[civilian]

I'm just shooting from the hip, but: Unless the posthole was actually on a fault line, there's no reason that an earthquake would wipe it out. The ground all moves together. Erosion & flooding can absolutely destroy them. But there's enough places that have been undisturbed since that time period.

When I was in Chengdu I visited Jinsha, which has lots of evidence of postholes: https://en.wikipedia.org/wiki/Jinsha_(archaeological_site) http://www.kaogu.cn/en/Chinese%20Archaeology/3/Discovery%20a... Granted, that's only 3000 years old.

A better example (which I just learned about-- I'm literally trying to google for the oldest post holes found :) ) is Monte Verde: https://en.wikipedia.org/wiki/Monte_Verde

> Radiocarbon dating of bones and charcoal in 1982 gave the site an average age of 14,800 years ago (calibrated), more than 1000 years earlier than the oldest-known site of human habitation in the Americas at that time.[2][8][9]

> In the initial excavation, two large hearths were found and many small ones as well. The remains of local animals were found, in addition to wooden posts from approximately twelve huts.