This usage seems nonstandard. The apparent meaning is "reduced" as in "seismicity has reduced." When I searched for "seismicity" and "laid down," I only found one text, a reading exercise for English learners.

If you lay down, you lower your body. You may be resting. You may be going to sleep. Also, "lay down your arms" is a common saying, meaning to drop your weapons and surrender in a battle of arms (war with weapons). So if the seismicity has laid down, it has backed down - at least decreased, maybe even stopped. Don't rely on a single dictionary to give you every possible interpretation of a word - much better to check multiple dictionaries via a search engine. Also, don't expect double-barrelled wording to appear in a traditional dictionary. Dictionaries usually focus on singular words.

FYI: This is the news posted in 2018. Title: Lava flow intensifies in Hawaii eruptions, spews 200 feet in air

It makes no sense. "Laid" is the past tense and also the past participle of the verb "to lay". "Lay" means to put or set down. The author of the above passage should have used "lain", not "laid". Every American is confused by the verbs "lie" and "lay". Even people who use the words every single day use them incorrectly. For example, nurses who need to use the words a dozen times every day will say stupid things like "Please lay down here while I take your pulse". Since they are professionals and the words are necessary for their job, you might suppose they would take ten minutes to learn how to use them correctly, but no.

Hi Shana. I’m thinking maybe he’s using it in the context of surrendering?