(http://www.cdc.gov/mmwr/preview/mmwrhtml/00001498.htm)
Wikipedia doesn't show many > 7 earthquakes. (https://en.wikipedia.org/wiki/Earthquakes_in_California#List...) (but maybe they have a weird inclusion criteria?)
Consider the 9.0 that hit japan in 2011. Despite 129,225 buildings totally collapsed, with a further 254,204 buildings 'half collapsed', and another 691,766 buildings partially damaged it killed under 20,000 people.
That said, we have invested a lot to drive the death totals down in large part because earthquakes and hurricanes are fairly common. In comparison we are spending far to much on terrorism and far to little on the rare but civilization ending mega disasters.
This stat is crazy. A low-end eruption (by far the most typical kind) at Yellowstone would kill few people in the U.S. The eruptions aren't that big and the Yellowstone downwind area is one of the least populated areas of the country.
PS: A great way of comparing threats is to multiply risk times death totals over a decade. On that scale terrorists and eathquakes are a joke.