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Host, Dana King: What are the underlying processes of the Earth?

Bill Ellsworth: The Earthquake Commission found that the San Andreas Fault

shifted along this length of more than 200 miles. How is that possible? How

could the Earth have shifted suddenly over such a great distance? There had been

no earthquake that had been seen like this before. And it was a question that

confounded scientists until the development of Plate Tectonics Theory in the 1960’s.

David Schwartz: Plate Tectonics is the overall model of how different parts of the

Earth’s crust are moving relative to each other. And the crust of the Earth is

broken up into major plates they slide past each other and they move on faults.

The San Andreas Fault is the major fault zone helping slide the North American

Plate past the Pacific Plate. And this movement occurs at about 2 and 1/2 inches

a year. It doesn’t sound like very much but over geologic time it builds up into a

lot of slip that has to be accommodated.

Host, Dana King: These great strides in understanding earthquake process track

directly back to the 1908 Lawson Report.

Bill Ellsworth: Perhaps one of the most important results that came out of this

study was the development of Harry Fielding Reid’s Theory of Elastic Rebound.

He used the information collected from land surveys that allowed him to measure

the changes in surface that had been caused by the earthquake. This was

something that had been understood in rough form before the earthquake but he

developed the first scientific theory about how forces accumulate along faults

only to be released in earthquakes. We call this “The Elastic Rebound Theory”

and it’s the foundation really of our modern understanding of the earthquake process.

Ross Stein: If you can think of the crust as a slab of rubber and you’re grabbing the

rubber from the two sides and moving it past each other if the fault itself were

Teflon the rubber would just move by and never be distorted. But, because the

fault has friction then the rubber gets distorted and stresses are built up on that

surface. So, that process is very very slow. An inch a year, and suddenly at the

time of the earthquake a very small piece of that highly fractioned surface is going

to let go and as it does so it starts slipping. And in the space of a few seconds that

slippage speeds up from an inch a year to 5000 miles an hour. And that process

then tears down the fault at these high speeds until it comes to a stuck patch

where it can hang there for several seconds. If it’s going to be a large earthquake

it’s going to burst through, speed up again to 5000 miles an hour and go flying


into the next knot in the piece of wood if you like. Hang there and then burst through again.

David Schwartz: In parts of California we just really have the San Andreas Fault.

It’s the major feature. But, in places like the Bay area, the fault splays; it’s like the

trunk of a tree. Comes into the Bay area, big branches come out; these are the

major faults like the San Gregorio and the San Andreas and the Hayward and

Rodgers Creek. The Calaveras, the Concord-Green Valley. And they accommodate

most of the movement and then even little faults come off of them and they’re

like smaller twigs. Each different sized fault produces different sized earthquakes

and here in the Bay area we have many faults of different sizes spread out across the entire region.

Mary Lou Zoback: Each of those faults could produce a damaging earthquake, in

fact taken together; we think a damaging earthquake is nearly twice as likely to

happen as not over the next 30 years.

Host, Dana King: Earthquake information for the Bay area, California and

worldwide QUAKE dot USGS dot GOV. Menu items link to things like “Did-You-

Feel-IT?” Shake Maps and the earthquake and information preparedness

handbook “Putting Down Roots”. Science now gives us all a way to clearly see

and understand the earthquake risk in our own lives.

Title: Earthquakes - Shock Waves

Description:

This short excerpt is from a USGS/Bay Area Earthquake Alliance produced television program “Shock Waves: 100 Years After the 1906 Earthquake”. This specific segment describes some of the history behind our modern understanding of the earthquake process. The program received numerous industry awards and was nominated for a regional Emmy Award in the Bay area. It aired twice on KPIX CBS5 and its affiliate station around the time of the April 18, 2006, 100 year anniversary of the Great San Francisco Earthquake.

Location: , Bay Area

Date Taken: 4/18/2006

Length: 4:55

Video Producer: Stephen M. Wessells , USGS and The Bay Area Earthquake Alliance

Note: This video has been released into the public domain by the U.S. Geological Survey for use in its entirety. Some videos may contain pieces of copyrighted material. If you wish to use a portion of the video for any purpose, other than for resharing/reposting the video in its entirety, please contact the Video Producer/Videographer listed with this video. Please refer to the USGS Copyright section for how to credit this video.

Additional Video Credits:

Department of the Interior, U.S. Geological Survey

A Production of:
USGS and The Bay Area Earthquake Alliance

Directed & Produced by:
Stephen M. Wessells

Host:
Dana King, KPIX, CBS5, San Francisco

Written by:
Donna Matrazzo

Primary Camera:
Haydon Lane

Editing:
Haydon Lane and Stephen M. Wessells

Audio:
Sandy Hain

On Camera Interviews:
Bill Ellsworth, David Schwartz, Ross Stein, Mary Lou Zoback

Surveyors:
Mt. Diablo Surveyors Historical Society, Michael J. Foley, Peter Friedman, Paul Lamoreaux, Cristino De La Paz

Full Credits Available At: Earthquake Hazards Program – Shock Waves

Full Program URL:Earthquake Hazards Program – Shock Waves

Source:

Full Program URL:Earthquake Hazards Program – Shock Waves

File Details:

• Quicktime
• Flash Video
• YouTube version *
• Text transcript

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Tags: Andreas BayAreaEarthquakeAlliance Fault GreatSanFranciscoEarthquake Plate San ShockWaves Shockwaves Tectonics Wessells anniversary earthquake earthquakes elastic quakes rebound usgs