Two Earthquakes Felt in San Francisco

(Photo : Pixabay)

A magnitude 4.7 earthquake shook the Central California area around 12:42 p.m. Tuesday, according to the U.S. Geological Survey. It came hours after a magnitude 4.5 quake hit the San Francisco Bay Area.

The epicenter is located in Tres Pinos, according to the reports. It is about 100 miles south of the site of the previous earthquake, which occurred on Monday evening with an epicenter in Pleasant Hill and Walnut Creek. The earthquake was also felt in the Monterey–Carmel area, Morgan Hill, Santa Cruz, Gilroy, King City, and Salinas areas.

Weak shaking was felt in Oakland, San Francisco, and San Jose during the first quake, according to USGS. A mild shock with a magnitude 2.5 likewise took place 10 minutes earlier near Pleasant Hill. The epicenter of the latest earthquake was located near a rugged mountain range and agricultural areas.

Ann Marie Baltay of the USGS told CBS News that the two earthquakes that happened in the San Francisco area were too far to be related. She said it's a "pretty typical event" for the fault, noting that it was a "very moderate ground motion." There were reports of damage or injuries on the tremor.

The earthquakes took place days before the 30th anniversary of the 1989 Loma Prieta earthquake, which killed 63 people and injured thousands. A magnitude 3.5 earthquake also shook the Bay Area in the town of Colma, located just south of San Francisco, a few months ago.

Why does San Francisco Bay Area get so many earthquakes?

The San Andreas Fault is a continental fault that runs some 750 miles through California. It serves as a tectonic boundary between the North American Plate and Pacific Plate.

There are seven "significant fault zones" in the area, according to the USGS. These are San Andreas, Calaveras, Concord–Green Valley, Greenville, Hayward, Rodgers Creek, and San Gregorio Faults.

California is also included in the Pacific Ring of Fire where the majority of the world's earthquakes and volcanic eruptions occur.

When is the next "Big One"?

The Great San Francisco Earthquake, which happened in 1906, continues to be the most powerful earthquake in Northern California's recorded history. Around 700 people passed away during the earthquake.

The earthquake in San Francisco, which happened in 1989, had a magnitude of 6.9. It caused 63 deaths, with an estimated US$6 billion in damage.

According to the U.S. Geological Survey, the earthquakes similar to the 1906 incident may happen again in about 200 years based on the long-term slip rate of San Andreas fault and the amount of offset that the fault occurred.

The Working Group on California Earthquake Probabilities said there is only about two percent chance that such an earthquake could occur in the next 30 years due to the time needed to gain slips equivalent to a 20-feet offset.

They added that the "real threat" to the Bay Area over the next 30 years would come from magnitude 7 earthquakes from the faults of Rodgers Creek, Peninsula segment of San Andreas fault, or the Hayward fault.