Earthquakes:Faults, Hazards, &
the Bay AreaRichard Sedlock, SJSU Geology/BAESI
408 924-5020 [email protected]
http://www.baesi.org -- click on “Workshop Links”
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
Earthquake Waves
Frequency0.1 Hz to 10 Hz (outside human sensory
range)
Types of Motion
P waves
S waves
surface waves
velocity amplitude
How does “slip” on the fault happen?
Elastic rebound
Alternative idea: Displacement pulse (“the rug”)
How do geologists.... Find faults?
How do geologists find faults?
How do geologists.... Find faults?
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
Some of the Bay Area’s active faults
Spaced-based measurements (VLBI and GPS)
show that PAC-NA motion in CA is ~50 mm/yr.
Earthquakes in California & Nevada, 1970-2003
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 ± 1 mm/yr, N35W)
discrepancy
0
10
20
30
40
102030
velocity of PAC to W (mm/yr = 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.
Seismic Hazards and Risks
Seismic hazard: A natural process caused by an EQ
Seismic risk: The potential for casualties andproperty loss due to a seismic hazard or hazards.
Example 1: Landslide in built-up Los Gatos vs.landslide in the Sierra Nevada backcountry
Example 2: Tsunami at Los Angeles harbor vs. tsunami at the Big Sur coastline.
Surface rupture
Landslides
Dust & Disease
Tsunamis
Liquefacton & subsidence
Ground shaking
Natural Seismic Hazards
Surface RuptureEarth’s surface breaks and shif
Only affects construction built directly atop the fault.
Affects the smallest area of all the seismic hazards.
Includes creep and earthquakes.How could you evaluate the level of
riskdue to surface rupture?
ts at an active fault.
LandslidesDownslope movement of Earth materials due to gravity
Most affect soil or loose sediment, but some affect rock.
May be triggered by earthquakes or heavy rainfall.
How could you evaluate the level of risk
due to landslide?
TsunamiDestructive sea waves (NOT tidal waves)
Caused by processes that abruptly moves ocean water:
earthquake submarine volcanic eruptioncoastal/submarine landslide ET impact
Time of arrival can be predicted—public warnings
How could you evaluate the level of risk
due to tsunami?
Possible tsunami“run-up” zones
LiquefactionAccompanies M6+ EQs.Water escape often causes the ground surface to subside.Subsidence can damage pipes, foundations, etc.How could you evaluate the level of
riskdue to liquefaction & subsidence?
Ground ShakingDue to passage of P, S, and surface waves
Ground type controls amplitude and duration of shaking:
Bedrock shakes least, shortestMud-rich soil shakes most, longest
How could you evaluate the level of risk
due to ground shaking?
Effect of Ground Type on Shaking
Seismograms for a
M4 aftershockof the 1989 Loma Prieta earthquake.