Transcript December 7, 2005: Earthquake hazards

Spaced-based measurements (VLBI and GPS)
show that PAC-NA motion in CA is ~50 mm/yr.
Earthquakes in California & Nevada, 1970-2003
40
discrepancy
Red arrow: predicted
motion: 50 mm/yr
Blue arrows: subsets
of the motion that
“add up” to the
predicted motion.
predicted at latitude
of central CA
(51 ± 3 mm/yr, N32W)
Basin & Range
(12 ±К1 mm/yr, N35W)
30
20
San Andreas fault zone
(35 ± 4 mm/yr, N34W)
10
32°
This diagram applies
at the latitude of
Bakersfield or
San Luis Obispo.
30
20
velocity of PAC to N (mm/yr = km/Ma)
The San Andreas is NOT “the PAC-NA plate boundary.”
North
0
10
velocity of PAC to W (mm/yr = km/Ma)
West
just like on a map
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 determine
individual rates.
Let’s add up the slip
on faults along four
paths to see whether
we’ve found the ~36
mm/yr.
4-8?
9
23
>6
~Stockton
9
Farallon
Islands
17-23
2-5?
6
17
9
7-10?
1-3?
15
17? 23?
Seismic Hazards and Risks
Seismic hazard: A natural process caused by an EQ
Seismic risk: The potential for casualties and
property 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.
Natural Seismic Hazards
Surface rupture
Landslides
Dust & Disease
Tsunamis
Liquefaction & subsidence
Ground shaking
Surface Rupture
Earth’s surface breaks and shif
ts at an active fault.
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 risk
due to surface rupture?
Landslides
Downslope 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?
Tsunami
Destructive sea waves (NOT tidal waves)
Caused by processes that abruptly moves ocean
water:
earthquake
submarine volcanic eruption
coastal/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
Liquefaction
Accompanies M6+
EQs.
Water escape
often causes the
ground surface to
subside.
Subsidence
can
damage pipes,
foundations, etc.
How could you evaluate the level of risk
due to liquefaction & subsidence?
Ground Shaking
Due to passage of P, S, and surface waves
Ground type controls amplitude and duration of
shaking:
Bedrock shakes least, shortest
Mud-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 aftershock
of the 1989 Loma
Prieta earthquake.