An earthquake is caused by the sudden movement of parts of the Earth's crust and results in shaking and trembling of the ground. There are over one million earthquakes each year, with around twenty having the potential to cause severe damage. Earthquakes are caused by faults in the Earth's crust, such as the San Andreas Fault in California where the land west of the fault moves north and the land east moves south. Underwater earthquakes can generate destructive tsunamis. The focus is the depth below the surface where the earthquake occurs, and the epicenter is directly above on the surface. Seismographs detect seismic waves that travel out from the epicenter, including fast P waves and slower S waves.
13. S waves
Slower than P waves
Travels through solids but
not liquids and gases
Speeds up in denser
material
Movement is side to side
14.
S waves: do "the wave" as they would
in a football stadium, raising their arms
and lowering them as the next person
raises his or hers.
P Waves: gently grip each other's
shoulders and having one pull or push
the next person in line.
16. Seismograph – is an instrument
that detects and measures
seismic waves
Seismologist – scientists who
study earthquakes, can
determine the strength of them
