Earthquake Hazard Assessment in the Pacific Northwest: Site Response Thomas L. Pratt U. S....
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Transcript of Earthquake Hazard Assessment in the Pacific Northwest: Site Response Thomas L. Pratt U. S....
Earthquake Hazard Assessment in the Pacific Northwest:
Site Response
Thomas L. Pratt
U. S. Geological Survey
School of Oceanography
University of Washington
Earthquakes are inevitable:How do we deal with their effects?
Design buildings to an appropriate level of safety
Prepare emergency services to respond
Inform citizens how to prepare and respond
Need to know:
How often are different types of earthquakes likely to occur?
How strong will the shaking be?
How will the shaking vary across the region?
At what frequencies will the ground shake?
Estimating the ground shaking from earthquakes
Probabilistic Hazard MapsPeak Ground Acceleration likely in a given time interval
Example:• 2% chance of 0.25g acceleration being exceeded in
a 50-year time span
Plate motions
GPS
EQ monitoring (PNSN)
Geophysics (SHIPS)
Geology
LIDAR
Paleoseismology
Earthquake Hazard Analysis
Probabilistic
Hazard Maps
Washington State EQs
Probabilistichazard map
Buildings and Resonant Frequency
35-story building ~ 3.5 sec = 0.3 Hz [e.g., Smith Tower]
1-story building (house) ~ 0.1 sec period = 10 Hz
10-story building ~1 sec period = 1 Hz [e.g., library]
75-story building ~ 5 sec = 0.13 Hz [e.g., Columbia Tower]
ProbabilisticHazard Maps
Site ResponseEstimates
Ground Motion
ModelingBuildingDesign
Spectral ratios(to eliminate source signature)
Simple Spectral Ratio – site/bedrock site
H/V ratio (horizontal/vertical recording)
Horizontal site Horizontal site
Horizontal bedrock Vertical site
Simple Spectral Ratio – site/bedrock site
H/V ratio (horizontal/vertical recording)
site response curve
G7009
0
2
4
6
8
10
0 10 20
frequency (Hz)
am
plif
ica
tion
. eq 6
eq 18
eq 28
average
noise
resonance,impedancechanges
A=Ir/Is=rvr/svs
Fr=1/4T
Resonance models
Focusing
Focusing in Seattle Basin
Surface Waves
movie
Puget Lowland
1999 “Dry” SHIPS
Geophysical data
Chi-Chi earthquake
Local earthquake, M2.8
0
2
4
6
8
10
12
14
16
18
20
480000 500000 520000 540000 560000 580000 600000 620000
UTM easting (m)
time
(se
c) -
X/6
.8
P
S
2852
2768
2689
2645
2609
2570
2525
2491
2451
2410
2360
2338
2296
2144
2121
1374
1343
1296
1251
1082
1038
1002
2262
Low freqs:large overSeattle Basin
High freqs:Small overSeattle Basin
Seattle SHIPS 2002
Seattle SHIPS (site response)
90 seismometers (3-component) recording continuously from Jan 27 to May 24, 2002
Identical instrumentation with 2 Hz sensors
Local: magnitude 2.0 to 2.1 or greater visible across array (~every 3 days?)
Teleseisms: magnitude 7.0 to 7.3 or greater visible across array (~15-20/year)
Seattle SHIPS 2002
46.5
47.0
47.5
48.0
48.5
121.50122.00122.50123.00123.50
Latitu
de
Vertical N-S E-W
Chi-Chi earthquake(surface waves = long durations)
Kingdome SHIPS
Kingdome demolition
Movie
Nisqually Earthquake
Effects of surface waves
Relatively modest ground motionsMaximum PGA of 0.27g
Non-linear soil responseAmplifications much smaller for strong ground motions (mainshock versus aftershock)
nisquallyampvariationwith time
0.5 Hz Amplification Factor
5
21
0.5 Hz Amplification Factor 15-55 sec after OT0.5 Hz Amplification Factor 40-65 sec after OT
5 km
Nisquallyamp vsafter amp
3.3 Hz Amplification Factor
5
21
3.3 Hz Amplification for Mainshock 3.3 Hz Amplification for M3.4 Aftershock
5 km5 km
Results from SHIPS/NisquallySeattle Basin characterized by amplifications of 8 to 12 at low frequencies(0.3 to 0.8 Hz (3.33 sec to 1.25 sec periods)At higher frequencies (5 to 10 Hz) the basin causes decreased shakingAmplification is time variant
(direct arrivals versus surface waves?)
Amplifications of 5 could come from resonance in the shallow deposits (<500 m)Amplifications from focusing likely <2Surface waves within the basin cause amplifications of as much as 16Non-linear effects come into play at moderate ground accelerations
END