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 ground during an EQ seismometer is the internal 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 Magnitude Under 2 Magnitude 3 Magnitude 4 Magnitude 5 Magnitude 6 Magnitude 7-8 Magnitude 8+ Damage "Micro Quake" - Generally not felt Normally not felt Often felt, damage is rare Felt widely, normally only slight damage Poorly constructed buildings are damage Very serious damage occurs "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) 1909 (M 6.0) Location Map 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) 1946 (M 6.4) What is the return period? 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 LiDAR survey of Nooksack River revealed Kendall Scarp (Haugerud et al, 2005) 10m DEM from 1:24K contours
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