lecture 4: earthquakes and seismic waveslecture 4: earthquakes and seismic waves key questions 1....
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Lecture 4: Earthquakes and Seismic Waves
Key Questions
1. What are the sources for EQs in the PNW?
2. What is a seismograph and seismogram?
3. What is the difference between Richter magnitudes and Mercalli intensities?
4. Where can information about PNW seismic stations be found?
5. How does seismology contribute to risk assessment of EQs?
6. Why is the frequency of occurrence of EQs important?
7. What is the “return period” of the great Cascadia subduction zone EQ?
Rock material at plate boundaries can bend, slide, fracture and hence, generate earthquakes
Tectonic plate motions, crustal blocks, and shallow earthquakes in Cascadia
R.E. Wells1, R.J. Blakely, R.W. Simpson, C.S. Weaver, R. Haugerud, and K. Wheeler
What causes earthquakes in the Pacific Northwest?
PNW EQ Sources1. Subduction Zone EQ
Earthquakes produced by slip along the subduction thrust fault or by slip on faults within the down-going ocean crust as a result of bending and extension as the plate is pulled into the mantle.
Nisqually EQ was 52.4 km below the surface
PNW EQ Sources2. Benioff Zone EQs
PNW EQ Sources
3. Shallow, Crustal EQs
Tectonic plate motions, crustal blocks, and shallow earthquakes in Cascadia
R.E. Wells1, R.J. Blakely, R.W. Simpson, C.S. Weaver, R. Haugerud, and K. Wheeler
What causes earthquakes in the Pacific Northwest?
compression
4. EQs due to volcanic activity
Mt. Rainier
Mt. Rainier
Energy is released in two forms:
1. Heat (~ 50 %)
2. Waves (~ 50 %)
P-wave: primary, compressional motion, fastest
S-wave: shear, transverse motion, slower than P-wave
R-wave: Rayleigh, surface wave, slowest (Love wave is a surface wave too).
See http://web.ics.purdue.edu/~braile/edumod/waves/WaveDemo.htm
seismograph
seismogram
seismometer
instruments used to record the motion of the ground during an EQ
seismometer is the internal part of the seismograph
Richter Magnitude Length Analogy 1 1 millimeter 2 1 centimeter 3 10 centimeters 4 1 meter 5 10 meters 6 100 meters 7 1 kilometer 8 10 kilometers 9 100 kilometers 10 1000 kilometers
Richter Magnitude Damage
Magnitude Under 2 "Micro Quake" - Generally not felt
Magnitude 3 Normally not felt
Magnitude 4 Often felt, damage is rare
Magnitude 5 Felt widely, normally only slight damage
Magnitude 6 Poorly constructed buildings are damage
Magnitude 7 - 8 Very serious damage occurs
Magnitude 8+ "Great Quake" - tremendous destruction and loss of life
FEBRUARY 28, 2001 NISQUALLY EARTHQUAKE
The science of seismology is essential for the assessment of earthquake risk. Why?
The science of seismology is essential for the assessment of earthquake risk. Why?
• locations• depths• magnitudes• frequencies• episodic activity
Seismic monitoring allows seismologists to determine EQ
How often do earthquakes occur in the Pacific Northwest?
http://www.pnsn.org/INFO_GENERAL/faq.html#1
Over 1000 earthquakes with magnitude 1.0 or greater in Washington and Oregon occur every year. About 25 are large enough to be felt.
Most Recent EQ
In the 20th century, there were about 14 earthquakes of magnitude 5 or greater that have occurred near Puget Sound (most are Benioff):
1904 (M 5.3) 1909 (M 6.0) 1932 (M 5.2) 1939 (M 6.2) 1945 (M 5.9) 1946 (M 6.4) 1949 (M 7.0) 1965 (M 6.5)1990 (M 5.0) crustal (Deming, WA) 1995 (M 5.0) crustal1996 (M 5.3) crustal2001 (M 6.8)2001 (M 5.0)
Location Map
How often do earthquakes occur in the Pacific Northwest?
http://www.pnsn.org/INFO_GENERAL/faq.html#1
Over 1000 earthquakes with magnitude 1.0 or greater in Washington and Oregon occur every year. About 25 are large enough to be felt.
In the 20th century, there were 13 earthquakes of magnitude 5 or greater that have occurred near Puget Sound (most are Benioff):
1904 (M 5.3) 1909 (M 6.0) 1932 (M 5.2) 1939 (M 6.2) 1945 (M 5.9) 1946 (M 6.4) 1949 (M 7.0) 1965 (M 6.5) 1990 (M 5.0) crustal (Deming, WA)1995 (M 5.0) crustal1996 (M 5.3) crustal2001 (M 6.8)2001 (M 5.0)
What is the return period?
The last Cascadia subduction zone EQ occurred in 1700
Evidence suggests they occur every 400 to 500 years
The science of seismology is essential for the assessment of earthquake risk. Why?
• locations• depths• magnitudes• frequencies• episodic activity GSC, Stanford, PNGA
Seismic monitoring allows seismologists to determine EQ
What other geologic research contributes to our understanding of EQs and hence contributes to risk assessment?
• Surface mapping with LiDAR• Monitoring plate movements with GPS• Sea-floor mapping• Mapping
scanner+GPS+IMU+computer
Light Detection And Ranging
LiDAR
7.5’ Topo Map
10-meter DEM from contours
12-foot Bare-earth DEM from LiDAR
What other geologic research contributes to our understanding of EQs and hence contributes to risk assessment?
• Surface mapping with LiDAR• Monitoring plate movements with GPS• Sea-floor mapping• Mapping
10m DEM from 1:24K contours
Maple Falls
Kendall
LiDAR survey of Nooksack River revealed Kendall Scarp
(Haugerud et al, 2005)
What other geologic research contributes to our understanding of EQs and hence contributes to risk assessment?
• Surface mapping with LiDAR• Monitoring plate movements with GPS• Sea-floor mapping• Mapping
What other geologic research contributes to our understanding of EQs and hence contributes to risk assessment?
• Surface mapping with LiDAR• Monitoring plate movements with GPS• Sea-floor mapping• Mapping
What other geologic research contributes to our understanding of EQs and hence contributes to risk assessment?
• Surface mapping with LiDAR• Monitoring plate movements with GPS• Sea-floor mapping• Mapping
http://earthquake.usgs.gov/research/hazmaps/interactive/cmaps/custom2002_2006.php
What is a ground shaking hazard map & who uses it?