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Earth 20: Earthquakes and the San Andreas Fault
Transcript of Earth 20: Earthquakes and the San Andreas Fault
San Andreas Fault
The San Andreas Fault is a transform Fault that marks a piece of the boundary between the Pacific and North American plates in California
1200 km (750 mi) long
Fault behavior varies in different regions
San Francisco 1906
April 26, 1906, Magnitude 8.0
Destruction greatest in areas built on artificial fill
Aftershocks and fires continued to ravage the city
Broken water lines hindered many rescue efforts
Earth 20: Earthquakes and the San Andreas Fault
By: Omar de la Cruz, Emily Angelo, Diana Velasquez, and Satchel Cronk
Introduction to Earthquakes
is the sudden slip on a fault. The ground shaking movement and radiated seismic energy is caused by the slip.
Take this into consideration:
Plate boundaries are responsible for most earthquakes.
Different types of faults creates different types of stresses.
Earthquakes produce seismic waves
When two plates
from each other.
When two plates
literally past each other.
Did you know:
1. Shallow EQ occur at transform and divergent boundaries.
2. Intermediate- Deep EQ occur at convergent boundary
When a fault slips, it releases energy in seismic waves that pass trhough the whole body of the planet (
) and others that pass near the surface only (
A. Body Waves
i. Primary waves
ii. Secondary waves
B. Surface Waves
i. Love Waves
ii. Rayleigh Waves
The different types of waves:
*These waves are responsible for shaking
Not many large earthquakes
Offset of sedimentary rock layers indicates that large earthquakes have occured here in the past
i. Primary Waves: 1st one to arrive, the fastest. Moves in a compression (push) and extension (pull) manner.
through any material:
ii. Secondary waves: 2nd to arrive. Transverse waves that propagate by shearing or shaking particles in their paths. Travel
i. Love Waves: similar to S waves, except it is from side to side in a horizontal plane. Travel faster than Rayleigh waves.
ii. Rayleigh Waves: Move in a backward-rotating, elliptical motion (ocean waves). Shaking produced by R waves can cause both horiztonal and vertical movement. Long periods.
North of Los Angeles
Not many earthquakes
Locked fault behavior
As a result, earthquakes are not common here
South San Francisco
Produces small to moderate sized earthquakes
Nothing exceeding magnitude 6
Fault creeps here
Major Earthquakes on the San Andreas Fault
Filthy conditions due to damage reported in the months following the earthquake
Caused epidemics of illness
Over 1/2 of the population lost their homes
Total death toll: 5,000
United States lost almost 2% of Gross National Product in 1906
Rebuilding efforts focussed on quickness, not safety
Loma Prieta 1989
October 17th, 1989, Magnitude 6.9
Pushing between the North American and Pacific plates caused 42 km rupture in the southernmost section of the fault
Epicenter: Loma Prieta, CA
Earthquake zone was in a seismic gap, where movement had not occurred since 1906
Locked zone had to catch up with creep zone through large fault movement
Seismic waves amplified on soft artifical sediments
Ground motion here 10 times stronger than on solid rock
Liquefaction of artificial sediments caused major damages
Earthquake left 67 dead, 3,757 injured, and and 6 billion dollars in damage costs
Main cause of building failure design flaw
THE BIG ONE!
Predicted to take place in Southern California, mainly in L.A.
Dip-Slip faults are inclined fractures where the crust shifts mostly vertically.
Normal Fault: The crust is extended and the hanging wall moves downward relative to the footwall.
Reverse Fault: The crust is shortened and the hanging wall moves up relative to the footwall.
Thrust Fault: Similar motion to a revere fault but the fault plane is at less than a 45 degree angle.
Predominantly horizontal offset parallel to the fault line.
The fault surface is usually near vertical, and the footwall moves left or right or laterally with very little vertical movement.
Sinistral Faults: left-lateral motion.
Dextral Faults: right-lateral motion.
These are as defined by the movement of the ground on the opposite side of the fault as a viewer.
Strike Slip Faults
A special type of strike-slip fault that forms at a plate boundary, usually at spreading centers such as a mid-ocean ridge.
Less commonly found within the continental lithosphere, for example the San Andreas Fault.
There is a 99% certainty of a major quake happening in SoCal in the next 35 years
The seismic gaps in Southern California (Coachella Valley) have usually lasted 150 years (for the last 1,500 years).
Seismic Gaps- The period of time a fault has remained inactive
This area has not seen major movement for
Northern California is less at risk because the last major quake occurred less than 100 years ago (1906 EQ)
There is still, however, a risk. Creeping segments are not as stable as previously believed.
The LA basin would experience intense shaking for 2-3 minutes
Fires would devastate the area
2,000 dead in LA metro area, 50,000 injured, $200 billion of damages
No water, electricity, internet service, telephone connections, sewage system, or transportation
LA Concrete Pipes:
Los Angeles has water pipelines that cross the SA fault itself and approximately 70% of the water pipes of made of brittle concrete
LA/Long Beach Pipelines
LA and Long Beach are the biggest ports in the U.S. and they have oil pipelines that run to Arizona and Nevada. If damaged, they could fuel large fires
Santa Ana Winds:
Also known as the “Devil Winds,” they are typically extremely dry and hot down-slope winds that sweep through Southern California and originate at the Great Basin and Mojave Desert. They are notorious for causing wildfires.
The aim is to make 90% of buildings earthquake safe which means at best, 10% of buildings WILL fail
If the SAF movements trends continue, in a couple million years LA will be next to San Francisco.