the noun ett skalv. Since skalv is an ett word, the adjective must take the neuter -t agreement ending. While adverbs in Swedish also end in -t, this word is functioning purely as an adjective in this sentence.

How would I use varför to add a consequence, such as '...which is why we left'?