global seismicity

Global seismicity

Earthquake epicenters (locations) are shown by the colored dots. Note locationsand concentrations of activity. Compare with volcano locations.

Plate collisions and volcanoes

• Ocean-ocean

• Ocean-continent

• Continent-continent

India-Asia continent-continent collision

Rock Behavior

• How rocks respond to applied forces (stress) • Stress – force per unit area (lbs per sq in)• Response is termed “strain”– Elastic deformation (reversible)– Ductile deformation (flow)– Brittle deformation (fracture)

Rock breakage

• Fracture– Separation only; no vertical movement

• Fault– Vertical and/or horizontal motion

Note offset in rocks

Footwall block

Hanging wall block

Note different fault motions

Strike-slip faultNormal or dip-slip fault

Reverse fault

Strike-slip motion on the plates

Note relative position of features

Subduction process

Shallow-focus: 0 to 70 km depth

Intermediate-focus: 70 to 300 km depth

Deep-focus: 300 to about 700 km depth

P- and S-wave motion

Note changes in amplitude of the three wave arrivals

Seismogram of EQ near recording station

Selected seismic stations in the US

Earthquake magnitudes

• Measures the “size” of an EQ• Four types of measurements– Local (ML) – original Richter scale– Body-wave (Mb) – P-wave amplitude– Surface-wave (Ms) – Rayleigh-wave amplitude– Moment (Mw) – considers amount of strain

energy release along entire fault rupture.

Comparison of magnitude scales

The Big Ones

Japan Mar 20119.0

Intensity scale

• Measures damage caused by seismic energy• Established by Mercalli (Italian) in 1902;

modified in 1931 to reflect enhanced building standards in US

• Uses Roman numerals (I – XII)• Values depend on EQ magnitude, distance

from source, bedrock type, building material and style, duration of shaking

Isoseismal map of Dec 1811 EQ near New Madrid, MO

Earthquakes don’t kill people- buildings do

• Many deaths in older regions on Earth due to poor quality construction, especially through trans-Mediterranean/Asiatic belt

• Secondary events (aftershocks) destroy already weakened structures

• Surface waves produce the greatest damage

Benefits of EQs

• Changing natural resource paths– Groundwater– Oil and natural gas– Exposures of minerals

• Natural mitigation– Small events lessen likelihood of large events

Short-term predictions

• Precursors– Events that imply an EQ; usually small magnitude

events, often in swarms– Foreshock (sometimes) – main shock – aftershock

• Ground deformation• Water level changes in wells• Seismic gaps– Greatest potential for large events in the gap

Sample seismogram showing P, S, and surface waves

Effects of EQs

• Shaking and ground rupture• Liquefaction• Regional elevation changes• Landslides• Fires• Disease

Humans cause earthquakes by

• Crustal loading by dams and reservoirs• Injection of liquid waste• Underground nuclear explosions

Human caused Eqs near Denver, CO

Earthquakes in the United States

Where do most EQs occur in US and why?

What’s happening in eastern & central US

Subduction zones in western North America

• Alaska– Subduction of Pacific

Plate• Pacific Northwest (BC-

WA-OR-CA)– Subduction of Juan de

Fuca Plate and smaller Gorda Plate

– Cascadia Subduction Zone

– Volcanoes on land

Subduction Zone Earthquakes

• Largest EQs worldwide– 9 of the 10 largest earthquakes (1904-2008)

were related to plate subduction• One in Tibet was due to India hitting Asia

– These 9 occurred along Circum-Pacific “Ring of Fire”

– Five EQs were located in northern Pacific [Japan-Kamchatka-Aleutians]

Examples of Subduction Zone Earthquakes

• Chile 1960 (Mw = 9.5): Nazca Plate diving under South American Plate; tsunami producer

• Alaska 1964 (Mw = 9.2): Pacific Plate dives beneath North American Plate; tsunami producer

• Mexico 1985 (Ms = 8.1): Cocos Plate dives beneath North American Plate

• Indonesia 2004 (Mw = 9.1): India Plate dives under Burma Plate; major tsunami producer

• Japan 2011 (Mw = 9.0): Pacific Plate dives under North American Plate; major tsunami producer

Seismic Gap, Mexico, Sept 19,1985 Ms = 8.1

• Mexico earthquake filled Michoacan seismic gap

• Guerrero gap remains

• Major aftershocks of Ms =7.5 and 7.3 within a month

Fig 2.20a

Average annual worldwide frequency of EQs magnitude 6.0 or greater

• M 6.0 – 6.9 Strong 100

• M 7.0 – 7.9 Major 15-20

• M 8.0+ Great 1 every 2-3 years

San Andreas Fault System

• Movement occurs on many faults

• Displacement is distributed over a wide zone

• Right-lateral strike-slip motion

San Andreas: Earthquake Probabilities

• Probability of major earthquake (1988-2018)

• Use historical and sag-pond data to calculate recurrence intervals

Loma Prieta, Oct 17,1989 [World Series EQ]

• Magnitude 6.9; 67 killed• Epicenter at Loma Prieta, highest

peak in Santa Cruz Mountains• 100 km SSE of San Francisco• Section of San Andreas that moved

in 1906 EQ ruptured again• Marina district in SFO was built on

rubble from 1906 EQ; mud was pumped in to fill holes; very unstable “land”

• Game 3 halted by Commissioner; after 10-day recess, series continued in Oakland – Oakland swept series 4 games to 0.

Fig 2.18a

Seismic Wave Amplification near Oakland

Plate boundary is not a discrete line, but rather a zone the width of the Bay Area

Notice the manymajor faults thatare parallel to the San Andreas fault

Location of Hayward fault

Univ of California stadiumin Berkeley

The trace of the Hayward fault runs through goal posts

Left side is moving N [top of image]

30 yrs of activity; crosses areepicenters

New Madrid, Missouri, 1811-2

• Series of three earthquakes in Mississippi River valley (Dec 1811-Feb 1812)

• 4 main events, 3 w/ magnitudes > 8.0 !

• Knocked down chimneys 400 miles away!

• Shook windows 800 miles away!

• Neotectonic analysis indicates earlier EQs in 500, 900, 1300, 1600 AD recurrence interval of 200-400 years.

• Why did it affect such a large area?

EQs associated with volcanoes

• Those connected with active subduction such as western U.S.– Much larger magnitudes due to brittle rock

• Isolated volcanism such as Hawaii– Usually lower magnitude due to molten rock

Foci of Hawaiian EQs