What Is An Earthquake?
•A wave-like Sudden vibration or trembling in the Earth
•The motion is caused by the quick release of slowly
accumulated energy in the form of seismic waves.
•Travels away from a point of sudden energy release
•Focus is the point of sudden energy release
•Epicenter is located at the earth’s surface immediately above
the focus
•Earthquakes are a form of wave energy that is transferred through
bedrock, (Bedrock is a Rock at or near the Earth's surface that is solid
and relatively unweathered).
• Motion is transmitted from the point of sudden energy release, the
earthquake focus, as spherical seismic waves that travel in all directions
outward (Figure 1).
•The point on the Earth's surface directly above the focus is termed the
epicenter.
•Two different types of seismic waves have been described by
geologists: body waves and surface waves.
•Body waves are seismic waves that travel through the
lithosphere. Two kinds of body waves exist: P-waves and Swaves.
• Both of these waves produce a sharp jolt or shaking.
•P-waves or primary waves are formed by the alternate
expansion and contraction of bedrock.
• P-waves also have the ability to travel through solid, liquid, and
gaseous materials.
•When some P-waves move from the ground to the lower
atmosphere, the sound wave that is produced can sometimes be
heard by humans and animals.
•How Are Earthquakes Measured?
•Richter scale: measures the magnitude of energy released at
the focus
•Open-ended scale - logarithmic increase
•Richter 3 to Richter 4 is 30 X
•Modified Mercalli scale: measures the intensity of an
earthquake according to damage
• observed Scale I-XII
•How are Seismic Waves Recorded?
•Seismograph Record , Seismic Waves
What Causes Earthquakes?
•Most earthquakes are caused by faults
•Volcanoes
•Atomic explosions
Where Do Earthquakes Occur?
•Most earthquakes occur in linear belts
•80% occur around the Pacific Ocean Basin
•Plate boundaries
Global Distribution of Magnitude 4.5+ Earthquake Activity, 1990-1995
Divergent Boundaries / Rift Zones
volcanic activity, shallow focus earthquakes
block faulting (normal faults)
Convergent Boundaries, subduction zones
deep ocean trenches, volcanic arcs: continental arcs & island arcs
shallow, intermediate & deep focus earthquakes,
folding & reverse faults
continental collisions
SUMMARY
Scientists now have a fairly good understanding of how the plates move and
how such movements relate to earthquake activity.
•Most movement occurs along narrow zones between plates where the results
of plate-tectonic forces are most evident.
•continental drift evolved into the theory of plate tectonics
•earthquakes & volcanic activity, mountain building and Folding and Faulting
occurs along tectonic plate boundaries.
•Divergent boundaries/constructive boundaries -- where new crust is generated
as the plates pull away from each other. Rift Zones.
•Convergent boundaries -- where crust is destroyed as one plate dives under
another.
•convergent boundaries: subduction zones and continental collisions, ocean to
ocean convergence
•Transform boundaries -- where crust is neither produced nor destroyed as the
plates slide horizontally past each other.