The 1906 SF Earthquake

Recently, I’ve read two statements in the newspaper presented as fact:

  1. The 1906 San Francisco Earthquake measured 7.9 on the Richter Scale.

  2. The Richter Scale was invented in 1935.

My response: Huh?

I’m assuming that it is statement #1 that is in error, and that it should read “…estimated at 7.9 on the Richter Scale.”

How would anybody make that estimate? Do they go by eyewitness accounts? Geological traces? How can 7.9 be anything but a wild guess?
(PS-If you got a couple extra bucks you might want to get in touch with a local mosque or the Red Cross. The latest est. death toll in Turkey is now in the range of 40,000!)

I don’t know the whole answer but one indication of the strength of an earthquake is the amount of damage it does. At a certain intensity cans fall off store shelves, at another your walls crack, at another brick buildings collapse, etc. I’m assuming they back-calculated from the damage done.

“non sunt multiplicanda entia praeter necessitatem”

A WAG, but maybe they were able to get a fairly accurate estimate based on how far away from the epicenter the effects were felt.

Earthquakes are judged by intensity and magnitude. Intensity is based upon the observable effects of the quake to buildings, people, etc. The Mercalli scale measures earthquake intensity. Magnitude is a measurement of the seismic energy released at the center (or thereabouts) of the quake. This is what Richters’ scale measures.

Although Richter invented his scale in 1935, seismic activity had been measured long before that. In fact, scientist John Winthrop (who is called the father of seismology, btw) began studying seismic activity in the late 1700’s.

I have seen estimates of the SF quake between 7.6 and 7.9 which leads me to believe these numbers are based upon translation of cruder methods of measurement to the Richter scale.

Simply put you are correct. They should have said “estimated.” I have seen the SF earthquake quotes go from 7.5 to 8.9.

Damage is an OK way to estimate but it must be done correctly and researched thouroughly. Often a smaller earthquake, in a third world city, where buildings aren’t up to snuff, will look worse than a larger earthquake in a city like SF where building codes are tougher.

I just wrote a mailbag answer about earthquake aftershocks. Not sure when it will appear on the site, though.

It surprised me a little to find that the 1906 earthquake was recorded at 96 seismograph stations around the world. Its surface expression was studied extensively at the time. So there is plenty of data available to retroactively apply the Richter Scale. An informative site:

The Richter Scale estimates of the 1906 Quake (San Franciscans prefer you call it “The Fire”) are just that estimates.

A better measure for that quake would be the Mercalli Scale, which measures earthquakes in terms of damage. It looks at the end result and not just the power of the earthquake. The highest mark (which I don’t remember know) is “complete and total devastation.” I think that may be XII on that scale.

You may live in an area prone to seismic activity, but some people live in areas where damage is minimized and in other places it is maximized (usually because of soil or building conditions.)

I’m sure some geologist can explain this better.

After the Loma Prieta quake in 1989, someone wrote that it was an aftershock of the 1906 quake!
In this case, later was * not * better than never!! :frowning:

Well im thinking they guessed the magnitude from the reports of damage. For instance, here in the Monterey area, we even had damage from the quake in 1906. Some chimneys from the old Del Monte Hotel collapsed,and a wall collapsed in Salinas. We are about 100 miles south of San Francisco, so that quake had to be a very strong quake to cause damage here.