Earth Sciences 240A Lecture 14
Earthquakes II: North
American West Coast
Plate Tectonic Scene
for
Pushed by spreading
Pushed by mid-Pacific spreading
Pacific Plate (west) – Farallon
Plate (east)
Eastern side:
Rapid subduction
Breakup of
spreading center as parts subducted
Fragments of Farallon:
Juan de Fuca
Plate
Rivera Plate
Cocos
Plate
Development of
Transform faults
Catastrophic because of population
density
Major break: San Andreas
1200 km long; Motion
4.8 cm/y; Some creep; mostly periodic
1906 SF (ML 8):
6m horizontal motion; Fault 430 km
x 15-20 km
Clearly visible trench
Paleoseismology
Aim: Assess future dangers
Age dating: C14 if organic material
present
Only suitable for M>~6.5
Decipher order, amount of motion
Pallet Creek
1906:
Foreshock ~20 min. prior to main
Population ~ 400, 000
No previous knowledge of San
Andreas or of large earthquakes
Totally destructive
Moment scale: 8.2; Mod. Mercalli
Scale: VII to IX
Official deaths: 700; probable 5000
Devastation by fires
Current
M 6-7: every ~3 years; M 7-8: every
~100-150 years
Loma Prieta Eq.: 1989: “The World
Series Earthquake”
Most of the city on sediments
Eq.
probability: 67% of M7 by 2020 (Still in effect!)
The Event
Candlestick Park
(
60,000 fans for W.S. game; TV on
M 7.1 at Loma
Prieta (
Thunder sound, motion followed; 11
seconds duration
67 people killed
Pacific Plate moved:
2 km/s over 42 km
length
Extension of
segment moved in 1906
Academic interests
Occurred at a bend
Focus deeper than normal
Vertical motion more than normal
Very short time of motion
Very unusual release pattern
Damage
Nimitz
Freeway
Type of basement foundation
critical
Probability raised 10% by Loma
Prieta
Historical records
Is there a tectonic style change?
Combined 67% of M7 within 15-20
years
Expect dramatic devastation
Northridge:
M 6.7 plis
hundreds of aftershocks
Largest eq.
in LA on record
$20 billion damage
Reverse fault NOT transform!
No relief to eq.
stress
Valley Fever.
Next
Misc. Eq. examples
Last lecture before mid-term