HAYWARD, Calif. (KRON) – Predicting where and when the next “big one” will hit the San Francisco Bay Area is a difficult task for seismologists. Five major fault lines traverse the Bay Area: San Andreas, Calaveras, San Gregorio, Hayward, and Rodgers Creek.
A fault is defined by the US Geological Survey as “a fracture or zone of fracture between two blocks of rock.” Errors allow the blocks to move relative to each other. This movement can be rapid in the form of an earthquake – or slow in the form of creep.”
Without sufficient creep, pressure builds up until an earthquake occurs.
Tuesday’s 5.1-magnitude earthquake originated on the Calaveras Fault and had its epicenter east of San Jose. Luckily, San Jose escaped without significant damage or injury. Still, the USGS warns, “We live in an earthquake country and we should all be prepared for the next big quake.”
The main Bay Area fault lines (image courtesy of USGS)
Which Bay Area fault will rupture next with a major earthquake?
“The Hayward Fault will erupt violently again, and perhaps soon,” USGS scientists wrote in their 2018 study. “The fault could be poised to trigger another 6.8 to 7.0 magnitude earthquake.”
Seismic waves from the Hayward Fault triggered a powerful earthquake on October 21, 1868. His strength was estimated at 6.8. Since seismographs were not yet invented, there are no records of the quake.
“The 1868 Hayward Fault earthquake is a reminder of the tremendous force lurking beneath the earth’s surface in the San Francisco Bay Area,” USGS scientists wrote.
“The strong shaking lasted more than 40 seconds and devastated several towns in the East Bay. Brick buildings, walls, and chimneys also collapsed in Oakland, San Francisco, Santa Rosa, and San Jose, and there was severe damage in Napa and Hollister. Numerous witnesses reported that the ground moved in waves. Aftershocks shook the Bay Area for weeks. Although the region was sparsely populated at the time, the 1868 quake killed about 30 people,” USGS scientists wrote.
In 1868, only about 24,000 people lived in Alameda County. Today, the county has a population of 2.4 million, making the Hayward Fault the most urbanized earthquake fault in the United States.
The 1868 Hayward Fault earthquake caused the second story collapse of the Alameda County Courthouse in San Leandro (photo courtesy Bancroft Library, University of California).
The Bay Area’s most famous fault is the San Andreas Fault. It was responsible for both the Great San Francisco earthquake of 1906 and the Loma Prieta earthquake of 1989.
Like the 1868 Hayward Fault tremor, Loma Prieta’s magnitude 6.9 rocked the ground dramatically. The epicenter of the quake was in the Santa Cruz Mountains along the San Andreas Fault. Violent shaking lasted 15 seconds. On October 17, 1989, freeways collapsed and buildings collapsed on victims. Some residents were so scared that they did not return to their homes for weeks, sleeping outside on mattresses or in tents. More than 60 deaths were directly caused by the quake and almost 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 broke out in San Francisco. Earth shook from the San Andreas Fault with so much energy it was felt from Los Angeles to Oregon. “Violent shaking underscored the strong shaking that lasted about 45 to 60 seconds,” the USGS wrote. Hundreds of victims were killed.
Prior to Tuesday, the last notable Calaveras Fault earthquake occurred around the same time of year, in October 2007, when the magnitude 5.4 quake ruptured the alum rock. Calaveras also caused the 1984 magnitude 6.2 Morgan Hill earthquake.
The Working Group for California Earthquake Probability has given an 11% chance that the Calaveras Fault would produce a magnitude 6.7 earthquake or greater within the next 30 years.
Why does California have so many earthquakes? The answer can be traced back to 200 million years ago.
The USGS states: “This region of the United States has been tectonically active since the supercontinent Pangea broke up about 200 million years ago, in large part because it lies close to the western boundary of the North American Plate. Since the formation of the San Andreas fault system 25 to 30 million years ago, the juxtaposition of the Pacific and North American plates in California has created many faults that allow lateral movement between the plates.”
