Earth Sciences 240A  Lecture 14

Earthquakes II: North American West Coast

Plate Tectonic Scene for Pacific Ocean

Pushed by spreading Atlantic

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: San Francisco: April 19

Foreshock ~20 min. prior to main

Population ~ 400, 000

No previous knowledge of San Andreas or of large earthquakes

Totally destructive Raleigh waves

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”

San Francisco area between San Andreas and Hayward Faults

Most of the city on sediments

Eq. probability: 67% of M7 by 2020 (Still in effect!)

The Event

Candlestick Park (5:04 p.m.)

60,000 fans for W.S. game; TV on

M 7.1 at Loma Prieta (Santa Cruz Mtns.)

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

Crystal Springs Reservoir Gap

Probability raised 10% by Loma Prieta

Historical records

Is there a tectonic style change?

San Francisco

Hayward Fault plus San Andreas

Combined 67% of M7 within 15-20 years

Expect dramatic devastation

Northridge: 17 January, 1994

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