EQ Mechanisms Bay Area FaultsEQ Magnitude

Earthquake Waves

Frequency0.1 Hz to 10 Hz (outside human sensory

range)

Types of Motion

P waves

S waves

surface waves

velocity amplitude

What is an Earthquake?A release of energy stored on a fault

What is a fault?A roughly planar surface where rock has broken and

separated

Why does an earthquake happen?Built-up energy exceeds frictional resistance on the fault

How does “slip” on the fault happen?

Elastic rebound

Rocks accumulate stress as two sides of fault move past each otherElastic strain is built up in rocks as they deform

Stress = force per unit areaStrain = change in shape of rocks due to stressesElastic = returns to original shape when released

Remember the fence

from: http://www.seismo.unr.edu/ep/nvguide/sbg1.html and http://eqseis.geosc.psu.edu/~cammon/HTML/Classes/IntroQuakes/Notes/faults.html

Fault Geometry

(focus)

Normal Fault Reverse Fault

Strike-slip Fault Thrust Fault

Faults Types from: http://www.tinynet.com/faults.html

How do geologists.... Find faults?

How do geologists find faults?

How do geologists.... Find faults?

The Hayward Fault is open for you to walk down into until October 31, 2006

See: http://1906centennial.org/activities/calendar/?id=135

How do geologists....determine whether a fault is active?

State of California (A-P act): An active fault is one that has slipped once in the last 11,000 years (or 2 or more

times in the last 700,000 years)

4,000 yr

1 m.y.

80 m.y.

Consider this schematic roadcut/seacliff:

fault #1fault #2 fault #3

Spaced-based measurements (VLBI* and GPS)

show that PAC-NA motion in CA is ~50 mm/yr. *Very Long Baseline Interferometry

Earthquakes in California & Nevada, 1970-2003

Some of the Bay Area’s active faults

The San Andreas is NOT “the PAC-NA plate boundary.”

This diagram applies at the

latitude of Bakersfield or

San Luis Obispo.

Red arrow: predicted

motion: 50 mm/yrBlue arrows:

subsetsof the motion that

“add up” to the predicted motion.

.

predicted at latitude of central CA

(51 ± 3 mm/yr, N32W)

San Andreas fault zone(35 ± 4 mm/yr, N34W)

Basin & Range(12 ±Ź1 mm/yr, N35W)

discrepancy

0

10

20

30

40

102030

velocity of PAC to W (mm/yr = km/Ma)

velo

city

of

PA

C t

o N

(m

m/y

r =

km

/Ma)

32°

just like on a map

North

West

23

94-8?

17-23

9>6

17 96

2-5?

17? 23?

15

7-10?1-3?

FarallonIslands

~Stockton

About 36 mm/yr happens on the San Andreas in central CA, but northwest of Hollister, things are a LOT messier.

The ~36 mm/yr must be divided up

on many faults. Geologists study

each to determineindividual rates.Let’s add up the slip

on faults along four paths to see

whether we’ve found the ~36

mm/yr.

from: http://pubs.usgs.gov/fs/2003/fs039-03/fs039-03.pdf