Earthquakes
Definitions Seismology The study of earthquakes dates back almost 2000 years to the Chinese Fault A crack in the Earth along which movement or sliding occurs
What is an earthquake? Focus vs. epicenter Foreshocks and aftershocks Fault creep Slow movement Slippage Elastic rebound
Offset of 2.5 m (8.5 ft) from San Francisco earthquake, 1906
How it happens Elastic rebound Rocks on both sides of an existing fault are deformed by tectonic forces Rocks bend and store elastic energy Frictional resistance holding the rocks together is overcome
Elastic Rebound
Seismographs Records the movement of Earth in relation to a stationary mass on a rotating drum or magnetic tape
Seismographs
Modern electronic seismograph uses a heavy magnet. Seismic movement generates electric current. Voltage of current represents size of motion
Seismic Waves Interior Waves P waves S waves Surface waves Love waves Rayleigh waves
P waves Pressure waves Compressional waves Travel in a straight line Fastest travel time Travels through liquids and solids Travels faster through more dense material
P waves
S waves Secondary waves Sine waves Shearing Slower than p waves Arrive at station second Travel through solids only
S waves
P and S waves
Rayleigh Waves Surface wave Ground moves up and down
Love Waves Surface wave Ground shears sideways
Locating the source of a quake Finding the epicenter Travel time graphs Triangulation
Locating the source of earthquakes
Earthquake Belts Earthquakes and Plate Tectonics Ring of Fire Circum-Pacific
Locating the source of earthquakes Earthquake depths Earthquakes originate at depths ranging from 5 to nearly 700 kilometers Earthquake foci arbitrarily classified as shallow (surface to 70 kilometers), intermediate (between 70 and 300 kilometers), and deep (over 300 kilometers)
Worldwide locations of earthquake epicenters
Zones of earthquake foci in 1985 in the vicinity of the Tonga Islands
Interior of continents New Madrid, Missouri upstate New York Charleston, South Carolina Charleston, SC August 31, 1886
Measuring the size of an earthquake Two measurements that describe the size of an earthquake are Intensity a measure of the degree of earthquake shaking at a given locale based on the amount of damage Magnitude estimates the amount of energy released at the source of the earthquake
Intensity and Magnitude Modified Mercalli intensity scale:rates damage Richter Scale magnitude: measure of amount of energy released logarithmic scale increase of 1 on the scale = increase of 10 in wave amplitude increase of 1 in magnitude = increase of ~32 times more energy released example: magnitude 6.5 releases 32x more energy than a 5.5 and 1,000x more energy than a 4.5
Measuring the size of an earthquake Intensity scales Modified Mercalli Intensity Scale was developed using California buildings as its standard The drawback of intensity scales is that destruction may not be a true measure of the earthquakes actual severity
New Madrid, Mo. earthquake
Zones of intensity: Mercalli Scale Zones of destruction associated with the Loma Prieta, CA earthquake of 1989 Zone VIII (near epicenter) experienced the highest intensity
Measuring the size of an earthquake Magnitude scales Richter magnitude - concept introduced by Charles Richter in 1935 Richter scale Based on the amplitude of the largest seismic wave recorded Accounts for the decrease in wave amplitude with increased distance
How it feels
J.C. Penny Building Anchorage, Alaska 1964 Good Friday earthquake measured 8.3 on the Richter scale
Earthquake destruction Destruction from seismic vibrations Ground shaking Regions within 20 to 50 kilometers of the epicenter will experience about the same intensity of ground shaking However, destruction varies considerably mainly due to the nature of the ground on which the structures are built
Effects of shaking
Destruction from Seismic Vibrations Amount of damage depends on: intensity duration nature of material structures sit upon unconsolidated material vs. bedrock design of structure
1989 Loma Prieta quake
