On the 25th anniversary of the Northridge earthquake, the second temblor in two days has woken up the Bay Area, with the last seismic event a magnitude 3.5 centered in the Oakland-Berkeley Hills.
The latest temblor struck at 6:11 a.m., with an epicenter less than a mile west of the western edge of the Caldecott Tunnel — about 2 miles southeast of the UC Berkeley campus and 4 miles northeast of downtown Oakland.
An earlier quake, a magnitude 3.4, hit a day earlier at 4:42 a.m.
The U.S. Geological Survey reported light shaking, or Intensity Level 4 shaking, on the Modified Mercalli Intensity Scale. Shaking of this type generally is felt indoors by many, and rattle dishes and windows and cause walls to make a cracking sound. It can feel like a heavy truck striking a building, and rock standing cars noticeably.
The East Bay is threatened by the Hayward fault, which has been called a “tectonic time bomb.” A landmark report by the USGS last year estimates that at least 800 people could be killed and 18,000 more injured in a hypothetical magnitude 7 earthquake on the Hayward fault centered below Oakland.
The Hayward fault is so dangerous because it runs through some of the most heavily populated parts of the Bay Area, spanning the length of the East Bay from the San Pablo Bay through Berkeley, Oakland, Hayward, Fremont and into Milpitas.
Out of the region’s population of 7 million, 2 million people live on top of the fault, and that proximity brings potential peril.
The fault has shaped the history of the Bay Area. Old city halls in Hayward and Fremont have been abandoned because they lie on the fault. At Memorial Stadium at UC Berkeley, seating was recently broken up and rebuilt so that the facility’s western half could move 6 feet northwest from the other side. In the hypothetical earthquake scenario, half of Memorial Stadium moves 2 feet northwest during the main earthquake, another foot over the next 24 hours, and yet another foot or so over the next few weeks or months.
The so-called HayWired scenario envisions a scale of disaster not seen in modern California history — 2,500 people needing rescue from collapsed buildings and 22,000 being trapped in elevators. More than 400,000 people could be displaced from their homes, and some East Bay residents may lose access to clean running water for as long as six months.
In some respects, the HayWired scenario would be at least 10 times as bad for the Bay Area as the magnitude 6.9 Loma Prieta earthquake, despite the similar magnitude. The 1989 earthquake is blamed for about 60 deaths and produced $10 billion in damage; the HayWired scenario envisions $82 billion in property damage and direct business losses, with fire following the earthquake potentially adding $30 billion more.
A Hayward fault earthquake could trigger significant aftershocks on other faults for up to half a year after the main shock. In the HayWired scenario, a large aftershock comes nearly six months after the main quake — a magnitude 6.4 close to Cupertino, the home of Apple’s headquarters, followed in close succession by a magnitude 6.2 temblor near Palo Alto, a key city in Silicon Valley, and a 5.4 back in Oakland.
The Hayward fault is one of California’s fastest moving, and on average produces a major earthquake about once every 150 to 160 years, give or take 70 or 80 years. The last major earthquake on the Hayward fault, a magnitude 6.8, had its 150th anniversary on Oct. 21.
In December, Oakland passed a law requiring some of its most vulnerable buildings — so-called “soft-story” apartments with flimsy first stories, often for garages — to be retrofitted. San Francisco, Berkeley and Fremont have similar laws, but many other Bay Area cities in the heart of California’s booming tech region, including Palo Alto and Burlingame, have not acted. Hayward, a city that shares its name with the Hayward fault, also has not passed a mandatory retrofit law for soft-story buildings.
No, these small earthquakes won’t prevent ‘The Big One’
You probably know earthquake weather is bogus and that no warning system will give you time to safely escape a quake, but you may still have questions about how minor quakes work to relieve stress on the San Andreas or Hayward faults.
The truth is that minor quakes do relieve pressure from our tectonic plates, but seismologists do not believe the effect is enough to prevent the larger magnitude earthquakes.
The large earthquakes are needed to release the energy being loaded into the Bay Area by the motion of tectonic plates, explains Andrew Michael, a research geophysicist with the U.S. Geological Society. There are 10 times more earthquakes every time you go down one unit in magnitude, and so lots of little tremors are happening deep below the Earth’s surface.
“But every time you go down one unit in magnitude, each earthquake releases 1/32 as much energy,” explains Michael. “So the total energy released by small earthquakes is less than what is released by the largest events.”
California sits on an active zone where the Pacific tectonic plate is continuously sliding past the North American plate. The San Andreas Fault, which has experienced two magnitude 7.8–7.9 earthquakes in recorded history, is the fastest slipping fault along the plate boundary, but in the Bay Area there are also the Hayward, Rodgers Creek, Calaveras, and San Gregorio and Concord Faults, and the list goes on.
The region has actually seen a period of seismic calm in the past four years and the last truly damaging earthquake struck the Napa area in 2014, but the Earth could come alive at any time.
An earthquake this large is capable of causing widespread damage, and tomorrow, during the national Great ShakeOut event, schools, hospitals, banks and other organizations will be practicing earthquake readiness drills and reviewing preparedness plans throughout California and the world.
This is also a good time to review what we know about earthquakes. To help you do that, check out the gallery above, where we debunk some of the most common myths about earthquakes.