Geography Essay Earthquakes
EARTHQUAKES
Introduction and Terminology
• Vibrations in the earth’s crust that occur when strain in the crust is suddenly released by displacement along a fault line • Focus = point at which an earthquake originates inside the earth. May be many km below surface • Epicentre = the point on the earth’s surface directly above the focus. Strongest shock normally felt here. Most damage usually occurs here. • Seismology = the study of earthquakes – from Greek – seio – to shake • Seismologists record earth tremors and earthquakes on seismographs • These record the passage of seismic waves through the ground • An analysis of the pattern reveals a lot about the underlying rock structures • Primary waves (p) – fastest – 7.5km/sec • Secondary waves (s) – bigger and slower – 4.3km/sec • Time difference between p and s waves reflects distance between seismograph and epicentre of earthquake • If this distance can be calculated for at least 3 widely spaced seismographic sites – precise location of epicentre can be determined
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Earthquakes – Causes and Mechanisms
• Explained in terms of Plate Tectonics and associated movement of fault lines e.g. San Andreas Fault • When movement is restricted – fault is locked and stores up strain energy • This energy is ultimately released as an earthquake • Immediate cause = sudden breaking and displacement of rock • As stress area ruptures – strain is relieved causing sudden ground movement around the fault • Aftershocks occur as fault settles back
Locations
• Major zone = around Pacific – Andes, California, Alaska, Japan, New Zealand • Other main areas of deep earthquakes usually close to crustal boundaries e.g. Great African Rift System
[pic]
Earthquakes – Magnitude and Intensity
1. Magnitude usually expressed using the Richter Scale(Charles Richter-1935) see below 2.
Introduction and Terminology
• Vibrations in the earth’s crust that occur when strain in the crust is suddenly released by displacement along a fault line • Focus = point at which an earthquake originates inside the earth. May be many km below surface • Epicentre = the point on the earth’s surface directly above the focus. Strongest shock normally felt here. Most damage usually occurs here. • Seismology = the study of earthquakes – from Greek – seio – to shake • Seismologists record earth tremors and earthquakes on seismographs • These record the passage of seismic waves through the ground • An analysis of the pattern reveals a lot about the underlying rock structures • Primary waves (p) – fastest – 7.5km/sec • Secondary waves (s) – bigger and slower – 4.3km/sec • Time difference between p and s waves reflects distance between seismograph and epicentre of earthquake • If this distance can be calculated for at least 3 widely spaced seismographic sites – precise location of epicentre can be determined
[pic]
Earthquakes – Causes and Mechanisms
• Explained in terms of Plate Tectonics and associated movement of fault lines e.g. San Andreas Fault • When movement is restricted – fault is locked and stores up strain energy • This energy is ultimately released as an earthquake • Immediate cause = sudden breaking and displacement of rock • As stress area ruptures – strain is relieved causing sudden ground movement around the fault • Aftershocks occur as fault settles back
Locations
• Major zone = around Pacific – Andes, California, Alaska, Japan, New Zealand • Other main areas of deep earthquakes usually close to crustal boundaries e.g. Great African Rift System
[pic]
Earthquakes – Magnitude and Intensity
1. Magnitude usually expressed using the Richter Scale(Charles Richter-1935) see below 2.