A swarm of at least six earthquakes reaching up to magnitude 2.9 rattled San Ramon near San Francisco, the U.S. Geological Survey reports. The other quakes in the Saturday, Dec. 13, swarm ranged from magnitude 1.3 to 2.3, according to the USGS.

Aftershocks have been recorded for both earthquakes. On Dec. 8, Japanese authorities warned an even more powerful earthquake could strike within the next week and called on residents from the northernmost island of Hokkaido down to Chiba prefecture, east of Tokyo, to be on high alert.

The clusters of earthquakes have all been underneath the East Bay suburb of San Ramon, which is close to the Calaveras fault. There were at least 19 earthquakes of magnitude 2 or greater on Monday alone.

California's seismic risk is increasing with supershear earthquakes, which are faster and more destructive than typical earthquakes.

Officials said the megaquake advisory is not a prediction and the probability of a magnitude 8 or larger quake is only about 1%, but there's hope it will serve as a wake-up call.

A nagnitude-5.7 quake hits 67 miles northeast of Kuji, Japan, rattling coastal towns just days after a tsunami warning.

Several earthquakes, the largest measured at magnitude-3.3, were reported Thursday morning in Ventura County. The earthquakes were centered near Fillmore, about 25 miles northeast of Ventura. Shaking was reported in Fillmore, Moorpark, Ventura and Santa Paula.

A minor, 3.1-magnitude earthquake struck in Southern California on Thursday, according to the United States Geological Survey. The temblor happened at 10:27 a.m. Pacific time about 8 miles northwest of Fillmore, Calif., data from the agency shows.