Lecture 4 Earthquakes and Seismic Waves by luy15016

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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 Sources
1. Subduction Zone EQ
PNW EQ Sources
2. Benioff Zone EQs

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 Sources
3. Shallow, Crustal EQs                                              compression




                           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?
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                              seismometer




instruments used to record the motion of the     seismometer is the internal
ground during an EQ                              part of the seismograph


                                    seismogram
http://www.iris.edu/hq/programs/gsn/maps
The Pacific Northwest Seismic Network
Richter Magnitude
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?

Seismic monitoring allows seismologists to determine EQ

 • locations
 • depths
 • magnitudes
 • frequencies
 • episodic activity
How often do earthquakes occur in the Pacific Northwest?
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)                  Location Map
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) crustal
1996    (M 5.3) crustal
2001    (M 6.8)
2001    (M 5.0)                           http://www.pnsn.org/INFO_GENERAL/faq.html#1
How often do earthquakes occur in the Pacific Northwest?

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)         What is the return period?
1946    (M 6.4)
1949    (M 7.0)
1965    (M 6.5)
1990    (M 5.0) crustal (Deming, WA)
1995    (M 5.0) crustal
1996    (M 5.3) crustal
2001    (M 6.8)
2001    (M 5.0)
                                            http://www.pnsn.org/INFO_GENERAL/faq.html#1
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?

Seismic monitoring allows seismologists to determine EQ

 • locations
 • depths
 • magnitudes
 • frequencies
 • episodic activity GSC, Stanford, PNGA
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
       Light Detection And Ranging
                LiDAR



scanner
+
GPS
+
IMU
+
computer
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
                                                             Maple Falls

                                Kendall




                                                        10m DEM from 1:24K contours
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
What is a ground shaking hazard map & who uses it?




        http://earthquake.usgs.gov/research/hazmaps/interactive/cmaps/custom2002_2006.php

								
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