The earthquake hit back in '59 and you have to remember that particular range of mountains also contains Mt Rainier, which was caused to erupt back during the GGD and the range remains an active spot, so this might not have been intentionally caused and just a bit of ongoing rumbling.
If it was done intentionally, I have never come across any credible groups claiming responsibility.
Plus from a game POV, it created a nice little Barrens area now filled with vagrants, ghouls and other oddities making a perfect area for more shadowy business, whether offloading that t-bird or just laying low for a while.
runner havens pg 71
sixth world almanac pg 187