Damage vs. substrate Soft substrate Buildings sway through large amplitudes Foundations sink into the ground liquefaction Hard substrate Sway through smaller amplitudes Foundations don t sink
Effects of shaking
Effects of shaking
Damage continued Liquefaction Landslides/mudslides Ground subsidence Fires
Liquefaction Unconsolidated materials saturated with water turn into a mobile fluid
San Francisco, CA Structures built on landfill collapsed during 1989 Loma Prieta quake
Turnagain Heights, Alaska 1964
Damage caused by the 1964 Anchorage, Alaska earthquake
Results of liquefaction Mexico, 1985
Landslide from 1964 Alaskan quake
Gas lines break causing fires
Fires -San Francisco, 1905
Sand volcanoes and Disruptive Bedding
Records of paleoearthquakes
Ground Subsidence
And the effect on water... Seiche Rythmic, back and forth motion of water in enclosed or semi-enclosed bodies of water Can be felt thousands of miles from the epicenter Ex: 1964 Alaska earthquake generated 2 m waves off the south coast of Texas Can be dangerous when occurring in bodies of water with damns
Earthquake destruction Seiches The rhythmic sloshing of water in lakes, reservoirs, and enclosed basins Waves can weaken reservoir walls and cause destruction
Tsunami Destructive waves that are often inappropriately called tidal waves Caused by vertical displacement due to an underwater earthquake Can travel thousands of miles from epicenter Result from vertical displacement along a fault located on the ocean floor or a large undersea landslide triggered by an earthquake
Tsunami In the open ocean height is usually less than 1 meter In shallower coastal waters the water piles up to heights that occasionally exceed 30 meters Can be very destructive
Tsunami! Normal faulting Reverse faulting creates a void pushes the water up equal to displacement
Tsunami travel times to Honolulu, Hawaii
Tsunami hitting Hilo, Hawaii
Aftereffect of tsunami that hit Kodiak, Alaska after the 1964 earthquake
Expected levels of ground shaking in U.S. over next 50 years
Probability of earthquakes in Ca. along San Andreas over the next 30 years
Seismic Gaps
Seismic Gaps
Earthquake prediction Short-range predictions Goal is to provide a warning of the location and magnitude of a large earthquake within a narrow time frame Research has concentrated on monitoring possible precursors phenomena that precede a forthcoming earthquake such as measuring uplift, subsidence, and strain in the rocks
Short range predictions To give warning of impending quake No reliable methods exist Study precursors of quakes Uplift Subsidence Strain in rocks along fault Changes in groundwater levels Electrical conductivity in rocks Anomalous animal behavior
Probabilities of a major quake between 1988 and 2018 along the San Andreas Fault, CA (or why not to live in Parkfield, CA)
Long range forecasts Time scales of 30 to 100 years Give the probability of a certain magnitude earthquake occurring on a time scale of 30 to 100 years, or more
Earthquake prevention and engineering A: wrapping bridge columns with cable and bolting span to the column B: making the building wider at the base and adding cross beams inside the structure C: placing the entire structure on rollers http://www.mines.utah.edu/~schuster/gg103/res/resonance.html
Taking cover
Tsunami
The coastline of Khao Lak, Thailand, before and after
The event not only affected coastlines (shown here is Indonesia), but also key shipping lanes. Digital Globe / AFP / Getty
Banda Aceh, IndonesiaThis is an aerial view on Dec. 29 of the ruined city of Banda Aceh, the capital of Aceh province in Sumatra. It was taken by a first Royal Australian Air Force plane, which delivered humanitarian aid and a medical assessment team to the devastated area.
Islets take the place of what used to be part of Banda Aceh, which was swamped by an earthquake-triggered tsunami on Dec. 27. The picture was taken from a commercial plane.
images help illustrate the huge task of remapping entire areas of the Indian Ocean after Digital Globe / AFP / GettySatellite
Volunteers helping to gather the dead and look for survivors gaze out from the lobby of a demolished beachfront hotel near Khao Lak, Thailand.