Lecture 4: Earthquakes and Seismic Wavesgeology.wwu.edu/rjmitch/L4_EQsources.pdf · Lecture 4:...

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

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?

• 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?