2G's From A M6.3 Quake

"If the dart lands right on you":

[T]he highest ground acceleration recorded was greater than 2G, or twice the acceleration of gravity –- which would make Tuesday’s quake among the most powerful in terms of ground-shaking acceleration on record, said Hough. It was strong enough to throw objects in the air.

“There’s only a handful of records of shaking” as strong as 2G, Hough said. “That’s quite extreme shaking.… Some earthquakes just have stronger shaking that affects buildings.”

By contrast, the ground shaking in the 6.7 Northridge quake in 1994 was less, recorded at 1.7G, Hough said. Hough has estimated that the strongest ground shaking in Port-au-Prince in the Haiti earthquake in 2010 was only about 0.5G.

...Hough said Tuesday’s quake offers a lesson in humility to California, showing what can happen when a quake is centered so close to the city center and populated by dense structures, some relatively new, and some historic.

...Another question is how very tall buildings in California would fare in a big earthquake, particularly the long period of booming and shaking, roughly similar to the low tones in music. “We don’t have a lot of observations of how modern tall buildings will perform in really big earthquakes,” Hough said.

There are other buildings in California whose vulnerabilities in quakes are well-known, such as multi-unit condos and apartment buildings with tuck-under parking. They are vulnerable because their first floor lacks a wall that would help keep the building from toppling in a quake, Hough said.