By Lucy Jones
On this date in 1906, the Big One hit northern California: An earthquake ruptured the San Andreas fault from south of San Jose all the way to Cape Mendocino. In San Francisco and Santa Rosa, the majority of their buildings were lost during the quake and the massive fires it triggered.
What made that the Big One? Early in my career as a seismologist, I answered that question quite literally: It was a strike-slip earthquake on the San Andreas fault, the largest we'd expect in California, and the type that happens, on average, every 100 years or so.
Over time and from talking to countless others, however, I have come to a more human-scale understanding. A Big One isn't defined by the initial event, but what happens afterward. What made the San Francisco Earthquake a Big One wasn't the 7.8 magnitude shaking, but that the region was so damaged that it took Bay Area communities decades to recover.
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