I’ve been awakened by an earthquake, but it was only M 4. I can’t imagine what it would have been like to be in San Francisco 103 years ago this morning, when a M 8-ish earthquake struck at 5:12 am. Shaking ten times stronger than the Loma Prieta earthquake – I couldn’t stand up in M 7 quake; would a M 8 knock a person out of bed?
And it was one thing to experience a M 7 earthquake in the daytime, as a geologist who knew just how nearby the Pacific Plate was, and how fast it moved, and what it could do. Did people in 1906 remember the Hayward quake of 1868, nearly 40 years before? Or would the foreshock have been the only warning, a rumbling bring people out of dreams and into a nightmare?
Image source: USGS photographic library. Photo by G.K. Gilbert, from USGS Bulletin 324.
During the earthquake, the San Andreas Fault broke from south of San Jose to the Mendocino triple junction, 296 miles to the north. The USGS estimates that offset varied from around 8 feet along the southern part of the fault to as much as 28 feet near the north end. Today, the actual rupture is hard to pick out – when I led Stanford intro geology students to see the fault in the early 90’s, I had to wave my arms vaguely and try to convince the students that somewhere in the poison oak was something that would have been really impressive in 1906. Point Reyes National Seashore has a trail with a re-created fence showing the offset. (It’s fun to stand with one foot on the North American plate and one on the Pacific plate, and try to imagine the movement.)
It’s fun to imagine the slip. But I wouldn’t want to be there the next time it goes.