eq mechanisms bay area faults eq magnitude
DESCRIPTION
EQ Mechanisms Bay Area Faults EQ Magnitude. Earthquake Waves. Frequency. 0.1 Hz to 10 Hz (outside human sensory range). Types of Motion. P waves. velocity. S waves. amplitude. surface waves. What is an Earthquake?. A release of energy stored on a fault. What is a fault?. - PowerPoint PPT PresentationTRANSCRIPT
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