GSU GEOL 1122K - Study Guide Chapter 8 (15 pages)

Essentials of Geology 4th edition by Stephen Marshak 2013 W W Norton Company Chapter 8 A Violent Pulse Earthquakes 1 A Violent Pulse Earthquakes 2 Introduction Earth shaking caused by a rapid release of energy Energy moves outward as an expanding sphere of waves This waveform energy can be measured around the globe Earthquakes are common on this planet They occur every day More than a million detectable earthquakes per year Most earthquakes result from tectonic plate motion 3 Introduction Most earthquakes are small Large earthquakes however Destroy buildings and kill people 3 5 million deaths in the last 2 000 years Several hundred per year 4 Introduction Most earthquake damage is due to ground shaking Earthquakes also spawn devastating tsunamis December 26 2004 Indian Ocean tsunami March 11 2011 eastern coast of Japan What Causes Earthquakes Seismicity earthquake activity occurs due to Sudden motion along a newly formed crustal fault Sudden slip along an existing fault A sudden change in mineral structure Movement of magma in a volcano Volcanic eruption Giant landslides Meteorite impacts Nuclear detonations Fault slip is the most PowerPoint slides prepared by Rick Oches Professor of Geology Environmental Sciences Bentley University Waltham Massachusetts common cause 5 1 6 What Causes Earthquakes Hypocenter focus the place were fault slip occurs Usually occurs on a fault surface Movement of magma in a volcano Volcanic eruption Giant landslides Meteorite impacts Nuclear detonations Essentials of Geology 4th edition by Stephen Marshak Fault slip is the most 2013 W W Norton Company common cause Chapter 8 A Violent Pulse Earthquakes 6 What Causes Earthquakes Hypocenter focus the place were fault slip occurs Usually occurs on a fault surface Earthquake waves expand outward from the hypocenter Epicenter land surface right above the hypocenter Maps often portray the location of epicenters 7 Faults in the Crust Faults are crustal breaks where movement occurs Displacement is a measure of movement Fault trace is the ground surface expression of a fault On a sloping fault crustal blocks are classified as Footwall block below the fault Hanging wall block above the fault 8 Faults in the Crust The fault type is based on relative block motion Normal fault The hanging wall moves down relative to the footwall Results from extension pull apart or stretching 9 Faults in the Crust The fault type is based on relative block motion Reverse fault The hanging wall moves up relative to the footwall Results from compression squeezing or shortening The slope dip of fault is steep 10 Faults in the Crust The fault type is based on relative block motion Thrust fault A special kind of reverse fault The slope dip of fault surface is much less steep Common fault type in compressional mountain belts PowerPoint slides prepared by Rick Oches Professor of Geology Environmental Sciences Bentley University Waltham Massachusetts 11 Faults in the Crust The fault type is based on relative block motion Strike slip fault One block slides laterally past the other block There is no vertical motion across the fault 2 Faults in the Crust The fault type is based on relative block motion Thrust fault A special kind of reverse fault th edition 4The slope dip of fault surface is much less steep Chapter 8 Essentials of Geology A Violent Pulse Earthquakes by Stephen Marshak Common fault type in compressional mountain belts 2013 W W Norton Company 10 11 Faults in the Crust The fault type is based on relative block motion Strike slip fault One block slides laterally past the other block There is no vertical motion across the fault The fault surface however is nearly vertical 12 Faults in the Crust Displacement the amount of movement across a fault During earthquakes fault blocks move Displacement also called offset is shown by markers Displacement is cumulative over time 13 Faults in the Crust Faults are found in many places in the crust Active faults ongoing stresses produce motion Inactive faults motion occurred in the geologic past A fault trace shows the fault intersecting the ground Displacement at the land surface creates a fault scarp Not all faults reach the surface Blind faults are invisible 14 Generating Earthquake Energy Earthquakes occur as the result of fault motion Energy creating earthquakes originates when Rocks break to form a new fault or A preexisting fault is reactivated Once created a fault remains a zone of weakness Generating Earthquake Energy Tectonic forces add stress push pull or shear to rock The rock bends slightly without breaking elastic Continued stress causes cracks to develop and grow PowerPoint slides prepared by Rick Oches Professor of Geology Environmental Sciences Bentley University Waltham Massachusetts Eventually cracking progresses to the point of failure Stored elastic energy is released at once creating a fault 15 16 Generating Earthquake Energy Rocks slide past one another along a fault Fault motion cannot occur forever 3 Once created a fault remains a zone of weakness Generating Earthquake Energy Tectonic forces add stress push pull or shear to rock The rock bends slightly without breaking elastic Chapter 8 Essentials of Geology 4th edition A Violent Pulse Earthquakes by Stephen Marshak Continued stress causes cracks to develop and grow 2013 W W Norton Company Eventually cracking progresses to the point of failure Stored elastic energy is released at once creating a fault 15 16 Generating Earthquake Energy Rocks slide past one another along a fault Fault motion cannot occur forever Fault motion is arrested by friction Friction is the force that resists sliding on a surface Friction is due to bumps along the fault 17 Generating Earthquake Energy Slip on a preexisting fault causes earthquakes Faults are weaker than surrounding crust Over time stress builds up leading to slip along the fault This behavior is termed stick slip behavior Stick friction prevents motion Slip friction is briefly overwhelmed by motion 18 Generating Earthquake Energy Elastic rebound theory Rocks bend elastically due to accumulated stresses Rock snaps back after slip along fault releases stress 19 Generating Earthquake Energy A major earthquake may be preceded by foreshocks Smaller tremors indicating crack development in rock May warn of an impending large earthquake Aftershocks usually follow a large earthquake May occur for weeks or years afterward Amount of Slip on Faults How much does a fault slip during an