earthquakes. what is an earthquake? an earthquake is the vibration of earth produced by the rapid...
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What is an earthquake?
An earthquake is the vibration of Earth produced by the rapid release of energy
• Energy radiates in all directions from its source, the focus
• Energy moves like waves • Seismographs record the event
Deformation
Deformation is a general term that refers to all changes in the original form and/or size of a rock bodyMost crustal deformation occurs along plate marginsDeformation involves
•Stress—Force applied to a given area
Deformation
How rocks deform•General characteristics of rock deformation
– Elastic deformation—The rock returns to nearly its original size and shape when the stress is removed
– Once the elastic limit (strength) of a rock is surpassed, it either flows (ductile deformation) or fractures (brittle deformation)
Folds
During crustal deformation rocks are often bent into a series of wave-like undulations called foldsCharacteristics of folds
•Most folds result from compressional stresses which shorten and thicken the crust
FoldsCommon types of folds
•Anticline—Upfolded or arched rock layers
•Syncline—Downfolds or troughs of rock layers
•Depending on their orientation, anticlines and synclines can be described as
– Symmetrical, asymmetrical, or recumbent (an overturned fold)
Faults
Faults are fractures in rocks along which appreciable displacement has taken placeSudden movements along faults are the cause of most earthquakesClassified by their relative movement which can be
•Horizontal, vertical, or oblique
Earthquakes and faults • Earthquakes are associated with faults• Motion along faults can be explained by
plate tectonics
Elastic rebound• Mechanism for EQ’s explained by H. Reid
– Rocks on sides of fault are deformed by tectonic forces
– Rocks bend and store elastic energy
– Frictional resistance holding the rocks together is overcome by tectonic forces
Elastic rebound• Earthquake mechanism
– Slips starts at the weakest point (the focus) occurs
– Earthquakes occur as the deformed rock “springs back” to its original shape (elastic rebound)
QuickTime™ and a Cinepak decompressor are needed to see this picture.
Elastic Rebound Movie in Mapview
Aftershocks
The change in stress that follows a mainshock creates smaller earthquakes called aftershocks
The aftershocks“illuminate” the that ruptured in the mainshock
Red dots show location ofaftershocks formed by 3earthquakes in Missouriand Tennessee in 1811/1812
San Andreas: An active earthquake zone
San Andreas is the most studied fault system in the world Displacement occurs along discrete segments 100 to 200 kilometers long
• Most segments slip every 100-200 years producing large earthquakes
• Some portions exhibit slow, gradual displacement known as fault creep
Seismology
Seismometers - instruments that record seismic waves
• Records the movement of Earth in relation to a stationary mass on a rotating drum or magnetic tape
Types of seismic waves • Surface waves
– Complex motion, great destruction – High amplitude and low velocity – Longest periods (interval between
crests) – Termed long, or L waves
Types of seismic waves• Body waves
– Travel through Earth’s interior – Two types based on mode of travel – Primary (P) waves
– Push-pull motion– Travel thru solids, liquids & gases
– Secondary (S) waves– Moves at right angles to their
direction of travel – Travels only through solids
Locating the source of earthquakes
Focus - the place within Earth where earthquake waves originate
Epicenter – location on the surface directly above the focus Epicenter is located using the difference in velocities of P and S waves
Locating the epicenter of an earthquake• Three seismographs needed to locate an
epicenter • Each station determines the time interval
between the arrival of the first P wave and the first S wave at their location
• A travel-time graph then determines each station’s distance to the epicenter
Locating the epicenter of an earthquake• A circle with radius equal to distance to the
epicenter is drawn around each station • The point where all three circles intersect is
the earthquake epicenter
Earthquake belts• 95% of energy released by earthquakes
originates in narrow zones that wind around the Earth
• These zones mark of edges of tectonic plates
Depths of Earthquakes• Earthquakes originate at depths ranging
from 5 to nearly 700 kilometers • Definite patterns exist
– Shallow focus occur along oceanic ridges – Deep earthquakes occur in western
Pacific east of oceanic trenches
Measuring the size of earthquakes
Two measurements describe the size of an earthquake
• Intensity – a measure of earthquake shaking at a given location based on amount of damage
• Magnitude – estimates the amount of energy released by the earthquake
Intensity scales • Modified Mercalli Intensity Scale was
developed using California buildings as its standard
• Drawback is that destruction may not be true measure of earthquakes actual severity
Magnitude scales • Richter magnitude - concept introduced
by Charles Richter in 1935 • Richter scale
– Based on amplitude of largest seismic wave recorded
– Each unit of Richter magnitude corresponds to 10X increase in wave amplitude and 32X increase in energy
Magnitudes scales• Moment magnitude was developed
because Richter magnitude does not closely estimate the size of very large earthquakes
– Derived from the amount of displacement that occurs along a fault and the area of the fault that slips
Earthquake destruction
Amount of structural damage depends on • Intensity and duration of vibrations • Nature of the material upon which the
structure rests (hard rock good, soft bad) • Design of the structure
Tsunamis, or seismic sea waves • Destructive waves called “tidal waves” • Result from “push” of fault block or
undersea landslide on water• In open ocean height is > 1 meter • In shallow coast water wave can be > 30
meters • Very destructive
Can earthquakes be predicted
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
Earthquakes cannot be predicted
Short-range predictions • Currently, no method exists for making
short-range earthquake predictions
Long-range forecasts • Calculates probability of a certain
magnitude earthquake occurring over a given time period