Two Earthquakes Strike California on Tuesday Night


On Tuesday, December 26th, two earthquakes struck along the Calaveras Fault. Fortunately, neither generated any reports of casualties. Residents noticed the tremors but did not experience significant disruptions.

An Earthquake Early in The Evening

The first tremor occurred at 7:19 p.m. The 3.1 magnitude earthquake occurred some 5.6 miles northeast of San Martin. The United States Geologic Survey reported the seismic activity occurred approximately 7.5 miles underground.

A Second Quake

A few hours later, at 10:32 p.m. a slightly stronger quake struck a short distance to the north. Registering at a magnitude of 3.9, this second quake hit 5 miles northeast of Alum Rock, in the South Bay Area. It occurred closer to the surface, and the USGS estimates the quake began only 5.5 miles underground.

A Complex Geology

The South Bay Area sits amidst an area of geologic interest to seismologists. The USGS has plotted several fault lines in Central California. The fault lines mark places where moving plates in the Earth's crust adjoin one another. Geologists have discovered different segments of the Calaveras Fault drift at different rates of speed.

Different Rates of Movement Along The Calaveras Fault

The northern portion of the Calaveras Fault moves, or "drifts," only 2 to 3 millimeters every year. It crosses an area of low seismic activity. However, the southern segment of the Calaveras Fault moves considerably faster, at a rate of around 14 millimeters annually. The middle section of the Calaveras Fault moves about 6 millimeters every year north of its intersection with the Hayward Fault, and 15 millimeters annually just south of the intersection.

Earthquakes Along The Calaveras Fault

The region which experienced the two earthquakes on Tuesday night has often witnessed seismic disturbances in the Magnitude 4 range in the past. The disruptions rarely cause significant structural damage to buildings in the area. However, the USGS has calculated an 11% likelihood that sometime during the next three decades the Calaveras Fault will witness an earthquake along the fault with a magnitude of 6.7 or greater. Higher magnitude earthquakes pose a more serious risk to residents.