Nicholas George looks under a buckled highway just outside of Napa, Calif. (Josh Edelson/AFP)

The earthquake that rocked California’s wine country on Sunday was followed by moderate aftershock Tuesday morning, as authorities continue to warn that a larger aftershock remains possible.

Sunday’s quake in Napa had a magnitude of 6.0, making it the strongest to hit the Bay Area in a quarter of a century. The 3.9-magnitude aftershock on Tuesday morning, a little more than two days after the Napa quake, produced moderate shaking in the Bay Area, according to the U.S. Geological Survey.

An aftershock registering at 5.0 or stronger could still occur in the next week, even though that is relatively unlikely, the Northern California Seismic System reports. (The USGS warns that the 3.9 shock doesn’t negate the possibility of a larger aftershock to follow, by the way.)

The Napa quake and the ensuing aftershocks were yet another reminder of the dangers earthquakes pose to people in California and beyond. Even so, only one in 10 people in California actually buys earthquake insurance, according to the Insurance Information Institute. There are reasons for this, of course. Such insurance “is not inexpensive,” as Glenn Pomeroy, chief executive of the state’s Earthquake Authority, told KPCC.

Still, despite California’s reputation as earthquake central, it is worth remembering that California is actually the state with the second-highest number of quakes in a given year. Alaska tops that list, according to the USGS. And Alaska was also the site of the strongest quake in U.S. history: The 1964 Great Alaska Earthquake, a monster 9.2-magnitude quake, easily eclipsed the strongest quake in California history; that occurred in 1867 in Fort Tejon, Calif., and had a magnitude of 7.9 (slightly stronger than the deadly 1906 San Francisco quake, which had a 7.8 magnitude). Alaskans noted the 50th anniversary of that disaster earlier this year.

In any event, even though Alaska has more quakes each year, California’s quakes are more damaging because they occur in an area with more people and infrastructure. The Alaskan quakes typically occur in areas where few people live, such as the 8.0-magnitude quake that occurred off the Alaskan coast in June.