Pick Our Brain - April 2011

How much larger was the recent Japanese earthquake than the 1906 San Francisco one?

The March 2011 Japanese earthquake off the coast of Honshu attained magnitude 9.0 on the Richter scale, while the 1906 San Francisco earthquake was 7.8. That makes the Japanese earthquake about 60 times bigger than the San Francisco one on a linear scale called the seismic moment.

Could the San Andreas fault produce a giant earthquake, of magnitude 9? Probably not.

According to the experts, to make a giant earthquake it takes a giant fault break in terms of length, width, and slip. For both the Japanese earthquake and the San Francisco one the fault break was long, as much as 300 miles. But the Japanese break was 100 miles wide with 50 feet of slip, while the San Francisco break was just 10 miles wide with only 15 feet of slip.

Northern California has a giant earthquake in its past. This 9.0 earthquake in 1700 which Delta Independent Science Board member Brian Atwater helped discover, occurred on the Cascadia subduction zone, a fault that resembles the Japanese one. The exact date and time of this massive earthquake - January 26, 1700 in the evening - was determined using forensic science. For further information, see Giant Quakes in Pacific Northwest story.

2011 Japanese Earthquake
2011 Japanese Earthquake
(Photo courtesy of AP Photo/Nexco East Japan via Kyodo News)
1906 San Francisco Earthquake
1906 San Francisco Earthquake
(Photo courtesy of USGS Photo Library)