Earthquakes

Focus/Hypocenter

the point of initial breakage and movement along a fault, where seismic waves originate

Where are the most important concentrations of earthquakes?

Most important concentrations in circum-Pacific and Mediterranean-Himalayan belts

Benioff Zones

Nearly all intermediate- and deep-focus earthquakes occur in Benioff zones inclined seismic activity associated with descending oceanic plate at subduction zones

P Wave

P wave - compressional wave in which rock vibrates parallel to the direction of wave propagation Fast (4 to 7 kms/sec) wave that is the first or primary wave to arrive at recording station following earthquake Pass through solids and fluids

Relationship between P Waves and S Waves

P- and S-waves leave earthquake focus at the same time P-wave gets farther and farther ahead of the S-wave with distance and time from the earthquake

Permanent displacement of land

Permanent displacement of the land surface can occur, leaving fractures and scarps

How to locate epicenter

Plotting distances from 3 stations on a map, as circles with radii equaling the distance from the quake, locates earthquake epicenter

Rayleigh Waves

Rayleigh waves - ground moves in an elliptical path opposite the direction of wave motion Extremely destructive to buildings

Where are Earthquakes common in the United States?

Earthquakes occur throughout the U.S., but are much more common in the western states and Alaska

Seismic Waves

Energy is released during earthquakes in the form of seismic waves. Released from a position along a break between two rock masses (fault)

How do fires occur because of earthquakes?

Fire is a problem just after earthquakes because of broken gas and water mains and fallen electrical wires

Effects of Earthquakes?

Ground Motion, Fire, Landslide, Liquefication, Permanent displacement of land

Ground Motion

Ground motion is the familiar trembling and shaking of the land during an earthquake Can topple buildings and bridges, most people are killed by falling debris Adherence to proper building codes minimizes damage and loss of life

How are landslides triggered during/after earthquakes?

Landslides can be triggered by ground shaking, particularly in larger quakes

Where are the largest seismic risks or hazards?

Largest seismic risks or hazards exist near the plate boundary along the U.S. Pacific coast (e.g., San Andreas fault), and around New Madrid, Missouri

Liquefication

Liquefaction occurs when water-saturated soil or sediment sloshes like a liquid during a quake

Love Waves

Love waves - side-to-side motion of the ground surface Can't travel through fluids

Magnitude

Magnitude Measure of Energy Released in an earthquake An earthquake has one magnitude Richter standardized a magnitude scale in 1935

Moment Magnitude

Moment magnitude - more objective measure of energy released by a major earthquake Uses rock strength, surface area of fault rupture, and amount of movement Smaller earthquakes are more common than larger ones

Where do most earthquakes occur?

Most earthquakes occur in narrow geographic belts which mark tectonic plate boundaries

S Wave

S wave - shear wave in which rock vibrates perpendicular to the direction of wave propagation Slower (2 to 5 km/sec) wave that is the secondary wave to arrive at recording station following earthquake Pass through solids only

Modified Mercalli Scale

Scale with a list of qualifications to determine intensity

How is seismic risk determined?

Seismic risk determined based on the assumption that large future earthquakes will occur where they have occurred in the past

Seismograms

Seismograms - paper (or digital) records of the earthquake vibrations, used to measure the earthquake strengths

Seismographs

Seismographs: recording devices used to produce a record of the motion detected by seismometers

Seismometers

Seismometers: used to measure seismic waves

Surface Waves

Slowest type of seismic waves produced by earthquakes

Paleoseismology

studies that determine where and when earthquakes have occurred and their size Average intervals between large earthquakes and the time since the last one occurred can also be used to assess the risk (over a given period of time) that a large quake will occur

Causes of Tsunamis

-generally produced by magnitude 8+ earthquakes ("great" earthquakes e.g., 9.3 Indonesia, 2004) -May also be generated by large undersea landslides or volcanic explosions -Travel across open ocean at speeds of >700 km/hr -Reach great heights in coastal areas with gently sloping seafloor and funnel-shaped bays

Earthquake Prediction Techniques

Accurate and consistent short-term earthquake prediction not yet possible, three methods assist in determining probability that an earthquake will occur: Measurement of changes in rock properties may serve as precursors to earthquakes, such as: magnetism, electrical resistivity, seismic velocity, and Porosity. Animal behavior, Foreshocks, and Surface Tilting have been used successfully in some case, but these methods don't work consistently. Studies of the slip rate along fault zones, using GPS and InSAR (interferometric synthetic aperture radar) If the slip rate along a fault is 5 cm/yr and the last earthquake resulted in 5 m of slip, we would expect the earthquake recurrence rate to be every 100 years.

Convergent Boundaries

At convergent boundaries, compressional forces produce shallow- to deep-focus quakes along reverse faults

Divergent Boundaries

At divergent boundaries, tensional forces produce shallow-focus quakes on normal faults

Transform Boundaries

At transform boundaries, shear forces produce shallow-focus quakes along strike-slip faults

Body Waves

Body waves - travel outward from the focus in all directions through Earth's interior

Depth of Earth's Surface

Depth of focus beneath Earth's surface can also be determined Shallow (85%): 0-70 km Intermediate (12%): 70-350 km Deep (3%): 350-670 km

Elastic Response

Each side of the fault snaps to a new, low-strain position

Richter Magnitude

Each unit increase in magnitude is a 10-fold increase in the maximum ground motion M=6 is 10-times the amplitude of the vibrations than M=5 M=5 is 10-times the amplitude of the vibrations than M=4, and so on  Each 10-fold increase in the size of the vibrations is caused by a 32-fold increase in energy released, so a M=5 earthquake releases 32 times more energy than a M=4 earthquake.

Plate Boundaries

Earthquakes are caused by plate interactions along plate boundaries Plate boundaries are identified and defined by earthquakes Earthquakes occur at each of the three types of plate boundaries: divergent, transform, and convergent

Plastic Strain

Strain energy builds up as opposite sides of a fault move

Seismic Gaps

Strain energy is released by earthquakes. Segments of faults that go for long periods of time without earthquakes build up strain. Seismically quiet zones indicate where large earthquakes are likely.

Surface Waves

Surface waves - travel along Earth's surface away from the epicenter

Travel Time Curve

Travel-time curve used to determine distance to focus based on time between first P- and S-wave arrivals

Tsunami

Tsunami (seismic sea waves) - very large sea waves caused by sudden upward or downward movement of the sea floor during submarine earthquakes

How are tsunamis reported?

Tsunami travel with long wavelengths over great distances. All circum-Pacific shorelines are susceptible Ocean-bottom recorders transmit data to surface buoys Surface buoys relay data to satellites

Brittle Failure

When strain exceeds the "elastic limit" of the rock the rock "breaks" along the fault.

Intensity of Earthquake

a measure of the effects an earthquake produces (on both structures and people) Measure of ground shaking during an earthquake Depends on 1) Distance from epicenter 2) Geologic material that is being shaken An earthquake has many intensities

Earthquake

a trembling or shaking of the ground caused by the sudden release of energy stored in the rocks beneath Earth's surface Tectonic forces within the Earth produce stresses on rocks that eventually exceed their elastic limits, resulting in brittle failure

Elastic rebound theory

earthquakes are a sudden release of strain progressively stored in rocks that bend until they finally break and move along a fault