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