Global seismicity Earthquake epicenters (locations) are shown by the colored dots. Note locations and 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 Fault Separation only; no vertical movement Vertical and/or horizontal motion

Note offset in rocks

Footwall block Hanging wall block

Note different fault motions Normal or dip-slip fault Strike-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 2011 9.0

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 Natural mitigation 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 Figure 5.1

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 Figure 4.8

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 See also Figure 4.4

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 Al

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 many major faults that are parallel to the San Andreas fault

Location of Hayward fault

Univ of California stadium in Berkeley The trace of the Hayward fault runs through goal posts Left side is moving N [top of image]

30 yrs of activity; crosses are epicenters

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