HAYWARD, Calif. (KRON) — Predicting where and when the next “big one” will strike the San Francisco Bay Area is a tough task for seismologist. Five major fault lines cross the Bay Area: San Andreas, Calaveras, San Gregorio, Hayward and Rodgers Creek.

A fault is defined by the U.S. Geological Survey as “a fracture or zone of fractures between two blocks of rock. Faults allow the blocks to move relative to each other. This movement may occur rapidly, in the form of an earthquake – or may occur slowly, in the form of creep.”

Without enough creeping, pressure will build up until an earthquake results.

Tuesday’s 5.1 magnitude earthquake originated from the Calaveras Fault and its epicenter was just east of San Jose. Fortunately, San Jose emerged without significant damages nor injuries. Still, the USGS cautions, “we live in earthquake country and we should all be prepared for the next big quake.”

The Bay Area’s major fault lines (Image courtesy USGS)

Which Bay Area fault will rupture with a major earthquake, next?

“The Hayward Fault will rupture violently again, and perhaps soon,” USGS scientists wrote in its 2018 study. “The fault may be ready to produce another magnitude 6.8 to 7.0 earthquake.”

Seismic waves from the Hayward Fault unleashed a powerful earthquake on October 21, 1868. Its magnitude was estimated to be 6.8. Because seismographs had not yet been invented, there are no recordings of the quake.

“The 1868 Hayward Fault earthquake is a reminder of the tremendous power that lurks beneath the Earth’s surface in the San Francisco Bay region,” USGS scientists wrote.

“Strong shaking lasted more than 40 seconds, devastating several East Bay towns. Brick buildings, walls, and chimneys were also shaken down in Oakland, San Francisco, Santa Rosa, and San Jose, and there was serious damage in Napa and Hollister. Numerous witnesses reported seeing the ground move in waves. Aftershocks rattled the Bay Area for weeks. Even though the region was only sparsely populated at the time, the1868 quake killed about 30 people,” USGS scientists wrote.

In 1868, there were only about 24,000 residents living in Alameda County. Today, the county has 2.4 million, making the Hayward Fault the single-most urbanized earthquake fault in the United States.

The 1868 Hayward Fault earthquake caused the second story of the Alameda County Courthouse in San Leandro to collapse (Photo courtesy of the Bancroft Library, University of California)

The Bay Area’s most famous fault is the San Andreas Fault. It was responsible for both the Great 1906 San Francisco Earthquake and the 1989 Loma Prieta Earthquake.

Like the Hayward Fault 1868 quake, Loma Prieta’s 6.9 magnitude also caused the ground to dramatically roll. The quake’s epicenter was located in the Santa Cruz mountains along the San Andreas fault. Fierce shaking lasted for 15 seconds. On October 17, 1989, freeways collapsed and buildings caved in on top of victims. Some residents were so terrified that they would not re-enter their homes for weeks, and slept outside on mattresses or in tents. More than 60 deaths were directly caused by the quake and nearly 4,000 victims were injured.

Buildings collapsed along the Pacific Garden Mall in Santa Cruz in 1989. (Photo by USGS)

The Great 1906 San Francisco Earthquake had a catastrophic magnitude of 7.9. Massive fires ignited in San Francisco.  The earth shook with so much energy from the San Andreas fault that it was felt from Los Angeles to Oregon. “Violent shocks punctuated the strong shaking which lasted some 45 to 60 seconds,” the USGS wrote. Hundreds of victims were killed.

Before Tuesday, the Calaveras Fault’s last notable earthquake happened around this same time of year, October of 2007, when the 5.4 magnitude Alum Rock quake ruptured. Calaveras also generated the 6.2 magnitude Morgan Hill earthquake in 1984.

The Working Group for California Earthquake Probability assigned an 11% probability that the Calaveras Fault would produce a magnitude 6.7 or larger earthquake in the next 30 years.

Why does California have so many earthquakes? The answer can be traced back to 200 million years ago.

The USGS explains, “This region of the United States has been tectonically active since the supercontinent Pangea broke up roughly 200 million years ago, and in large part because it is close to the western boundary of the North American plate. Since the formation of the San Andreas Fault system 25-30 million years ago, the juxtaposition of the Pacific and North American plates has formed many faults in California that accommodate lateral motion between the plates.”