General

1
Puget Sound Seismicity A brief introduction
Sarah Paulsen
2
Puget Sound Seismicity
Recorded earthquakes since 1872

• Why?

• Where?

• How often?

• How strong?

3
Pacific Northwest Seismicity
Juan de Fuca plate compressed in east-west
direction
4
Pacific Northwest Seismicity
North American plate
North American plate compressed in north-south
direction
5
Pacific Northwest Seismicity

• Three primary sources of earthquakes
• Crustal sources
• Shallow (0-25 km)
• Up to M7.4?
• Seattle fault, unknown faults,

Seattle Fault Eq. 1100 ybp M 7.4 1872
crustal eq.
6
Pacific Northwest Seismicity
Puget Sound Area Earthquake Sources
7
Pacific Northwest Seismicity
Past Earthquakes 1949 Olympia Earthquake Magnitud
e 7.1
8
Pacific Northwest Seismicity
Past Earthquakes 1965 Seattle-Tacoma
Earthquake Magnitude 6.5
9
Pacific Northwest Seismicity
Past Earthquakes 2001 Nisqually Earthquake
Facts and Figures Date February 28,
2001 Time 185431 UTC (105431 a.m.
PST) Magnitude (Mw) 6.8 Focal depth 52
km Epicentral location 47.17N 122.73W
10
Instrumental Intensity
Nisqually, M6.8
11
The Big One

• Subduction zones
• Large area
• High compressive forces
• Brittle rock

• Juan de Fuca subduction zone
• 8 million years old
• Convergence rate 35 mm/yr
• 1,100 km (700 miles) long
• 90 km (60 miles) wide

But where are the earthquakes?
12
The Big One
Two possibilities

• Juan de Fuca plate is steadily
• sliding under the North American
• plate

• Juan de Fuca plate is stuck, and
• energy is building up

13
The Big One
If the plates were stuck, what would we see?
If the plates became unstuck, what would we see?
Wheres the evidence?
14
The Big One
Answer In the field
Buried peat
15
The Big One
Answer In the field
16
The Big One
Answer In the field
17
The Big One
Current understanding
Locked zone is predominantly offshore
Size and frequency of large earthquakes still
unknown likely up to M9.2
Moderate level of shaking in Puget Sound, but
very long duration
18
The Bigger One
19
The Bigger One
LIDAR (Light Distance and Ranging) Laser energy
emitted 30,000 times/sec from airplane Reflections
recorded Tree canopy produces reflection Ground
produces reflection
20
The Bigger One
LIDAR (Light Distance and Ranging)
21
The Bigger One
LIDAR (Light Distance and Ranging)
22
The Bigger One
LIDAR (Light Distance and Ranging)
23
The Bigger One
The Seattle Fault
24
The Bigger One
The Seattle Fault
Maximum magnitude?
25
The Bigger One
The Seattle Fault
Maximum magnitude?
26
Rick Blakely, USGS
27
Rick Blakely, USGS
28
10-meter DEM from 124K-scale contours
1 km
Southern Bainbridge Island
Restoration Point
Ralph Haugerud, USGS
29
12-ft LIDAR DEM
1 km
Southern Bainbridge Island
Restoration Point
Ralph Haugerud, USGS
30
Seattle fault evidence
Brian Sherrod, USGS
Vasa Park, Bellevue
31
Seattle fault evidence
Lake Sammamish
Peat
Qoutwash
Deformation front
Lineament
Scarps
Mvseds
Qtill
Mvseds
Landslide headscarp
Qtill
Lake Washington
Olseds
Qalluvium
Olseds
Inferred fault traces
Evolc
Scarps
32
Seattle fault evidence
S
N
33
Seattle fault evidence
34
Seattle fault evidence
Lake Sammamish
Trench
X scarps
35
Seattle fault evidence
36
Seattle fault evidence
37
Current understanding
38
All of em
Current Projections - approximate 50-yr
probabilities
Cascadia, M 9 Seattle Fault, M gt 6.5 Deep
intraplate, M gt 6.5 Random shallow, M gt 6.5
(within Puget Sound basin)
10 14 5 84 15